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-3b- CUMULATIVE HYDROLOGIC IMPACTS OF MINING Because of their proximity to each other, the Allen and the Maxweii 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, depressions 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 Allen refuse pile. The total dissolved solids levels are expected to equal or exceed 2600 mg/1 (Exhibit 6, Table 13 of the permit application). This ground water would spread as a plume down dip. The effect of having two mining operations affecting strata in hydrologic conmunication with one another would be to decrease dilution within the aquifers and increase the distance a detectable mine water plume would spread. The degradation of water quality in the coal aquifers will not impact ground water use in the general area, since the poor quality and depth limited use originally.