2020-06-25_REVISION - C1981019 (6)

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RULE 2 PERMITS • Base flow in Good Spring Creek will be reduced by up to 7% during and for 45 years after mining. • Total dissolved solids in the base flow of Good Spring Creek will increase by 1.6%to 13.5% for several hundred years after mining has been completed,with sulfate the dominant increasing ion. • Base flows of Taylor Creek will not be reduced, and peak flow of Taylor Creek will be reduced 2%by the South Taylor pit. • No other statistically significant changes to surface water quality or quantity are anticipated. These consequences are discussed in the following subsections. Potential Impacts to Springs and Seeps Springs in the Colowyo Mine area result from three general sources: 1) typified by a relatively deep soil accumulation immediately upslope and shallow bedrock downslope of the point of discharge, 2)discharge within valley bottom deposits, and 3) from sheer bedrock faces on hillsides (CDM 1985b). The first two of these sources may mask or contribute to bedrock sources of the springs. The seeps and low volume springs flow generally in response to snowpack accumulation and subsequent melting resulting in seasonal flows. A total of 8 springs, which maintained flow for the month of July, contribute to base flows in the receiving streams adjacent to South Taylor, and were determined as a critical component of the hydrologic balance. Seeps and springs relevant to this permit revision are shown on Map 10. The majority of the springs, with bedrock sources, appear to be contact springs. A contact spring results from the infiltration of water from the surface to a porous zone (such as sandstone) above a horizontal hydrologic barrier (such as shale) where the water preferentially flows along the contact to the exposure. This type of spring is common in areas where alternating sequences of lithologies exist that exhibit differential hydraulic conductivities, such as the Williams Fork Formation. Springs that have a potential to be impacted by mining activities include 3-93-17-142, 3-93-17-432 (Taylor Creek), WFS-1 and -IA, WFS-2, WFS-4, WFS-5, and WFS-7 (West Fork Good Spring Creek), and GSCS-1 (Good Spring Creek). Springs that will be eliminated by the South Taylor pit include 3-93- 20-212 and 3-93-17-432 (Taylor Creek), 3-93-20-213, and 3-93-20-214 (West Fork Good Spring Creek). The FW source is an artesian well completed in the Trout Creek Sandstone that flows through a cracked wellhead and not a natural water discharge point. Table 2.05.6-1 lists the springs found in the vicinity of the South Taylor mining area. The locations of the investigated springs and seeps are presented on Map 10. The elevations of the springs were compared to the elevation of the confined groundwater of the Williams Fork Formation in well 84-0-013. The water level in this well was 7,054 feet above mean sea level in October 1984 (CDM 1985a). Of the base flow springs, GSCS-I, WFS-2, and WFS-2A are below this elevation and may result from confined groundwater recharge from the Williams Fork Formation. Data collected for the springs contributing to the base flow of the surface water system and that have a potential to be impacted by mining are summarized in Table 2.05.6-2. During peak flow, typically April or May, seven springs contribute a combined approximately 130 gallons per minute (gpm) [equivalent to 0.3 cubic feet per second (cfs)] into the West Fork Good Spring Creek. About 20 gpm (0.04 cfs) is contributed during base flow periods. Potential Impacts to Bedrock Groundwater Quantity No impacts are anticipated to the quantity of groundwater in the Williams Fork Formation or the Trout Creek Sandstone of the Iles Formation. Drilling and mining by Colowyo in the area identified very South Taylor/Lower Wilson—Rule 2,Page 78 Revision Date: 6/22/20 Revision No.: MR-220