ML24045A067
| ML24045A067 | |
| Person / Time | |
|---|---|
| Site: | WM-00073 |
| Issue date: | 02/14/2024 |
| From: | Hayes K NRC/NMSS/DDUWP/URMDB |
| To: | Frazier B US Dept of Energy, Office of Legacy Management |
| References | |
| Download: ML24045A067 (6) | |
Text
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Bill Frazier, P.E., LM Site Manager U.S. Department of Energy Office of Legacy Management 2597 Legacy Way Grand Junction, CO 81503
SUBJECT:
U.S. NUCLEAR REGULATORY COMMISSIONS STAFF REVIEW OF THE NOVEMBER 2020 DRAFT GROUNDWATER COMPLIANCE ACTION PLAN (GCAP) WORK PLAN FOR THE TUBA CITY, ARIZONA, DISPOSAL SITE
Dear Bill Frazier:
The U.S. Nuclear Regulatory Commission (NRC) staff is writing in response to the U.S.
Department of Energy, Office of Legacy Management (DOE-LM) document entitled, Draft Groundwater Compliance Action Plan (GCAP) Work Plan for the Tuba City, Arizona, Disposal Site, dated November 2020, (Received May 2023; Agencywide Documents Access and Management System [ADAMS] Package Accession No. ML23135A912). The following comments are provided:
1.
Section 1.2.1, Groundwater Contamination, states that background groundwater quality characterization for the N-Aquifer was presented in the Phase I GCAP (DOE 1999; ML20211C493) and the historical concentrations from the upgradient MW-901 monitoring well are provided in Table 1 of the current submittal. Table 1, titled Background Water Quality of the N-Aquifer, lists the background concentration of nitrate as 13 mg/L (which exceeds the current 10 mg/L standard for the contaminant).
Although, the MW-901 well, screened within the A horizon of the aquifer, has exhibited lower nitrate concentrations over the past two decades, generally between 2 and 5 mg/L, the deeper screened MW-910 (B horizon) and 911 (F horizon) wells located immediately adjacent to MW-901, also exhibit similar nitrate concentrations over the same period. This set of nested wells is located approximately 2,000 feet north (upgradient) of the site. Review of the 1998 Final Site Observational Work Plan for the site (ML20153D665) indicates the presence of four N-Aquifer supply wells for the former facility, 968, 970, 971, and 972, situated in a triangular layout surrounding the MW-901-910-911 well cluster. The approximately 700- to 900-foot-deep supply wells collectively provided approximately 200 gallons per minute (GPM) of production for the facility with production intervals extending from approximately A horizon to the M horizon of the N-Aquifer.
The historical nitrate concentrations in the supply wells are of interest with one of the highest concentrations, 35 mg/L, observed in the 971 well located over 4,000 feet north of the site in 1985. The 971 well nitrate concentration was almost three times higher than the background nitrate groundwater concentration listed in Table 1. Supply February 14, 2024
B. Frazier wells 968 and 970, located in closer proximity to the site (approximately 2,000 and 3,000 feet north or upgradient of the site, respectively), exhibited much lower nitrate concentrations during the same period (3 and 17 mg/L, respectively). The unusually elevated nitrate concentration in the distant 971 well may indicate the presence of preferential flow pathways connecting the production zone of the supply well to the developing site source area plume during plant operations. Although other nitrate sources may have contributed to the 971 historical nitrate concentration and concentrations have shown a reducing trend in the nearby MW-901-910-911 well cluster, the presence of elevated nitrate in the former supply well may indicate the need for additional evaluation of the vertical extent of the plume in the direction of the 971 well and the former wellfield cone of depression.
NRC recommends DOE-LM consider this site data as the proposed investigation takes shape, particularly with respect to Nuclear Magnetic Resonance (NMR) geophysical logging locations, the use of Passive Flux Meters (PFMs) in long-screened wells, vertical delineation of the nitrate plume, and defining the overall extent of impacted groundwater. Of course, the results of the most recent drilling, downgradient well installations and sampling events, not yet provided to NRC, will also guide the proposed investigation. NRC looks forward to reviewing the most recent downgradient nitrate groundwater data for the site.
