ML20042D735

Draft Technical Evaluation Rept for DOE Proposed Remedial Action Ambrosia Lake Umtrap Site,Nm

ML20042D735
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Issue date:	03/31/1990
From:	NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION IV)
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{{#Wiki_filter:-... - h I d* ""% p (> l i Draft Technical Evaluation Report i j for DOE's Proposed Remedial Action \\ l AMBROSIA LAKE UMTRA PROJECT SITE New Mexico \\ March 1990 l by Uranium Recovery Field Office j Region IV Nuclear Regulatory Commisssion l [ gor. : 90o379 WM-67 PDC i

? i f I J f TABLE OF CONTENTS l M i 1

1.0 INTRODUCTION

1 1.1 EPA Standards............................................... I

1. 2 Site and Proposed Action....................................

2 1.3 Review Process.............................................. 3 1.4 TER Organization............................................ 4 l 1.5 Summary of Open and Confi rmatory Issues..................... 4 i i 2.0 GEOLOGIC STABILITY............................................... 6 2.1 Introduction................................................ 6 2.2 Location.................................................... 6

2. 3 Geology.....................................................

6 2.3.1 Stratigraphic Setting............................. 6 I 2.3.2 Structural Setting................................ 7 2.3.3 Geomorphic Setting................................ 7 2.3.4 Seismicity........................................ 8 2.4 Geologic Stability.......................................... 8 2.4.1 Bedrock Suitability............................... 9 2.4.2 Geomorphic Stability............................... 9 e 2.4.3 Seismotectonic Stabilit 10 l Conclusion.......................y..........................

2. 5 12 3.0 GE0 TECHNICAL STABILITY...........................................

13 j 3.1 Geotechnical Site Characterization.......................... 13 3.2 Soil Properties............................................. 13 3.3 Slope Stability............................................. 14 3.4 Liquefaction................................................ 15 3.5 Settlement.................................................. 16 6

3. 6 Construction Criteria.......................................

16 4.0 SURFACE WATER HYOROLOGY AND EROSION PROTECTION................... 18 i 4.1 Hydrologic Description and Site Design...................... '18 l 4.2 Flooding Determinations..........,.......................... 18 4.2.1 Probable Maximum Precipitation (PHP).............. 18 [ 4.2.2 Infiltration Losses............................... 19 4.2.3 Time of Concentration............................. 19 4.2.4 PMP Rainfall Distributions........................ 19 4.2.5 Computation of PMFs.......................... 19 4.2.5.1 Drainage Swa1es............................... 19 4.2.5.2 Pile Top and Embankment Outslopes............. 19 4.3 Erosion Protection 0esign................................... 20 4.3.1 Erosion Protection of the Reclaimed Pile.......... 20 t 4.3.2 Erosion Protection of the North and 9 East Swa1es...................................... 20 i 4.4 Rock Durability...................,......................... 21 i 4.5 Conclusions................................................. 21 1 i

+ i TABLE OF CONTENTS (cont.) Pag 5.0 WATER RESOURCES PROTECTION....................................... 22 5.1 Introduction................................................ 22

5. 2 Hydrogeological Characterization............................

22

5. 2.1 Hydr 0 stratigraphy and Ground-Water Occurrence.....

22 5.2.2 Geochemical Conditions and Water Use.............. 24

5. 2. 3 Extent of Contamination...........................

25 5.2.4 Tailings Characterization......................... 26 5.3 Design features for Water-Resources Protection.............. 26 l 5.4 Disposal and Control of Residual Radioactive Material....... 26 d 5.4.1 Ground-Water Protection Standard.................. 26 5.4.1.1 Applicability of Supplemental Standards. 28

5. 4.1. 2 Proposed Supplemental Standards.........

28 5.5 Cleanup and Control of Existing Contamination............... 29 5.6 Conclusions................................................. 29 l

6. 0 RADON ATTENUATION...............................................

30 l

7.0 CONCLUSION

S...................................................... '31 i

j I I l

1.0 INTRODUCTION

The Ambrosia Lake, New Mexico, site is one of 24 abandoned uranium mill j tailings piles designated to receive remedial action by the U. S. Department of Energy (DOE) under the Uranium Mill Tailings Radiation Control Act of 1978 i (UMTRCA). UMTRCA requires, in part, that the U. S. Nuclear Regulatory Commission (NRC) concur with DOE's selection of remedial action, so remedial l action meets standards promulgated by the U. S. Environmental Protection Agency l (EPA). This draft Technical Evaluation Report (TER) documents NRC staff's i review of the DOE preliminary final remedial action plan (DOE,1989a) and outlines the conclusions and outstanding issues resulting from this review. Information in this TER is freely cited from DOE submittals. Other sources are l cited as appropriate. l l 1.1 EPA Standards i i As required by UMTRCA, remedial action at Ambrosia Lake must comply with standards established by the EPA in 40 CFR Part 192, Subparts A-C. To summarize 1 (1) The disposal site shall be designed to control the tailings and other residual radioactive material for up to 1000 years to the extent reasonably achievable and, in any case, for at least 200 years I (40 CFR 192.02(a)), t (2) The disposal site design shall prevent radon-222 fluxes from residual radioactive materials to the atmosphere from exceeding 20 picoeuries/ square meter /second (pCi/mts) or from increasing the annual average concentration of radon-222 in air by more than 0.5 picoeuries/ liter (pci/1) [40 CFR 192.02(b)]. (3) The remedial action shall ensure that radium-226 concentrations in land that is not part of the disposal site averaged over any area of i 100 square meters do not exceed the background level by more than 5 picoeuries/ gram (pCi/g) averaged over the first 15 centimeters of soil below the surface and 15 pCi/g averaged over any 15-centimeter thick layer of soil more than 15 centimeters below the land surface [40 CFR 192.12(a)]. l. On September 3, 1985, the U. S. Tenth Circuit Court of Appeals remanded the ground-water standards (40 CFR part 192.2(a)(2)-(3)) and stipulated that EPA promulgate new ground-water standards. EPA proposed these standards.in the l form of revisions to Subparts A-C of 40 CFR Part 192 in September 1987. The proposal consists of two parts: standards applying to the cleanup of contamination that occurred before the remedial action of the tailings, and standards governing the control of future ground-water contamination that may i occur from tailings piles after remedial action. 1 I

i s 2 1.2 Site and Proposed Action The Ambrosia Lake mill and tailings site is located in McKinley County, l 25 miles north of Grants, in western New Mexico. The site is found on an alluvial slope of the Rio $an Mateo valley (Figure 1), and the area is used 7 mostly as open range land. The site includes 195.6 acres, 111 of which are occupied by the tailings pile, The tailings volume is 2.7 million cubic yards. Another 1.1 million cubic yards of soil have been contaminated by windblown tailings. The mill was constructed in 1957, and operated by Phillips Petroleum from June 1958, until ^ March 1963. The mill buildings have recently been demolished. Rubble is segregated for future disposal in and out of the tailings pile. Remedial action at the Ambrosia Lake site is designed and implemented by DOE, The design objectives are: t (1) Consolidate and stabilize contaminated materials on the existing site. (2) Reduce the radon flux from the stabilized tailings and associated contaminated materials to the atmosphere to levels not greater than 20 pCi/m2s. (3) Design controls to be effective for up to 1000 years, to the extent reasonably achievable, and at least for 200 years. (4) Prevent inadvertent human or animal intrusion into the tailings. (5) Ensure that existing or anticipated uses of ground and surface waters are not adversely affected by the stabilized tallings and contaminated materials. (6) Reduce contaminant levels in areas to be released for unrestricted use to levels which do not exceed 5 pCi/g Ra-226 above background values in the top 15 centinieters of soil, and 15 pCi/g in any 15-cm layer below that depth. (7) Minimize the size of the restricted final disposal site. f (8) Prevent the release of contaminants from the site during j construction. (9) Minimize the area disturbed during construction. (10) Minimize exposure of workers and the general public to contaminated r materials, f DOE's remedial action will consist of cleanup, consolidation, and stabilization l of most contaminated materials in the tailings pile. Other uncontaminated or slightly contaminated rubble is destined to be disposed of in a trench north of i \\

1 ~ 3 1 the former mill. This TER focuses on those aspects of the site characteristics and site remediation design which may affect meeting EPA standards. The proposed remedial action is comprised of the following major phases: (1) Consolidation of contaminated subsoils, windblown contaminants, and demolition debris in the existing tailings pile. (2) Moving tailings and subsoils from the north side of the pile to the top, filling a concave depression. The resulting shape will assure l runoff of rainfall from the pile, (3) Use windblown materials to buttress the sideslopes. Thus, excavation and recontouring of the existing sideslopes will t,e avoided, and weak slime zones in the pile's perimeter will not be disturbed. (4) Place a radon / infiltration barrier atop the consolidated contaminated materials. The radon barrier material will consist of weathered Mar.cos Shale, excavated from a borrow site one mile north of the tailings pile. The current design calculations indicate a radon barrier 3.5-feet thick is required to reduce radon flux to 20 pCi/m8s or less. 00E proposes to conduct further tests on the radon barrier i material and adjust the cover's thickness as needed. (5) Cover the embankment with erosion resistant rock and bedding layers l designed to withstand a Probable Maximum Precipitation (PMP) event on the pile as well as Probable Maximum Flood (PMF) events around the embankment, l Af ter completion of remedial action, the disposal site will be fenced and I posted with appropriate warning signs to discourage human intrusion. In. addition, the site will be s'urveyed and monitored periodically by a custodial j agency under an NRC license. 1.3 Review Process Reviews were performed to ensure DOE provided information required in " Standard l format and Content for Documentation of Remedial Actions Selection at Title ~I l Uranium Mill Tailings Sites"'(SFC)(NRC, 1989a). NRC's review consisted of i comprehensive assessments of DOE's proposed creliminary final design and ',l remedial action plan as described in the Standard Review Plan (NRC, 1985). The information assessed by NRC staff during this review was provided primarily in j DOE submittals: (1) Remedial Action Plan and Site Design (RAP) - Preliminary Final (00E, 1989a). (2) Design Calculations (DOE, 1987a; 1987b). (3) Main Construction Subcontract (DOE, 1987c). -).

