2023-04-07_REVISION - M2001046 - Laserfiche WebLink

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Nix Site Groundwater Contamination Potential March 31, 2023 implementation of water quality standards established by the WQCC. The 2022 annual report submitted by DRMS notes that, "As of this reporting period, the Minerals Program requires approximately 31 mine sites to conduct some type ofgroundwater quality monitoring. Of these sites,26 are hard rock mining operations, and 5 are construction material extraction operations."Considering that hard rock mining operations have substantially greater potential to impact water quality, particularly with respect to water quality constituents that have more severe health concerns, it makes sense that DRMS has focused these monitoring requirements on hard rock mines. Of the five construction materials sites listed in the report, four sites involved extraction of limestone for cement production and only one site (permit no. M2017-036) involved sand and gravel extraction. It is important to note that the M2017-036 site is directly adjacent to denser residential, commercial, and industrial areas than the Nix Site and is also not directly adjacent to several other aggregate mine sites that are not subject to the same monitoring requirement. 3. GROUNDWATER FLOW AND CONTAMINATION POTENTIAL Cell 1A will be unlined and used to supply water to the concrete plant to be located in the area of Cell 6. Cells 2-6 will be surrounded by low permeability slurry walls prior to mining operations in those cells. The slurry walls will be keyed into the underlying low permeability bedrock to effectively hydrologically isolate the area from the surrounding groundwater. By virtue of the slurry walls and low permeability bedrock,the potential for groundwater contamination by operations in these areas is minute. This means that any significant potential for contamination to the groundwater system would be based on the potential for contamination at the unlined ponds in Cell 1. Based on RMCC's monitoring data collected from eight monitoring wells (Figure 3) near the permit boundaries,the direction of groundwater flow in the area is generally to the west-northwest with an average groundwater gradient across the site of approximately 0.004 to 0.005. Using a hydraulic gradient of 0.0045 and a hydraulic conductivity of 350 ft/day based on aquifer data in the Colorado Division of Water Resources Decision Support System (CDSS),the groundwater flux through the permit boundary is about 325,000 ft3/day,which would act in-part to dilute any diffuse discharges of pollutants that could occur at the site. It should be noted that water levels in monitoring wells near the western and southern edges of the permit boundary are likely significantly affected by dewatering and/or cell lining occurring at the neighboring sites (DRMS Permit Nos. M 1999-006 and M2015-033). Dewatering operations will make the water levels lower than the typical static water level absent the dewatering,while lining can tend to make water levels higher on the upgradient side of the liner. It seems apparent from data from Well MW5 that the depression in the water table from dewatering at the M2015-033 site coupled with the clay DINATALE WATER CONSULTANTS 4