ML20042G870
| ML20042G870 | |
| Person / Time | |
|---|---|
| Issue date: | 05/10/1990 |
| From: | NRC OFFICE OF NUCLEAR MATERIAL SAFETY & SAFEGUARDS (NMSS) |
| To: | |
| Shared Package | |
| ML20042G867 | List: |
| References | |
| REF-WM-62 NUDOCS 9005160207 | |
| Download: ML20042G870 (61) | |
Text
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. ;.. e DRAFT TECHXICAL EVALEATIOX REPORT for the PROPOSED REM 3 DIAL ACTICX of the
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1 RIFLE ERANIUM MILL TAILIXGS i
SI"E 6
Division of Low-Level Waste and Decommissioning U.S. Nuclear Regulatory Commission t
) 51 90051o WM PDC
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2 TABLE OF CONTENT Section Page
1.0 INTRODUCTION
6 1.1 E PA S t a nd a r d s................................................
6 1.2 S ite and P roposed Act ion..................................... 6 1.3 Review Process..............................................
11
- 1. 4 TE R O rg a n i z a t i o n............................................
11
- 1. 5 S umma ry of O p en I s s ue s......................................
12 l
2.0 GEOLOGIC STABILITY..............................................
16 2.1 Introduction...............................................
16 2.2 Location...................................................
16 2.3 Geology.......................
............................ 16 2.3.1 Stratigraphic Setting................................
16
- 2. 3.2 St ru c tu ral S etti n g...................................
18
- 2. 3. 3 Geomorph i c Se tti ng...................................
19 2.3.4 Seismicity...........................................
20 2.4 G e o l o g i c S ta b i l i ty......................................... 21 2.4.1 Bedrock Suitability..................................
21 2. 4. 2 G e omo r p h i c S t a b i 1 1 ty................................. 21 2.4.3 Sei smotectoni c Stabi lity.............................
23 2.5 Conclusions................................................
24 3.0 GE0 TECHNICAL STABILITY..........................................
25 3.1 Introduction...............................................
25 3.2 Site and Material Characteriz6 tion.........................
25 3.2.1 Geotechnical Investigation...........................
25
- 3. 2; 2 T e s ti ng P rog r am...................................... 2 6 3.2.3 Groundwate r Condi ti on................................ 26 3. 2. 4 S tra t i g ra p hy......................................... 2 7 3.3 Geotechnical Evaluation....................................
28 3.3.1 Sl ope S ta bility Ev a lua ti on........................... 28 3.3.2 Sett1erient...........................................
28 3.3.3 Liquefaction Potentit1...............................
30 3.3.4 Cover Design.........................................
30 f
3.4 Construction Details........................................
33 3.4.1 Construction Methods and Features....................
33 3.4.2 Testing and Inspection...............................
33 3.5 Conclusion.................................................
33 4.0 SURFACE WATER HYDROLOGY AND EROSION PROTECT 10N..................
35 4.1 Hydrologic Description and Site Conceptual Design...........
35 4.2 Flooding Determinations.....................................
36 4.2.1 Probable Maximum Precipitation.......................
37 1
4.2.2 Infiltration Losses..................................
37
- 4. 2.3 T inic of Concent ra t i on................................ 37 4.2.4 PMP Rainfall Distributions...........................
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4.2.5.1 Top and Side ?. opes.........................
38 4.2.5.2 Interceptor Ditch...........................
38 4.2.5.3 T o e D i t ch...................................
3 9 i
4.3 Water Surf ace Profiles and Channel Velocities.............. 39 l
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- 4. 3.1 To p a nd S i de S10 )es................................. 39 4.3.2 Interceptor Ditc1...................................
39 4.3.3 T o e D i t ch........................................... 3 9
- 4. 4. E ros i on P rotec ti on......................................... 4 0 4.4.1 Top Slopes and Side S1 opes..........................
40 4.4.2 Toe Ditch...........................................
40 4.4.3 I nte rceptor D i t ch................................... 40 4.4.4 Slopes Above T ail i ng s............................... 40
)
4~4.5 Summa ry of R i p ra p - D es i g n............................ 41 4.4.6 Ro c k Du ra b 111 ty...........................
4........ 41 4.5 U p s t re am D am F a i l u re....................................... 4 3 4.6 Co W usions................................................
43 l
5.0 W AT E R RESOURCES PT,0TE CTI ON...................................... 44 1
5.1 I n t rod u c t i o n............................................... 4 4 5.2 Hy dregevlog i c C haracteriz ati on............................. 44 i
5.2.1 Identification of Hydrogeologic Units................
44 5.2.2 Hydraulic and Transport Properties................... 46 5.2.3 Geochemical Conditions and Extent of Contamination... 47 1
5.2.4 Water Use............................................
48 5.3 Conceptual Design Fet.tures to Protect Water Resources...... 49 5.4 Disposal and Control of Residual Radioactive Materials..... 50' 5.4.1 Water Resources Protection Standards For Disposal.... 50 5.4.1.1 Haza rdou s Const ituents........................
50 5.4.1.2 Con centrati on L imi ts.......................... 51
- 5. 4.1.3 Poi nt of Compl iance........................... 51
-5.4.2 Performance Assessment...............................
51 5.4.3 C losure Perf ormance Demonstration..................... 51 5.4.4 Groundwater Monitoring and Corrective Action Plan....
52 i
5.5 Cleanup and Control of Existing Concentration............... 52 5.6 Conclusion.................................................53 6.0 RAD 0N ATT ENUAT ION AND SITE CLE AN-UP.............................
55 6.1 I n t ro du c t i on............................................... 55 6.2 Radon Attenuation..........................................
55 6.2.1 Selection of Parameters..............................
55 6.2.2 Calculation Methcdology and Design Results...........
57 6.3 Site C1ean-up...............................................
57 6.3.1 Radiological Site Characterization...................
57 6.3.2 S tandards Used f or C1ean-up..........................
57 6.3.3 Verif ication of C1ean-up.............................
58 6.4 Conclusions................................................
58 7.0 RE F E RENC ES/C I BL IOGR AP HY......................................... 60
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LIST.0F FIGURES.
Figure-Page
-t 1.1 R i f l e S i t e A re a M a p.............................................. 8 t
1.2 Old Rifle Processing Site........................................
9 1.3 New Rifle Processing S1te.......................................
10
' 2.1 Generalized Geologic Cross-section of the Rifle Aree............
17 3.1 Proposed Des'ign of the Tailings Pile at Estes Gulch.............
29 3.2 Proposed _ Cover Cross-section....................................
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' 1.1 Su ma ry o f O pe n I s su e s..........................................
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5.1 Hyd ra u l i c Co n du c t i v i ty......................................... 46
- 5. 2 L n e a r V e o c ty.................................................. 47 i
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l 5.3 Corrective Action P lan Suma ry.................................. 53
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5 6.1 Parameters Used in Radon Attenuation Analysis...................
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1.0 INTRODUCTION
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!The Rifle site was designated as one of 24 abandoned uranium mill tailings Uranium Mill Tailings Radiation Control Act of 1978 (gy (DOE) under the piles to be remediated by the U.S. Department of Ener UMTRCA) - UMTRCA requires, in part, that NRC concur with DOE's selection of remedial action, such that the remedial action meets appropriate standards promulgated by the U. S. Environmental Protection Agency (EPA). This draft Technical Evaluation Report (TER) documents the NRC staff's review of the DOE preliminary final design and remedial action plan and outlines the resulting open issues.
..a 1.1 EPA Standard r
As, required by UMTRCA, remedial action at the Rifle site must comply with regulations established by the EPA-in 40 CFR Part 192, Subparts A-C.
These-regulations may be summarized as follows:
1.
The disposal site shall be designed to control the tailings and other residual radioactive materials for 1000 years to the ' extent reasonebly j,
achievable and, in any case, for at least 200 years (40 CFR 192.02 (b)).
2.
The dispesci site design shall prevent radon-222 fluxes from resideal radioactive materials to the atmosphere f rom exceeding 20 picoeuries/ square.
meter /second or from increasing the annual average concentration of.
radon-222 in air by more than 0.5 picocuries/ liter (4U CFR 192.02 (b)).
3.
The remedial action shall ensure that radium-226 concentrations in land that is not part of the disaosal site averaged over any area'of 100.
s square meters do not exceed the Jackground level by more.than 5 picocuries/ gram averaged over the first 15 centimeter of. soil.below the surface and 15 picocuries/ gram averaged over any 15 centimeter thick la of soil more than 15 ccntimeters below the land surface (40 CFR 192.12(yera))..
~
On September 3,1985, the U.S. Tenth Circuit Court of Appeals remanded the groundwater standards (40 CFR Part 192,2(a)(2)-(3)) and stipulated that EPA promulgate new groundwater standards.
EPA proposed these standards in the a
forn of revisions to Subpart A-C to CFR Part 192 on September 24, 1987.
The proposed standards consists of two parts; a first part, governing the-l control of any future groundwater contamination that may occur from tailings piles after remedial action,.and a second part, that applies to the clean up of contamination that occurred before the remedial action of the tailings. ;In accordance with UMTRCA Section 106(a)(3), the remediel. action shall comply with the' EPA proposed standards until such time as the final standards are
. promulgated. At that time, DOE has committed to re-evaluate its groundwater protection plan and undertake such action as necessary to ensure that the final EPA standards are met.
- 1. 2 Site and Proposed Action LThe Rifle, Colorado site actually consists of two separate t'ilings sites a
adjacent tc the city of Rifle as shown in Figure 1.1.
The eastern site, i
known as Old Rifle, and the western site, known as New Rifle, are located, respectively.5 uiles southeast and 2 miles southwest of the center of the city of Rifle. The Old Rifle site covers 22 acres, consisting of the 15 acre i
g 7g; 4
-7 tailings pile and the nine acre mill area, including ore storage and milling facilities. The contaminated materials at-the site are approximately 340,000 cubic yards -(cy) of tailings, 168,000 cy of subpile contaminants and 160,000 cy
.of windblown and mill area contaminants.
Figure 1.2 depicts the general features of the Old P,ifle tailings site prior to the initiation of remedial action. The New Rifle site covers 142 acres, consisting of 33 acres of tailings, a mill facility, water retention pond and-two ore storage ponds. The contamination at the New Rifle site consists of approximately 2,415,000 cy of~
tailings, 575,000 cy of subpile contaminants and 684,000 cy of windblown-and mill area contaminants.
Figure 1.3. depicts the general features of the New Rifle tailings site prior to remedial action. The designated disposal site for-
-the Rifle mill tailings is the Estes Gulch site located approximately 6 miles north of the city. Figure 1.1 shows the Old and New Rifle sites and the
-proposed Estes Gulch disposal site.
The remedial action proposed by DOE consists of the following major activities:
i 1.
Relocation and stabilization of contaminated materials from the Old Rifle and New Rifle processing sites to a disposal embankment at Estes Gulch. The disposal' cell will cover approximately 95 acres and will contain approximately 4.1 x 10 cubic yards (cy) of contaminated material.
2.
The stabilized entankment will be constructed partially below the existing ground surface. The excavation for the below-grado portion of the embankment will extend to within a few feet of the bedrock and/or may extend two-to three feet into the highly weathered bedrock of the Wasatch Formation.
3.
A three foot thick radon / infiltration barrier, consisting of a 36-inch-thick layer of compacted silty and sandy clay, the upper 12
-inches of which will be mixed with bentonite, will be constructed over 1
the contaminants.
4.
A 36-inch-thick frost-barrier will be constructed over the radon / infiltration barrier. A six-inch-thick drain layer of relatively coarse material will be constructed between the frost barrier and radon / infiltration barrier. The drain will prevent'the build up of hydaulic head over the radon / infiltration barrier and the drain layer gradation will prevent the migration of the finer materials from the frost and radon barriers into the drain.
5.
The erosion protection for the topslope of the embakment will consist l
of a 12-inch-thick-layer of Type A riprap, with a D-50 minimum of 3.5 and a D-100 maximum of 6 inches. The erosion protection for the embankment sideslopes including the toe ditch and berms will consist of a 12-inch-thick layer of Type B riprap with a D-50 minimum of 5.7 inches and a 0-100 maximum of 10 inches. The entire riprap is underlain by a six-inch-thick drain layer to facilitate drainage of cny surface runoff that may percolate down into it.
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6.
All buildings and structures at the Old Rifle and New Rifle processing j
sites will be demolished.
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posted with appropriate warning signs to discourage human intrusion.
In addition, the site will be surveyed and monitored periodically by a custodial
' agency under a NRC license.
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1.3 Review Process The NRC staff review was performed in-accordance with the Standard Review Plan for UMTRCA Title 1 Mill Tailings Remedial Action Plans (Ref.6 ) and consisted of comprehensive assessments of DOE's proposed preliminary final design and renedial action plan. Staff review of preliminary final data and designs submitted by DOE indicate that there are still open issues as outlined in Section 1.5 and discussed in further detail in Chapters 2 through 6 of this TER. All open issue must be addressed before concurrence with the
- proposed remedial action can be granted by NRC. The NRC will review all appropriate data submitted by DOE in this regard. Upon resolution of the open
-issues, the NRC staff will revise this TER into final form to include evaluations and conclusions with respect to the additional information -
submitted by DOE.
The remedial action information assessed by NRC staff was provided in the following documents:
1.
DOE,1990 " Remedial Action Plan and Site Design for Stabilization of the inactive Uranium Mill Tailings Site at Rifle, Colorado",
UMTRCA-D0E/AL-050506.0000 (Ref.1) 2.
DOE,1989, "Information for Reviewers", Uranium Mill Tailings RemedialActionProject, Rifle, Colorado (Ref.2) 3.
DOE,1989, " Design Calculations", Volumes 1-VI, Uranium Mill TailingsRemedialActionProject, Rifle, Colorado (Ref.3) 4.
00E,1989, "Information for Bidders", Volumes I-IV, Uranium Hill Tailings Remedial Action Project Rifle, Colorado (Ref.4) 5.
DOE,1989, " Subcontract Document", Uranium Mill Tailings Remedial ActionProject,P,1fle, Colorado (Ref.5)
It is DOE's intent that in the future, Remedial Action Inspection Plans (RAIPs) will be submitted with remedial action docuinentation, and the reviews will be performed concurrently. However, this is not the case for.this site, since the documents were already on separate schedules. The inspection plan and procedures will be reviewed separately when the RAIP is submitted by DOE.
-1.4' TER Organization The purpose of this draft Technical Evaluation Report is to document the NRC staff review of DOE's areliminary final remedial action plan for the Rifle processing sites and the designated disposal site, Estes Gulch; and discuss the open issues resulting from this review. The following sections of this
' report have been organized by technical discipline relative to the EPA standards in 40 CFR Part 192, Subparts A-C.
Section 2, 3, and 4 provide the technical basis for the NRC staff's conclusions and identification of open items with respect to the long-terni stability standards in 192.02(a).
Section 5, Water Resources Protection, summarizes the NRC staff's conclusions and open items regarding the adequacy of DOE's compliance demonstration with respect to EPA s
< groundwater protection requirements in 40 CFR Part 192. Section 6 provides the basis for the staff's conclusions and identification of open items with respect to the radon -control standards in 192.02(b) and soil cleanup in 192.22.
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.1.5 Sunnary of Open Items-s :,
The HPsC staff review of the proposed DOE areliminery final design and remedial.
