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A. Ground Water The Trinidad Sandstone is a significant regional aquifer in the Canon City area. However, it lies approximately 600 feet stratigraphically below the strata affected by mining, and is separated mainly by poorly-permeable sedimentary rocks. As a result no impact on its water supply was anticipated during the previous permit term, and none is expected in the future. The alluvial aquifers associated with Oak Creek and its tributary, Lewis Gulch, could also be potentially impacted by mining activities. The Twin Pines No. 2 operation began pumping mine discharge water into Lewis Gulch in October, 1987 at an average rate of 24 to 27 GPM. The water is first passed through a sedimentation pond, and meets all NPDES effluent limitations. Laboratory analysis of this discharge indicates a sodium-sulfate-bicarbonate water with a TDS value of 844 mg/1. Although the town of Rock vale draws its municipal water supply from the alluvium of Oak Creek, the wells are located above the confluence of Lewis Gulch, effectively isolating them from the discharged mine water. Considering these facts and the absence of beneficial use of ground water from either the Lewis Gulch alluvium source or downstream from the confluence with Oak Creek, no significant impact to alluvial ground water sources is expected. Inflows to the Twin Pines No. 2 Mine could also affect ground water levels in lenticular sandstone beds and coal seams of the Yermejo Formation, which are potential aquifers in the area. Due to the uniformly poor aquifer properties in these strata however, local water supply wells tend to be completed in abandoned mine workings in order to gain acceptable yield. The Caldirola Mine Well provides emergency water supplies for the town of Coal Creek, and is completed in workings adjacent to the Twin Pines operation. The flooded area of the mine tapped by this well is however, hydrologically separate from that immediately adjacent to the Twin Pines No. 2 Mine. Semi-annual monitoring of this well indicates that it has not been impacted during the previous permit term. No future impact is expected. Other adjacent workings, such as the abandoned Cedar Canyon and Twin Pines No. 1 Mines, have been partially dewatered by the Twin Pines No. 2 operation, however, they are also not related to any established water uses. Water levels in monitoring well TP-Wl, completed in a Vermejo Formation sandstone bed a few feet below the Brookside seam, have not varied substantially during the previous permit term. Water quality in this well has remained fairly good; however, levels of sodium have been rising steadily. Although the quality of water in this well has possibly been impacted by mining, the aquifer is unrelated to any established local water use. Upon closing of the Twin Pines No. 2 operation, the workings are likely to flood as a new potentianetric surface becomes established. This will create a new local source of ground water. Areas of lower-quality water probably will occur in the workings however, the extremely low hydraulic conductivity of the surrounding Vermejo Formation strata should limit such degradation to within a few thousand feet of the mine. -9-