2021-04-20_REVISION - M1983033 (20)

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Walstrum Spur Amendment, Exhibit G— Water Information April 2021 between Highway 6 and Clear Creek, at the east end of the Texaco Placer. Pending final design and approval by CDOT, the structure will likely be constructed in late 2021. The quarry only discharges water after heavy precipitation events. These stormwater discharges are managed under the State of Colorado approved discharge permit #COG500268. The site is covered by a stormwater management plan in accordance with the State permit. Any stormwater detained will be discharged within 72 hours or accounted for in the water accounting reports. Overall, the q�arry has little to no effect on the water quality of the watershed, Clear Creek Habitat, or aquatic life in the area. This is due to investments by AFS in infrastructure and BMPs to reduce or eliminate the discharge of potential pollutants. In addition, AFS has voluntarily promoted habitat restoration throughout the watershed, including supporting reclamation of 19th century legacy metal mines, habitat enhancement by the Clear Creek Watershed Association, and West Denver Trout Unlimited projects. 8.0 GROUNDWATER The movement and occurrence of groundwater in the Spur Amendment and surrounding area were evaluated utilizing the following key elements: Performance of a detailed hydrogeologic reconnaissance including review of test pit data Review of well records from the Colorado Division of Water Resources (DWR) Review of existing information including previous Walstrum Quarry DRMS permits and published geologic maps prepared by the USGS The results from implementing the key elements are described below. 8.1 Hydrogeologic Reconnaissance During the period from April through September 2020, a hydrogeologic reconnaissance was performed of the Spur Amendment, the Existing Quarry, and surrounding watershed. The investigation included an assessment of conditions indicative of the potential for flow and included a survey of surface expression of groundwater seepage and an evaluation of vegetative patterns on the landscape. Evidence for groundwater at the surface was evaluated along the Tributary of North Clear Creek, North Clear Creek, and Clear Creek between the mine entrance and the intersection of Highway 119 and Highway 6 at the confluence of the two creeks. The evaluated locations are shown in Figure G-5 and are summarized in the following sections. An annotated set of photographs for the survey is provided in Appendix G-1. Evaluation of the possibility of shallow groundwater in the colluvium at the surface was performed with a series of test pits dug at six locations (Figure G-2) in June 2020. No evidence of saturated or moist conditions was observed in any of the test pits. The presence of grass in drainages suggests that seasonally there may be sufficient surface moisture to support growth. However, there is insufficient water to produce saturated conditions. 8.1.1 Unnamed Tributary of North Clear Creek The Tributary is present in Gilpin County north of the Spur Amendment. No direct indication of seepage which could result from a regional water table in the form of springs was visible in the Tributary. However, a small, approximately 1 to 2 gallons per minute (gpm) flow was observed in the upper reach of the Tributary. Observations made during the hydrogeologic reconnaissance indicated that flow did not increase in the downstream direction. The Tributary was dry from a distance of approximately 1,000 feet continuing downgradient to the confluence with North Clear Creek. Photograph locations 1 through 6 in Appendix G-1 show the flow present in the Tributary. Moss was observed above the north bank of the Tributary in a few areas indicating shallow soil moisture. The source of moisture supporting the moss is likely the seasonal melting of the snowpack with associated slow soil drainage and the north-facing orientation of the hillside. Based on this, flow in the surface drainage is likely higher during the spring and early summer as the shallow moisture from the melting snow upslope saturates the shallow soils near the base of the hill, discharging in the form of temporary surface flow. The Spur Amendment does not cross the drainage divide at the top of this slope. Consequently, the proposed disturbance area should not interrupt this small amount of water flow. � TETRA TECH P a g e 5 � 7