4 l 4 i (4) Information for Bidders (Morrison-Knudsen Engineers, Inc.) (00E, 1987d) i (5) Environmental Assessment of Remedial Action (DOE, 1987e). (6) Responses to NRC comments by DOE (DOE, 1989b). l 1.4 TER Organization The purpose of this draft TER is to document NRC staff's review of DOE's l preliminary final remedial action plan and design for the Ambrosia Lake site and to address the open issues resulting from this review. The following sections of this report have been organized by technical discipline to address the EPA standards in 40 CFR Part 192, Subparts A-C. Sections 2, 3, and 4 provide the technical basis for the review's conclusions and identification of ( open items with respect to the long-term stability standard in 192.02(a). Section 5, Water Resources Protection, summarizes conclusions and any open items regarding the adequacy of DOE's demonstration of compliance with respect to EPA's ground-water protection requirements in 40 CFR Part 192. Section 6 l provides the bases for conclusions and identification of any open items with respect to the radon control standards in 192.02(b). I 1.5 Summary of Open and Confirmatory Issues i Review of DOE's preliminary final-design and remedial action plan has i identified open issues. The issues are explained in detail in the following chapters. A summary of the issues is found in Table 1. j F i (

it' l 5 Table 1 - Summary of open issues for NRC's concurrence in proposed remedial actions and site design for the Ambrosia Lake UMTRA Project site. Explanation TER Section Citation 1. Moisture specifications for the buttress fill 3.0 40 CFR 192(A) are not determinea. 2. DOE has not established the potential trend

5. 7. 2 40 CFR 192(C) of ground-water contamination in the Tres Hermanos-B sandstone member.

3. DOE has not proposed a ground-water corrective

5. 5 40 CFR 192(C) action plan.

4. Radon barrier design, including testing. 6.0 40 CFR 192(A) is not complete. J

s 6 2.0 GEOLOGIC STABILITY 2.1 Introduction EPA standards in 40 CFR 192 do not include generic or site-specific requirements for characterization of geological conditions at UMTRA Project sites. Rather, 40 CFR 192.02(a) requires control shall be designed to be ef fective for up to 1000 years, to the extent reasonably achievable, and, in any case, for at least 200 years. This standard is interpreted to mean that certain geological conditions must be met in order to have reasonable assurance that long-term performance objectives will be achieved. Guidance regarding these conditions is specified in the SFC (NRC. 1989a). This section of the TER documents NRC staff's review of geological information for the proposed remedial action. Background information for this section of the TER is derived primarily from DOE's RAP (DOE, 1989a), subcontract documents and calculations, observations during site visits and data reviews, and independent sources as cited. 2.2 Location The Ambrosia Lake site is located along an alluvial slope formed between San Mateo mesa and a small basin tributary to Rio San Mateo. The site is found in the Colorado Plateau physiographic province near its transitional zone with the Basin and Rang ( The site is part of the Grants mineral belt which has produced uranium for several decades. A large percentage of the site overlies abandoned mine shafts. Several other younger and larger uranium mills and tailings piles are found within 20 miles (Figure 1). Detailed geologic site characterization was completed by DOE for the valley area, which is underlain by Mancos Shale and bounded by high cuestas composed of Dakota Sandstone (southwest) and sandstones of the Mesaverde Group (northeast). 2.3 Geology 2.3.1 Stratigraphic Setting DOE characterized regional and site stratigraphy, referencing published work and original field investigations as recommended in the SFC (NRC,1989a). The Ambrosia Lake site lies in a broad strike valley underlain by Mancos Shale. The abandoned mill tailings overlie 15-50 feet of unconsolidated alluvium. The alluvium is locally derived, mostly from Mancos Shale, but also contains some coarse sand and gravel derived from Dakota ard Mesaverde Sandstones (00E, 1989a). The alluvium forms a slope graded from San Mateo Mesa southwestward to Arroyo del Puerto. Bedrock strata beneath the alluvium consists of various members of the Mancos Shale (Figure 2). The main body of the Mancos is interbedded with silty sandstone members called the Tres Hermanos Sandstone. The uppermost sandstones subcrop into the alluvium beneath the tailings pile. At least three sandstone members occur beneath the site, and their local outcrop pattern has been mapped by Santos and Thaden (1966). The mapped units each consist of yellowish-brown l l 1

4 7 or grey fine-to medium grained sandstone overlying a thicker unit of dark grey shale typical of the main Mancos Shale. The Mancos in turn is underlain by Dakota Sandstone. The Dakota is then underlain by the Morrison Formation and older strata which are not significant to the remedial action. Detailed site investigation by DOE included completion of numerous ground-water monitoring wells during 1985 and 1989. The wells penetrate the alluvium, weathered Mancos, Tres Hermanos units A, B, and C, and the Dakota Sandstone. Lithologic and geophysical logs of the holes provide a detailed view of stratigraphic conditions underlying the tailings pile. Tres Hermanos C is hydraulically connected to the alluvium (see Section 5.0). i 2.3.2 Structural setting DOE characterized the region's structural setting by referring to published j regional geologic maps, and conducting serial reconnaissance, field observation, and mapping of features critical to assuring the long-term stability of the remedial action. These studies were recommended in SFC Section 2.2.3 (NRC, 1989a). The site is found in a structurally transitional j area between the Zuni uplift to the southwest and the San Juan basin to the l north. The transitional area is known as the Chaco Slope. All these structural elements are part of the Navajo Section of the Colorado Plateau physiographic province. The Ambrosia Lake region is near the Plateau's l transitional zone, bordering on the Rio Grande rif t to the east. The zone is t characterized by a broad belt of faults and volcanic fields known as the Jemez Lineament. Normal-slip faults are prolific in the site area. The faults generall only 1\\ y trend north northeast. Rock strata in the Chaco Slope typically dip degrees (Figure 2). However, some fault blocks near the site are i tilted to 6 degrees or more. Some of the major faults in the region are known I to display as much as 400 feet offset. Most of the faulting is interpreted to be related to Laramide deformation occurring about 65 million years ago. Some of the faults, however, are known to have been active concurrently with Mt. Taylor volcanism. For example, 8 miles southeast of the site, basalts 2.8 million years in age overlying La Jara Mesa are offset by some faults. For a discussion of the potential existence of capable design faults, see Section 2.4.3 of this TER. 2.3.3 Geomorphic Setting I DOE characterized the region's physiography by referring to published i literature and topographic maps, as recommended in SFC Section 2.3.3 (NRC, 19998). $lte geomorphic conditions were characterized by aerial photographic interpretation and field observations. Ambrosia Lake is located in the Navajo l Section of the southeastern Colorado Plateau physiographic province (Hunt, 1974). Arroyo del Puerto, 2 miles southwest of the site, is the nearest water course to the site. The channel currently appears to be entrapped in a narrow strike i valley formed by the shale separating the Tres Hermanos-A and -B sandstone members. The mill and tailings pile, meanwhile, lie on an alluvial slope l l i

o 8 underlain by the main body of the Mancos and Tres Hermanos-C sandstone. The slope exhibits a gradient of 0.011 from the arroyo to the nill site, but rapidly increases to 0.04 approaching the sandstone cuesta beneath San Mateo Mesa. The alluvial slope is overlain by muddy alluvium derived locally from the Mancos, as well as sandy and gravelly sediment from the mesa. Sand dunes and sand sheets are common across the valley and the region. Drainage in the Arroyo del Puerto and on the alluvial slope is very poorly integrated and ephemeral. Major channels heading on San Mateo Mesa are incised to approximately the change in slope., Below this point, most runoff appears to become distributary. As such, no actual drainage basin above the Ambrosia Lake tailings pile is discernable. 2.3.4 Seismicity DOE characterized regional seismicity by obt.aining earthquake data bases provided by the National Oceanographic and Atmospheric Administration (NOAA), by applying accepted techniques to deterniine earthquake magnitudes, and by employing methods suggested in SFC Section 2.2.4 (NRC, 1989a) for calculating peak horizontal ground acceleration generated by a design-basis event. Ambrosia Lake is located in a transitional, more seismically active border zone of the Colorado Plateau (DOE, 1989a). The Plateau's border zones are associated with elevated terrestrial heat flow, crustal stress, gravity, and seismicity. DOE's analysis of potential earthquake magnitude for the interior Colorado Plateau included determining both the Maximum Earthquake (ME) and the Floating Earthquekt (FE) for the region. Sparse seismic records for the Colorado Plateau suggests the ME value could be between 6.2 and 6.8. The average, 6.5, is the value adopted previously by DOE for other UMTRA Projects, and appears very conservative considering that events of magnitude 5.0 or greater have been scarce on the plateau and its border zones. An FE magnitude, resulting from an earthquake unassociated with known tectonic structures, is generally less than an ME magnitude for a given seismotectonic province. 00E 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. 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 magnitude 6.2 occurring 15 km from the site was chosen as the design earthquake. This event would result in horizontal ground acceleration of 0.21g (DOE, 1989a), based upon Campbell (1981) (see Section 2.4.2).

