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The second ground water system identified near the permit area is the Hay Gulch alluvial aquifer. This aquifer system may be potentially affected by surface runoff from the mine bench to the sediment pond or discharge from the sediment pond infiltrating into the alluvial body. This infiltration could cause localized increases in total dissolved solids or other dissolved constituents within the alluvium. To document that the sediment pond does not degrade the quality of the Hay Gulch aquifer, a monitoring well will be completed immediately downgradient from the sediment pond. This well will be monitored for depth-to-water and water quality as explained in Section V - Ground Water Hydrology of this document. The La Plata No. 1 Mine will operate a leach field within the bounds of the Hay Gulch alluvial valley floor (AVF). La Plata Coal Corporation has demonstrated to the satisfaction of the San Juan Basin Health Unit that the area in which the leach field will be constructed does not contain ground water within eight feet of the AVF surface. As the leach field is small in size and the depth-to-water is more than two feet below the bottom of the leach field, the Division has concurred with construction of this facility. In order to verify the water table elevation in this area, La Plata Coal Corporation has committed to constructing a ten-foot deep, two-inch diameter monitoring well ten feet downgradient from the leach field. Depth-to-water measurements will be taken during periods of springtime snowmelt runoff and during the wet months of September or October. B. Cumulative Hydrologic Impact Assessment The Division has made an assessment of the probable cumulative hydrologic impacts of anticipated and permitted mining operations within the ground water and surface water study areas. The purpose of the cumulative hydrologic impact assessment is to determine the cumulative impacts to the hydrologic system from all existing and anticipated coal mining operations within the ground and surface water study areas. The surface water study area is defined as the Hay Gulch drainage basin from about Hesperus, Colorado, to its confluence with the La Plata River near Kline, Colorado. The ground water study area is defined as the Menefee Formation and Hay Gulch alluvial aquifer in the immediate vicinity of the existing and anticipated mining activities. The coal mining operations which are considered in this cumulative assessment include the La Plata No. 1 Mine (permit application submitted; inactive), King Coal Mine (permit issued; active), and the Blue Flame Coal Mine (permit issuance pending; inactive). 1. Surface Water Effects An evaluation of the cumulative effects of mining on surface water is limited because the receiving water, Hay Gulch, is an alluvial body and not a flowing stream. Each of the three mines contained within the surface water cumulative impact area are located within ephemeral drainages immediately adjacent to the valley of Hay Gulch. Surface water discharges occur as a result of periodic discharges of effluent from sediment ponds during storm events. No baseflow, either as streamflow or alluvial ground water, is evident at any of the three -19-