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_29_ cavity, but flow would be very slow away from the mine due to the low permeabilities of the coal, causing the flushing action to persist over an extended period of time. Since the coal seam aquifer contains poor quality water, 806 to 1100 mg/1, and is not used as a water supply in the general area, mining within this aquifer will not materially damage the qua]ity of ground water used off site. Post-mining effects of the New Elk Mine on the water quality in the stream-alluvial aquifer system would be expected to be less than during operations, because mine water would no longer be discharged from the underground workings. Therefore, seepage from the refuse piles would be the only source of pollutant and would increase the dissolved load during low flow in the Middle Fork of the Purgatoire River by approximately 16 mg/1 (Exhibit 6, Table 7 of the permit application). This increase would be much less under average or high flow conditions. CUMULATIVE HYDROLOGIC IMPACTS OF MINING Because of their proximity to each other, the New Elk and Golden Eagle mines are expected to have cumulative hydrologic impacts on surface and ground water systems. Possible combined impacts include dewatering of the coal seam and overburden aquifers, depression of the piezometric surfaces in these aquifers, deterioration of the quality of ground water flowing through abandoned coal workings into coal aquifers, depletion of flow in the stream/alluvial aquifer system, and a deterioration of water quality in the stream/alluvial aquifer system. Water quality impacts to the Purgatoire River would be greatest during operations when mine water is discharged from the two mines. The Allen and Maxwell coal seams lie close to one another in the stratigraphic column, separated by approximately 20 feet of interburden material. Disturbance of roof, floor, and interburden material during mining, and subsidence after mining, will result in the breakdown of the interburden strata. As a result, hydrologic communication is expected to occur between the two coal seams, and ground water impacts are expected to be cumulative. The following assessment considers the two coal seams as one aquifer. During operations, dewatering would occur in the coal aquifer and the drawdown of the piezometric surface could extend outward to approximately 3 miles from each mine. Since the mines are adjacent to each other, a portion of these affected areas would overlap. Within that zone, drawdown effects would be additive. After the cessation of operations, mine water would no longer be discharged and the abandoned workings would fill with water. The ground water level and piezometric surface would partially recover. A permanent depression in the piezometric surface of the coal aquifer would exist in the vicinity of the flooded workings of the mines. The depression would not be large, but it may extend 3 miles from each mine. Approximately twice as much ground water would flow through the flooded mine workings than flowed through the undisturbed coal prior to mining. This mine water flow would, however, be restricted by the low permeability of the undisturbed coal surrounding the mine workings. Ground water would stagnate in the mine workings and react with fragmented roof, floor, coal, rock dust, and other materials in the mine cavity. This ground water would have a poorer quality than water currently pumped from underground workings; which is similar to ground water observed in the New Elk refuse pile. The total dissolved solids levels are expected to equal or .....-esa ~~nn mn/1 (Fvhihit fi Tah1P 1"1 of the narmit annliratinnl Thie nrni~nrl