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9 <br />According to a study conducted by the USGS in 1994 at the nearby Seneca Mine, elevated (higher <br />than pre-mining) TDS and sulfate concentrations may potentially persist in backfilled mine aquifers. <br />USGS study estimates indicate that higher sulfate and TDS concentrations from some of the backfill <br />aquifers at Trapper Mine may, in the absence of any ameliorating factors, last for several hundred <br />years under a worst-case scenario in which the entire mass of pyrite available in the spoil body is <br />oxidized. <br />Four wells monitor water quality in mine backfill aquifers (GD-3, GF-5, GF-7 and GF-11). These four <br />wells were all included within the SL-4 Phase III bond release area. However, because they are used <br />as ash monitoring wells they are still sampled and are representative of backfill aquifer conditions <br />occurring in reclaimed blocks proposed for bond release with this application. Sites GD-3 and GF-11 <br />show moderate increasing trends in TDS and sulfate concentrations over time suggesting that the <br />processes of infiltration, carbonate and sulfate mineral dissolution, and pyrite oxidation are resulting <br />in the observed cumulative increases in water quality constituent concentrations but that these <br />processes are moderated by the influence of underburden inflows. At backfill well GF-7, the water <br />quality response has been more rapid suggesting that surface water infiltration processes are not <br />being offset significantly by underburden inflows in this location. In contrast, little to no degradation in <br />water quality is apparent at backfill well GF 5 suggesting that underburden inflows in this area <br />probably play a significant role. <br />Detailed discussions of selected groundwater quality constituents are presented in the Annual <br />Hydrologic Reports (Appendix W). These discussions take into account all of the most recent data <br />along with the historical information. <br />4.3.1. Impacts to Adjudicated Wells (see Trapper Permit C-81-010 Map M31) <br />No impacts to adjudicated wells are anticipated in the areas down gradient of the lands proposed for <br />Phase III bond release with this SL-10 application. <br />4.4. Post Mine Hydrologic Balance <br />Precipitation amounts are expected to be similar to pre-mine conditions after mining. The post- <br />mining evapotranspiration rate is similar to the pre-mine values as vegetation is now re-established. <br />If recharge to the spoils is larger than pre-mine values, direct surface runoff will be reduced <br />subsequent to mining. A larger net groundwater discharge to the surface system is likely in areas <br />where spoil springs have developed. These discharges should offset a decrease in direct surface <br />9