1
- action' plan has identified open issues, w11ch are discussed in more-detail in the following chapters. A brief summary of these open issues is provided in-Table 1.1.,
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-TABLE 1.1 SU MARY OF OPEN ISSUES Open Issues TER Subsect %
.................t.......................................................q;i.....
1)TheRAPisunclearastothepercentageofbentonite 3.3.4 to be mixed with the' radon / infiltration barrier material to achieve the desired permeability. The RAP must either identify a design percentage or percentage range based on testing performed at the same percentage or range of percentage or provide a commitment to perform a test fill l
that will determine the design percentage.
2).The low hydraulic conductivity of the underlying strata 3.3.4/5.2.5 as determined by DOE, results in a potential for buildup of.
perched infiltration within the disposal cell, if. the design permeability of the radon / infiltration barrier is not achieved.. DOE should perform an analysis of the perching g
potential, establish permeability specifications for the radon barrier, and cormit to the performance of field testing of the radon barrier to verify the actual permeability achieved. 00E must also discuss the long term aspects of
' maintaining the difference in cover and foundation hydraulic i
conductivity.-
- 3) DOE must provide adequate monitoring to demonstrate that 5.3.4 in the disposal cell p(erched water will not build u)
" bathtub"effect). This 'may >e accomplis 1ed by water level monitoring within the cell, monitoring of cover infiltration, or other applicable methodology.,
.4) DOE -must demonstrate that by deferring groundwater clean 5.4 up at the Rifle processing sitcs public health and safety will not be affected. DOE should provide the water quality date of wells-(on-site and domestic) at the processing sites to demonstrate that tiley are not currently, nor will be, affected by the existing contamination.
If contamination is confirmed and DOE proposes to defer cleanup, D0E shall provide remedial actions, such as providing alternate sources of water or other institutional measures, to correct the situation.
-5) Radon diffusion coefficient tests have not.been 6.2.1 performed on the bentonite-araended radon barrier material. DOE has indicated in the calculations l-lr i
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Open' Issues TER i
Subsection supportinD the RAP that these tests will be performed at a later date, and RAECOM will be rerun using the resulting' data. DOE'should provide the additional j
analysis.using a measured diffusion coefficient for the cover material with bentonite to demonstrate that j
there is no need to increase the barrier thickness.
- 6) The RAP contains conflicting information regarding 6.3.2 the fate of asbestos and hazardous materials (other than radioactive materials) resulting from the clean-up of the processing sites.
It is unclear
. where DOE intends to dispose of these materials.
Page C-6, Section C.3.3 and page 80, Section 4.4.7 indicate that hazardous materials and asbestos will be sent off the site to approved disposal a reas. Page 66, Section 4.2 indicates that hazardous materials will be sent off the site, but that asbestos
. will-be placed in containers and buried in the tailings cell. Section 02200 of the specifications, page 02200-12
. indicates that asbestos and hazardous materials shall be-laced in the lower lifts of the tailings cell. The RAP p(and specific 6tions) need to be revised to clearly and
. consistently identify what types of hazardous materials are present and what is propcsed for disposal of each type of raaterial.
7)-For uranium contamination, DOE proposes a supplemental 6.3.2 clean-upstandardof35pCi/g(uraniumtotal),-andcites a 1981 NRC staff position (Reference 16)-as recommending
- this value as a level for which no restrictions on burial method were required.
NRC has not recommended this application for clean-up of UMTRA Program processing sites or vicinity properties. The 1981 staff position was not intended to replace clean-up criteria for uranium mill
- tailings; rather, it was developed to be consistent with the EPA standards, which were, at that time, proposed standards and were more stringent than existing standards.
. Further, the 35 pCi/g' criterion for uranium corresponds to
- depleted uranium and is not appropriate for use in th'is case. Should DOE wish to impose a supplemental standard for uranium.that meets the intent of this staff position and is consistent with present EPA standards, the criteria to be used (after clean-up of Ra-226) would be 10 pCi/g o
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15-TABLE 1.1 - Continued
SUMMARY
OF OPEN ISSUES Open Issues TER
. Subsection
........z..............r.......................................................
1 total uranium in the top 15 cm of soil and 30 pCi/g total uranium in subsequent 15 cm layers. However, should DOE i
4 elect-to support the use of 35 pC1/g or another total uranium clean-up standard, then DOE can present justification under 40 CFR 192.21 and 192.22 for use of supplemental standards.
- The RAP ~ discussion on supplemental standards for uranium should be revised to reflect one of these options.
-8) DOE must demonstratt that the uppermost aquifer at the 5.0 Estes Gulch. site is protected in accordance with the EPA standards. As part of this demonstration, DOE must:
a) adequately characterize the hydraulic and transport properties of the uppermost aquifer, 1
b)
- adequately define the background water quality of the uppermost aquif er, a
c) adequately define the background concentration liuits for the uppermost aquifer, d)
-define-the Point of Compliance'for the uppermost L
aquifer, and-e) provide a groundwater monitoring and corrective action-plan for the uppermost aquifer.
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16 2.0 GE0 LOGIC STABILITY
- 2. I' Introductica This section.of the TER documents the staff's review of geologic information for proposed remedial action at the Rifle uranium mill tailings processing sites and the Estes Gulch disposal site. Since 00E has proposed to postpone cleanup:of the groundwater at the Rifle processing sites until the EPA final
' standard are promulgated, the emphasis-in this section is on the
. geologic information relative to the disposal of the uranium inill tailings at the Estes Gulch site. At the time that DOE provides the information for
= cleanup of the Old and New Rifle processing sites, additional information 4,
should be provided regarding their site-specific geology.
Background geologic information for this TER is derived from the Remedial Action Plan (Ref.1), the ancillary documents (Ref. 2 - 5), supplemental information provided during the '
review process, staff site visits, and independent sources, as cited.
2.2 Location The Rifle uranium mill tailings sites are two separate tailings sites adjacent-to the city of Rifle in Garfield County, Colorado. Both the Old Rifle and New Rifle sites are north of the Colorado River, approximately.3 and 2 miles, respectively, from the center of Rifle. The designated disposal site, Estes Gulch, is located 6 miles north of the city of Rifle, 7 miles north of-the Old Rifle tailings site and 9 miles north of the New Rifle tailings site.
2.3 Geology EPA standards listed in 40 CFR 192 do not include generic or site-specific requirements for characterization of geologic conditions at UMTRA project sites..Rather,-40 CFR 192.02 (a) requires control shall be designed to be effective for up to 1,000 years, to the extent achievable, and in any case for at least 200 years. NRC staff has interpreted this standard to mean that cert'ain geologic' conditions must be met in order to have reasonable assurance that the long-term performance objectives will be achieved.
Guidance with regard to these conditions is specified in the Standard Review Plan Guide (SRP)(Ref.7).
2.3.1 Stratigraphic Setting s
DOE characterizeo stratigraphy by refering to published and unpublished geologic literature and maps; reviewing site-specific subsurface geologic data, including logs of exploratory boreholes advanced in the site; and-
-conducting or.iginal field investigations as recommended in SRP Section 2.2.2.1 (Ref.7). -The' Estes Gulch disposal site is located on the western edge of the Piceance Creek Basin.5 miles west of the Estes Gulch tributary to Government Creek..The site lies at the head of a small drainage basin on a dissected southwest toward Government Creek from pediment and alluvial fan surface sloping (Figure 2.1).
the foot of the Grand Hogback monocline The Estes Gulch disposal site ranges in elevation from 5960 to 6200 feet above mean sea level.
The surficiel deposits in the site area consist of alluvial fan deposits of several ages, ridges of coarse pedirnent alluvium, eclian silts and sands, and minor recent debris-flows. These deposits are discussed in detail in Section 2.3.3 Geomorphic Setting.
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no WHITE, RIVER UPLIFT Axis OP PiCEANCE sASIN ESTES GULCH-SITE RIFLE OLDER SEDIMENTS SITE CotoRAco GRAPO HOGBACK
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OLDER SEDIMENTS
'lLLUSTRATIVE ONLY. NOT TO SCALE' l
PRECAMBRIAN COMPLEX t
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.i 1c Figure 2.1 Generalized Geologic Cross ection of the Rifle Region i
(Reference 1)
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In th'e' Rifle arid Estes Gulch area the bedrock immediately underlying the sites is the Wasatch Formation. This stratigraphic unit is divided into an upper 1
member, the Shire; a middle member, the Molina; and a lower member, the Atwell Gulch. The variegated claystones and siltstones of-the Shire member are t
approximately 1600 feet-thick at the Rifle processing sites and several i
thousands of feet at the Estes Gulch disposal site. The Molina, underlying the Shire, consists mainly of sandstones and thin, interbedded claystones and siltstones. The approximate thickness of the Holina member is 500 feet.
~ Underlying the Mo. lina is the approximately 600-foot-thick Atwell Gulch member, which is a series of shales and sandstones with thin, discontinuous interbeds of lignite and carbonous shale.
In the_ Estes Gulch disposal' area this strata along the bluff facing Government Creek dips from 10 to 20 degrees to the southwest, while the portion of the terrace that directly underlies the disposal site dips from 65 to 75 degrees southwesterly.
Underlying the early
- tertiary Wasatch Formation is the Ohio Creek Formation which consists of approximately 100 feet of massive conglomeratic sandstones comprised of chert and quartzite pebbles.
The early Tertiary formations lie unconformably upon the Late Cretaceous Mesaverde Group and Mancos Shale which are not of significance to the remedial action.
2.3.2 Structural Setting.
' DOE characterized the region's structural setting by refering to published regional geologic maps, aerial reconnaissance, field observations and mapping of features critical to assuring the long-term stability of the remedial action. These studies were recomended in SRP section 2.2.2.3 (Ref.7)._
The Rifle area is located on the extreme northeastern edge of the Colorado Plateau. The Plateau,_ characterized by broad uplifts and basins, is a structurally unique area in the western' United States in that it has been only moderately deformed in comparison with the surrounding, more tectonically active ard deformed regions during the Laramide orogeny (early Teriary). Each of the. major uplifts are bounded on one side by major monoclines along which most of the Plateau's structural deformation occurred.
- Within the Colorado Plateau, the the Estes Gulch disposal sitt is located on the western edge of the Piceance Basin. The Piceance Basin is an asymmetric structural downwarp, elongated northwest-southeast, and lying between the Uncompaghre Uplift to the south and the White River Uplift to the_ north and east.
l The Grand Hogback monocline marks the boundary between the Piceance Basin and l-the White River Uplift. The pertion of the basin near Rifle is characterized by numerous subparallel northwest-trending antic 11nes and synclines. Several i
northwest-trending normal faults with small displacements are also present in the northeastern part of the. basin. A fault zone, of possible Holocene age,-
L is located approximatcly 40 miles northwest of Rifle (Ref.14). At the Estes L
Gulch site,'a fault zone, revealed by slant drilled coreholes, has been located 500 to 800' feet-downslope of the proposed toe of the disposal site. The fault appears to be a-rupture along a tight fold of the Grand Hogback monocline and DOE-attributes the lack of surface evidence for the fault to the Laramide age of the structure. A more detailed discussion of faulting is provided in TER Section 2.4.3.
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2.3.3 Geomorphic Setting DOE characterized the region's physiography by referring to published literature and topogra ahic maps, as recommended in SRP Section 2.2.12.2 (Ref.7). Site geomorpiic conditions were characterized by aerial
' photographic interpretation and field observations., Rifle is located n
on the northeastern edge of the Colorado Plateau physiographic province, near its boundary with the Southern Rocky Mountains Province.
In the Rifle area, the Colorado Plateau is divided into the Canyonlands section to the. south and the Uinta Basin section to the north. These two. sections are separated from-one another by the Book Cliffs, a prominent topographic escarpment.
The Grand Hogback monocline marks the western boundary of the Southern Rocky Mountain physiographic province. The steeply tilted hogback forms a drainage barrier between the Colorado Plateau and the steep canyon and mountain region northeast of the White River Plateau. The Tstes Gulch site lies on a gently sloping pediment within the hood of a drainage basin extending northward from i..
the Government Creek Valley, a tributary to the Colorado River, and the Grand Hogback.
Surficial deposits within the Estes Gulch site are mainly the result of the processes of pediment formation, slope movement, erosion and alluviation
.during the Pleistocene and Holocene. At the site, a central flat surface of younger alluvial surface deposits lie within an erosional valley between the ridges of pediment alluvium. This central area consists of alluvial fan deposits of several ages, eolian silts and sands, and minor recent debris
' flows.
The fine-grained fan deposits consist primarily of fine-grained solian silt and sand. The trenches on the site also reveal a varying thickness of clayey eolian deposits mixed with sandy to gravelly fan deposits and alluvium.
The clayey deposits' vary in thickness from 5 feet at the northern end of the site to 35 feet near the south end of the site. Sandy alluvium layers about five feet thick occur near the base of the eclian deposits. At the far north end of the site the Wasatch Formation is overlain by a thin layer of sandy alluvium and possibly some minor landslide deposits. The thickest eclian and
' alluvial deposits are within the narrow wash passing through the center of the site.
The ridges on the edge of the disposal area are pediment alluvism consisting of clay, silt, and subrounded to angular cobbles. A slide debris up consisting of angular cobbles and large boulders of sandstone rest on the ridge tops of the pediment alluvium deposits on the west and east sides of the site. This bouldery debris forms a moderately resistant cap over the peridment alluvium ten to fifteen feet thick.
Landslide and debris flow deposits occur within the area of the Estes Gulch site, but DOE has stated that they do not affect the proposed tailings site L
area.
Further discussion of these processes is provided in TER Section 2.4.2.
Gully erosion of the older alluvial fan surf ace is the on-going geomorphic process in the site area. A network of gullies incise a three to five foot thickness of eclian silty sand, an underlying buriec soil, and sandy to gravelly alluvial and co11uvic1 deposits.
The depth and intensity cf the gullying is greatest along the south end of the site. Actively eroding gullies in this area are up to TC feet wide and 15 feet deep, usually cut down to Wasatch Formation bedrock or the angular sandstone alluvial layer everlying bedrock.
p,
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20 Well developed piping is present throughout the gully system.
Piping holes and cracks occur up to'15 feet from gully embankments and appear to be the main initial form of tributary gully development. Large gullies are also found through the center of the site with near vertical walls and depths to 15 feet.
These gullies are actively migrating headward as evidenced by recent bank
. undermining, piping, and recent exposure of tree roots. The incised gully at the head of Estes Gulch site basin cuts through the pediment alluvium capped bedrock ridge.
4 2.3.4" Seismicity DOE characterized the regional seismicity by obtaining earthquake data bases -
- provided by the National Oceanographic and Atmospheric Administration-(NOAA),
- employing methods suggested in SRP. Section 2.2.2.3 (Ref.7)gnitudes, and by by_ applying accepted techniques to determine earthquake ma for calculating.
peak horizontal ground acceleration generated by a design basis event.
The Rifle processing sites and the Estes Guich disposal site are located in the relatively stable Colorado Plateau, approximately 30 km northeast of its 1
c boundary with the Western Mountain Province. Historical and instrumental seismic events have been concentrated along the boundary zones of the Plateau, specifically, the Basin and Range, including the intermountain i
seismic belt 150 miles west of the Rifle sites, the Rio Grande Rif t, the Wyoming Basin' and the Western Mountain Province.(Ref.1). Of the 77 earthquake epicenters compiled within 200 km of the site, approximately 15'were artificially-induced through mining, two were associated with the stable interior of the Plateau, seven were associated with the boundary zone closest to the site (Western Mountain Province), and-the remainder were associat' d with e
the more: active and distant boundary zones. Most of the major structrual and tectonic features of the site region date to Laramide time and are considered inactive in-the present tectonic regime.
