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leaching of caved and fractured overburden material) would discharge to alluvial units and surface streams. During mining and recovery of potentiometric levels following mining, it is likely that discharge from the Wadge overburden will be minima]. This reduction in flow, primarily to Fish Creek as discussed in a previous section, is not expected to have any significant effect on water quality characteristics of the alluvial deposits or the surface streams. Once discharge from the Wadge overburden is re-established, there will be a period, calculated to be about I10 years, when the quality of the bedrock discharge will be similar to pre-mining conditions. Afrer this time, discharge of poorer quality water, which has been affected by increased solute leaching near the underground workings, will start to occur. The quality of this discharge is likely to be characterized by high sodium and sulfate concentrations with conductivities of 4000 to 7500 µmhos/cm. The rate of dischazge of this water is expected to be similar to pre-mining conditions as discussed in a Effects on Ground Water Quality of Aouifers. This rate has been calculated to be about 11,000 gpd (12.3 acre-fr per year) which is equivalent to about 0.02 cfs or about 0.5 percent of the average low flow in Fish Creek. This level of discharge is not expected to have any significant effect on the overall water quality of surface water since it constitutes such a small percentage of the total flow in the creek even under low flow conditions. Effects of Mine Discharees on Alluvial Water Quality ~' r{ ~ ~s The release of the mine discharge water into Foidel and Fish Creeks can be expected to affect the water quality in the alluvium in the creeks, downstream of the discharges. Due to mixing and diffusion, it would be expected that the water quality in the alluvium will change in accordance with the long-term average in-stream water that recharges ii. (Modeled mean water qualities of in-stream flows in Fish Creek (Exhibit 49, Tables E49-12 to E49-14) indicate `` during the year with maximum discharge the in-stream water quality will increase from a baseline of 588 µmhos/cm to 916 µmhos/cm. Therefore, the alluvial water will still be suitable for irrigation, as it will be less than the material damage level of 1500 µmhos/cm. The Fish Creek alluvium is not used for domestic purposes, sites are found in the 1998 Annual Hydrologic Report, Figures 49, 51, and 52. Modeled mean water qualities of in-stream flows in Trout Creek (Exhibit 49, Tables E49-12 to E49-14) indicate during the year with maximum discharge the in-stream water quality will increase from a maximum baseline of 527 µmhos/cm to 665 µmhos/cm. This is a small increase. In addition, the alluvial water will still be suitable for irrigation, as it will be less than the material damage level of 1500 µmhos/cm. The maximum modeled sulfate concentration for the mean flow conditions is less than the drinking water standard of 250 mg/I. The wells in the Foidel Creek alluvium, downstream of Site 109 are already affected by the spoil spring discharges from CY ce mme. The conductivities in Foidel Creek alluvial wells downstream of Site 109 already exceed ]00 µmhos/cm. Site 109 discharged from 1984 to 1996. During this period the spoil springs were also discharging. se upon plots of conductivity in Foidel Creek alluvial wells, the impact from Site 109 discharge could not be found. In addition, the effects of the reduced 1996 discharge and the elimination of discharge in 1997 and 1998 could not be found. (see 1998 Annual Hydrologic Report, Figure 38, 41, 45, and 47.) Subsidence Impacts on Ground Water Longwall mining of coal seams causes collapse, fracturing, bed separation, and bedding plane slip in the roof strata above the seam. All of these impacts on the overlying strata can result in changes to surface and ground water if a 7ajor water resource is within reach of the disturbance. w~ TR99-32 2.05-146 AP~R9PIE~ yea c s Zooo 1/3/00