Hydrologic Impacts of Multiple Seam Underground and Surface Mining: A Northern Appalachia Example

Abstract

An underground mine complex overlain by extensive surface mining in north-central Pennsylvania is drained principally by one discrete discharge point at which the flow rate (median of 2,167 L/min) increased significantly (67%) above background (median of 1,317 L/min) during a 3 year period. The source of this major discharge rate increase and other unusual hydrologic characteristics were investigated. Subsequent to background monitoring, about 440 ha of surface mining and reclamation (85% of the recharge area) occurred on numerous seams overlying the underground mines, which induced 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 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 mine is the main storage unit for mine water. The Middle Kittanning mine behaves like a perched aquifer system because of the moderate vertical hydraulic conductivity (a median rate of 1.0 × 10−7 m/s) of the thin (mean of 11.7 m) clay-rich shale and siltstone interburden and local structural features. During periods of low recharge, pool levels decline to a point where most of the mine water flowing downward from the Middle Kittanning mine to the underlying Lower Kittanning mine is diffuse in nature. The discharge rate is consistently in a narrow range of 1,745–2,381 L/min about a median of 2,040 L/min. When surface infiltration rates are high, the mine pool levels rise, and a portion of the recharge from the Middle Kittanning mine to the lower seam mine is apparently more channelized, flowing through the backfill 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.