2. 4 Geologic Stability Geologic conditions and processes are characterized to determine the site's l

[ 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 water. Geomorphic processes are considered for their potential impact upon long-term tailings stabilization and isolation. Potential geologic hazards, l

9 including seismic shaking, liquefaction, on site f ault 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 Even though surficial deposits and bedrock units beneath the tailings pile and proposed disposal site appear suitable for meeting EPA standards for long-term stability, adequate characterization of the site presented a concern in NRC's review process. Precise characterization of site stratigraphy is difficult because the rocks are not well exposed at the surface. 00E, however, relied heavily on data collection by drilling for geotechnical and ground water information. Many mining records are also available providing detailed information on the site's stratigraphy. Careful reviews of the Remedial Action Plan, reference to independent geologic literature, requests for additional information, and site visits and data reviews resulted in a relatively complete understanding of geological conditions at the site. Even though some details regarding site stratigraphy remain unclear at this time, it is concluded that DOE's level of characterization effort was suited to the information required and methods available. Small scale complexities of the site's underlying bedrock probably cannot be further characterized by well bore methods. The drilling program adequately characterized underlying 11thologies, and more detailed information is unnecessary, 2.4.2 Geomorphic Stability s DOE proposes to stabilize the Ambrosia Lake tailings pile in place. The pile location is subject to erosion due to runof f from three separate sources: (1) extreme flood flows in Arroyo del Puerto, (2) overland runoff from the upstream basin area to the northeast, and (3) runoff from the pile due to direct rainfall. See Section 4.3 of this TER for an analysis of erosion-protection requirements for runoff from these source areas. DOE identifies drainage-network rejuvenation'and extension as the only long-term geomorphic hazard to site stability. Many drainage basins in the western U.S., especially New Mexico, are notorious for their rapid rates of erosion in the past century. In addition, erosion processes are usually more severe in areas which have been altered by h' mans. u t. Analysis of geologic and topographic characteristics at the site shows that drainage channels exhibiting minor gullying occur near the tailings pile, and these channels potentially could headcut toward the stabilized pile. In addition, other geomorphic studies in this area indicates valley-floor channels are capable of backfilling with sediment and becoming reincised to a depth exceeding 5 meters within 2300 years. Studies by Patton and Schumm (1975) determined that a channel's potent?al to become incised is a factor of valley slope and upstream drainage area. To

,o 10 summarize, a steeper slope or a larger basin area is likely to result in an unstable drainage channel; one which will become gullied. A comparison of the Ambrosia Lake site with data for incised and unincised drainage channels in New Mexico (Wells and Gardner,1985) indicates that channels near the tailings pile are in a threshold condition to become unstable, if they do not already display incision. DOE's site selection and design have been prepared to take into account these long-term geomorphic hazards: (1) The drainage basin upgradient of the site is relatively small, 3.14 square miles (DOE, 1989a). Therefore, the potential for deep incision of channels is relatively low. (2) Drainage diversions are proposed to route runoff away from the pile. Although the diversions will not eliminate erosion, they will direct erosive runoff away from the pile. (3) DOE proposes rock aprons and toe trenches at the base of the pile on all four sides. The rock apron is 5 feet thick and exceeds 5 feet in width. The apron will provide armor'to the sides of channels which migrate or heaccut to the pile, inhibiting further channel changes and undercutting of the pile. (4) The north and east sides of the site will be recontoured to eliminate runoff concentrations. It is not possible to quantify the effect this design will have on long-term geomorphic processes, but short-term erosion will be minimized. In conclusion, there is reasonable assurance that long-term drainage channel changes are the only hazard to long-term stability of the disposal site. The potential for excessive geomorphic hazards is low, and DOE's proposed design should result in protection of the tailings pile adequate to meet EPA stability standards. 2.4.3 Seismotectonic Stability DOE's analyis of seismic hazards included searching for a design-basis f ault, selecting a design earthquake, calculating estimated peak horizontal ground acceleration, recognizing potential onsite fault rupture, and recognizing potential earthquake-induced geologic failures at the site. Delineating faults with recent movement in the site region consisted of low-sun angle aerial reconnaissance, interpreting aerial photographs in black and-white, color, and false-color infrared and LANDSAT imagery to a 65-km radius, and field reconnaissance mapping of faults within 15 km of the site. In addition, the DOE obtained and analyzed NOAA's list of instrumentally and historically recorded earthquake data for the Colorado Plateau and an area of 320 km radius around Ambrosia Lake. The main purpose of DOE's analysis of design-bases faults is to determine the existence of capable faults near the site. Briefly, a capable fault is one

j o 11 i I i exhibiting evidence of movement in the past 35,000 years, one which is associated with instrumented macroseismicity, or one which is structurally related to another capable fault. The definition for capable faults was l derived by NRC for nuclear power plant citing criteria, and is discussed in more detail in 10 CFR Part 100. Appendix A. l l DOE's literature search, aerial reconnaissance, and field studies failed to identify any unknown faults. Therefore, examining regional faults for design purposes concentrated upon faults already mapped and cited in the geologic literature. DOE (1989a, Plate 3) identified ten fault groups within 15 km of Ambrosia Lake to be closely examined for capable faults. The examination l included literature checks and field observation for evidence of Late Quaternary movement in any of the fault groups. DOE determined none of the fault groups are capable. Given a lack of capable faults within 15 km of the site, DOE based its evaluation of site seismic hazards on a general appraisal of Colorado Plateau i i seismotectonics and the available earthquake records. Because an earthquake of magnitude 6.2 can be expected to occur in the Colorado Plateau (see Section 2.3.4), and because a capable design-basis fault is not identified in s the site's region DOE adopted a Floating Earthquake of r.agnitude 6.2, l occurring 15 km from the site as the design earthquake. This specification is justified because: 1 (1) Table 1 of Algermissen and others (1982) indicates that the maximum l magnitude earthquake for the Colorado Plateau (Algermissen's source rone No. 16) is 6.1. Algermissen's relationship for magnitude as a function of intensity, M = 1.3 + 0.6(I), shows a magnitude 6.1 would be the equivalent of Modified Mercalli Intensity of I = 8. 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 performance is significantly longer than the historical period. (2) In its assessment of seismic hazards at potential sites for high-level radioactive waste repositories in the Paradox Basin. DOE (1984) identified structures capable of generating earthquakes with magnitudes as high as 6.5. The presence of such structures within the plateau, and evidence that fault-scarp expression can be reduced by only a few decades or centuries of erosion, indicates that such an earthquake may have occurred despite a lack of any existing surficial deformations. Despite such structures, it is considered that this magnitude is unjustifiably conservative for design purposes at Ambrosia Lake. (3) DOE's calculations of the Maximum Earthquake for other UMTRA Project sites in the Colorado Plateau are given as 6.2. I Appropriate methods of calculation show that peak horizontal acceleration at Ambrosia Lake resulting from a 6.2 magnitude earthquake at a distance of 15 km, using Campbell (1981) 84th percentile values, is 0.21g. Data inputs and these

s 12 results are reasonable and conservative for DOE's calculation of the seismic coefficient for the site. See Section 3,3 of this TER for applications of the seismic coefficient to the geotechnical stability of the remedial action design. 2.5 Conclusion Based upon review of the Final Remedial Action Plan, Final Design for Review, and DOE's response to NFC comments on drafts of these documents, there is reasonable assuranco that regional and site geologic conditions have been-characterized adequately to meet 40 CFR Part 192, Conditions hindering long-term stability of the site have been identified and mitigated by features in the remedial action design,