2.
Section 1.3.1 in the draft GCAP work plan report briefly discusses the draft 2016 Groundwater Flow Model for the Tuba City disposal site and how it might be utilized in the GCAP work plan. The NRC staff had previously reviewed the DOE report entitled Draft Groundwater Flow Model for the Tuba City, Arizona, Disposal Site dated February 2016 (ML16053A017) and had several comments (ML16075A139) to which the DOE responded (ML17118A155). NRC staff replied (ML18044A722) to DOE responses and stated that four of the DOE responses to NRCs 23 comments were considered adequate responses. The DOE stated in ML17118A155 that additional data to address data gaps, additional support to bolster assumptions, and further refinement of critical input parameters through sensitivity analyses would be used to support the next iteration of groundwater model development and the next revision of the groundwater flow and transport model report. NRC staff expects that the remaining 19 outstanding comments from ML16075A139 will be addressed after the activities listed above have been completed and that each comment will be adequately addressed in a response to NRC.
3.
For the estimates of historical and future water infiltration volume in Section 1.3.2, the estimated infiltration volume for the milling period differs between the minimum and the maximum infiltration cases, i.e., an estimated 381 million gallons were assumed to infiltrate to the subsurface through the footprint of the mill tailings for the minimum infiltration case while 514 million gallons was given as the estimated volume for the milling period in the maximum infiltration case. Since the rate of cover degradation is not relevant during the milling period, it is not clear why there is a difference in the estimated volume unless this difference represents the uncertainty of water used during the milling period. The NRC staff would be appreciative if the DOE could provide the basis for the difference.
4.
The color coding used to represent contaminant concentrations in some of the Appendix A figures is less than correct. For example, in Figure A-1, referenced in Section 3.1.1.1, the representation of the uranium plume around wells 267, 275, and 290 does not match
B. Frazier the concentrations shown in the legend. The same holds true for wells 290, 275, 1003, 1004, and 930 in Figure A-31, referenced in Section 3.1.3.1. The outline of the sulfate plume would differ from that shown in the figure if the color legend and interpolation had been followed. In Figure A-16, referenced in Section 3.1.2.2, it is unclear why the nitrate plume outline stretches so far to the northeast when most of the values, with the exception of the evaporation pond area wells, are below 10 mg/L. Please evaluate the referenced maps and provide explanations and/or corrections, as appropriate.
For future submittals, NRC highly recommends that contoured plume location slice maps at appropriate depths/elevations across the site be prepared for NRC review (including separate maps for the primary contaminants of concern). NRC recommends the maps include labeled monitoring wells, screened within the depth/elevation interval, with the appropriate contaminant concentrations included with each labeled monitoring well.
Nested vertical extent wells can also be shown on the maps with contaminant concentrations, for reference. Given the elevation change across the site, water table monitoring well maps and subsequently deeper slice maps may cross multiple horizons within the N-Aquifer. The requested depth specific plume maps should provide a much clearer visual representation of the degree of plume delineation provided by the site monitoring well network. Depth specific groundwater flow maps are also recommended for future submittals.
5.
In Section 4.2, Identify the Study Objectives, under study objective O2, Evaluate the Hydrology and Geochemistry of the Lower Terrace, study questions such as the following were proposed:
o Can zones of relatively higher and lower hydraulic conductivity be identified on the lower terrace?
o Are there preferential pathways of plume migration?
The evaluation of potential preferential pathways for plume migration appears to be warranted within the greasewood/slope, middle terrace, and upgradient areas as well.
