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As the mining activity in Twentymile Park is highly concentrated, and detailed <br />data collection been intensive since 1980, it is possible to define the <br />cumulative hydrologic impacts to each significant aquifer within the basin, as <br />they relate to the Seneca II mining operation. <br />The Trout Creek Sandstone, which underlines the mined units at Seneca II Mine, <br />will not be adversely impacted by mining operations. Due to the 70 foot thick <br />sequence of relatively impermeable siltstone, shale and lenticular sandstone, <br />it is not thought that there will be any hydrologic effects from mining to <br />this unit. It is unlikely that fracturing, due to unloading of the overlying <br />coal and associated overburden, will completely penetrate the Wolf Creek <br />underburden, which would induce vertical flow into backfilled mine pits. It <br />is also unlikely that the spoils aquifers which develop in the backfilled <br />portions of each mine would have sufficient head to penetrate the relatively <br />impermeable interburden. Therefore, it does not seem likely that any vertical <br />hydrologic communication between these aquifers will develop. <br />All of the mining operations in Twentymile Park are expected to impact the <br />quantity and quality of the Wolf Creek Coal, Wadge Coal and Wadge Coal <br />Overburden Aquifers (Middle Coal Group) to some extent. <br />The surface mines which rim the basin will all contribute to a depletion of <br />ground water flow in the vicinity of each mine. This depletion is usually due <br />to the extraction of the coal and its associated overburden, thus interrupting <br />the aquifer systems. <br />The interruption of the aquifer by coal extraction does not constitute a <br />regionally significant depletion of the aquifer. This is because the Middle <br />Coal Group, as a whole, transmits water very slowly. The length of time in <br />which the recharge potential is interrupted (from coal removal until a spoils <br />aquifer is established) is limited, so the preclusion of recharge to the <br />undisturbed aquifer is inconsequential. <br />Some surface mines will further deplete the adjacent aquifers by pumping <br />ground water which accumulates in the pits to the surface water system. <br />Underground mines will locally deplete ground water during the active life of <br />the operation. Underground workings function as a well and redirect ground <br />water flow paths toward the mine. Upon abandonment, the mine will become <br />flooded and an equilibria will be achieved, at which time the ground water <br />flow directions will approximate baseline conditions. <br />The short term affects of aquifer depletion include the impacts to local water <br />wells, and decreased base flow contribution to local streams and alluvial <br />systems. <br />Monitoring at the Seneca II Mine indicates that the head in the Middle Coal <br />Group aquifers is significantly affected by mining only in the immediate <br />vicinity of the operation. Therefore, it does not seem likely that wells <br />spatially removed from a surface strip operation will be significantly <br />affected by aquifer depletion. The same logic may be applied to the potential <br />_pg_ <br />