13 3.0 GEOTECHNICAL STABILITY j 3.1 Geotechnical Site Characterization DOE reported that the site and surrounding area is characterized by 281 borings (of which 208 are from previous studies), 126 piezocone tests, and 25 test i pits. The boring and sounding logs are provided in Information to Bidders (00E, 1987d). Fifteen geotechnical borings, Nos. 951 965, were drilled through the tailings pile as shown on drawing AMB PS-10-0417 (DGE 1989a). Standard penetration tests were performed on a nearly continuous basis through the soil materials of each of the fifteen geotechnical borings. Disturbed and undisturbed samples were obtained for laboratory testing from the exploration program. The i proposed radon barrier primary borrow was characterized by the 15 test pits located on drawing AMB-PS-10-0403 (DOE, 1989a). A secondary source area is also defined on the drawing, i The exploration program determined that the tailings pile is underlain by alluvial soils and weathered Mancos Shale. The alluvium soils consist of silty and clayey sands with some sandy clays. The eastern portion of the pile is underlain by the weathered Mancos Shale, while the western dike is undericin by up to 70 feet of alluvium soils overlying the bedrock. \\ The tailings disposal area was originally surrounded by starter dikes on all four sides. A second tier of dikes was reportedly constructed on southern and western sides using the upstream method. The eastern dike also may have been raised. The initial dikes were constructed of native materials, but the second tier is reported to contain some tailings. Construction records were not available. The tailings appear to consist predominantly of slimes and sand / slime mixtures, as would be expected at a carbonate-leach mill where tailings were spiggotted to the disposal area (figure 3). l 3.2 Soil Properties The geotechnical properties of the project soils were determined by performing various laboratory tests in accordance with applicable standards. Details of i the 16boratory testing results are discussed in RAP Appendix D, Sections 0.5, 0.6, and 0.7 (DOE, 1989a). t Seven of the samples of the foundation material were classified in accordance with the Unified Soil Classification System (USCS)(U.S. Department of the Interior,1977). These soils ranged from fat and lean clays (CH and CLs) to silty sands (SMs). Specific gravity tests were performed on eight samples, and in situ moistures and unit weights were determined for fourteen samples. The 'c"Tays (weathered shale) range in dry density from 98 pounds per cubic foot (pcf) to 116 pcf at saturations up to 100 percent. Alluvial material exhibited dry densities from 92 pef to 110 pcf with saturation values up to 84 percent, d Hydraulic conductivity, consolidation, and unconsolidated undrained and l consolidated undrained triaxial step tests were performed on samples of foundation alluvium and weathered shales. In several cases, DOE's testing information was incomplete. However, results of these tests were l

i 14 representative of the material types tested, and are acceptable to support j design calculations. Capillary moisture tests were performed on disturbed l samples for use in the radon barrier analysis. l t The tailings were tested by a similnr program, adding compaction testing to design the relocated tailings. Pierocone soundings were used to identify and classify three material types within the pile. Nineteen samples were l classified by the USCS system; a " sand" (SM), four " slimes" (CL, CH), and fourteen " sand / slimes" (SM, SC, ML, SM-SC, CL-ML). M situ moisture contents of these materials indicated that some are very moist. Several samples exhibited h situ moistures higher than their laboratory liquid limits. Dry densities of the in-place tailings ranged from 64 pcf to 105 pcf. Selected samples of the proposed radon barrier were tested in the laboratory for compaction (four samples), dispersivity (two samples), hydraulic conductivity (three samples), consolidation (one sample), strength (five samples), and capi 11ery moisture (two samples). The secondary borrow source was also characterized by laboratory testing. Laboratory tests indicate that the material is generally classified as a non-dispersive CH material (weathered 1 shale) with permeabilities in the range of 10E-7 cm/s. The 15-bar capillary moistures were greater than the reported h situ moistures. Strength and consolidation parameters are within acceptable limits. Logs of test pits indicate that many of the samples tested were claystones or shales. The i testing program did not indicate how the soils were processed prior to laboratory testing. Because the material placed in the cover may not be broken down prior to placement to the same extent as normal laborhtory processes, the laboratory testing program that will be performed during construction should attempt to model conditions in the field. Testing of strength and consolidation properties resulted in acceptable design parameters based on the upper bounds of expected typical values associated with the material types identified. Minimal but adequate testing was performed to characterize the in situ tailings and foundation materials. Additional laboratory testing of the proposed radon barrier material will be required prior to review of the radon barrier thickness calculations (see Section 6.0).

3. 3 Slope Stability i

The DOE performed static and pseudo-static analyses to evaluate the stability of the reclaimed plie. Three identical sections were selected for analysis: the exposed 5:1 temporary cut on the north slope; the north slope finished embankment; and the west slope finished embankment. No attempt was made to identify and separate sands, sand / slime mixtures, and slimes in the models. A phreatic surface was assumed on the north model to simulate the existing l perched water identified in the area. Short-and long-term strength parameters were applied in association with assumed pseudo-static loads of 0.lg for short-term and 0.14g for long-term analysis. The ME for the site results in a recommended peak horizontal acceleration of 0.21g (see Section 2.4.3). One-half to two-thirds of the expected acceleration was utilized as " seismic coefficients" for short-and long-term studies, respectively. The resulting t

I 15 I minimum factors of safety are summarized on sheet 7. Calculation 16-439-06-01, (DOE, 1987a; Volume III). As indicated, the factors of safety for the long-term static and pseudo-static models are in excess of the minimum factors of safety recommended by NRC (1977). The short-term analysis of the temporary cut model indicated a factor of safety of 1.0, which is below the recommended t minimum factor of safety of 1.3. The other models resulted in minimum factors of safety in excess of those recommended for short-term static and pseudo-static models. The analyses are found to be conservative as DOE modeled (1) critical sections of the finished embankment; (2) simplified sections of the tailings using the weakest of the expected material types, slimes, as representative of the entire zone; (3) lower strength values than those resulting from the laboratory testing program for the majority of the sections; and (4) a phreatic condition that, when the cover system is in place to minimize infiltration, will tend to dissipate with time. The reported minimum factor of safety for the temporary cut section, which was lower than NRC recommended minimum, does not affect long-term stability. As the title implies, the cut is temporary and could present a problem during construction. Pseudo-static loading coefficients and strength values are considered reasonable. Therefore, the DOE stability analyses are representative and acceptable with respect to the acceptance criteria set forth in the SRP (NRC, 1985). 3.4 Liquefaction The DOE analysis of liquefaction used three different methods to evaluate the liquefaction potential of the reclaimed tailings pile: the modified Seed-Lee-Idriss method, the Chinese method, and the use of published cyclic strength data. The effect of earthquake induced settlement was also evaluated. These studies concluded that the liquefaction potential of the reclaimed site is "very low to none." The analyses also concluded that anticipated earthquake-induced settlement of the pile will be small and acceptable. Several areas in the liquefaction study are considered inaccurate or incomplete. For example, the use of average soil properties in the Seed procedure cannot be considered acceptable. The liquefaction analysis should identify specific zones with liquefaction potential rather than averaging the overall safety against liquefaction of a type of material. Additionally, sufficient laboratory testing was not reported to' perform an in-house analysis without using average properties. Further analysis was not requested, however, because an engineered buttress will be constructed around the existing and relocated tailings (see Figure 4). This buttress will contain any localized liquefaction. Also, the addition of a cover system will help ensure that infiltration will be minimized, thus reducing the chances of liquefaction occurring. The construction records should provide the necessary information to verify that the buttress constitutes an engineered fill and that the cover will minimize infiltration. I

h 16 i

3. 5 Settlement i

To accurately estimate settlements and to demonstrate the feasibility of single-stage construction, DOE constructed two test fills on the disposal site. Analyses based on the results of laboratory consolidation tests, empirical I studies, and actual field performance indicated that post-construction l ~ settlements would "not adversely effeet adequate performance of the embankment." Six settlement monuments will be installed for additional 1 verification. I The analysis indicates that the majority of the total settlement for the embankment and foundation will occur during the construction period. The estimated post-consolidation settlement was utilized to evaluate cracking of I the radon cover over the design life. A special study (DOE, 1989b) of the side slope areas was performed to better quantify tensile strains at the break in slope. The maximum post-construction settlement was estimated to be less than 10 inches, resulting in a predicted maximum tensile strain of about i 0.03 percent. DOE utilized commonly accepted procedures for the settlement analysis and reasonably conservative soil parameters. Therefore, the l settlement analysis performed by the DOE is considered adequate. 3.6 Construction Criteria i The specifications for the project are contained in DOE's RAP, Appendix F (00E, 1989a). The earthwork section, number 02200, describes acceptable material i types and properties, construction requirements, and identifies areas that will be included in the quality control prograni. The quality control program will i be further defined in the RAIP (00E, 1989c). Uncontaminated fill material shall contain no more than 5 percent organic l material, with a maximum particle size no greater than the compacted lift thickness. The radon barrier material shall consist of CL, CH, ML. MH, SC, or SM soils with a minimum of 25 percent passing the No. 200 sieve. Additionally, at least four samples tested out of five consecutive tests shall have at least i 40 percent passing the No. 200 sieve. The buttress fill will be comprised of windblown-contaminated natural soils that are predominantly silty sands. The buttress material and radon barrier will be compacted to 95 percent and 100 percent of the Proctor maximum dry density, respectively. All other subgrade preparation and embankment construction will be compacted to 90 percent of the proctor maximum density. Additionally, during compaction of the radon barrier material, moisture content shall be maintained at minus one to plus three percent of the optimum moisture. Although the overall design of the disposal area relies significantly on the buttress fill, no specific moisture control for material placement is included in the specifications. 00E indicated that the maximum density requirement would produce adequate results. This specification does not appear to be adequate, and moisture control of the buttress is considered an open item, t 9

s, 17 Demolished materials and debris stockpiled on-site will be disposed of in the tailings pile within the limits shown on drawing AM8-PS-10 0410 (DOE,1989a).

