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<br />-36- <br />low. Under low flow conditions, seepage from 16.6 acres of waste pile (8.6 <br />acres current pile, 8.0 acres historic pile to be reused) would increase total <br />dissolved solids in the river by approximately 16 mg/1. In combination with <br />discharge from the Allen underground workings, this results in a total <br />increase of 48 mg/1 at low flow. Under average or high flow conditions, the <br />increase would be less. <br />After the cessation of mining operations, mine water from the Allen Mine would <br />no longer be discharged, and the abandoned workings would fill with water, a <br />process taking up to 80 years. The piezometric surface would partially <br />recover. Because the underground workings lie 400 to 800 feet below the <br />surface, the piezometric surface of the coal aquifer would not be expected to <br />rise to a level where it would intersect the ground surface and, thus, <br />discharge. A permanent depression in the piezometric surface would exist in <br />the vicinity of the flooded mine workings. Given the assumption that the <br />Allen seam is hydrologically similar to the Maxwell seam, the depression may <br />extend to a distance of 2 to 3 miles from the edge of the mine workings. <br />When pumping of water from the workings is discontinued, the water that <br />accumulates is expected to be of lower quality than that currently pumped due <br />to increased residence time (Exhibit 6, Table 5 of the application). The <br />quality of the stagnant mine water should be similar to the quality measured <br />below the Allen waste piles (Exhibit 6, Table 13). Total dissolved solids are <br />expected to equal or exceed 2600 mg/1. This mine water would spread as a <br />plume of degraded quality ground water down gradient from the mine. Based on <br />the structural contours of the coal seam, water passing through' the flooded <br />workings would move in a northeasterly direction. It may spread to <br />approximately 2 to 3 miles from the edge of the mine workings. It is <br />anticipated that soluble materials would be readily flushed from the mine <br />cavity, but flow would be very slow away from the mine due to the low <br />permeabilities of the coal, causing the flushing action to persist over an <br />extended period of time. Since the coal seam aquifer contains poor quality <br />water, 806 to 1100 mg/1, and is not used as a water supply in the general <br />area, mining within this aquifer will not materially damage the quality of <br />ground water used off site. <br />Post-mining effects of the Allen Mine on the water quality in the <br />stream-alluvial aquifer system would be expected to be less than during <br />operations, because mine water would no longer be discharged from the <br />underground workings. Therefore, seepage from the refuse piles would be the <br />only source of pollutant and would increase the dissolved load during low <br />flow in the Middle Fork of the Purgatoire River by approximately 16 mg/1 <br />(Exhibit 6, Table 7 of the permit application). This increase would be much <br />less under average or high flow conditions. <br />CUMULATIVE HYDROLOGIC IMPACTS OF MINING <br />Because of their proximity to each other, the Allen and the Maxwell mines are <br />expected to have cumulative hydrologic impacts on surface and ground water <br />systems. Possible combined impacts include dewatering of the coal seam and <br />overburden aquifers, depression of the piezometric surfaces in these aquifers, <br />