DOE's analysis of the-potential earthquake magnitude for the interior Colorado Plateau,includeddeterminationofboththeMaximumEarthquake(ME)andthe FloatingEarthquake(FE)fortheregion. Due to the scarcity of earthquake data for the Colorado Plateau Interior, the data collected is actually E
representative of the boundary zones. The seismic records for the Colorado Plateau suggest the NE value could be between 6.2 and 6.8 (Ref.1). The 3
average, 6.5, is the value adopted by previous authors (Ref.13) and appears very ' conservative considering that events of magnitude 5.0 or greater have L
been scarce on the plateau and its border zones.
The earthquake epicentral maps of the Colorado Plateau region show a M
heightened level of seismic activity possibly coinciding with the border zone along the contact with the Western Mountain Province. DOE determined the ME n
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. magnitude for the Western Mountain Province also to be 6.5.
The maximum L
horizontal acceleration in rock expected at the site area, frora a possible NE l
event having a magnitude of 6.5. occurring within 30 km (19 miles) of the site p
area, is.139 An FE magnitude, resulting from an earthquake unassociated with known tectonic
' structures, is generally less than an ME magnitude for a given seismotectonic province. DOE suggests a range of 5.5 to 5.8 may represent reasonable values of FE magnitude, based on the historical record for the Colorado Plateau, o
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s Because the: range of'ME. magnitudes are higher,. DOE adopted 6.2 as a conservative design event.- Because no capable fault was identified, an FE event of masMtude 6.2 occurring 15 km (9 miles) from the site was chosen as
' the design earthquake. This event would result in a free-field, nonamplified, peak horizontal ground acceleration at the site of 0.219 2.4 Geologic Stability Geologic conditions and processes are characterized to determine the site's ability to meet 40 CFR 192.02(a).
In general, site lithologic, stratigraphic, and structural: conditions are considered for their suitability as a disposal foundation and their potential interaction with tailings leachate and ground l
water. Geomorphic processes are considered for their potential impact upon l
long-term tailings stabilization and isolation. Potential geologic hazards, including seismic shaking, liquefaction, on-site fault rupture, ground collapse, and volcanism are identified for the purpose of assuring-the long-term stability of the disposal cell and success of the remedial. action design.
2.4.1 Bedrock Suitability
~ DOE's proposed remedial-action is influenced mainly by characteristics of
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unconsolidated alluvial deposits at the Estes Gulch disposal site. The bedrock underlying = the alluvial deposits, as described in Section 2.3, a
consists of aaproximately 2500 feet of claystones, siltstones, and sandstones of the Wasatc1 Formation. An unmapped fault, revealed by slant drilled coreholes, has been identified by 00f, 500 to 800 feet downslope of the proposed toe of the disposal site. DOE has drawn the following conclusions regarding this fault; (1) the evidence suggests the fault occurs along the strike of.the Grand Hogback monoclinal fold and trends along its upturned limb, (2) the lack of surface evidence for the f ault is attributed to the Laramide age of the structure; also the erodability of the soft rocks in which the fault occurs differs little from the durability the disturbcd rock in the fault zone, (3) the fault, as well as-the monoclinal fold of the Hogback, is considered to be noncapableinthepresentseismo-tectonicstress-regime,and(4)geologicand geomorphic evidence indicate no potential for surface rupture, specifically due to the existence of undisturbed Quaternary pediment alluvium and alluvial fan deposits of at least 10,000-15,000 years and may be as old as 100,000 a
years. The NRC staff has reviewed the data presented by DOE in the P,AP and concurs with DOE's conclusions regarding non-capability and lack of potential for surface rupture.
The staff concludes that bedrock stratigraphic and structural conditions at the site should have no'effect on the design's ability to meet standards for long term stability of the remedial action.
Section 5.2 provides further discussion of the hydrogeologic characterization of the bedrock units and the 1
fault's potential as a hydrologic pathway at the disposal site.
2.4.2 Geomorphic Stability DOE has determined that the gtorrorphic hazards at the Estes Gulch disposal site are (1) gully erosion and arroyo encroachment, (2) wind erosion, and (3) i possible slope failure on the steep slopes.
The' major geomorphic hazard at the site is the surficial erosion by deeply
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C incised gullies and headward extension of those systems into the area of the proposed tailings pile. As discussed in Section 2.3.3 Geomorphic Setting, the location of the proposed disposal site and~the surficial materials at the site make gully erosion an active site process. Although a precise rate of gully erosion has not been determined for the Estes Gulch site, DOE has determined that-a properly engineered tailings pile will prevent further gully erosion-and assure appropriate surface-water runoff diversion.
In order to divert surface runoff, and prevent gully encroachment and undercutting of the tailings embankment, DOE's' design has encorporated the following design features, as dis:ussed in detail in TER Section 3.0:.
An interceptor ditch will be constructed to divert runoff from about six acres of the uppermost watershed. This ditch will be relatively stee) and deep and no erosion protection will be provided. This ditc1 will be allowed to progressively incise into the alluvium and eventually reach a stable elevation.
An erosion protection cover will be placed on the remaining 14 acres of upland drainage area. This will effectively eliminate all existing and future gullies. The graded area will be crowned slightly to shed part of the runoff away from the tailings embankment top.
j An armored toe ditch at the south end of the pile will collect the runoff from the embanknent surface and adjacent areas and convey the runoff westward. The outlet of this ditch will be founded in bedrock to eliminate or minimize long-term head cutting.
Along the east and west sides of the tailings pile, the embankrent top will be flush with the existing terrain slope of about 7%. The terrain adjacent to'the embankment will be graded to promote sheet-l flow and rock-filled key trenches will be provided at the interfact i
of the abankment and existing terrain.
DOE did not present data on wind speed, duration, or direction for Estes Gulch; however, no eolian dunes are currently present, nor do any occur in the local stratigraphy. DOE indicated that the properly armoured and vegetated I.
tailings pile surface would eliminate any hazard of wind erosion. The NRC staff concurs with DOE's conclusions on the effect of wind erosion.
Although lanuslide and debris flow depcsits occur within the southwest and northeast quarters of the site region, DOE has concluded that they do not j
-affect the area of the proposed stabilized tailings site. A very small, thin debris flow occurs along the southeast facing slope of the pediment alluvium ridge at the east side of Estes Gulch site.
The fine-grained silt, sand, and gravel material in this flow originated in a small, deeply incised gully in the H
L side of the ridge and was carried out onto the flat surface of the alluvial fan. 00E. considers the debris flow to be very localized with continued small movements of parts of the flow during rainstoms.
DOE considers this debris l:
flow to have no significant impact on the proposed tailings pile site.
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Slon failure presently is a small-scale but persistent process at Estes Gulch
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witi bare toil areas being subject to shalluu surface slips of fine-grained L
materials. llinor. debris flows hcve occurred at the mouths of small, steep l
gradient channels in the ridge along the west side of the site. DOE has l
concluded that the emplacement of the stabilized tailings pile surfcce as L
dcsigned would clin,inate the source of these surface failures.
The NRC staff I
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i has reviewed the data and'the proppsed design and concurs.with DOE's conclusion.
The NRC staff, based on the information provided by DOE in the RAP, has reasonable assurance that geomorphic conditions of the site have been adequately characterized and that the remedial actions listed above will mitigate the effects of long-term geomorphic changes.
For a discussion of-rock-size requirements, rock gradation, quanitities, durability and other aspects of erosion protection design details see Section 4.4 of this TER.
2.4.3 Seismotectonic Stability
- Studies by DOE to analyze seismic hazards included searching for a design-basis f ault, selecting of a design earthquake, calculating estimated peak horizontal ground acceleration, recognizing potential on-site fault rupure, and recognizing potential earthquake-induced geologic failures at the site.
4 Delineating faults with recent movenent in the site region consisted of L1 low-sun angle aerial reconnaissance. interpreting aerial photographs in L,
black and-white, color, f alse-color infrared, and LANDSAT imagery, and field reconnaissance mapping of faults within 65 km of the site.
In addition,_the 00E obtained and analyzed NOAA's list of instrumentally and historically recorded earthquake data for the Colorado Plateau and an area of 200 km radius around the Rifle, Colorado.
Epicenters locations within 65 km radius of the site.were plotted on a fault and seismicity base Map (Ref.1; Plate D 3.1).
As~a result of these geologic and seismologic investigations, DOE analysis concluded that all the faults and lineaments within the 65 km radius were not capable. The NRC-staff-has evaluated the data presented by DOE and concurs with the conclusion that there are no capable faults within 65 km.-
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Given a lack of ~ capable faults within 65 km'of the site, DOE-based its evaluation of site seismic hazards on a general appraisal of Colorado Plateau 1
seismotectonics and the available earthquake records.
Because an earthquake of magniture 6.2 can be expected to occur in the Coloredo Plateau (see Section 2.3.4), and because a capable design-basis fault is not identified in the h
site's region, DOE adopted a floating Earthquake of magnitude 6.2-occurring 15 km from the site as the design earthquake. NRC consider this a justified value because:
(a) Table 1 of Algermissen and others (Ref.30) indicates that the maximum magnitude earthquake for the Colorado Plateau (Algermissen's source zone no. 16) is 6.1.
Algermissen's relationship for magnitude as a function of intensity, M=1.3+0.6(1), shows a magnitude 6.1 would be equivalent of a Modified Mercalli Intensity of Vill.
This value is near or slightly above the maximum intensity ever observed in the Colorado Plateau.
It is not overly conservative to assume a somewhat higher magnitude value since the period of perfomance is significantly longer than the historical period.
(b) The neximum magnitude 6.2 of the floating earthquake is equal to the threshold magnitude of 6.0-6.2 at which fault scarps are produced at thc ground surf ace in the western United States.
i (c) DOE's calculations of the Maximum Credible Earthquake f or other l#4TRA Project sites in the Coloraao Plateau are given as 6.2 1
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. The'NRC-staff has reviewed the data presented by DOE in the Rifle RAP and concurs that the peak horizontal acceleration from a 6.2 magnitude earthquake at a distance of 15 km,~using Campbell's 84th percentile values (Ref.27) is.219 Staff; find data inputs and these results to.be reasonable-and
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conservative for DOE's calculation of the seismic coefficient for the site.
Section 3.0 of this.TER provides the calculations and applications of this seismic coefficient and the geotechnical stability of the remedial action design.
2.5 Conclusion Based upon review of the Preliminary Final Remedial Action Plan and Final Design for Review the staff has' reasonable assurance that regional and site geologic conditions have been characterized-adequately to meet 40 CFR Part 192.- Conditions tindering long-term stability of the site have been identified and mitigated by features in the remedial design.
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'y 25 3.0 GE0 TECHNICAL STABILITY:
'3.1 Introduction w
This section presents the NRC staff review of the geotechnical engineering
' aspects. of the proposed remedial actions-at the Rifle site and primarily consists of evaluations'of the processing and disposal site. characterization,
, the stability aspects of the proposed stabilized tailings cell, and the cover
-design. The staff. review of the related geologic, geomorphic, and seismic aspects of the site is presented in Section 2.0 of this report. The staff review of the groundwater conditions at this site is presented in Section 5.0 of this report.
4 3.2 Site and Material Characterization 3.2.1 Geotechnical Investigations 3
Th'e geotechnical investigations for the Rifle project include investigations of.
the Old Rifle tailings pile, the New Rific tailings pile, the proposed disposal site at Estes Gulch, and potential borrow sites for earthwork materials.
The tailings piles of the Old and New F,1fle sites have been characterized by several investigators, including. Mountain States Research and Development}, and
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i Colorado State University,.the UNTRA Technical Assistance Contractor (TAC the UMTRA Remedial Action Contractor (RAC).
In all, 191 borings and 37 test pits, for which logs are available, have been drilled or excavated on the Old and New Rifle tailings piles. The multiple investigations used both solid-stem and hollow-stem augers-to advance the boreholes. Samples were obtained using Shelby tubes and ring-lined split-barrel samplers.
Blow counts were recorded
-for most of the split-barrel sampling.
' Foundation soils at Estes Gulch have been characterized by standard geotechnical investigation procedures including hollow-stem auger borings and test pit excavations.
Field work was, performed by local.drillins and excavation companies under the supervision of the TAC or RAC field engineers and/or geologists. Samples were obtained from the 40 borings using ring-lined,
't split-barrel sampling)(Standard Penetration Tests), and thin-walled Shelby tube sampling (undisturbed ; The 25 test pits were excavated to delineate soil types and to obtain bull' samples that were tested to evaluate the suitability of the soil as radon cover material. The pits were advanced to about 12 feet using a tractur-mounted backhoe.
Since radon cover and f rost barrier materials are expected to be obtained from the Estes Gulch excavation, borrow sites are proposed only as sources of filter and erosion prctection materials. Several sand / gravel and rock quarries within a 50-mile radius of the Estes Gulch site have been identified as having acceptable-quality materials in adequate quantities for these uses.
The. staff has reviewed the details of the borings and test pits as well as the scope of the overall geotechnical exploration prcgrm. The staff concludes that the geotechnical investigations conducted at the Rifle processing and disposal sites have adequately established the stret: graphy and soil conditions to support assessment of the geotechnical stability of the proposed remedial l
a ctior..
Further, the geotechnical explorations are in general conformance with applicable provisions of Chapter 2 of the HEC Standard Revicw Plan (SRP)
(Ref.7).
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26 3.2.2, Testing Program The' NRC staff has. reviewed the geotechnical engineering material. testing program for the earthen materials associated with the remedial action at the-Estes Gulch (disposal site) gram has determined the properties of the in-situ Rifle site. The tes',ing pro foundation soil, the remolded foundation soil for the radon barrier.and frost barrier layers, the tailings materials, the subpile materials'under the tailings, and the offpile/ windblown materials.
cThe in-situ foundation soils'at the disposal site were tested for standard Lphysical properties for-use in soil classification and as a basis for other.
testing and' analyses. These tests included particle size distribution, 1
- Atterberg limits, specific gravity, and moisture content and dry density in the
. natural state.
In addition, hydraulic conductivity tests for see) age analysis,
-consolidation-tests for settlement. calculation, and shear strengt1 tests.for.
slope stability-analysis were conducted.
Excavated foundation soils at the Estes Gulch site are planned to be used for f
the' radon barrier and frost barrier layers in the cover design. Compaction characteristics (optimum moisture content and maximum dry density) of the radon barrier material were'obtained by performing standard proctor tests on bulk -
test pit samples. Samples of the foundation soils were remolded to expected field conditions and were tested for permeability, capillary moisture relationships, shear strength, and compressibility.
In addition, the radon
. barrier soils were tested for erodability by the crumb test, the double hydrometer test, and the pinhole test.
Classification tests (gradation, Atterberg limits, and specific gravity) were performed on the.in-situ tailings materials to aid in the determination of the distribution of sands, sand-slime niixtures, and slimes within each tailings pile.
In-situ nioistures and densities were also determined in order to assess the amount of moisture conditioning necessary and the amount of shrinkage that
.will occur. Standard aroctor tests were run on the tailings materials to determine compaction ciaracteristics, and then consolidation tests, strength tests, and hydraulic conductivity tests were performed on appropriately remolded samples.