Demolished materials shall be distributed uniformly on the tailings pile and placed in a manner that avoids nesting and minimites voids. All pipes greater than six inches in diameter shall be filled or crushed prior to placement. DOE's construction criteria and quality control program are consistent with standard engineering practice, and in accordance with the acceptance criteria set forth in the SRP (NRC, 1985). Therefore, it is acceptable, provided that appropriate moistura specifications are established for the buttress material. I l 'b l I I t l l L i

s 0 i ~ 18 i 4.0 $URFACE WATEP HYDROLOGY AND ERO$10N PROTECTION 4.1 Hydrologic Description and Site Desion l The Ambrosia Lake site is located in a valley within the Ambrosia Lake portion of the Grants Mineral belt, a major uranium producing area in New Mexico. The valley is drained by Arroyo del Puerto, an ephemeral stream which is the major water course in the Ambrosia Lake area. At its closest proximity, the arroyo C is about 1 mile away from the tailings pile, and 60-80 feet lower in elevation. Many smaller ephemeral streams are tributary to Arroyo del Puerto. The i tailings pile is located in the floodway of two of these small streams as shown in Figure 5. t DOE proposes to stabilize and reclaim the tailings pile in place. The reclaimed pile will be protected against erosion by a soil and rock cover. As shown in Figure 6, the pile top will slope from the center toward the sides ( at a maximum of 4 percent (25H:1V), and the embankment outslopes will be l 20 percent (5H:1V). Wide swales on the north and east sides of the pile will divert flood flows around the pile. Both the top and the outslopes will be i protected against erosion by a 1-foot thick layer of ror,k riprap. The toe of the outslopes will be protected by a rock-lined apron and toe trench. 4.2 Floodino Determinations To comply with EPA standards which require stability for 1000 years to the extent reasonably achievable (or for a minimum of 200 years), the design basis events used for erosion protection, include the ProbaMe Maximum Precipitation (PHP) rainfall event, and the Probable Maximum Flood (PMF). 00E considered the potential for flooding from two sources: t (1) Probable Maximum Precipitation (PMP) occurring directly over the reclaimed pile. (2) Probable Maximum Floods (PMFs) resulting from the PMP positioned critically over the two small drainage areas located north of the pile. Review of DOE's submittals leads to a conclusion that these are the only credible sources of potential flooding at the site, 4.2.1 Probable Maximum Precipitation (PMP) The PMP is the estimated depth of precipitation (rainfall) for which there is virtually no risk of being exceeded. A 1-hour PMP value of 10.5 inches was determined from Hydrometeorological Report No. 55 (HMR-55; U.S. Department of Commerce, 1984). This 1-hour PMP was then distributed into increments as small as 2.5 minutes, using the factors suggested in NUREG/CR-4620 (Nelson and others, 1986). Analysis of the rainfall computations shows that the PMP was acceptably derived.

H 19 4.2.2 Infiltration Losses When a PMP event occurs over a drainage area, not all of the rainfall results in runoff. A portion is lort due to infiltration, evaporation, and other causes. In computing the peak flow rate for the design of the rock erosion protection, DOE assumed that essentially no losses would occur, so the entire PMP would contribute to flood flows. This is a very conservative assumption and therefore is acceptable. 4.2.3 Time of Concentration The time of concentration (tc) is the time it takes for overland flow to travel from the most distant part in a drainage basin to the point at which a flood discharge is being estimated. Th? peak runoff for a given drainage basin is inversely proportional to the time of concentration for that basin. That is, if the time of concentration is conservatively estimated to be small, the peak discharge will therefore be conservatively large. Times of concentration for the pile top, embankment outslopes, and diversion swales were estimated by DOE using methods published by the U.S. Department of the Interior (1977). The procedures used for computing tc are representative of the small drainage areas present at the site and are therefore acceptable. 4.2.4 PMP Rainfall Distributions DOE derived rainfall distributions and intensities from HMR-55 (U.S. Department of Commerce, 1984) and NUREG/CR-4620 (Nelson and others, 1986). In the determination of peak flood flows, rainfall intensities for durations as short as 2.5 minutes were used. Reviews of this aspect of the flooding determination leads to a conclusion that the computed peak rainfall intensities are conservative, and therefore, acceptable. 4.2.5 Computation of Probable Maximum Floods (PMFs) 4.2.5.1 Drainage Swales The PMFs for the two drainage swales were estimated using Soil Conservation Service procedures, discussed in U.S. Department of the Interior (1977). The PMF for the North Swale (Figure 5) which has a drainage area of about 1555 acres, was estimated to be about 20,000 cfs. For the East Swale, the PMF was estimated to be about 7100 cfs for a drainage t.rea of 457 acres. ' Independent calculation of PMFs for the two swales result in PMFs of 19,300 cfs for the North Swale and 6600 cfs for the East Swale. On the basis of this close comparison of discharges, DOE's estimated PMFs for the North and East Swales are acceptable. 4.2.5.2 Pile Top and Embankment Outslopes s For the pile top and outslopes, DOE estimated PMF flows using the Rational Method (U.S. Department of the Interior, 1977), and the input parameters

20 discussed above. The estimated flood fl<vs m t v.anservative and acceptable. 4.3 Erosion Protection Desian Protection against erosion will be provided by rock layers on the pile top and embankment outslopes. A rock-lined apron and toe trench (Figure 6) will be provided at the toe of the outslopes to prevent undercutting. The swales to the north and east sides of the pile are designed to reduce flow velocities below those considered to be erosive. During PMF events, flood flows in the swales will flow against the pile. However, the riprap-lined apron and toe trench will prevent erosion at the toe of the embankment. 4.3.1 Erosion Protection of the Reclaimed Pile The top of the pile slopes from the center toward the four sides at a maximum slope of 4 percent. The sides (outslopes) of the pile are sloped at 20 percent. The riprap for the pile top was designed using the' Safety Factors Method (Simons and Senturk, 1977). For the outslopes, the Stephenson (1979) Method was used to size the riprap. Riprap sizes are as specified by the Department of Energy (DOE, 1987a; Volume IV). For the pile top, DOE proposes to provide a 12-inch thick layer of riprap having a median diameter (Dso) of 1.5 inches. For the outslopes, the proposed riprap is a 12-inch thick layer of 3.7-inch D o rock. To protect 3 against undercutting at the toe of the outslopes, an apron and toe trench with a Dso of 10.0 inches will be provided. The design of the apron and toe trench is as shown on Figure 6. DOE's calculations were reviewed and an independent analysis was performed to verify the adequacy of the riprap. This analysis was performed using procedures discussed in NUREG/CR-4651 (Abt and others, 1987), and resulted in a calculated Dso of 1.4 inches for the pile top and 3.5 inches for the outslopes. Because the DOE proposed Dso values of 1.5 inches and 3.7 inches for the pile i top and cutslopes respectively, the staff concludes that DOE's riprap design is acceptable. DOE riprap calculations for the outslope apron and toe trench were compared with several others developed the U.S. Army Corps of Engineers (1970). s On the basis of this comparison, the 10.0-inch Dso riprap and the overall design of the apron and toe trench are acceptable. 4.3.2 Erosion Protection of the North and East Swales The remedial design includes two swales to divert flood runoff away from the pile. DOE states that both swales will be excavated in weathered shale and ) sandstone, so riprap is not needed for erosion protection. The North Swale diverts runoff from an area of 1555 acres and the East Swale diverts a 457-acre area. The drainage areas are shown in Figure 5. Both swales are designed using the Manning equation. During a PMF, the maximum average velocities in the swales will not exceed 5 feet per second. During i

21 more frequent flood events such as the 100-year flood, velocities will not exceed 3 feet per second. These low velocities are considered to be ) non-erosive. The swales are far enough from the toe of the reclaimed pile so i normal flood discharges will not flow against the pile. The center line of the North Swale is 230 feet away and the East Swale is 150 feet away. During a PMF, however, flood waters will rise to the pile outslopes. As discussed in Section 4.3.1 above, the pile outslopes are protected with a 1-foot thick layer of riprap having a Dso of 3.7 inches. In addition, the toe of the outslope is protected with an apron and toe trench to prevent undercutting and erosion. Review calculations do not necessarily indicate that during a PMF there will be no erosion in the swales. However, since the swales are located away from the pile, and since the outslope toe has a riprapped apron and toe trench, any erosion that does occur will not affect the pile. The riprap on the outslope is designed for flows coming off the pile top and outslopes. PMF flows in the swales, however, will exert a horizontal erosive force on the riprap. Using the Safety Factors method (Simms and Senturk, 1977), DOE determined that the 3.7-inch Dso riprap on the outslopes provides a factor of safety of about 2.4 to 2.8 for PMF flows in the swales. The calculations provided by DOE were reviewed and an independent analysis was performed using the tractive force (shear stress) method (U.S. Army Corps of Engineers, 1970). This analysis indicated that a Dso of about 1 inch is sufficient to withstand the horizontal forces in the swales. Since the riprap being used is 3.7 inches, the riprap design of the outslopes is adequate to resist PMFs in the swales. 4.4 Rock Durability Rock to be used for erosion protection will be basalt, obtained from a borrow area in the Cibola National Forest. DOE (1987a; Volume I) presents gradation and rock durability criteria, including the results of several durability tests. The acceptability of the proposed rock was evaluated using the procedures developed by the Nuclear Regulatory Commission (NRC, 1989b). Based on this evaluation, the proposed rock is considered of good quality and 5 acceptable for use as erosion protection material. 4.5 Conclusions Based on review of the information submitted by DOE and on independelt evaluations, it is concluded that the site design meets the EPA stant*u ds of 40 CFR 192 with regard to flood design measures and erosion protection design.

