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Homestake Mining Company of California

Thomas Wohlford . Grants Project

U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555-0001

Mr. Ron C. Linton, Project Manager

19 October 2018

Uranium Recovery and Material Decommissioning Branch Division of Decommissioning, Uranium Recovery, and Waste Programs Office of Nuclear Material Safety and Safeguards Mail Stop: T-5AI0 U.S. Nuclear Regulatory Commission Washington, DC 20555

Closure Manager

RE: Homestake Mining Company of California - Grants Reclamation Project - NRC License SUA-1471, San Mateo Creek Basin Groundwater Model Update

Dear Mr. Linton: '

In March 2018, Homestake Mining Company of California (HMC) submitted a Groundwater Flow and Transport Modeling Work Plan (Work Plan) for the Grants Reclamation Project (GRP) to the U.S. Nuclear Regulatory Commission (NRC) (ML18093A641). The Work Plan outlined a recommended approach for development of a new groundwater flow and transport model spanning the entire San Mateo Creek (SMC) Basin, including the area of the GRP. This modeling effort's primary objective is to provide a tool that simulates current GRP groundwater restoration activities and can predict system changes in response to basin-wide remediation efforts, including an understanding of the long-term fate and transport of constituents of concern (COCs). This Groundwater Modeling Progress Update memorandum describes the status of model development and includes discussion of adjustments made during this process, which represent changes from the original Work Plan.

Geologic Model Development

As an initial step toward creating a groundwater flow and transport model, the Work Plan describes development of a 3-D geologic model that captures stratigraphy and faulting at both the site scale around the GRP and regionally within the SMC Basin. The geologic model is then used to create appropriate hydrostratigraphic layer structure within a USGS MODFLOW environment. Figure 1 provides a representative north-south trending cross section through SMC Basin that was generated using the 3-D geologic model.

Homestake Mining Company P.O. Box 98, Grants, NM 87020 Tele: (505) 287-4456 Fax: (505) 287-9289

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Figure 1

Letter to NRC RE: SMC Basin Groundwater Model Update

Representative Geologic Cross Section

Groundwater Flow and Transport Model Development

The Work Plan also described development of both a site-specific model and a simplified regional scale model that would be coupled using an advanced MODFLOW modeling technique known as Local Grid Refinement (LOR). MODFLOW-LGR iteratively couples local- and regional-scale flow model solutions to maintain solution consistency where the site-specific model domain intersects with the regional-scale model. After additional consideration, this methodology was deemed unnecessarily complicated.

An alternative approach was implemented where a single regional-scale model covering the entire SMC Basin has been developed. This construct will ultimately incorporate appropriate grid cell-size resolution at the GRP for simulation of both the fate and transport of COCs and prediction of remediation time frames for the GRP using the current groundwater pump and treat (and re-injection) approach. The model simulations will be performed using a recent improvement to the USGS MODFLOW-2005 code known as MODFLOW-NWT, which is effective at handling numerical instability often associated with the drying and rewetting of cells. A 3-0 visualization of the current model domain, layering, and hydrostratigraphy is provided in Figure 2 .

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Homestake Mining Company P.O. Box 98. Grants. NM 87020 Tele: (505) 287- 4456 Fax: (505) 287- 9289

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Figure 2

Letter to NRC RE: SMC Basin Groundwater Model Update

3-D Oblique View of Current Model

Initial efforts to develop the single model construct included 18 layers, which allowed very detailed representation of hydrostratigraphic units found beneath the GRP area and at the regional basin scale. Initial model layering was created as follows:

1. Alluvium 4. Mulatto 7. Dakota 10. Chinle 13. Middle 16. Chinle Tongue Sandstone Shale Chinle Shale

2. Point Lookout 5. Gallup 8. Morrison 11. Upper 14. Chinle 17. San Sandstone Sandstone Fm. Chinle Shale Andres Fm.

3. Crevasse 6. Mancos 9. Entrada 12. Chinle 15. Lower 18. Glorieta Canyon Fm. Shale Sandstone Shale Chinle Sandstone

Table 1- Initial Proposed 18-layer Model Hydrostratigraphic Units

Initial steady-state MODFLOW-NWT 18-layer model simulations led to lengthy run times and model solution nonconvergence. Based on comments from the third-party reviewer, HydroGeo Group, the Upper San Mateo Creek Basin bedrock units were lumped together, to reduce the overall initial complexity of the model. The simplified I 0-layer model honors critical hydrostratigraphic units at the GRP area but aggregates regional bedrock formations located north and north-east of the GRP. This 10-layer steady-state groundwater flow model is characterized by minimal run times and convergent solutions and is also currently reasonably calibrated to pre-l 950s groundwater elevations for both the alluvium and the San Andres/Glorieta (SAG) aquifer. The third-party reviewer comments are included as Attachment A .

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Homestake Mining Company P.O. Box 98, Grants. NM 87020 Tele: (505) 287- 4456 Fa x: (505) 287- 9289

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Letter to NRC RE: SMC Basin Groundwater Model Update

l. Alluvium 3. Chinle 5. Chinle 7. Chinle Shale 9. Chinle Shale Shale Shale

2. Bedrock above 4. Upper 6. Middle 8. Lower 10. San Andres Chinle Chinle Chinle Chinle /Glorieta

Table 2 - Current 10-layer Model Hydrostratigraphic Units

Next Steps

HMC is currently working to achieve a transient model calibration starting from the parameterization implemented within the pre-development period model. Current calibration efforts are focused primarily on the immediate GRP area while maintaining representative simulated groundwater elevations at the regional scale. This approach minimizes the possibility of an unreasonable regional solution negatively affecting the GRP area model solution.