Classification tests, standard proctor tests, strength tests, and permeability tests were also performed on samples of:the offpile/ windblown and subpile materials at the old and new Rifle processing sites.
_ Based on a review of the geotechnical engineering testing program for the l
L proposed Rifle rerredial action, the NRC staff concludes that the ' scope of the l-
- program is appropriate for support of the necessary engineering analyses and is L
_in general agreement with the Standard Review Plan.
3.2.3. Groundwater Conditions The alluvium at the Old Rifle site is approximately 20 feet thick, with depths tu groundwater ranging from 2 to 12 feet below the land surface. Groundwater L
levels-in the alluvium fluctuate more than seven feet during the year depending
's on the stage height of the Colorado River. The alluvium at the New Rifle site L
is 25 to 30 feet thick, and depths to groundwater range from 5 to 10 feet.
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' Groundwater levels fluctuate approximately five feet during the year. The L
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alluvial potentiometric surface at New Rifle does not extend into the tailings pile during high river stage, as it does at Old Rifle, but remains two to three feet below the pile.
Dewatering is proposed to facilitate excavation at both the processing sites.
A slurry wall will be constructed at the Old Rifle processing site to reduce the amount of Colorado River water that will seep into the excavation. After:
excavation, drying of= the tailings may be necessary prior to their transport to-the Estes Gulch disposal site. Groundwater exists beneatn the Estes Gulch site in fractures, joints, and sandstone strata of the Wasatch formation. A detailed discussion of the groundwater at the Estes Gulch disposal site is provided in Section 5.0 of this TER.
Based on its review, the staff concludes that groundwater conditions have been l
adequately characterized for the purposes of the geotechnical' engineering design and analyses.
.l 3.2.4 Stratigraphy The tailings -in the Old Rifle pile have been altered from their depositional stratigraphy and composition by reclamation activities which took place in 1967.
The major constituent of the pile is silty sand' tailings. Some sand tailings-'are present in the pile along with a purple slime zone (silt) in the southwest corner. The consistency of the tailings is very loose to medium dense. Most of the. tailings are partially saturated, with water content below five percent. The slimes zone near the bottom of the pile is near saturation with water content-as high as 60 percent.
The. tailings at the New-Rifle sitt reflect their depositional history.
-Discharge during operations'resulted in the coarser tailings being near the outer portion of the pile and the slimes being near the center of the pile.
Dbsed on borehole data, approximately 56 percent of the tailings are sands and 44 percent are ' slimes. The slimes are gray to brown, low to high plasticity silt. The tailings consistency ranges from very loose to medium dense.
-Moisture contents within the tailings pile range f rom 0.1 to 87 percent, and
.are controlled primarily by material type, although there is a wide variation within.each type, h
Borings at'the Estes Gulch proposed disposal site have been used to develop cross sections to show the underlying stratigraphic conditions. Surficial soils at the site range from a few feet deep on the ridges surrounding the
-guich to 43 feet deep toward the lower end of-the site. The surficial soils consist of sandy, silty clays interbedded with lenses of silty sands throughout the area mai; ly at depths directly overlying the bedrock. Occasional lenses ofgravelal: were found at various-locations and depths, but mostly close to L
the east and west ridges. The clay material contains some sar.d and silt, is i
classified by the Unified Soil Classification System (Ref.8) as a CL material, is calcareous, and possesses a low to medium plasticity. The silty
- sand is classified as an SM material, is predominantly fine-to medium-grained, 4
and exhibits a low plasticity. The bedrock underlying the surficial soils consists of variegated claystone, siltstone, and fine-grained sandstone of. the Wasatch formation.
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a-28 3.3 Geotechnical Engineering Evaluation 4
3.3.1. Slope Stability Evaluation The NRC staff has reviewed the analysis of slope stability associated with the Rifle disposal cell design, including the selected cross section, the selected design parameters, the methods of analysis, and the results of the analysis.
Since the disposal cell is proposed to be situated within the head of a i
drainage basin, there are no significant-slopes associated with the north, west, and east sides of the cell (see Figure 3.1).
In addition, there are no critical off-pile slopes associated with the disposal site design. Therefore,
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the south slo)e of the disposal cell is the only cross section to be analyzed for slope sta>ility. DOE has ap3ropriately represented the slopes (6.5% top, 20% side), the elevations, and tie material zoning in the cross section used in the analysis, in the cross section, DOE conservatively assumed that the southern embankment of the toe ditch did not exist.
The slope of the embankment and its foundation were modeled as 12 separate layers, each being defined by engineering properties established during the laboratory test program and the exploratory program. Unit weights for each material type were calculated, and design strength parameters for most materials were appropriately selected based on the results of laboratory
' triaxial and direct shear tests. Strength parameters for rock cover la bedrock were assumed using accepted raethods from the literature (Ref.9)yers and Appropriate methods of stability analysis have been employed and the likely adverse conditions to which the slope might be subjected have been addressed.
The analysis methods used were the Bishop's modified method for circular are:
failure, the Morscnstern and Price method for planar surface failure, and the i
infinite slope raethod for sliding surf ace parallel to the slope.
Factors of safety against failure of the slope-for static and seismic loading conditions have been evaluated for both the short-terr;. (end of construction). state and the
-long-term state.
Seismic stability was analyzed by the psk.0-static metnod using horizontal seismic coefficients of 0.11 for end of construction and 0.14-for long-term. The seismic coefficients were calculated in accordance with the guidance of the SRF, and are acceptabic to the staff.
l The calculated minimum factors of ::fety were all sufficiently in excess of the minimum requirement for the sy ocif u r.ondition analyzed. The most critical
-condition was the long-term seis, i loading, as analyzed by the modified Bishop 1
method. This analysis resultec' in a computed factor. of safety of 1.22, L
compared to e required minimum vclue of 1.1.
Therefore, the staff concludes L
that the proposcd design slopes would be stable under short-term and long-term conditions, and this aspect of the design would meet the EPA standard for stability.
1
'3.3.2 Settlement L
Calculations to estimate total settlement occurrin0 after placement of the radon barrier, and to evaluate the possibility of cover cracking due to differential settlement have been performed by DOE, Total settlement was E
L calculated at five profiles along a north-south cross section through the i
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i figure 3.1 - Proposed Design of the tailings pile at Estes Gulch
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-proposed disposal cell. The total settlement is a combination of primar
' consolidation settlement (conservatively assuming saturated conditions) yand
_j
~ secondary compression over the 1000-year design stability period. The maximum i
total settlement over the 60-acre disposal cell was calculated to be a> proximately 11 inches. The maximum differential settlement was found to be a>out 6.5 inches,-resulting in a maximum lateral strain of.076%. The calculatedtensilefailurestrainfortheradonbarriermaterial(Pbl5%)was
.095%.
5, DOE has concluded, and the staff agrees, that total and differential settlement of the materials comprising the proposed disposal cell will not have a significant' adverse effect on-the ability of the cell to meet the stability 1
-standards. Settlement will generally be small due.to the compaction of the materials in the cell and the granular nature of most of the material.
Differential settlement should not cause )onding concerns due to the sloping configuration of the cell. Cracking of tie cover due to settlement should not occur, since the resulting maximum strain is well below the calculated tensile failure strain.
3.3.3 Liquefaction Potential Based on the review of the applicabic geotechnical information, the NRC staff concludes that DOE has adequately assessed the potential for seismically induced liquefaction of the tailings or foundation soils at the Estes Gulch site. For liquefaction to occur, a soil must be loose, noncohesive, and saturated. -As discussed in Section 3.1.3, the groundwater at the site is i
located deep within the bedrock.
The foundation. soils will be mostly excavated. Those soils that will be left in place will be unsaturated and L
cohesive, and thus not susceptible'to liquefaction. The tailings materials themselves as they exist at the processing sites do include some saturated-slimes and sand-slime mixtures.
However, the tailings as placed at the-disposal-site will be mixed, moisture controlled, and_ compacted to a minimum of 90 percent of the raximuu dry density as determined by the ASTM D-698 test.
Therefore,'the tailings pile itself is not considered susceptible to-l:
liquefaction.
i-3.3.4 Cover Design The cover configuration and design have been reviewed and evaluated by the NRC i
staff. The cover consists of a multi-layered unit of natural materials (Figure l
3.2), selectively placed under controlled conditions over the total surface L
area of-the disposal cell. 'Beginning at the top of the contaminated material (i)the five layers proposed for the disposal cell cover. radon / infiltrat l
and proceedin upward l
will be~as fo lows:
L top foot. amended with bentonite, (2} type-D2 drain layer, 6 inches thick, (3) frost barrier..three feet thick, (4/ type-D1 drain layer, 6 inches thick, and l.
(5) types-A&B riprap, one foot thick. This cover system provides a total of 8
?
feet of cover over the contaminated materials, and collecthely is designed to l
limit infiltration of precipitation, protect the pile from catastrophic erosion, and control the release of radon from the cell.
The 3-foot-thick radon / infiltration barrier is designed to limit infiltration through the tailings and to limit raden emissions into the atmosphere. Details of the staff revien; related to infiltration are addressed in Section 5.0, and
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31 t
k LOWER FILTER LAYER 0.5' THICK RADON BARRIER 3.0' THICK INTERCEPTOR DITCH FROST BARRIER 3 0' THICK 6.6%
UPPER FILTER 1REDDING)
LAYSM 0.5' THICK p
/
EROSION PROTECTION f
LAYER 1.0' THICK TAILINGS AND
,/
CONTAMINATED 7
EXISTING OROUND MATERIALS
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a p-BELOW. GRADE EXCAVATION TOE DITCH sent.u Afic -
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L Figure 3.2 - Proposed Cover Cross-section l
C 32 review of the radon attenuation aspects of the cover is presented in Section 6.0.
DOE proposes to construct this layer by using compacted silty and sandy clays obtained from the disposal cell excavation and mixing the top foot with bentonite. Laboratory testing of the bentonite-amended soil to determine properties for design were conducted on samples with 14 yercent bentonite.
In the construction specifications require that tae bentonite content addition, be less than 14 percent by weight. However, the description of the shall not design in the RAP states tsat approximately 10 to 14 percent bentonite must be added to the barrier material to achieve the required permeability. DOE neeris to make it clear in the RAP either that 14 percent is the design value, or that the design percentage will be determined by a test fill.
If a desing range is specified, it should be based on the results of testing performed on the same range.
Vertical stepage will perch on a low bydraulic conductivity geologic unit wherever tht vertical seepage flux is greater than the drainage capacity of the geologic unit.
If this were to occur in the disposal cell, it could cause seeps of tailings leachate to develop on the south slope of the cell and would necessitate re-analysis of stability.
Based on constant head permeability tests conducted on four monitor wells at the Estes Gulch site. DOE has formation is 2 X 10'gerage hydraulic conductivity of the non-weathered Wasatch concluded that the a cm/s. Therefore to 3revent perching of tailings seepage onthelow-permeabilitystrataunderlyIngtiesite,thesteadystate
? X 10'gnal hydraulic conductivity of the radon barrier must be less than operati amended with bentonite will be on the order of 10'flity of the radon barrier as cm/s. DOE has indicated that the permeab cm/s. DOE has not established the permeabilit) difference between the radon barrier and the foundation strata necessary to prevent buildup of perched infiltration. The staff concludes that based on the low hydraulic conductivity of the underlying strata and associated potential for buildup of perched infiltration within the disposal cell, DCE should perform an analyte of the perching potential, establishe perneability specifications for the radon barrier, and commit to the perf oruance of fielo penteability testing of the radon barrier. Further discussions of the potential perching is provided in TER Section 5.2.5.
A lower crain layer is proposed to be constructed betwecn the radon barrier and the_ frost barrier.
Its prNary function is to shed water laterally off the pile, thereby provonting buildup of hydraulic head over the radon barrier, and thus minimizing infiltration. The c.aterial for the lower drain will bc a L
well-graded gravelly sand acquired from a local sand and gravel pit, and as designed, will hav.e a pernebility of from 0.05 to 0.1 cm/s.
The staff has reviewed D0'E's design calculatfons for the lower drain material ar.d concludes that the design graa tion has been appropriately established.
A-3-foot-thick f rost barrier will be placed above the lower drain layer to i
protect'the radon / infiltration harcier from degradation due to freezing and thawing. The material will be obtained from the disposal cell excavation and will consist primarily cf silty end sandy clay. The design thickness of the i
frost barrier was established based on computer analysis of W depth of f rost penetration through the multi-layer cover, using Estes Gulch weather data.
'~
Based on a review of this analysis, the staff concludes that the proposed frost barrier will provide adequate protection against frost heave dan 49e to the radon /infilte ation terrier.
I
a 33 0
A second drain layer will be placed between the frost barrier and the erosion i
protection layer. The drain will be comprised of a clean, well-graded sand and gravel and will serve to facilitate drainage of surface runoff that percolates down into it. This layer will also serve as a filter between the clays of the frost barrier cod the rock of the erosion protection layer. The upper drain as designed will have a permeability of from 0.08 to 0.36 cm/s. The staff has reviewed DOE's design calculations for the upper drain material and concludes that the design gradation has been appropriately established.
The top layer of the cover will consist of one foot of Type A riprap on the top slope and one foot of Type B riprap on the embankment sideslope.
Type A riprap is designed with a d of 3.5 inches, and Type B with a d of 5.7 inches.
Detailsoftheerosid0protectiondesignarediscussedinSection4.0, g
Based on its review of the information on the disposal cell cover design provided in the Remedial Action Plan, the NRC staff has identified two open issues which precludes our determination that the cover has been adequately designeu from a geotechnical engineering perspective to provide the necessary protection for the long term. DOE needs to provide additional assurance regarding the potential for infiltration buildup within the cell through connitment to a field perrteability testing progra:.: for the radon / infiltration barrier.
As a secor.d open issue, DOE needs to clarify the design percentage of bentonitt to be mixed into the upper foot of the radon / infiltration barrier.
3.4 Construction Details 3.4.1 Construction Methods and features The NRC staff has reviewed and evaluated the geotechnical construction criteria and drawings provided in Appendix F to the RAP.
Based on this review, the staff cor.cludes that the plans and drawings clearly convey the proposed remedial action design features.
In addition, the material specifications appropriately reflect the materici characterization and testing, and the placement specifications represent accepted standard practice.
3.4.2 Testing and Inspection In reviewing the quality control requirements provided in the Appendix f specifications, the staff noted certain deviations from the NRC Staff Technical Position on Testing and Inspection (Ref.10).
However, these will be addressed as part of the review of the testing and inspection quality control in the 1
Remedial Action Inspection Plan (RAIP), when submitted by DOE, 3.5 Conclusion Based on a review of the geotechnicci engineering aspects of the Rifle, Colorado proposed remedial action, the staff, with the exception of the issues identified in TER Section 3.2.4, has reasonable assurance that the long-tena stability aspects of the EPA standards (40 CFR Part 192.02(a))'will be met by this design as-selected by DOE.
Geotechnical engir.eering open issues that remain to bc addressed are as follows:
- 1) The RAP is unclear as to the percentage of bentonite to be reixed with the radon / infiltration barrier material to achieve the desireo permeability.
t e;
'4 34 The RAP must either identify a design percentage or percentage range based on testing performed at the same percentage or range of percentage or provide a commitment to perform a test fill that will determine the design percentage.