.. o l i 22 ] i 5.0 WATER PESOURCES PROTECTION 5.1 Introduction NRC staff reviewed the preliminary final Remedial Action Plan and Site Design (DOE, 1989a) for the Ambrosia Lake site for compliance with EPA's proposed ground-water protection standards in 40 CFR 192, Subparts A-C (52 FR 36000). Guidance for the review is found in NRC's Draft Technical Position of Information Needs to Demonstrate Compliance with EPA's Ground-Water Protection Standards in 40 CFR 192, Subparts A-C (NRC, 1988). DOE proposes application of supplemental standards under the provisions of draft 40 CFR Part 192, Subpart C, for ground-water protection because aquifers at Ambrosia Lake contain Class III ground water. Based on the review, DOE has provided adequate information to demonstrate that supplemental standards are applicable at the Ambrosia Lake site, and that the proposed remedial action complies with the site-specific supplemental standards, j r 5.2 Hydrogeologic Characterization 5.2.1 Hydrostratigraphy and Ground-Water Occurrence DOE has characterized the hydrogeologic characteristics in the vicinity of the disposal site using acceptable techniques, methods and approaches, and the assessment of hydrogeologic characteristia is adequate to support DOE's performance assessment to demonstrate compliance with the proposed supplemental standards. DOE determined that ground water beneath the Ambrosia Lake site occurs in alluvium, weathered Mancos Shale, and each of three Tres Hermanos sandstone members in the lower Mancos formation (Figure 2). In. addition, the deeper Westwater Canyon Member of the Morrison Formation is also saturated. The sandstone aquifers are separated by intervening shale'or claystone beds. Previous experience with the occurrence of Mancos Shale at other UMTRA Project sites, and inspection of cores and lithologic logs for Ambrosia Lake, provide evidence that the shale units serve as aquitards between the sandstone units. ) Most of these aquifers, however, have become connected locally by mine shafts and vents. Uppermost Aquifer i Based on regional geological data and detailed site lithologic logs, DOE (1989a) determined that the uppermost aquifer consists of surficial alluvium, weathered Mancos Shale, and the Tres Hermanos-C sandstone member (TRC) where it subcrops into the alluvium beneath the tailings pile. The alluvium underlying the pile ranges from 10 to 45 feet thick. Depths to the water table range from 15 to 45 feet from the land surface. The alluvial aquifer is not continuously saturated near the tailings pile, and the maximum observed saturated thickness is 15 feet. The Tres Hermanos-C sandstone is recharged by ground water in the alluvium. Two separate sandstone layers (called the C1 and C2) exhibit unconfined ground water, and each unit is only basally saturated. DOE (1989a) suggests that C1

- g o 23 and C2 may have been saturated prior to mining, but were depressurized due to seepage down mine shafts and vents. Based on measured ground-water elevations in seven wells screened in the alluvium, and eleven wells screened in the Tres Hermanos-C1 and -C2 members, DOE constructed maps of potentiometric surfaces to determine the direction and rate of flow. The predominant direction of ground-water flow in the alluvium and weathered Mancos Shale is southwesterly, toward Arroyo del Puerto, under an average hydraulic gradient of 0.025. The flow path is controlled by the erosional contact between the alluvium and bedrock, grading toward Arroyo del Puerto. The predominant direction of ground-water flow in the TRC sandstones is northeasterly, under an average hydraulic gradient of 0.026. This ground water is unconfined and flows along the dip of the sandstone beds. DOE conducted pump or slug tests in the alluvium / weathered shale aquifer in three wells upgradient and downgradient of the tailings pile. Results from pump tests resulted in a calculated hydraulic conductivity of 4.1 E-5 centimeters per second (cm/s). Discharge rate was 0.35 gallon per minute (500 gallons per day), but could only be sustained for 12 hours. Slug tests revealed hydraulic conductivity measurements between 1.9 E-5 and 1.1 E-3 cm/s. Slug tests in the TRC unit resulted in measured hydraulic conductivity ranging from 1.1 E-5 to 1.2 E-3 cm/s. t 00E (1989a) indicates that ground water within the alluvial aquifer is naturally recharged by surface infiltration of runoff from Roman Hill northeast of the site. DOE asserts that basal saturation occurs where the alluvium is underlain by shale beds. In addition, DOE states that ground water was artificially recharged by seepage from an unlined mill process (make-up) water pond, seepage of surface discharge from dewatering pumps'in the Ann Lee Mine, and seepage from the tailings impoundment. DOE further cites other published evidence that all alluvial ground water in the Ambrosia Lake' area is derived from seepage from uranium mill tailings piles or mine dewatering. The review of site geology, core samples, and DOE's 11thologic logs indicates that TRC sandstones are difficult to distinguish from more typical Mancos Shale. TRC sandstone is more silty and shaley than its name implies. The reviews support DOE's conclusion that TRC produces only small quantities of low quality ground water. i Tres Hermanos-B Aquifer Based on surface outcrop data and monitoring well logs, DOE determined that the Tres Hermanos-B sandstone member (TRB) lies approximately 50 feet below the C sandstone. Ground water within TRB flows to the northeast, following the structural dip, under a hydraulic gradient of 0.04. TRB varies in thickness from 30-50 feet. Monitor wells screened in the unit are sporadically or permanently dry, leading 00E to conclude that unit TRB exhibits only basal saturation. 00E cites measurements made for Quivira Mining Company, indicating TRB's hydraulic conductivity is 1 E-3 to 2 E-5 cm/s.

24 l [ The Tres Hermanos-B member is recharged by surface infiltration and seepage from the alluvial aquifer upgradient from the site. Therefore, there is no potential recharge from the site into TRB. Observations of site geology and core samples, and review of DOE's lithologic logs indicates that TRB contains relatively well sorted and permeable sandstones. However, the cleanest sandstone is likely lenticular and interbedded with silty and shaley material. The highest hydraulic conductivities in TRB are likely to be discontinuous. Intervening Mancos Shale Aquitards Previous experience with characterizing the Mancos Shale indicates it is actually a hard, dense, dark grey claystone with abundant partings at fossils i and sporadic pyrite mineralization. Only when encountered near the surface does weathered Mancos display shaley partings. Porosity in the claystone typically is a result of secondary fracturing. Observations of DOE's core samples indicates the shale is difficult to distinguish from the Tres Hermanos sandstones in many instances. The stratigraphic contacts are gradual, and the " sandstones" typically are muddy sandstone or siltstone. 00E cites measurements by other authors giving hydraulic conductivities for Mancos in the Ambrosia Lake area ranging between 4.3 E-8 and 1.4 E-6 cm/s. These values generally agree with measurements in Mancos Shale conducted for other UMTRA Project sites in the Colorado Plateau. In addition, there is no evidence that the Mancos locally exhibits increased permeability due to severe i fracturing. Lower Aquifers DOE has not characterized the Tres Hermanos-A, Oakota, or Westwater Canyon aquifers in detail. Based on the current understanding of site's stratigraphy, it appears unlikely that seepage from the tailings pile could impact these aquifers. Contaminants from tailings leachate are most likely to migrate downgradient with ground water in the uppermost or TRC aquifers, and least likely to migrate downward through intervening shale and claystone aquitards. A possible exception to downgradient migration of contaminants is vertical infiltration through mine shafts and vents which penetrate all the aquifers to the Westwater Canyon Member. See section 5.4 for a discussion of the potential impacts of this contaminant pathway.