Reviews of geologic logs for site monitoring wells in areas ranging from the upgradient MW-901-910-911 well cluster to source area, vertical extent, and downgradient wells indicate the potential presence of preferential flow pathways within each of the logged N-Aquifer horizons (A through I). Examples include:
o Middle Terrace monitoring well MW-251 - Driller reports borehole interval at approximately 4920-foot elevation in C horizon near the KNTZ upper contact making noticeable amount of water for several feet.
o Greasewood Area C horizon monitoring well MW-930 logging multiple horizontal fractures cutting across crossbedded sandstone within the screened interval (approximately 4933-to 4903-foot elevation) as well as unconsolidated sands (SP) interbedded with the sandstone at intervals of 1 to 3 feet.
o Possible loss of drilling fluids within the E horizon (approximately 4836-foot elevation) of the MW-903 borehole located on the slope between the middle and lower terrace.
o Poor core recovery, drilling washout zones, and/or low RQD values in multiple boreholes potentially indicating unconsolidated sands and/or highly fractured rock zones.
o The logged core in upgradient monitoring well MW-911 described heavily fractured rock intervals (horizontal to inclined fractures); interbedded
B. Frazier unconsolidated sands with cross bedded sandstone intervals; soft, weakly cemented sandstone intervals; and vertical fractures ranging from 0.25 to 1 foot in length within most of the horizons from A through F. A 0.5-foot vertical fracture was noted in the core at the approximate contact between the A and underlying B horizon.
The use of NMR logging and PFMs will hopefully provide some helpful insight into the groundwater flow regimes in the subsurface (matrix vs. fracture flow; unconsolidated sands vs. well cemented, less permeable rock, etc.).
NRC recommends DOE-LM consider the use of vertical groundwater profiling, likely using a sonic drilling rig, to collect samples at discrete, isolated zones for laboratory analyses at frequent intervals for a more detailed mapping of the groundwater plume concentrations with depth. The vertical sampling intervals can provide multiple contaminant concentrations within each aquifer horizon (as appropriate) at critical locations around and on the site to better delineate the contaminated zones within the N-aquifer. The NMR, PFM, and most recent groundwater sampling results can be used to determine the appropriate groundwater sampling depths and the surface locations of the vertical profiling boreholes. The vertical profiling could be conducted in conjunction with the drilling and laboratory work for the solid-phase subsurface sampling described in Section 5.1.1 of the submittal. The vertical profiling groundwater results can then be used to select appropriate locations and screened intervals for the placement and construction of permanent monitoring wells at the site.
6.
Section 5.1.2 states that, Construction of 24 wells of 4-inch diameter up to a total depth of 200 ft is preliminarily proposed as the conservatively high level of effort required for plume delineation on (1) the northern extent of the lower terrace downgradient of the current sentinel wells, (2) the west and southwestern margins of the current nitrate plume, (3) the northeastern margin of the current nitrate plume, and (4) beneath the disposal cell. NRC staff strongly agrees with these preliminary proposals but would like to suggest modifications to two of the four activities, and one additional activity, in order to gather risk-significant data which may be crucial for constructing a conceptual model of the Tuba City site.
o With regards to the 1st activity (i.e., installing monitoring wells at the northern extent of the lower terrace downgradient of the current sentinel wells), NRC staff would suggest to linearly expand these proposed lower terrace monitoring wells along the boundary with the middle terrace to the northeast and to the southwest.
The northeast wells should be downgradient of the evaporation pond, specifically the well 290 (B horizon screened well, 10-foot screen) which has been recorded to have higher levels of contaminant concentrations. The southwest wells should be downgradient of the wells 267 and 281. Appropriately placed, nested vertical extent monitoring wells will also be needed to define the dissolved plume. The vertical profiling of groundwater recommended for consideration in the first comment can provide valuable information on the number of permanent monitoring wells needed and the screened intervals required for delineation.
Monitoring wells downgradient of existing wells 267 and 281 would complement the 2nd activity on delineating the west and southwestern margins of the nitrate and sulfate plume.
o With regards to the 4th activity (i.e., installing monitoring wells beneath the disposal cell), NRC staff would also suggest installing monitoring wells
B. Frazier beneath/immediately adjacent to the evaporation pond due to the higher concentration values around the pond and due to the risk significance of such a potential leak in the liner of the pond.
o In addition, NRC staff would suggest an activity that specifically targets potential preferential flow in the Tuba City area as discussed in Comment 2, e.g., pumping tests, etc. Section 5.1.2 does not include an activity that would collect data to determine the source of the higher contaminant values in wells 930, 1003, 1004, and 691. Such data may assist in the construction of a conceptual model that would explain the cause of these relatively high values.