- 2) The low hydraulic conductivity of the underlying strata as determined by DOE, results-in a potential for buildup of perched infiltration within the disposal cell, if the design permeability of the radon / infiltration l
barrier is not achieved, DOE should perform an analysis of the perching potential, establish permeability specifications for the radon barrier, and commit to the performance of field testing of the radon barrier to verify the actual permeability achieved. DOE must also discuss the long term aspects of maintaining the difference in cover and hydraulic conductivity.
f e
4 a
35 l
4.0 SURFACE WATER HYDR 01.0GY AND EROSION PROTECTION
?
4.1 Hydrologic Description and Site Conceptual Design The existing tailings in the city of Rifle, Colorado will be moved from two locations in the floodplain of the Colorado River to the Estes Gulch site. The Estes Gulch site is located approximately six miles north of Rifle and is situated on an alluvial surface. Small ephemeral streams are located in the imediate site vicinity.
In order to comply with epa standards, which require stability of the tailings for a 1000-year period, DOE proposes to stabilize the tailings in an engineered embankment to protect them from flooding and erosion. The design basis events for protection of the embankment slopes included the probable maximum precipitation (pMp) and the probable maximum flood (pMF) events, both of which are considered to have low probabilities of occurrence during the 1000-year stabilization period.
The tailings will be consolidated into a single pile which will be protected by layers of rock riprap. The slopes of the pile will be variable, ranging from a minimum of about 61 percent on the top slopes to a maximum of 20 percent on the side slopes.
The drainase area north of the tailings disposal area consists of about 20 acres of moderately steep terrain.
Ground cover and vegetation are sparse, and the area is subject to gully crosion.. DOE proposes to divert flood flows from the uppermost 6 acres of this drainage area into an interceptor ditch.
This ditch is located about 1500 feet upstream of the tailings area and will be designed to discharge into an adjacent stream. The ditch will not be protected by riprap and will.have a steep slope to promote erosion toward the adjacent drainage, rather than erosion towa: 1 the tailings disposal area.
The remaining drainage area between the interceptor ditch and the tailings will be protected by riprap.to prevent gully formation and flow concentrations which could adversely affect the stability of the tailings. The riprap is designed to minimize the potential for gully erosion and to encourage the continuation of sheet flow as the flow progresses downstream toward the tailings. The slope t
of the rock cover in this area ranges fron' about 12 to 16 percent, and the
-cover will be slightly crowned to promote lateral dispersion of the flow.
There will be no tailings placed under this rock cover.
Runoff from the disposal area will be directed to a toe ditch which will be constructed at the toe of the 20 percent slope at the extreme downstream (southern) end of the pile. The ditch will have a slose of 0.005 ft/ft and will be trapezoid 61 in cross-section. The ditch will se lined with riprap, and the outlet will be excavated into bedrock to a depth of about 10 feet at its western termination point. The total contributing drainage areas te the toe ditch is about 95 acres. The ditch intercepts runoff from the tailings disposal area and directs the flow peraendicular to its original direction.
Thus, the existing gullies and dry wasles downstream from the disposal area will not receive any runoff from their original upland drainage areas and will not be susceptible to further headward erosion.
c
.c 36
+
4.2 Flooding Determinations The computation of peak flood design discharges for various design features at the site was performed by DOE in several steps. Thesestepsincluded(1) selectionofadesignrainfallevent,(2)determinationofinfiltrationlosses, (3) determination of times of concentration, and (4) determination of appropriate rainfall distributions, corresponding to the computed times of concentration.
Input paraneters were derived from each of t1ese steps and were then used to determine the peak flood discharges to be used in water surface i
profile and velocity modelling and in the final determination of rock size for erosion protection.
A.
Selection of Design Rainfall Event.
1 One of the most disruptive phenomena affecting long-term stability is surface water erosion. DOE has recognized that it is very important to select an
-appropriately conservative rainf all event on which to base the flood protection designs. DOEhasconcluded(Ref.18)andthehkCstaffconcurs(Ref.19)that the selection of a design flood event should not be based on the extrapolation of limited historical flood data, due to the unknown level of accuracy associated with such extrapolations.
Rather, DOE utilized the Probable Maximum Precipitation (PMP)),which is computed by deterministic methods (rather than statistical methods and is based on site-specific hydroneteorological
)
characteristics. The PMP has been defined as the most severe leasonably possible rainfall event that could cccur as a result of a combination of the most severe meteorological conditions occurring over a watershed.
No i
recurrence interval is normally assigned to the PMP; however, DOE and the NRC staff have concluded that the probability of such an event being equalled or exceeded during the 1000-year stability period is small. Therefore, the PMP is considered by the KRC staff to provice an acceptable design basis.
I Prior to deterr.iining the runoff from the drainage basin, the flooding analysis requires the determination of PHP Amounts for tie specific site location.
Techniques for determining the PHP have been developed for the entire United States priraarily by the Nationel Oceanographic and Atmospheric Administration (NOAA) in the f orm of hydrereteorological reports for specific regions. These
. techniques are widely used and piovide straightforward procedurfes with minimal variability. The staff, therefore, concludes that use of these reports to derive PHP estiraates is acceptabic.
D.
Infiltration Losses.
Determination of the peak runoff rate is dependent on the amount of precipitation that infiltrates into the ground during the occurrence of the rainfall.
If the ground is saturated f rom previous rains, very little of the rainfall will infiltrate and most of it will become surface runoff. The loss rate is highly variable, depending on the vegetation and soil characteristics of the watershed.
Typically, all runoff models incorporate a variable runoff coefficient or variable runoff rates. Commonly-used models such as the Rational formula (Ref.
- 14) incorporatc e runoff coefficient (C); a C value of I represents 1001 runoff and no infiltration. Other models such as HEC-1 (Ref. 12) separately compute infiltration losses within a certain reriod of time to arr te at a runoff artcunt during that tirne period.
For this site, DOE used the rational formula;
37 l
DOE's use of this method and a runoff coefficient of I resulted in conservative infiltration estimates and is, therefore, considered to be acceptable, j
C.
Time of Concentration.
The time of concentration is the amount of time required for runoff to reach the outlet of a drainage basin from the raost remote point in that basin. The peak runoff for a given drainage basin is inversely proportional to the time of r-concentration of that basin.
If the time of concentration is computed to be small, the peak discharge will be conservatively large. Times of concentration and/or lag tirnes are typically computed using empirical relationships such as those developed by federal agencies (Ref. 14).
Velocity-based approaches are also used when accurate estimates are needed. Such approaches rely on estit.ates of actual flow velocities to determine the time of concentration of a drainage basin.
Staff review of the methods used by DOE indicate that the methods are appropriate for the staa11.drainagc basins at this site. The methods are velocity-based and produce conservatively low estiinates of the time of concentration, and are, therefore, acceptable, j
D.
r,ainfall Distributions.
Af ter the PMP is determined, it is necessary to determine the rainfall intensities corresponding to shorter times of concentration. A typical PMP value is derived for periods of about one hour.
If tht time of concentration is less than one hour, it is necessary to extrapolate the data presented in the various hydrometeorological reports to shorter time periods. DOE utilized a precedure recor;vnended by NOAA and endorsed by the NRC staff. This )rocedure involves the determination of rainfall amounts as a percentage of tle one-hour PMP, and computes r61nfa11 ancunts for a very short periods of time.
DOE and the NRC staff have concluded that this procedure is conservative.
L l
4.2.1 Probable Maximum Precipitation (PMT)
A N!P rainf611 c'epth of approxirnately 7.4 inches in one hour was used by DOE to compute the PMF for the stra11 drain 6ge areas at the site. This rainfall estiuate has developed Ly DOE using Hydrometeorological Report (HMR) 49 (Ref.
25). The staff perforraed an independent check of the PHP value, based on the procedures given in HMR 49. Based on this check of the rainfall computations, the staff concludes that the PHP was acceptably derived for this site.
4.2.2 Infiltration tosses In computing the peak flow rate for the design of the rock riprap erosion protection, DOE used the rational formula.
In this fornola, the runoff coefficient (C) was assuraed by DOE to Le unity; that is DOE assumed that no infiltration losses would occur.
Based on a review of the con.putations, the staff concluces'that this is & very conservative assumption, as discussed above, and is, therefore, accept 6ble.
4.2.3.
Tir..e of Concentration 1
Within a computerizeo oesigt, procedure (Ref.EL) that estimates riprap sizes, the times of-concentration for the pile top and sides were estimated by s
I 38 computing the actual flow velocity and dividing the length of the design segment by that velocity. Such a velocity-based method is considered by the staff to be appropriate and very precise for estimating times of concentration.
Based on the precision and conservatism associated with such a method, the staff concludes that the tc's have been acceptably derived.
The staff further concludes that the procedures used for computing tc are representative of the small stee) drainage areas present at the site. For ver small drainage areas with very s1 ort times of concentration the use of low tc'y s
i by DOE is considered conservative.
4.2.4 PMP Rainfall Distributions Using the one-hour PMP estimate as discussed above, DOE derived rainfall distributions and intensities using procedures recommended by the NRC staff.
In the detemination of peak flood flows, rainfall ;stensities for durations as short as 2.5 minutes were used. The distribution of FMP rainfall is derived by plotting and extrapolating the following relationships that were recommended by the NRC staff:
Rainfall Duration
'A of 1-hr PMP (min) 2.5 27 5
45 lb 74 30 89 45 95 60 100 Thc staff checked the rainfall amounts for the short durations associated with small drainage basins. Based on a review of this aspect of the flooding 3
determination, the staff concludes that the computed peak rainf all intensity of about 43 inches / hour (corresponding to a tc of 3.9 minutes) is conservative, and therefore, acceptable.
4.2.5 Computation of PMF 4.2.5.1 Top and Side slopes The peak runoff rates for the top and side slopes were estimated using a computerized design procedure for riprep sizing (Ref 25),(which iteratively computes the riprap size required. The rational formula Ref.11) forms the basis for computing the peak sheet flows down the slopes.
Based on a review of the calculations presented, the staff concludes that the peak rates of runoff have been conservatively derived.
4.2.5.2 Interceptor Ditch The PMF for the interceptor ditch was estimated using the rational formula, which provides a standerd method for estimating flood discharges. The PMF for the ditch, which has-a draina p area of about 6 acres, was estimated to be a> proximately 275 cfs.
Based cn staff review of the-computational procedure, t1is (ttimate is considered to be conservative.
39 4.2.5.3 Toe Ditch The pMF for the toe ditch was estimated by dividing the ditch into several segments and cornputing the peak flow in each segment.
Velocity-based methods were used to estimate tc, and the rational formula was used to estimate the peak flow. The design procedure is computerized and is discussed in UMTRA design procedures (Ref.25), which the staff finds acceptable. Review of these i
computations and computer output indicates that the peak flood estimate of j
approximately 1800 cfs at the extreme downstream end of the ditch is acceptable.
1 4.3 Water Surface Profiles and Channel Velocities i-following the determination of the peak flood discharges, it is necessary to determine the resulting water levels, velocities, and shear stresses associated with that discharge. These parameters then provide the basis for the determination of the required riprap size and layer thickness needed to assure stability during the occurrence of the design event.
In determining ripra'; requirements for this site, DOE utilized a computerized design procedure. T1e procedure is iterative in nature and first assumes a Various parameters, such as tirne of concentration, rainf all trial D intensih, size.and flow velocity, are then computed for individual slope segments.
The procedure assumes the occurrence of sheet flow on a one-foot-wide stri) of a givt n slope length. The Safety factors Method is used for slopes less taan 10 percent, and the Stephenson Method is used for slopes greater than 10 porcent. The validity of these two design approaches was verified by the imC l
8 staff through the use of fiume tests at Colorado State University.
It was determined that the selection of an appropriate design procedure dcpends on the magnitudeoftheslope(Ref.17). The staff therefore concludes that the procedures and design approaches used by DOE are acceptable and reficct state-of-the-art methods for designing riprap crosion protection.
4.3.1 Top and Side Slopes Water surface profiles, velocities, and shear stresses used in designing rock-protected slo)es, were computed using computerized UNTRA design procedures. The Ns0 staff chccked DOE's computer outaut to deteririne the accuracy of thc con.putations. B6 sed on this check,11e staff concludes that the estimates are acceptable.
4.3.2 Interceptor Ditch Water surface )rofiles and velocities for the interceptor ditch were developed by DOE using tie flanning Equation (Ref. 31) and sciving for normal depth and velocity. Based on a check of the computations, the staff concludes that the peak velocity estimate has been acceptably derived.
4.3.3 Toe Ditch DOE using a Velocities and shear stresses in the toe ditch were calculated by(Ref.25).
computerized design progren, outlined in UMTRA design procedures The NRC staff checked the results of several computations and the computer output to deterraine the acceptaldlity of the computations. Based on these checks, the staf' concludes tant the tauimum computed velocities and sheer stresses
. have beo. ;ceptably derived.
l-
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7 O
i 40 4.4 Erosion protection i
After the peak flood levels, shear stresses, and velocities are determined, it is necessary to calculate the rock size necessary to withstand these forces.
As previously stated, DOE utilized a computerized, iterative procedure to l
determine rock requirements.
In addition to computing all of the required input parameters, the procedure iterates the D r n.i size until the assumed en size is sufficient to withstand the forces produced; the iterations are necessary because the shear forces vary non-linearly with the rock size, while the resisting force of the rock is generally a linear function of the rock size. These methods have been reviewed by the staff and compared to other design methods, such as those of the U. S. Army Corps of Engineers.
Based on these comparisons, the staff has determined that the methods are appropriate (Also, see Section 4.3, above).
4.4.1 Top Slopes and Side Slopes DrE proposes Type A riprap with a De0 rock size of 3i inches for the top slopes of the pile and Type B riprap with 8 D rock size of about 6 inches for the 0
side slopes of the pile. Both types of riprop will have a layer thickness of 12 inches. The rock sizes for the top slopes and the side slopes were determined using the Safety Factors Method and the Stephenson Method, respectively, as discussed above. The staff reviewed the computer output 4
provided by DOE.
Based on this review and a comparison with the rock size information arovided by Reference 17, the staff concludes that the computed roc < sizes are adequate.
4.4.2 Toe Ditch DOE proposes that Type B riprap with a D n of 6" inches will be placed in the g
toe ditch. The required average rock sitt of 4.0 inches was determined using the Safety Factors Nethod.
By providing Type B rock in the ditch, DOE has satisfied the need to safely protect the ditch frou the flows occurring on the pile side slopes.
In this case, the forces produced on the side slope of the ditch from flows occurring down the slope are greater than the forces produced from flows in the ditch. Based on staff review of the computational procedures and assumptions applied, the calculated riprap size is acceptable.
4.4.3 Interceptor Ditch The interceptor ditch upstream of the tailings area will not be protected with riprap. The ditch will be designed to incise progressively deeper into the In order to promote erosion toward soil and to eventually (stabilize itself.rather than toward the tailings), the ditch will be i
the adjacent drainage constructed with progressively steeper slopes (20 to 40 percent) at the downstream end where it discharges into the adjacent drainage. This concept will ensure that maximum velocities occur at locations where erosion is desired.
l 4.4.4 Slopes Above Tailings 00E proposes that Type A rock will be provided to completely cover approximately 12-14 acres of drainage area above the stabilized tailings. This design effectively eliminates the formation of potential Eullies and flow i
k
41 concentrations and promotes desind sheet flow across the pile top slopes. The required rock size of 2.7 inches was calculated using the Stephenson Method for these~12-16 percent slopes. Thus, Type A rock (used for the slopes which directly cover the tailings), with an average size of 3.5 inches is adequate to protect these slopes.
