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• concentrations in the backfill aquifer decline. This mechanism is described in more detail under the <br />Groundwater Impacts discussion, Section 4.8.3.2. <br />No measurable changes in water quality attributable to mining are anticipated in the Yampa River. The <br />surface water flow rates typically associated with spoil spring discharges are very small during base flow <br />periods and insignificant as compared to the Yampa River flow. The majority of the base flows from the <br />Trapper Mine drainages will not make it to the Yampa River due to infiltration and evaporative losses <br />along the way. <br />Major constituents in the base flow in Johnson Gulch are being affected by discharge from the backfill <br />aquifer in this area. The total dissolved solids (TDS) concentrations have increased roughty 1000 mg/1 <br />over baseline conditions. Typical base flow in Johnson Gulch is approximately 20 gpm (0.045 cfs). <br />Combining this water with typical base flow conditions for the Yampa (200 cfs and 280 mgll TDS; based <br />on data from USGS Gauging Station 09247600, Yampa River below Craig, CO) results in an estimated <br />TDS baseline concentration of 280.2 mg/I in the Yampa River below Johnson Gulch. <br />TDS Yampa River = 1000 mall(20aoml + 200 cfs 1448.8 aomlcis11280 mall) = 280.2 mgll <br />• Below Johnson Gulch (20 gpm + (200 cis) (448.8 gpmlcfs)) <br />This represents a baseline addition of 0.2 mg/I TDS in the Yampa. This potential variation will be <br />undetectable in the Yampa River due to the much larger natural variations in TDS concentrations that <br />occur. Trapper might develop additional drainages that will contain increased TDS concentrations in spoil <br />springslerived base flows. However, some of the backfill aquifers at Trapper contain water that is <br />essentialty the same quality as the baseline surface water quality (see backfill well GF-5 as compared to <br />the No Name drainage surface water data). Trapper projects that under a worst case scenario where <br />multiple drainages wntribute TDSaffected base lbws to the Yampa River, TDS concentrations in the <br />Yampa might be increased by approximately one tenth of one percent. The actual increase will probably <br />be significantly less than this due to the likelihood that such small flows from the mine will never reach the <br />Yampa River. <br />The Annual Hydrologic Reports (Appendix V~ present summaries and discussions of surface water <br />quality for selected water quality constituents at monitoring locations throughout the project site. <br />4.8.3.2 Groundwater Impacts <br />• The following sections present discussions of the probable hydrologic consequences due to mining on <br />groundwater quantity and groundwater quality. These sections are followed by a discussion of the <br />expected impacts to adjudicated wells in the vicinity of the project area. <br />Revision: TiK' lt'y <br />ApprovecpCT 2 3 2Uii`1 <br />4-235 <br />