5. 2. 2 Geochemical Conditions and Water Use Uppermost Aquifer DOE determined that background water quality in the uppermost aquifer is

'I existing quality because saturated conditions were created by infiltration of milling solutions and mine-dewatering runoff. Preoperational ground-water quality data are nonexistent for the Ambrosia Lake site. Concentrations of uranium, chromium, cadmium, molybdenum., nitrate, selenium, lead, silver, and

t e j 25 [ I activities of gross alpha and radium-226 and -228 exceed EPA's Maximum Concentration Limits (MCLs) (DOE, 1989a; Tables 0.8.12 and D.8.13). Alluvial monitoring wells exhibiting the influence of tailings seepage are numbers 674, 675, 792, and 793. Wells completed in the TRC and exhibiting contamination are 676, 677, 779, 782, 784, 785, and 787. In all cases, the contamination indicator is sulfate. Analytical data are summarized in DOE's Remedial Action Plan (DOE, 1989a). i Based on an inventory of ground-water users near Ambrosia Lake. DOE determined that no domestic wells are completed in any Tres Hermanos sandstone member or in the alluvium within a 3-mile radius. Tres Hermanos-B Sandstone I Ground water in the Tres Hermanos-B member appears to be unaffected by tailings seepage. With two exceptions, all constituents are at or below EPA's proposed MCLs. One well (678) exhibits elevated nitrate, and two (678, 777) exhibit elevated selenium. DOE (1989a) reports the geochemical data are either inconsistent or are based on only an initial sampling round. Therefore, determining whether unit TRB exhibits a trend for exceedance of MLCs remains an. open item. Westwater Canyon Member DOE monitors the Westwater Canyon Member of the Morrison Formation because it receives seepage from higher, contaminated aquifers through mine shafts and vents. The Westwater is the primary ore zone in the Ambrosia Lake area. Due to extensive impacts of mine development and dewatering, it is difficult to t quantify which geochemical effects in this aquifer, if any, are attributable to tailings seepage from the Ambrosia Lake site. DOE cites 1980 and 1983 data for samples of mine dewatering discharge. EPA's proposed MCLs are exceeded for cadmium, chromium, lead, molybdenum, selenium, silver, and uranium, and activities of radium-226 and 228, and gross alpha. A number of domestic and stock wells are completed in the Westwater Canyon member, including a municipal well for the town of San Mateo,10 miles southeast of the site (DOE, 1989a). Nine active wells are located within 5 miles of the site. Four are completed in the Westwater, while the fifth may penetrate deeper. Some wells northwest (upgradient) of the site have gone dry due to mine dewatering. Other wells downgradient are used for stock and domestic purposes, even though they supply poor quality water. c. 5.2.3 Extent of Contamination Based on existing ground water data and contamination trends, it appears only the uppermost aquifer is affected by tailings seepage. In addition, lack of ground water in the alluvial aquifer prior to mining and milling, led DOE to conclude that existing ground-water quality and background are one and the J same.

s i 1 1 ) 26 Migration of contaminants is poorly understood, but appears to occur in circular or elliptical plumes beyond the site boundaries. Concentrations in excess of MCLs for both molybdenum and uranium have migrated from the tailings pile (see DOE, 1989a; Figures 0.8.21-0.8.26). The extent of migration exceeds 1000 feet for uranium through both the alluvium and TRC sandstone, and for molybdenum through the alluvium. 5.2.4 Tailings Characterization DOE characterized tailings fluid by sampling pore water from two lysimeters and I nine well points. Constituents exceeding EPA's proposed MCLs in tailings pore water are summarized in Table 2. DOE provides no analysis of hazardous constituents existing in the tailings in solid phase. ~ 5.3 Desian Features for Water Resources Protection i In accordance with draft 40 CFR 192.02(a)(3), DOE specifies features of the proposed disposal unit design needed for ground-water protection: (1) a contoured cover promoting sheet flow of runoff from the pile, (2) a 3.5-foot thick radon / infiltration barrier constructed of compacted weathered Mancos Shale, having a saturated hydraulic conductivity of 1 E-7 cm/s, and (3) an overlying filter layer with hydraulic conductivity 1 E-1 cm/s. 5.4 Disposal and Control of Residual Radioactive Material 5.4.1 Ground-Water Protection Standard Under Title I of the Uranium Mill Tailings Radiation Control Act of 1978, as amended, EPA's proposed ground-water protection standards in 40 CFR Part 192, Subparts A and C, require that disposal units be designed to control residual radioactive material in conformance with site-specific ground-water protection standards. EPA's proposed standards in Subpart C of 40 CFR Part 192 include provisions for supplemental standards as an exemption from the primary standards in Subpart A in special cases where compliance with the primary standard is not necessary to protect human health and the environment. Supplemental standards for ground-water protection may be implemented if any criteria in 40 CFR Part 192.21 apply. In order to propose supplemental standards, DOE should: (1) demonstrate that supplemental standards are applicable using the criteria in 40 CFR Part 192.21; (2) specify the supplemental standards that it proposes in lieu of the primary standard in Subpart A; (3) demonstrate that the proposed remedial action complies with the proposed supplemental standards; and (4) demonstrate that the proposed remedial action comes as close to meeting the otherwise applicable standard as is reasonable under the circumstances.

} ) 27 i i Table 2 - Maximum observed concentrations of EPA MCL constituents t in lysimeter and well points in Ambrosia Lake tailings and underlying unsaturated alluvium (adapted from 00E, 1989a; Table 0.8.11). Constituent MCL (ma/1) Maximum Arsenic 0.05 0.30 Barium 1.0 0.30 Cadmium 0.01 0.008

• I Chromium 0.05 0.11 Lead 0.05 0.74 Mercury 0.05 0.0001*

Molybdenum 0.1 247.0 Nitrate 44,0 3600.0 Selenium 0.01 0.403 Silver 0.05 0.02

• Uranium 0.044 14.7 Gross Alpha 15 pCi/1 N/A Radium-226/-228 5 pC1/1 220.0
• = not in exceedance of MCL i

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c 28 1 5.4.1.1 Applicability of Supplemental Standards DOE hes proposed application of supplemental standards for the Ambrosia Lake site in lieu of the primary standard based on the " Class III" applicability criterion in EPA's proposed standards in 40 CFR Part 192.21(g). As defined in the standards, Class III ground water is not a current or potential source of drinking water for any of three reasons, including inability of the aquifer to provide a sustained yield exceeding 150 gallons per day. DOE determines that the upper aquifer, composed of alluvium, weathered Mancos Shale, and the Tres Hermanos-C sandstone members, is incapable of sustained yield of 150 gallons per day. DOE's determination is based on the following: (1) A pump test was performed on well 675. It produced 0.35 gallons per [ minute (500 gallons per day). The water level was drawn down 13 feet in 12 hours, and the pumping could not be sustained. (2) Well 675 has the greatest saturated thickness of any wells completed in the uppermost aquifer, 'and is screened to the base of the saturated zone. Therefore, it probably produces the maximum available water. (3) DOE calculated that long-term output of 150 gallons per day at well 675 could not be sustained for more than 1 day. DOE further justifies application of supplemental standards at Ambrosia Lake because water in the uppermost aquifer is of limited extent, and is derived solely from infiltration of mining and milling water sources. Therefore, there is reasonable assurance that DOE is eligible to apply for supplemental standards because the uppermost aquifer at Ambrosia Lake contains Class III ground water, i

5. 4.1. 2 Proposed Supplemental Standards i

In lieu of naturally occurring ground water beneath the site, DOE indicates that existing water quality in the upper aquifer and background water quality are one and the same. DOE indicates the existing level of saturation in the upper aquifer will not be sustained after remedial action is completed. Therefore, 00E does not propose concentration limits for each of the hazardous constituents identified in 40 CFR 264.93. In addition, DOE proposes neither ground-water monitoring nor a point of compliance. Because the uppermost aquifer discharges in the subsurface into mine vents and shafts, all contaminants migrate downward into the Westwater Canyon Member. This migration has no affect on ground-water quality in that unit. Therefore, DOE's proposed supplemental standards for Ambrosia Lake are acceptable.

a. e 29 i I

5. 5 Cleanup and Control of Existing Contamination DOE is required to demonstrate compliance with proposed EPA standards in 40 CFR Part 192, Subparts B and C for cleanup and control of existing i

contamination. DOE's cleanup evaluation should consist of a (1) ground-water cleanup standard, (2) cleanup demonstration and (3) cleanup monitoring program. NRC may allow deferral of cleanup if DOE demonstrates that the disposal may proceed independently of cleanup. DOE has deferred demonstration of compliance because EPA's standards are not final. In addition. DOE is continuing hydrologic characterization of the site to provide a more thorough compliance data base. Based upon the determination that any water in the upper aquifer is Class III, it appears unlikely aquifer restoration will be required. In addition, discharge of the aquifer through mine shafts and vents means it is slowly being dewatered naturally. It is concluded that DOE's deferral of ground-water cleanup is acceptable until after EPA promulgates final ground-water protection standards. However, the deferral leaves final concurrence in the corrective action program an open issue.

