Hydrologic Characterization of Multiple Seam Underground and Surface Mining in Northern Appalachia

Abstract

Abstract.A hydrologic assessment of an underground mine complex overlain by extensive surface mining in north central Pennsylvania has been conducted. The mine complex is drained principally by one discrete discharge point which the flow rate (median of 2,167 L/min.) increased significantly (67%) above background (median of 1,317 L/min.) during the period it took to construct a treatment plant. The source of this major discharge rate increase and other unusual hydrologic characteristics were investigated. Subsequent to background monitoring, about 440 hectares of surface mining and reclamation (85% of the recharge area) occurred on numerous seams overlying the underground mines which greatly increased infiltration rates. A direct correlation was observed between the surface mined area and increased recharge to the underlying deep mines.Atypically, in-mine water storage does not exist to any substantial degree in the basal Lower Kittanning underground mine from which the main discharge emanates. The overlying Middle Kittanning (Shawmut) mine is the main storage unit for mine water. The Middle Kittanning mine behaves like a perched aquifer system because of a moderate vertical hydraulic conductivity (median rate of 1.0 x 10 -7 m/s) of the thin clayrich shale and siltstone interburden (mean of 11.7 m) and local structural features. During periods of low recharge, pool levels decline to a point where the majority of the mine water flowing downward from the Middle Kittanning mine to the underlying Lower Kittanning mine is widespread and diffuse in nature. The discharge rate is consistently within a narrow range of 1,745 to 2,381 L/min. about a median of 2,040 L/min. When surface infiltration rates are high causing the mine pool levels to rise, a portion of the recharge from the Middle Kittanning mine to the lower seam mine is more channelized flowing through the backfill from outcrop surface mining over the buried highwalls and into the underlying Lower Kittanning mine. During these periods, the flow ranges more broadly from 5,725 to over 11,356 L/min. about a median of 8,328 L/min.The openness of the Shawmut mine workings and the water quality of the mine pool make it viable to inject iron sludge. A test injection of 2.8 million liters of iron sludge into the mine was successful. Testing of the mine workings indicates that sufficient space exists to accommodate iron sludge disposal for more than 100 years.