HMC will be finalizing the calibration datasets for the Lower San Mateo Creek Basin, which will include selected locations in the alluvium, upper, middle and lower Chinle and the SAG. If sufficient information is available, we will be aggregating the calibration datasets into training, validation and testing subgroups. We will specify in our next update the calibration locations, targets and metrics used to establish calibration. It is also our goal to continue to add additional layers representing the individual Upper San Mateo Creek Basin bedrock formations.

If you have any questions, please do not hesitate to contact me via e-mail at twohlford@homestakeminingcoca.com or via phone at 505.290.2187.

Respectfully,

Thomas P. Wohlford, CPG Closure Manager Homestake Mining Company of California Office: 505.287.4456 x34 I Cell: 505.290.2187

Copy To:

A. Winton, NMED (electronic and hard copy) M. Purcell , EPA (electronic copy) M. McCarthy, Barrick, Salt Lake City, Utah (electronic copy) C. Burton , Barrick, Henderson, Nevada (electronic copy) G. Hoffman, Hydro-Engineering, Casper, Wyoming (electronic copy) R. Whicker, Environmental Restoration Group, Albuquerque, New Mexico (electronic copy)

Homestake Mining Company P.O. Box 98, Grants. NM 87020 Tele: (505) 287-4456

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Letter to NRC RE: SMC Basin Groundwater Model Update

ATTACHMENT A-Third Party Review Update

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P.O. Box 98. Grants, NM 87020 Tole: (505) 287-4456 Fax: (505) 287-9289

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HGG I HYDROGEO G R O U P

Capitalizing on Innovation, Technology, & the Evolution of Knowledge

Dear Tom:

Memo Homestake Mining Company (HMC)

Grants Reclamation Project: Model Review and Validation

Octo ber-2018

TO: Thomas Wohlford, Closure Manager

Homestake Mining Company

FROM: Michelle Smilowitz, President

HydroGeo Group

CC: Johnny Zhan Ph.D., Senior Manager,

Hydrology I Barrick Gold

DATE: October 19th, 2018

In April 2018, I joined the HMC modeling team to provide third-party review and guidance during the model development process for the Grants Reclamation Groundwater Modeling project. My involvement includes participating on conference calls, review of modeling files, model validation, recommendations and general oversight. These progress reports are intended to maintain transparency and achieve consensus within all groups as we move forward with the model development.

The following is a summary of my involvement and progression of recommendations.

On April 261h, 2018, I met with Adam Arguello onsite at the Homestake Mining Company's (HMC) Grants Reclamation Project Facility (GRP Facility). After that meeting, I became involved with model development and review .

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HGG I HYDROGEO G R O U P

Capitalizing on Innovation, Technology, & the Evolution ef Kn0111ledge

The first iteration of the numerical model followed an 18-layer schema. As a result of the complex design, the model was unstable and convergence issues delayed progress. I made a recommendation to the modeling team to simplify model layering to achieve stable solutions that converge quickly. Complexity will be added after.

Although simple models do not capture every aspect of the local systems, typically most groundwater systems can be described in gross terms when classified by Era. With most groundwater flow models, if the geology is accurate, when water is added to a model that correlates with Era, the model will typically converge quickly. The underlying groundwater flow systems beneath GRP can be described as Cenozoic, Mesozoic, and Paleozoic deposits, which correlate with the alluvial system, the Chine group, and the SAG, respectively (Figure 1) .

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HGG I HYDROG EO G R O U P

Capitalizing on Innovation, Technology, & the Evolution of Knowledge

Shallow groundwater comes from upgradient mountain front recharge and flows downgradient within the alluvium along the natural relief of the upper shales of the Chinle Formation, which at the GRP Facility is to the south and southeast. As water moves through the alluvium, it moves vertically and horizontally along the path of least resistance. Groundwater flow in the Chinle Formation and the SAG follows the dip of the formations and generally flows downward to the northeast and southeast, respectively.

The transition to a simple model design resulted in a stable model that converged quickly. Once that directive was accomplished, complexity was added to better define the Upper, Middle, and Chinle aquifers. This complexity was added to improve the distribution of recharge in the model.

I have also been performing model validation concurrently to guide my recommendations and to continue methods for maintaining transparency throughout the project.

MODEL VALIDATION

The first set of model validation includes taking 16 published cross sections and comparing those to model layer assignments. The results of this validation and a presentation of selected cross section templates with the new 10-layer construct will be presented to the NRC in a separate, subsequent deliverable. This process is designed to establish correlations with published geology and HRC model layering and provide an added level of confidence in the model construct. These cross section templates will also be used throughout the modeling to document calibration in forms of cross sections and animations and to provide a transparent approach and simple approach for presenting our progress.

We look forward to maintaining an open line of communication through this project and welcome feedback along the way.

Warm est regards,

Michelle Smilowitz, MES HydroGeo Group, President

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