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Wattenberg Mine, Exhibit G—Water Information May 2023 3.0 HYDROGEOLOGIC SETTING Geology The geology mapped at land surface beneath the site consists of alluvial deposits comprised of sand, silt, and gravel primarily(Figure G-2)of the Piney Creek and Post-Piney Creek Alluvium units. On site wells(Figure G-3)were installed in 2002, however no boring logs were found. The depth of the wells on site indicates up to 25 feet of alluvial material overlaying bedrock. Most wells appear to be highly transmissive. Water levels measured in the wells are generally less than 10 ft deep and approach the ground surface in some cases(see hydrographs in Appendix G). The shallow bedrock of the Laramie-Fox Hills aquifer consists of weathered, consolidated sedimentary rock varying from claystone to siltstone. In comparison to the alluvium, the weathered bedrock is expected to be several orders of magnitude less transmissive due to the consolidated, finer-grained properties. Groundwater Groundwater at the site represents a combination of water that flows through the high-permeability alluvial deposits parallel to the South Platte River and water that infiltrates due to the agricultural fields that dominate the land use to the west of the river. As groundwater approaches the river, groundwater flows in a direction that is near parallel to the river due to the constant interaction with the river stage within the porous sands and gravels. The direction of regional shallow groundwater flow is therefore toward the northeast but changing to a near northerly direction near the river. This is especially apparent during the winter when compared to the end of the agricultural season in late August(Figures G-4 and G-5, respectively) and may be due to the influence of agricultural runoff coming from west of the site. Monitoring well MW-6 has limited groundwater elevation data as this well was inaccessible and not monitored prior to 2020. The small amount of available data shows limited water in this well compared to the saturated thickness of the other site wells (Appendix G). This well was refurbished in August 2021 which resulted in an additional 3 feet of water in the well.Assuming the well was installed correctly and is not damaged, it is possible this discrepancy is due to the upgradient slurry wall around the Parker-Dersham Pond, which may be casting a groundwater shadow on this well. In addition, while dewatering has taken place previously, the most recent dewatering of the unlined Struck Pond began in late 2022. The resulting water is routed to the unlined Stillwater Ski Lake, where the effect of additional water into this lake is noticeable as shown in Appendix G and Figure G-4. Existing Monitoring Wells A network of monitoring wells was installed in 2002 to characterize the groundwater conditions at the Wattenberg Lakes property (Figure G-3). The boreholes for the wells were drilled to the bedrock contact and typically have a high saturated thickness. These wells generally show the direction of groundwater flow to be in a northeasterly to northerly direction, generally in equilibrium with the river water surface elevations, although as discussed above MW-6 may be exhibiting signs of a localized groundwater shadow caused by the upgradient Parker-Dersham Pond slurry wall. Well Inventory In April 2023, a well inventory of the Affected Area and adjacent areas was conducted to identify wells near the site. The inventory included a search of the State of Colorado Office of the State Engineer database of wells located within 1 mile of the Affected Area (Table 1). The well inventory identified 442 wells within 1 mile of the Affected Area. Figure G-6 shows the Affected Area and the constructed well locations on file with the Colorado Division of Water Resources, where the numbers shown on Figure G-6 correspond to the first column numbers shown in Table 1. The majority of these wells are screened in the shallow alluvial material, although a handful are noted to be screened in the Laramie-Fox Hills aquifer and the Lower Arapaho aquifer. Wells in this area are noted to be used for irrigation, domestic use, stock water, monitoring, industrial, and municipal use. OTETRA TECH P a g e 3 1 8