4.4.5 Sunenary of Riprap Design The design of erosion protection features for the Estes Gulch disposal site is summarized in the sunnary table for several areas, as follows:
1 - Area 1 above tailings, between interceptor ditch and tailings
' 2 - Area 2 above tailings, between interceptor ditch and tailings 3 - Top Slopes 4 - Side Slopes 5 - Toe Ditch i
D Type Nethod Used Area Slope tc (min)
Q (cfs)
(in/hr)50(inches) 1 0.16 4
0.8 43 2.7 A
Stephenson 2
0.12 7
1.1 33 2.5 A
Stephenson 3
0.065 14 1.6 24 3.2 A
Safety Factors 4
0.20 16 1.6 21 5.7 B
Stephenson 5
0.005 18 1800 19 4.0 B
Safety Factors where:
Slope = Slope of the feature to be. protected with rock Tirc of concentration (minutes) tc
=
I Design discharge (cfs/ft), except for toe ditch where Q = total Q
=
discharge in cfs Rainfallintensity(inches /hr) i Required average rock size (inches)
D
=
The Rock type to be used, with A being 3.5 inches and B being 6 inches.
=
4.4.6 Rock Durability The epa. standards require that control of residual radioactive materials be effective for up to 1000 years, to the extent reasonably achievable, and, in any case, for at least 200 years. The previous sections of this report examined the ability of the erosion protection to withstand flooding events reasonably expected to occur in 1000 years.
In this section, rock durability is considered to determine if there is reasonable assurance that the rock itself will survive and remain effective for 1000 years.
Rock durability is defined as the ability of a material to withstand the forces of weatherinc.
Factors that affect rock durability are (1) chemical reactions with. water,((2)saturationtime,(3)temperatureofthewater,(4)scourby sediments, 5) windblown scour, and (6) wetting and drying.
DOE conducted investigations tc identify acceptable sources of rock within a ressenable distence of the site. The suitability of the rock as a protective
[..
o.
42 cover was then assessed by laboratory tests to determine the physical characteristics. The results of these tests were used to classify the rock's quality and to assess the expected long-term performance of the rock. The i
tests included:
i 1.
Petrographic Ensination (ASTM C29S). Petrographic examination of rock is used to determine its physical and chemical properties. The examination establishes if the rock contains chemically unstable minerals or volumetrically unstable materials.
l 2.
Bull Specific Gravity (ASTM C127). The specific gravity of a rock is an indicator of its strength or durability; in general, the higher the specific gravity, the better the quality of the rock.
3.
Absorption (ASTMC127). A low absorption is a desirable property and indicates slow disintegration of the rock by salt action and mineral hydration.
4.
SulfateSoundness(ASTMCBS).
In locations subject to freezing cr exposure to salt water, a low percentage loss is desirabic.
5.
Freeze-Thaw ( AASHTO 103). A low percentage loss is indicctive of
^
resistance to weathering resulting from the crystallization process.
6.
Schmidt Rebound Hammer. This test measures the hardness of a rock and can be used in either the field or the laboratory.
7.
Los Angeles Abrasion (ASTM C131 or C535). This test is a measure of rock's resistance to abrasion.
8.
Tensile Strength (ASTM D3967). This test is an indirect test of a rock's tensile strength.
All samples for testing were taken in accordance with Standard Practices fer Sampling Aggregate (ASTM D75). DOE used a step-by-step procedure for rock durability, in accordance with procedures recouraended by the evaluating (Ref.19),asfollows:
NRC staff Step 1.
Test results from representative urt es were scored on a scale of 0 l
to 10.
Results of 8 to 10 are considered " good"; results of 5 to 8 are considercd " fair"; and results of 0 to 5 are considered " poor".
Step 2.
The score was multiplied by-a weighting factor. The effect of the weighting factor is to focus the scoring on those tests that are the most applicable for the partiolar rock type being tested.
Step 3.
The weighted scores were totaled, divided by the maximum possible score, and multiplied by 100 to determine the rating.
Step 4.
The rock quality scores were then compared to the criteria which determirts its acceptability, es defined in the hRC scoring procedures (Ref.19).
1
43 For the rock to be placed in the ditches and on the pile, gradation and rock durability criteria were presented. DOE has identified a rock source at Glenwood Springs, where rock of acceptable quality can be found. DOE has also identified river gravels at the Com, Frei, and Casey pits, which are also of acceptable quality. The rock sources were evaluated using the NRC scoring criteria (Ref.19).
Following is a partial summary of the rock quality evaluation for typical samples from each potential quarry:
Rock Rock Specific Absorp-Sodium L. A.
Final Source Type Gravity tion Sulfate Abrasion Rating
(%)
(% Loss)
(% Loss)
Glenwood Sprs. Limestone 2.68 0.20 2.1 29.8 81 Casey Pit Mixed Gravel 2.64 0.71 0.6 24.4 88 Corn Pit Mixed Gravel 2.65 0.70 0.6 29.7 87 Frei Pit Hixed Gravel 2.79 0.60 1.6 25.1 91 Based on a comparison of the data with the NRC scoring criteria, we conclude that the rock durability criteria proposed are adequate to assure that rock of acceptable quality will be provided. Any of the rock sources identified appear to be acceptable, assuming that DOE has tested representative samples from each source.
4.5 Upstream Dam F611ures There are no impoundments whose failure could potentially affect the site, 4.6 Conclusions Based on its review of the information submitted by DOE, the staff concludes
-l that the site design will meet EPA requirements as stated in 40 CFR 192 with regard to flood design measures and erosion protection. An adequate hydraulic design has been provided to reasonably assure stability of the contaminated
'l material at Estes Gulch for a period of up to 1,000 years.
i l
l i
n 1.
3
o 44 5.0 WATER RESOURCES PROTECTION 5.1 Introduction The NRC staff has reviewed the Remedial Action Plan (Ref.1) and auxiliary documents for the Rifle, Colorado UMTRA Project site for compliance with EPA's proposed groundwater protection standards in 40 CFR Part 192, Subparts A-C.
The NRC staff perforned the review in accordance with EPA's proposed Groundwater Protection Standards (Ref.12) and relevant portions of the NRC staff's Standard Review Plan for UMTRCA Title ! Mill Tailings Remedial Action Plans (Ref.6).
DOE has concluded that the proposed remedial action complies with the EPA standards because the tailings contaminents will not migrate to the Point of Compliance (P00) in 1000 years. DOE reached tais conclusion by treating an upper saturated zone in the Wasatch Formation as the uppermost aquifer. DOE estimated the horizontal travel time from the edge of the tailings to the P0C in the saturated zone to be over one thousand years and thus concluded that the proposed remedial action plan is in compliance with EPA's groundwater protection standards. DOE also proposes to defer clean up of existing groundwater contamination in the processing sites until EPA promulgates the groundwater clean up standards.
The fiRC staff disagrees with DOE's ap) roach that treats an aquitard as an equifer. The EPA standards require t16t the uppermost aquifer be protected.
Therefore, the NRC staff concluded that DOE's proposed remedial action plan has not been shown to be in compliance with We EPA groundwater protections standards.
Consistent with EPA's groundwater protection standeds, the NRC staff distinguishes between the disposal of residual radicative materials at the disposal site and the cleanup of existirig groundwater cwtamination at the processing sites. The NRC staff cannot acce>t DOE's proposal to defer groundwater cleanup until DOE cemonstrates t1at public health and safety will not be impacted by the existing contamination at Rifle processing sites.
5.2 Hydrogeologic Characterization 5.2.1 Identification of Hydrogeologic Units A.
Procet ing Sites The Old and New Rifle processing sites are affected hydrologically by the two uppermost hydrogeologic units, the floodplain alluvium and the Wasatch forn.a tion. They are described below:
(a) The uppermost hydrogeologic unit is the floodplain alluvium which has a thickness of approximately 20 feet at Old Rifle and between 25 to 30-feet at New Rifle. The alluvium is comprised of relatively pervious silts, sands, gravels and cobbles; it is within the floodplain and is hydraulically connected to the Colorado River. Groundwater in the alluvium is shallow, it is un& r water table condition, and it fluctuates between two to twelve feet below ground surf ace. DOE reported that at times the tailings in thc Olc Rifle site were inundated, but no inundation was
45 observed at New Rifle. Groundwater movement in the alluvium genere11y parallels the direction of river flow and eventually discharges to the river some distance downstream of the site. The discharge and recharge relationship of the alluvial aquifer is complex as it varies with the seasons, the river stages and local conditions. DOE notes that a drainage culvert discharges upgradient of the Old Rifle Site resulting in transient groundwater mounds and reversal of gradients. During spring and early summer when the river stage is high, the alluvium is recharged by the river and discharges to the underlying Wasatch formation. The role is reversed when the river stage recedes. Such reversal of gradicots due to natural and artificial reasons affect the migration of contaminants in the alluvium and the underlying Wasatch formation.
(b) The Wasatch formation underlies the floodplain alluvium and has a thickness of ap3roximately 3,000 feet at Rifle. The formation is comprised of a 1eterogeneous series of interbedded shales, claystones, i
siltstones and sandstones dipping at an angle of five to ten degrees to the west and southwest.
The formation is further divided into three zones: The upper Shire Member is 1600 feet thick near Rifle and consists of variegated claystones, siltstones, and some lenticular sandstones. The middle Molina Member is 500 feet thick and consists arimarily of sandstone with thin, interbedded claystones and siltstones. T1e lower Atwell Gulch
. Member is approximately 600 feet thick and contains a series of shales and sandstones with thin, discontinuous interbeds of lignite and carboniferous shale. Hydrologic 611y, the more transmissive sandstone Molina Member is the uppemost aquifer confined above and below by the less transmissive Shire and Atwell Gulch Members. The NRC staff reviewed the depths of the monitor wells in the Wasstch formation. All wells are less than 200 feet deep and hence only penetrate the upper portion of the Shire Member. The potentiometric surface elevation of the Shire Member is nearly the same as the water table elevation in the alluvium. Groundwater flow directions also follow closely those of the alluvial aquifer, suggesting strong hydraulic connection of the two units. There is a slight downward vertical gradient between the alluvial aquifer and the Wasatch formation at New Rifle, and a slight upward vertical gradient at Old Rifle. The vertical gradients, however, are reversible in response to river stage variations and other local factors.
B.
Disposal Site The Estes Gulch disposal site is located approximately six miles north and 700 feet above Rifle on the southwestern flank of the Grand Hogback monocline. The geologic setting is similar to that at the Rifle processing sites discussed above except that it is outside the alluvial floodplain, and the depth to groundwater is considerably greater.
The site is covered by 5 to 35 foot thickness of unconsolidated, fine-grained pediment alluvium and eolian clay. Much of the unconsolidated materials will be removed to construct the disposal cell covers.
Beneath the alluvium is the Shire Member bedrock of the Wasatch formation.
Based on regional geology, the Shire Member is estirated to be at least 2000 feet thick at Estes Gulch.
It overlies the Molina Member sandstone which, as described previously, is the uppermost aquifer. Actual depth to the uppermost seuifer is undefined as the deepest well at Estes Gulch is only 545 f eet.
46 It is equally difficult to define the depth to groundwater because the Shire Member is very impervious. DCE installed a total of 13 monitor wells and all but four were dry. The dry wells are either too shallow or the screen lengths too short to intercept sufficient saturated materials. The wet wells have screen lengths exceeding 100 feet, but the water recovery rates are extremely j
slow and have not reached equilibrium. Based on unstabilized water levels, DOE estimates the saturated zone to be located at 160 feet below the ground surface, j
As discussed in Section 5.3.5., DOE's groundwater protection analysis treats 1
the upper saturated zone as the uppermost aquifer, although it is, in reality, an aquitard.
Bedding planes beneath the site dip at 65 to 75 degrees and decreases abruptly to 10 to 20 degrees 500 to 800 feet downslope of the proposed toe of the tailings pile. This abrupt change in slope occurs along a fault parallel to the Grand Hogback monocline. DOE states that this fault, along with several minor faults encountered, is filled with clay gouge and does nct appear to be a
,F significant groundwater transport pathway. A constant head permeability test was perfomed at Well 965 which intercepts this f ault and shows that it is nearly water tight.
In a letter of October 3,1909, NRC Technical Assistance Contractor, Pacific Northwest Laboratcries (PNL) (Ref.11), expressed concern about a 50-foot sandstone /siltstone unit that crops out in the ephemeral drainage west of the proposed disposal site. This' unit appears to be connected to tae fault intercepted by Well 965 and it may act as a preferential groundwater pathway.
Based on independent reviews of well logs and permeability test data, the staff concludts there is no evidence of near surface saturation or preferential groundwater pathways. The conclusion is based on the following observations:
a (a) No moist materials were encountered for at least 100 feet beyond.the surficial soil (b)thesurficialsoilhasameanin.situwatercontentofonly 8 percent, (c),all permeability tests indicate the formation is extremely impervious,and(d)nospringsorseepswereobservedorreportedinthe vicinity of the site.
5.2.2 Hydraulic and Transport Properties Based on informetion from DOE, the hydraulic conductivity and linear velocity calculations at the processing and disposal sites are summarized below:
Table 5.1 HYDRAULIC CONDUCTIVITY f t/d (cm/s)
Unit Old Rifle New Rifle Estes Gulch Method Alluvium 200(7.1E.2) 70(2.4E-2)
Pump test Wasatch
.027 (9.5E.6) 46 (1.6E.4)
Slug test Wasatch 6.7E.4 2.4E-7 Core Samples Wasatch 4.5E-3 1.6E-6 Packer tests Wasatch E.0E-5 2.0E-0 Constant head tests L
fa*
47
. Table 5.2 LINEAR VELOCITY Site Gradient Porosity K
Velocity OldRifle(Alluvium) 0.003 0.27 200 ft/d 840 f t/yr OldRifle(Wasatch) 0.003 0 10 0.03 ft/d 0.3 ft/yr NewRifle(Alluvium) 0.003 v.27 70 ft/d 280 ft/yr NewRifle(Wasatch) 0.003 0.15 0.46 ft/d 3 ft/yr EstesGulch(Wasatch) 1.000*
0.20 6E-5 ft/d 0.1 f t/yr
- assumed conservatively The NRC staff defers comment on the calculations for the processing sites because DOE proposes to defer groundwater clean up until EPA promulgates the final clean up standards.
The _NRC staff disagrees with DCE's approach of addressing the EPA standard by treating an equitard as the Popermost aquifer. Therefore, DOE has not adequately characterized the hydraulic and transport proterties of the uppermost aquifer at the Estes Culch site. The rtRC staff considers this an open issue.
5.2.3 Geochemical Conditions And Extent Of Contamination A.
Processing Sites Based on assessment from DOE, the entire 20 to 30 foot thickness of the alluvium beneath-the Old and New Rific processing sites is contaminated. The Wasatch formation is contaminated to a depth of approximately 100 ft but the lateral extent is smaller. The uranium contamination plume in the alluvium covers an area of 1000 by 3000 feet at Old Rifle and 2800 by 7800 feet at New Rifle.
In general, the extent of contamination is more extensive at New Rifle, and the concentrations exceed the EPA MCL by a greater margin. DOE also presented the background water quality data and estimated natural flushing times to remove the contauinants.