5. 6 Conclusions i

In conclusion, DOE proposes application of supplemental standards under tne provisions of 40 CFR Part 192, Subpart C, for ground-water protection because the uppermost aquifer contains Class III ground water. It is concluded that DOE has provided adequate information to demonstrate that supplemental standards are applicable to Ambrosia Lake. Also, DOE has adequately justified that the proposed supplemental standards are protective of human health and the environment. The deferral of ground-water cleanup is acceptable, but remains an open issue. j i I

i l t 30 i 6.0 RADON ATTENUATION DOE intends to submit the design of the radon barrier after obtaining actual field construction data. The current design of the barrier is based on an i analysis presented in the Draft Remedial Action Plan (DOE, 1985; Appendix B). The DOE analysis utilizes a cross section similar to the generalized cross section shown in Figure 4. It models a 15-layer tailings system, each 2.5-foot thick layer being assigned an appropriate activity, covered by 10.0 feet of relocated tailings and 7.6 feet of offsite contaminated material (windblown). The RAECOM computer code (Rogers and others, 1984) was used to optimize the cover thickness. Activities and emanation coefficients were based on testing of the materials. Long-term moisture contents were based on engineering judgement, taking into consideration experience, in situ and placement moistures, and results of 15-bar testing. To verify DOE's analysis, the RADON computer code (NRC,1989c) was used to model a three layer tailings and cover system. Depths, activities, and long term moisture contents of the relocated tailings and windblown materials were taken from the DOE analysis. All other parameters for the tailings and windblown materials were conservatively assigned NRC " default" values programmed into the computer code due to the minimal amount of testing that had been performed. Due to the depth of in situ tailings below the cover, the tailings were not considered to signiTTcantly contribute to the flux and were i therefore not modeled. Average soil properties from four samples of the designated borrow were used to model the barrier's density and porosity. The long term moisture content used in the model was the average of the 15-bar test i results from two samples; both of which had reported in place moistures less than the 15-bar results. An average of all the available in place moistures, however, resulted in a moisture content greater than the average 15-bar moisture, so it was considered conservative to use the 15-bar results. This analysis resulted in a required barrier thickness of 2.3 feet, about i 1 foot less than DOE's analysis Therefore, DOE's estimated barrier thickness of 3.5 feet is considered reasonable for preliminary acceptance. The design of the radon barrier will be evaluated when it is submitted for review to ensure that the EPA standards for release of radon-222 to the atmosphere are met. Radon attenuation will remain an open item. j

31 7.0

SUMMARY

This Technical Evaluation Report (TER) summarizes the NRC staff review of the proposed remedial action for inactive uranium mill tailings at Ambrosia Lake, New Mexico. Open issues requiring resolution are discussed in this report. The deficient areas are discussed in sections 3.0, 5.0, and 6.0, and the issues are summarized in Table 1. Reviews of additional information provided by DOE will be presented at a later date as a Final TER, or as TER supplements. NRC's concurrence position on the proposed remedial action will be presented at that time. 5 i j

l e l 32 i REFERENCES Algermissen, S.T., Perkins, D.M.. Thenhaus, P.C., Hanson, S.L, and Bender. B.L.,1982, Probabilistic estimates of maximum acceleration and velocity in rock in the contiguous United States: U.S. Geological Survey Open File Report 82-1033. Abt. S. R., Khattak, M. S., Nelson, J. P., Ruf f, J. F., Shaileh, A., Wittler, R. J., Lee, D. W., and Hinkle, N. E.,1987, Development of Riprap Design Criteria by Riprap Testing in Flumes: U.S. Nuclear Regulatory Commission, NUREG/CR-4651, September 1988. Campbell, K.W., 1981, Near-source Attenuation of Peak Horizontal ground motion: Bulletin of the Seismological Society of America, v. 71, p. 2039-2070. DOE, 1989a, Remedial Action Plan and Site Design for the Stabilization of the Inactive Uranium Mill Tailings at Ambrosia Lake, New Mexico - Preliminary Final: DOE document number UMTRA-DOA/AL 050516.0000, t July 1989. DOE, 1989b, letter and response to comments, Mark Matthews, DOE to Edward Hawkins, NRC, August 4, 1989. 00E, 1989c, MK-Ferguson Company; UMTRA Project Ambrosia Lake Remedial Action Inspection Plan, September 1985. DOE,1987a, Uranium Mill Tailings Remedial Action Project, Ambrosia Lake, New Mexico; Design Calculations, Volumes I, II, III, IV, and V, February 1987. 00E, 1987b, Uranium Mill Tailings Remedial Action Project, Ambrosia Lake, New Mexico; Design Calculations, Volume VI, August 1987. 00E, 1987c, Uranium Mill Tailings Remedial Action Project, Ambrosia Lake, New I Mexico; Main Construction Subcontract, March 1987. DOL, 1987d, UMTRA Project - Ambrosia Lake Information for Bidders: by Morrison-Knudsen Engineers, Inc., San Francisco, Volumes I - V, October 1987. DOE, 1987e, Environmental Assessment; Remedial Action at the Ambrosia Lake Uranium Mill Tailings Site, Ambrosia Lake, New Mexico: Albuquerque, DOE /EA-0322, June 1987. DOE, 1985, Draft Remedial Action Plan and Site Conceptual Design for Stabilization of the Inactive Uranium Mill Tailings at Amorosia Lake, New Mexico: DOE document number UMTRA/00E-AL 05016.0000, December 1985. 00E,1984, Draf t Environmental Assessment, Davis Canyon site, Utah: DOE Document Number DOE /RW-0010.

33 Hunt, C.B., 1974 Natural Regions of the United States and Canada: San Francisco, W.H. Freeman and Company, 725 p. Nelson, J.D., Abt, S.R., Hinkle, N.E., Staub, W.P., Volpe, R.L., and van Zyl, D., 1986, Methodologies for Evaluating Long-Term Stabilization Designs of Uranium Mill Tailings Impoundments, Phase I Report: U.S. Nuclear Regulatory Commission, NUREG/CR-4620, July 1986. NRC, 1989a, Standard Format and Content for Documentation of Remedial Action Selection at Title I Uranium Mill Sites: Washington, D.C., Division of Low-Level Waste Management and Decommissioning, February 24, 1989. NRC, 1989b, Draft Staff Technical Position, Design of Erosion Protection Covers for Stabilization of Uranium Mill Tailings Sites: Washington, D.C., Division of Low-Level Waste Management and Decommissioning. NRC, 1989c, Calculation of Radon Flux Attenuation by Earthen Uranium Mill Tailings Covers: NRC Regualtory Guide, June 1989. NRC, 1988, Information Needs to Demonstrate Compliance With EPA's Proposed Groundwater Protection Standards in 40 CFR 192, Subparts A-C: Washington, D.C., Draft Technical Position Technical Branch, Division of Low-Level Waste Management and Decommissioning, NMSS. NRC,1985, Standard Review Plan for UMTRCA Title I Mill Tailings Remedial Action Plans: Washington, D.C., Division of Low-Level Waste Management and Decommissioning, October 1985. NRC, 1977, Design Construction, and Inspection of Embankment Retention Systems for Uranium Mills, Revision 2: NRC Regulatory Guide, December 1977. Patton, P.C. and Schumm, S. A.,1975, Gully Erosion: a Threshold Phenomenon: Geology, v. 3, p. 88-90. Rogers, V. C., Nielsen, K. K., and Kalkwarf, D. R., 1984, Radon Attenuation Handbook for Uranium Mill Tailings Cover Design: Washington, D.C., U.S. Nuclear Regulatory Commission NUREG/CR-3533. Santos, E.S. and Thaden, R.E., 1966, Geologic Map of the Ambrosia Lake Quadrangle, McKinley County, New Mexico: U.S. Geological Survey Geologic Quadrangle Map GQ-515. Simons, D.B., and Senturk, R., 1977, Sediment Transport Technology: Fort Collins, Colorado, Water Resources Publications. Stephenson, D., 1979, Rockfill in Hydraulic Engineering: Developments in Geotechnical Engineering, v. 27.

a i 34 U.S. Department of Commerce, 1984, Probable Maximum Precipitation Estimates, United States Between the Continent Divide and the 103th Meridian: National Oceanic and Atmospheric Administration, Hydrometeorologicai 7 Report No. 55. U.S. Department of the Interior, 1977, Design of Small Dams: Washington, D.C. U.S. Bureau of Reclamation, Water Resources Technical Publications. U.S. Army Corps of Engineers, 1970, Hydraulic Design of Flood Control Channels: Engineering Manual EM 1110-2-1601. Wells, S.G. and Gardner, T.W.,1985, Geomorphic Criteria for Selecting Stable Uranium Mill Tailings Disposal Sites in New Mexico: New Mexico Energy Research and Development Institute, NMERDI 2-69-1112, 353 p. F

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ooa o WM-67/140E/JPG/90/03/15/L DISTRIBUTION Docket File WM-67 'PDR/DCS-URF0 r/f ABBeach, RIV LLO Branch, LLWM JGrimm DJacoby RGonzales BGarcia, RCPD, NM EMontoya, EID, NM l l CONCURRENCE: DATE: 3//4[9, JGrimm/URF0/1v h f[f0 RGonzales/URF0 DJacoby/URF0 M,- J//9/@ EHawkins/URF0 'h S /9[93 4 1 5 ! /. 9. 0 REHall/URF0 ..y i t I i .j}}