7.
An additional cluster of monitoring wells with relatively high contaminant concentration values can be found around the evaporation pond. For example, Figures A-1, A-16, and A-31 show monitoring wells 268, 275, and 290 with relatively high values. It is worth noting that the three previously listed wells are screened within N-Aquifer horizons varying from relatively shallow to deep with well 268 screened across the D, E, and F horizons with a 100-foot-long screen, and the 275 and 290 wells screened across the D and B horizons, respectively, with screened interval lengths ranging from 10 to 20 feet.
Section 5.1.2 does not seem to present a plan to collect data to find the source of these higher values, although it does propose to determine the boundary or outline of water with higher contaminant concentrations. The NRC staff encourages the DOE to include an activity to determine the source of the relatively high contaminant concentration values detected in monitoring wells 268, 275, and 290. A leaking evaporation pond could significantly reduce the time it takes for contaminants to reach the Moenkopi Wash and therefore should be considered a risk-significant issue and be included in the GCAP work plan.
8.
Preferential pathways between the terrace and floodplain for contaminant flux is listed as a bullet under Section 5.2.2; however, it is not clear by what method site and contaminant source characterization data will be collected to support or refute a conceptual site model involving preferential pathways. Section 5.1.7 also does not present a plan to collect data that may either confirm or refute fast pathways as a potential method of contaminant transport in the Tuba City disposal area. NRC staff had previously commented on this topic in Comment 1.a. of ML15209A624 which discusses a possible fracture-dominated hydraulic connection between the disposal cell area and the lower terrace. In addition, the DOE-LMs 2013 annual ground water report [Annual Groundwater Report April 2012 Through March 2013 Tuba City, Arizona, Disposal Site August 2013] discussed the potential for a hydraulic connection to the disposal cell by way of a fracture zone (see Section 3.1.2 and again in the section entitled Lower Terrace Monitoring Wells on page 22).
Information from the draft GCAP work plan again shows relatively elevated concentration values for uranium, nitrate, and sulfate for the monitoring wells 691, 1003, 1004, and 930. DOE-LM should determine if a preferential pathway connection exists and if it is impacting the groundwater near the site. DOE-LM should determine the source of the relatively high contaminant concentration values detected in monitoring wells 691, 1003, 1004,and 930. A preferred contaminant pathway or pathways could significantly reduce the time it takes for contaminants to reach the Moenkopi Wash and therefore should be considered a risk-significant issue and be included in the GCAP work plan. NRC requests that DOE-LM consider the technical suggestions provided in
B. Frazier the previous comments, including vertical groundwater profiling and targeted aquifer testing, in the evaluation of preferential pathways in the N-Aquifer beneath the site.
In accordance with Title 10 of the Code of Federal Regulations 2.390, Public inspections, exemptions, requests for withholding, of the NRCs Agency Rules of Practice and Procedure, a copy of this letter will be available electronically for public inspection in the NRC Public Document Room or from the Publicly Available Records component of the NRCs ADAMS.
ADAMS is accessible from the NRC Web site at https://www.nrc.gov/reading-rm/adams.html.
If you have any questions concerning the NRC review of the work plan, please contact me at 301-415-0549 or by email at Kevin.Hayes@nrc.gov.
Sincerely, Kevin R. Hayes, P.G., CPG, Hydrogeologist Uranium Recovery and Materials Decommissioning Branch Division of Decommissioning, Uranium Recovery, and Waste Programs Office of Nuclear Material Safety and Safeguards Docket No. WM-00073 cc: Gunnison ListServ List Signed by Hayes, Kevin on 02/14/24
Ltr ML24045A067 OFFICE NMSS/DUWP/URMDB NMSS/DUWP/URMDB NAME RVonTill KHayes DATE Feb 14, 2024 Feb 14, 2024