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4.3. Ground Water Impacts The northern edge of the mined area in each of the drainages included in this proposed Phase III bond release application is typically several thousand feet south of the shallow alluvial aquifer systems identified to the north of the mine permit area. Geologic structure generally controls the groundwater movement in these drainages. (Permit C-81-010, Map M32). Upper Williams Fork units that discharge to the shallow aquifer systems typically do not extend south into the mine areas with the exception of the Third White Sandstone, which extends into the northern pit limits of Trapper's historic Browning pit on the northwest portion of the proposed bond release area. Mining removed some of the Third White Sandstone in the northern portion of this pit. Recharge to the Third White Sandstone is through the backfill aquifer, which feeds the Third White Sandstone aquifer along the northern margin of the pit. Recharge to the backfill aquifer is likely to be slightly greater than the present recharge to the Third White Sandstone. Therefore, the quantity of water available in the Third White Sandstone following recovery of the backfill aquifer will be similar to or greater than that observed in pre-mine conditions. According to a study conducted by the USGS in 1994 at the nearby Seneca Mine, elevated (higher than pre-mining) TDS and sulfate concentrations may potentially persist in backfilled mine aquifers. USGS study estimates indicate that higher sulfate and TDS concentrations from some of the backfill aquifers at Trapper Mine may, in the absence of any ameliorating factors, last for several hundred years under aworst-case scenario in which the entire mass of pyrite available in the spoil body is oxidized. Four wells monitor water quality in mine backfill aquifers (GD-3, GF-5, GF-7 and GF-11). These four wells were all included within the SL-4 Phase III bond release area. However, because they are used as ash monitoring wells they are still sampled and are representative of backfill aqufer conditions occurring in reclaimed blocks proposed for bond release with this application. Sites GD-3 and GF-11 show moderate increasing trends in TDS and sulfate concentrations over time suggesting that the processes of infiltration, carbonate and sulfate mineral dissolution, and pyrite oxidation are resulting in the observed cumulative increases in water quality constituent concentrations but that these processes are moderated by the influence of underburden inflows. At backfill well GF-7, the water quality response has been more rapid suggesting that surface water infiltration processes are not being offset significantly by underburden inflows in this location. In contrast, little to no degradation in water quality is apparent at backfill well GF-5 suggesting that underburden inflows in this area probably play a significant role. Detailed discussions of selected groundwater quality constituents are presented in the Annual Hydrologic Reports (Appendix W ). These discussions take into account all of the most recent data along with the historical information.