The NRC staff defers comment on the geochemistry assessment of the processing sites because DOE proposes to defer groundwater clean up until EPA promulgates the final clean up standards. Because of the dynamic nature of the contamination plumes, DOE must redefine the plumes at a later time when the remediation of the aquifer is to take place.
B.
Disposal Site DOE presented the background water quality data for the Estes Gulch site and proposes concentration limits based on MCLs or backgrcund levels, whichever are higher, for post-disposal surveillance purposts. The follovting naturally occurring inorganic constituents were identified:
arsenic, barium, cadmium, chromium, leed, mercury, molybdenum, nitrate, selenium, silver, uranium, radium 226 and 228 combined, gross alpha, antimony, beryllium, cobalt, ccpper, cyanide, nickel, thallium, tin, vanadium and zinc.
n
48 Since NRC cannot agree with DOE's use of an aquitard as the uppermost aquifer, the water quality data provided for the aquitard is not relevant to the establishment of background. In order to comply with the EPA standard, DOE must define the background water quality of t1e uppermost aquifer. The NRC considers this an open issue, 5.2.4 Water Use A.
Processing Sites DOE. indicates the Lolorado River water is the primary source of drinking water at Rifle. A municipal water intake of the City of Rifle is located approximately 0.5 miles upstream of the Old Rifle site. DOE does not identify tie location of the next intake below Rifle and thus the potential impact on the next downstream user from the processing sites cannot be assessed. The NRC staff reviewed the surf ace water quality data and considers there is no indication of contamination at the river; however, due to high variability of surface water quality, further testing under low flow conditions is needed to confirm it. One special note is that, despite discharge of contaminated
)
groundwater, the dilution effect of the river flow will likely reduce the contaminant concentrations to non-detection levels.
Outside the city limits, residents obtain water by domestic wells. There are 26 to 39 domestic wells, respectively, within 2 miles from New and Old Rifle.
DOE has not provided hydrologic or water quality data from these wells.
It is not known whether these wells were completed in the alluvium or in the Wasatch formation, and whether the capture zones have been or will be affected by the contamination plumes. The NRC staff overlaid the uranium plume maps onto the domestic wells location map in Figure D.7.32 and observed that two wells at New Rifle, and one well at Old Rifle are very close to, if not alrendy within, the contaminutich plumes. As discussed in TER Section 5.4, the NRC staff considers this an an open issue. Therefore, DOE should provide hydrologic and water quality data of these wells to demonstrate that they are not currently, nor will be, affected by the existing contamination.
If contamination is confirmed. DOE shall provide remedial actions, such as providing alternate sources of we.ter or other institutional measures, to correct the situation.
l B.
Disposal Site
(
There are 8 deniestic wells within 2 miles from the Estes Gulch disposal site.
Three wells are screened ir. the Wasatch formation approximately 0.75 miles west of the disposal site. DOE states that these wells are completed in a different hydrogeologic stratum different from the disposal site.
Recharge to these L
wells is stated to be frora local rainfall and creek infiltration in limited -
localized sandstone strata. Water quality data provided by DOE show that the l
L domestic well water is of much higher quality, and is significantly different from the groundwater beneath the disposal site. By independent review of l
the geology and water quality data, the HRC staff concludes that there is no hydraulic connection between the groundwater beneath the disposal site and the nearby dorrestic wells.
l Based on population forecast and the fact that surf ace water is readily available, DOE forecests there will be limited increase in groundwater usage in the riext 50 years at both the processing and disposal sites.
6
49 5.3 Conceptual Design Features to Protect Water Resources DOE proposes to relocate the tailings and contaminated materials from the Rifle processing sites for disposal at the Estes Gulch site.
Construction dewatering will be required at the Old and New Rifle sites to excavate the contaminated soil below the water table. Additionally, a slurry trench will be constructed along the river and upgradient of the Old Rifle site to limit infiltration from the river. At the completion of the excavation, windows will be
>unched in the slurry wall to restore natural flushing. At the disposal site, t1e surficial unconsolidated materials will be excavated to bedrock to provide low permeability materials to construct the radon and frost protection covers. The foundation will then be prepared to receive the tailings and contaminated materials. The tailings and contaminated materials will be placed in successive lifts and compacted at near optimum moisture level. Temporary drainage ditches, retention ponds, and decontamination pads will be constructed at all three sites to control surface and construction water during construction.-
Treatment facilities will be provided to treat the contaminated surface, dewatering and construction water prior to discharge.
The cover design of the disposal cell consists of, in ascending order: (a) a 3-foot radon barrier constructed from remolded clay obtained from the foundation excavation. The top one foot is amended with sodiun, bentonite to further reduce the hydraulic conductivity. The design overall saturated hydraulic conductivity is IE-8 cm/s; (b) a 6-inch filter layer with a design hydraulic conductivity of 0.1 cm/s; (c) a 3-foot low permeability frost barrier constructed with the same materials as the radon barrier with design hydraulic conductivity of IE-6 cm/s; (d) a 6-inch upper filter layer with design hydreulic conductivity of 0.4 cm/s; and (f) a one foot riprap layer for erosion protection.
A design censideration that must be addressed is the potential for perching of contaminated water in or beneath the disposal cell. DOE states that the tailings leachate will not perch beneath the disposal cell because the design hydraulic conductivity of the radon barrier is less than the hydraulic conductivity of the underlying bedrock. The design hydraulic conductivity of the radon barrier is 1E-6 cr/s. The hydraulic conductivity of the bedrock averages between IE-6 to IE-7 cm/s in the upper weathered zones, and IE-8 cm/s in the lower unweathered zores.
i Certuse of the great depth to the uppermost aquifer and poor water quality of euting saturated zones in the Wasatch, the NRC staff is more concerned with the potential perching of contaminated groundwater within the disposal cell than within the underlying bedrock, perching within the disposal cell could cause seeps of tailing leachate and instability of the pile. Thus, it is important to maintain a differential hydraulic conductivity between the radon barrier and the foundation. While the NRC staff considers the design hydraulic conductivity of the rador. barrier adequate, the staff is concerned with the following factors:
(a) Except for the top one foot, the radon barrier has essentially the same properties and hydrevlic conductivity 6s the foundation as it is constructed of the same materials.
(b) the foundation will bc corpacted during tailings placement tnd will be under greater lere term consolidation stress than the cover materials; f
j
io...
50 (c) the radon barrier is more susceptible to weathering due to wind and water thermal and dehydration stresses, freeze and thaw cycles, and other erosion, face forces; and near sur (d) the as-built hydraulic conductivity may not acquire the design value determined in the laboratory due to construction defects, incomplete mixing, dehydration cracks and other hydraulic defects.
In a thousand years design life, these factors may tend to reduce the hydraulic conductivity of the foundation material, and increase that of the cover.
If the cover hydraulic conductivity increases above that of the foundation,
>erching within the cell, or " bathtub" effect, may occur. This will eventually auild up hydraulic pressure on the downslope side of the cover.
Although the above scenario may be unlikely because of the design difference in hydraulic conductivity between the cover and the foundation, it is dependent or,1) achieving that difference and 2) maintaining that difference. This is an open issue DOE must address. DOE should conduct field tests, such as infiltrometer tests, or other appropriate analysis to verify the design hydraulic conductivity, and to provide reasonable assurance the radon barrier is, and will remain, less pervious than the foundation. DOE must also discuss the long term aspects of meintaining the difference in cover and foundation hydraulic conductivity as part of this open issue.
Further discussion of this topic f rom the stability perspective is covered in Section 3.2.4 5.4 Disposal And Control Of Residual Radioactive Materials epa's proposed standards in Subparts A and C of 40 CFR Part 192 require DOE to demonstrate that the disposal of residual radioactive material complies with site-specific groundwater protection standards and closure performance standards in four areas: Water Resources Protection Standards for Disposal (Subsection 5.3.1),terformanceAsscssment(Subsection 2.3.2), Closure Performance Standards (5.3.3), and Groundwater Monitoring and Corrective Action Program (Subsectin 5.3.4).
5.4.1 Water Resources Protection Standards for Disposal The site-specific standards for disposal consist of three elements: (a) a list of hazardous constituents, (b) a corresponding list of concentratien limits, and(3)aPointofCompliance.
5.4.1.1 Hr.zardous Constitutnts Based on the characterization cf the tailings and pore fluids at the Rifle processing sites, DOE identificd the following organic and inorganic hazardous j
constituents in the tailings: arsenic, barium. cadmium, chromium, lead, molybdenum, nitrate, selenium, silver, uranium, radium 226 and 228 combined, gross alpha activity, antimony, beryllium, cobalt, copper, nickel, sulfide, thallium, tin, vanadium, zinc, benzo entracene, benzo pyrene, chrysene, fluoranthene, indero pyrene, pyrene, toluenc, 2,4-D, 2,4,5-T and 2,4,5-TP.
L The I:RC staff has reviewed DOE's assessments of the hazardous constituents using the following three criteria to select hazardous consitituents: (1) l
(
whether the constituents are resonably expected-to be or derived from the 1
i
0
$1 tailings, (2) whether they are listed in Appendix VIII of 40 CFR Part 261, and (3)whethertheyweredetectedinthetailingsorgroundwateratthesite (Ref.29). Based on en independent analysis of the information provided by DOE, the NRC staff concurs with the hazardous constituents proposed by DOE.
s 5.4.1.2 Concentration Limits DOE proposes to meet Maximum Concentration Limits (NCL) or background concentrations (BKG),whicheverarehigher. Since the uppermost aquifer has l
not been sampled, the NRC staff cannot concur with the BKGs established by DOE.
As described in TER Section 5.2.3B, the NRC staff considers this an open issue.
DOE must define the background concentrations to be met based on the aackground water quality of the upper aquifer.
- 5. 4.1. 3 Point Of Compliance Even though the Shire Member of the Wasatch formation beneath the Estes Gulch disposal site is not an aquifer, DOE treats it as the uppermost aquifer in its groundwater protection analysis. 00E identifies the Point Of Compliance (POC) as the vertical plane extending downward into the Wasatch formation from the hydraulically downgredient limit of the riprap as shown in Figure E.3.1.
Since the uppermost aquifer has not been defined, the NRC staff cannot concur with DOE's proposed P0C. The NRC considers this an o pen issue. DOE should define the '0C for the uppemost aquifer as required 'y the EPA standard.
a 5.4.2 Performance Assessment DOE must deuenstrate the performance of the disposal unit will comply with
' EPA's groundwater prettction standards in 40 CTR 192 Subparts A and C.
In attempting to show compli6nce with the standards, DOE focused on a saturated zr 91 thin the Shire Member of the Wasatch, which is an equitard and treated j
uppermost aquifer. DOE cstimated the horizuntal travel time from 11 L o
the cov
- the tailings to below the limit of the riprap in this saturated zone to.c over 1000 years, based on horizontal distance of 100 feet and linear velocity of 0.1 f t/yr as explained in TER Section 5.2.2.
As a result DOE R
concluded that the EPA standards would be met becausc the concentration of hazardous constituents at the POC will be below the concentration limits for greater than 1000 years.
The NRC staff disagrees with addressing the EPA standard by treating an aquitard at the uppermost aquifer. The NRC staff reviewed EPA's standards, NRC's technical position paper and other relevent documents and concludes that there is no provision, from a groundwater protection aspect, to substitute the protection of an aquifer by an aquitard. DOE must demonstrate that the uppermost aquifer, and not the equitard, is protected in order to show couplier.ce with the EPA standard.
5.4.3 Closure Performance Demonstration in accordance with the closure performance standards of 40 CFR 192.02(a)(4),
l DOE is required to demonstrate that the proposed disposal design will (1) minimize and control groundwater contaminction, (2) minimize the need for furthcr maintenance, and (3) meet initial performance stancards of the oesign.
L
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l
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52 As discussed in Section 5.2.5, the NRC staff considers the more likely failure i
mode of the design is perching within the disposal cell, and not perching beneath the cell. Therefore, in order to minimize long term maintenance and i
meet initial performance standards, DOE must provide further deraonstration that perching within the cell will not occur.
This open issue is discussed in detail in TER Section 5.2.5.
5.4.4 Groundwater Monitoring And Corrective Action Plan Pursuant to the proposed EPA groundwater protection standards in 40 CFR part 192.02(a) and (b), DOE is required to implement a groundwater monitoring and corrective action program to be carried out during the post-disposal period.
DOE is to provide a Surveillance and Maintenance Plan which includes a groundwater nonitoring program for concurrence by NRC.
t J
D0E describes the conceptual groundwater monitoring program as follows: After the disposal cell is constructed, DOE will install a series of nonitor wells in the colluvium and the Wasatch formation to monitor water quality and levels of saturation downgredient and cross-gradient to the tailings. Data from these wells and existing wells will be used to determine changes in water quality at the POC in the Wasatch forination and to detect perching of tailings seepage on the underlying bedrock.
As stated previously, the NRC staff disa5rees with DOE's use of an aquitard as the uppermost aquifer, and therefore corcludes that DOE must demonstrate that the groundwater raonitoring and corrective action >1an will protect the uppermost aquifer at the Estes Gulch site, as required by tie EPA standard. This is considered an open issue by the NRC staff.
As discussed in Section 5.2.5, the NRC concluded that the more likely failure mode is a " bathtub
- effect inside the disposal cell. Therefore, the surveillantc monitoring must include monitoring of the disposal cell to ascertain that no perched water.is building up within it. The NRC staff concludes that, as an open issue, DOE must provide adequate monitoring to dersonstrate that perched water is not building up in the disposal cell.
This racy be accomplished by water level raonitoring within the cell, monitoring of cover infiltration by lysimeters or other applicable methodology.
DOE is to provide a detailed corrective actich plan for concurrence by NRC in theSurveillanceandMaintenacne(S&M) Plan. DOE briefly outlined in the RAP a number of failure secnarios as shown in Table 5.3 and described the available reraediation options.
NRC staff reviewed the failure scenarios, considers the list coraplete, and concludes that practical remediation options exist for every scenario identified.
5.5 Clean Up And Control of Existing Contanination DOE needs to demonstrate compliance with the EPA standards listed in 40 CFR Part 192, Subparts B and C for cleanup and control of existing contatination.
The NRC staf f considers that groundwater cleanup may be def erred, as provided for in UMTRCA amendment of 1982.
However, in order to def er clear, up of the Rific processir.g sites, DOE must derchstrate that (1) clean up of the processing sites will nct be impacted by the disposal and thus is separable from the disposal actions, and (2) that public heath and safety vill be protected.
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4:
53 Ly L
, Table 5.3 i
CORRECTIVE ACTION PLAN
SUMMARY
Failure Scenario Remedial Action j
Contaminated seepage emerges Modify cover to eliminate excess J
below the pile, infiltration Groundwater quality deteriorates (1) Invoke supplemental standards
- due to tailing seepage and perform a risk assessment (2) Modify cover and apply institutional controls (aquiferrestoration impractical i
Radon barrier cracks due to Replace filter laycr with lower dessiccation, permeability layer.
S11tation affects erosion No action needed unless siltation 4
protection layer.
increases infiltration or induces vegetation.
Vegetation threatens integrity Apply biointrusion barrier of radon barrier Animals intrude into the pile Modify rock cover Frost heave / Cover erosion Not realistic / included in design DOE proposes to defer the compliance demonstration for clean up of existing contamination.
By virtue of moving the tailing to the Estes Gulch site, DOE has demonstrated that the disposal of the tailings will not impact groundwater cleanup of the Rifle processing sites. However, DOE has not addressed the potential effect to public health and safety of leaving the l
contaminated groundwater at the processing sites. As discussed in TER Section l
5.P.4, the NRC steff'is concerned that the domestic wells in the vicinity of I-the arocessing site might have already been, or potentially will be, impacted by tie existing contamieJtion. There is also concern that future water use development near the proceuing sites might be. impacted by the existing contamination prior to its cleanup. The NRC considers this an open issue and n
cannot concur on deferral of groundwater clean up at the Rifle processing sites
~
i until this issue is resolved.
5.6 CONCLUSION
S Based upon the review of the Preliminary Final Remedial Action Plan, the NRC staff concludes that DOT's proposed remedial action has not been shown to be j
in. con.plience with the EPA groundwater standards. The following open issues j
remain to be resolve 6:
)
1; DOE should conduct field tests, such as infiltt or..eter tests, or other appropriate analysis to verify the design hydraulic conductivity, and to provide
((..
54 reasone.ble assurance the radon barrier is, and will remain, less pervious than the foundation in order to avoid the " bathtub" effect. DOE must also discuss the long term aspects of maintaining the difference in cover and foundation hydraulic conductivity.
(Section 5.2.5/3.2.4.)
2.
DOE must provide adequate monitoring to demonstrate that ;)erched water willnotbuildupinthedisposalcell(" bathtub"effect). T115 may be accomplished by water level monitoring within the cell, monitoring of cover infiltration, or other applicable methodology.
3.
DOE must demonstrated that by defering groundwater clean up at the Rifle processing site >ublic health and safety will not be affected. DOE should provide t1e water quality data f rom wells (on-site and donestic) at the processing sites to demonstrate that they are not currently, nor will be, affected
)y the existing contamination.
If contamination is confirmed. DOE shall provide i
remedial actions, such as providing alternate sources of water or other measures, to correct the situation.
i 4.
DOE must demonstrate that the uppermost aquifer at the Estes Gulch site is protected in accordance with the EPA standards. As part of this demonstration, DOE must:
a) adequetely characterize the hydraulic and transport properties of the upperniost aquifer, b) adequately define the background water quclity of the uppermost
- aquifer, I
c) adequately define the background concentration limits for the uppermost equifer, d) define the Point of Compliance for the uppermost aquifer, and e) provide a groundwater monitoring and corrective action plan for the uppermost aquifer.
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I
.s 55 6.0 RADON ATTENUATION AND SITE CLEANUP 6.1 Introduction This section of the TER documents the staff evaluation of the radon attenuation design and the radiation survey plan to assure compliance with the EPA standards for radon emissions and cleanup of lands and buildings.
6.2 Radon Attenuation Attenuation of radon to levels meeting the EPA standards is proposed to be accomplished with an earthen cover (radon / infiltration barrier) placed and compacted over the entire disposal cell. The material for the barrier will be 511ty and sandy clays obtained from the disposal cell excavation, with the top foot amended with bentonite.
The review of the cover design for radon attenuation included evaluation of the pertinent design parameters for both the tailings and redon barrier soils, and calculations of tie minimum radon barrier thickness required for this site.
The design pararaeters evaluated for acceptability included material layer thicknesses in the disposal cell, radon diffusion coefficient, porosity, bulk density, radium concentration, radon emanation coefficient, and long-tern.
moisture cer. tent. The analysis technique utilized the RAECOM computer code (Ref.15) to calculate the redon barrier thickness based on the above parar:iet ers.
6.2.1 Selection of Paraueters DOE has analyzed two cases, i.e., radon barrier with and without bentonite, and sumarized the parameters used in each analysis (see Table 6.1 below). Case 1 (without bentonite) includes the following four material layers as they will be 31 aced in the. disposal cell: 1) the tailings f rom both the Old Rifle and New L!ifle processing sites, 2) the itew Rifle subpile materials, 3) the New Rifle offpile/windblownmaterials,and4)theradonbarrieritself. Case 2 included the same materials, but with the redon barrier separated into an additional 1 foot thick (30.5 cm) layer with bentonite to control infiltration.
Table 0.1. Pararneters Used in Radon Attenuation Analysis Parameter OR&NR UR-
- NR Radon Rad Bar Tailings Subpile Offpile Barrier w/ Bent 30.5 913 126 215 Thickness (cm)2.sec)
Diff Coeff (cra
.022
.020
.017
.006
.005 Porcsity 44
.40
.42
.36
.39 Ra-226 Conc (pCi/g) 680 244 97 Ernanation fraction
.36
.42 45-3 1.52 1.61 1,58 1.72 1.67 Bulk Density (g/cm ) )
Long. Term Moisture (%
10.0 11.0 11.4 12.9 15.0 i
7 0
56 The material layer thicknesses are based on the calculations of the volumes of the various contaminated materials at the processing sites. The volumes were i
estimated from boring and contaminated-soil analysis data. The Table 6.1 summary does not include a layer thickness for the racon barrier, since the analysis is directed toward determining the thickness required to meet the redon standard.
Based on review of the layer thickness calculations, the values determined for use in the radon attenuation analysis are reasonable representations of the expected field conditions and are accepteble to the staff.
The bulk density and the specific gravity for each of the material layers was i
determined by laboratory tests on samples obtained from borings and test pits, and the corresponding porosity was calculated. The staff has reviewed the computation of these parameters, and concludes that density and porosity values sresented in Table 6.1 are representative of the average values that should ;e achieved in the fic1d during construction.
The radon emanating fractions for the various contaminated materials were determined by ove. raging the results of many laboratory tests on samples obtained from the p.ocessing sites. Based on a review of the average values in Table 6.1 and comparison to other values in the literature, the staff concludes that the values selected are appropriate for use in the analysis.
The radium content (Ra-226) of the contaminated materials at the two processing sites was measured by extensive sampling by various contractors.
Valucs of radium content were determined by averaging the results of hundreds of sample measurerrents for each type of contaminated traterial.
Since the Old Rifle tailings, New Rifle tailirgs, Old Rifle offpile, and Old Rifle subpile will all be mixed, a voluix-weighted average was determined from the average values for these materials. The staff concludes that the characterization of the radium content of the contaminated inaterials has been adequate.
The values used in the design for the long-terra moisture contents (see Table 6.1) are based on the laboratory determination of the 15-bar inoisture content.
These values were checked by comparison to values obtained from the computer program NOIST. Since the radon barrier will be placed at moisture contents from about 15 to 18 percent end the frost barrier will provide additional protection against drying, the staff concludes that the selected long-term moisture for this layer (12.9%) is reasonable. Based on a review of applicabic laboratory data and computer results, the staff cor.cludes that the values selected for the other layers are acceptable for use in the RAEC0ft analysis.
The design radon diffusion coefficients for the contaminated materials and the radon barrier were derived from correlation curves of moisture saturation fractiers versus radon diffusion coefficients, based on the estimated long-term moisture content of the material. The curves were developed using diffusion coefficient and moisture content laboratory test data. The data was obtained f rom tests performed cn samples from test pits excavated at the processing sites and the Estes Gulch proposed disposal site.
Hcwever, radon diffusion coefficient tests have not been performed on the bentonite amended radon barricr naterial. 00E has indicated that these tests will be performed at a later-date, and RAECOM will be rerun using the resulting data. The sttff
. concludes that, with the exception of the bentonite aianded layer, the redon diffusion coefficients have been adequately dctermined and are acceptable for L
use in the design.
1
...: 7 0
57 6.2.2 Calculation Methodology and Design Results The radon barrier thickness necessary to comply with the radon flux limit was calculated using the RAECOM com> uter code. Use of the RAECOM computer code is an acceptable methodology that tas been used in all previous UlGRA project designs. Two separate calculations were run; one without any bentonite and one with the top foot of the radon barrier amended with bentonite. The design thickness of the radon barrier without any bentonite is 2.7 feet, while the designghicknesswithbentonite,usinganassumeddiffusioncoefficientof
.005 cm -sec is 2.6 feet. Based on these results, DOE has proposed a radon barrier 3.0feetthick. The staff notes that the analysis was done neglecting the effect of the frost barrier in further reducing radon release.
The staff concludes that the cover as designed should reduce radon releases to levels that are in compliance with the EPA standard. However, when DOE performs an additional analysis using a measured diffusion coefficient for the cover material with bentonite, the results should be provided to demonstrate that there is no need to increase the barrier thickness.
6.3 Site Clean-up i
6.3.1 Rcdiological Site Characterization Field sampling programs were conducted at the Old and New Rifle sites to deterruine background radiation levels, the subsurface boundary of the tailings piles, and the depth and area of mill yard, ore storage, windblown, and waterborne contamination. Subsurface evaluations were perforued using gamma well logging techniques and by analyzing cores from boreholes, in general, boreholes and surface measurements and samples were raade on grids ranging from 100 by 100 feet to 200 by 200 feet. Additional measurements were performed in areas of radiological interest.
Buildings on both Rifle sites were surveyed for gamira ar.d alpha contamination.
The results of the characterization surycys are being used to plan the control monitoring for the excavation cf contaminated materials, as well as the final verification survey, 6.3.2 Standards Used for Clean-up DOE has committed to clean-up of the processing sites in accordance with the epa standards.
Excavation control monitoring will be perforraed to ensure that the 5 pCi/g (surface) and 15 pCi/g (subsurface) Ra-226 standards are met.
If uranium or Th-230 are encountered in significant concentrations af ter the Ra-226 has been cleaned up DOE proposes to impost supplemental standards under criterion (f) of 40 CFR 192.21.
For Th-230, DOE would apply a supplemental standard ~of either 15 pCi/g projected Re-226 concentration in 1000 years or a calculated projected radon daughter concentration in a slab-on-grade house of 0.02 WL in 1000 years, for uranium contamination, DOE proposes a supplemental clean-up standard of 30 pC1/g (uranium total), and cites a 1981 NRC staff position (Ref.16) as recomraending this value as a level for which no restrictions on burial method were required.
NRC Fat not recommended application of Reference 16 for clean-up of UllTRA Program processing sites or vicinity. properties.
The 2001 staff positior, was not intended to replace cletn-up criteria for uranium mili tailings; rather, it was developed to be consistent with the EPA standards, which were at that time proposed standards
o 1
58 and were more stringent than existing standards. Further, the 35 pCi/g criterion for uranium corresponds to depleted uranium and is not appropriate for use in this case. Should DOE wish to impose a supplemental standard for uranium that meets the intent of Reference 16 and is consistent with present EPA standards the criteria to be used (after clean-up of Ra-226) would be 10 pC1/g total uranium in the top 15 cm of soil and 30 pCi/g total uranium in subsequent 15 cm layers. - Under the assumption of secular equilibrium and p
decay of total uranium to Ra 226 the values of 10/30 pCi/g U-total would eventuallyresultintheRa-720levelsrequiredbythestandard.
- However, should DOE elect to support the use of 35 pCi/g or another total uranium clean-up standard, then DOE can present justification under 40 CFR 192.21 and i
192.22 for use of suppleraental standards, All buildings and structures at the two processing sites will be demolished and the resulting debris and nonhazardous materials are proposed to be buried in the Estes Gulch disposal cell.
The RAP contains ccoflicting information regarding the fate of asbestos and hazardous materials (other than radioactive materials) resulting froin the clean-up of the processing sites.
It is unclear where DOE intends to dispose of these raaterials. The RAP (and specifications) need to be revised to clearly and consistently identify what types of hazardous materials are present and i
what is proposed for disposal of each type of material, 6.3.3 Verification of Clean-up In general, the final radiological verification survey for land clean-up will be based on 100-square-meter areas. DOE may use a variety of measurement techniques, depending on particuler circumstances.
The st6ndard raethod for Ra-226 verific0 tion is by composite soil samples, followed by analysis by gamma spectrometry. Other Ra-226 verification methods, such as the RTRAK gamma scanning tractor or a nine-point composite gamma raeasurement technique, may be used at the sites. No on-site structures at the processing sites will require radiological verification, since they all are to dennolished and buried at Estes Gulch.
6.4 Conclusion Based on a review of the radon ettenuation aspects of the proposed remedial action, the staff concludes that the radon barrier as designed should meet the applicable EPA standards contained 11 40 CFR Part 192.02(b). However, additional analysis using a racasured diffusion coefficient for the bentonite amended layer remains to be provided.
the proposed site clean-up, and the plans for verification accepta>1e.the staff finds th rograra, formation In general In on disposal of asbestos and hazardous raat e ials, and on proposed supplemental standards for branium remains to be provicto.
Radon attenuation and soil clean-up issues that remain to be addressed are as follows:
l
- 1) Radon diffusion coefficient tests have not been perfortiec' cr the bentonite-araended radon barrier matcrial.
DOE has indicated in the calculations supporting the FIP that these tests will be perfortied et a 1
1
lj SA a
9 59 later date, and RAECOM will t% rerun using the resulting data. DOE should provide the additional analysis using a measured diffusion coefficient for the cover material with bentonite to demonstrate that there is no need to increase the barrier thickness,
- 2) The RAP contains conflicting information regarding the fate of asbestos andhazardousmaterials(otherthanradioactivematerials)resulting from the clean-up of the processing sites.- It is unclear where DOE intends to dispose of these materials.
Page C-6, Section C.3.3 and page 80, Section 4.4.7 indicate that hazardous materials and asbestos will be sent off the site to approved disposal areas. Page 66, Section 4.2 indicates that hazardous katerials will be sent cff the site, but that asbestos will be pla ad h containers and burie6 in the tailings cell. Section 02200 of the sp cifications,
) age 02E00-12 indicates that asbestos and hazardous ma;.erials shall >e placed in the lower lifts of the tailings cell. The RAP (and specifications) need to be revised to clearly and consistently identify what types of hazardous materials are present and what is proposed for disposal of each type of-n.aterial.
- 3) For uranium conteuination, DOE proposes a supplemental clean-up standard of 35 pC1/g (uranium total), and cites a 1981 NRC staff position (kef.16) as recommending this value as a level for which no restrictions on burial method were required. NRC has not recommended application of Reference 16 for clean-up of UllTRA Program processing sites or sicinity properties. The 1981 staff position was not intended to replace clean-up criteria for uranium mill tailings; rather, it was developed to be consistent with the EPA standards, which were, at that time, proposed standards and were more stringent than existins standards. Further, the 35 pCi/9 criterion for uranium corresponds to depleted uranium and is not appropriate for use in this case. Should DOE wish tc impose a supplen. ental standard for uranium that meets the intent of Reference 16 and is consistent with present EPA standards, the criteria to be used (after clean-up of Ra-226) would be 10 pCi/g total uranium in the top 15 cm of soil and 30 pC1/g total uranium in subsequent 15 cm layers. However, should DOE elect to support the use of 35 pCi/g or another total uranium clean-up standard, then DOE can present justification under 40 CFR 192.21 and 192.22 for use of supplemental standards. The RAP discussion on supplemental standards for uranium should be revised to reflect one of these options,
Y 3.s-
'8 60 7.0 BIBLIOGRAPHY / REFERENCES 1.
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3.
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i 6.
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[
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.)
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8..
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j
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11.
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e l
41 pye y
61 w
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t 20.
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U.S. Army Corps of Engineers, Hydrologic Engineering Center, Flood Hydrograph Package, HEC-1, continuously updated and revised
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