Site-specific variation in the potentiometric response to subsidence aboveactive longwall mining

Longwall underground coal mining produces major changes in the hydraulic pr operties and groundwater levels of overlying shallow aquifers because of th e fracturing associated with mine subsidence. Many aspects of these impacts remain unclear and unpredictable, particularly the variations in response between different sites, Our detailed, long-term studies of subsidence stra ins and hydraulic responses at sites in Illinois address this and other asp ects of the conceptual model. At a study site in Saline County, inconsisten t permeability changes and large rapid head drops, without significant reco very, were observed in a low transmissivity sandstone over an active longwa ll panel. Corresponding head changes in overlying glacial drift units refle cted localized drainage from the drift to the bedrock. In comparison to our earlier study at a site in Jefferson County, Illinois, in which full recov ery occurred in a moderately transmissive sandstone aquifer affected by lon gwall mining, the Saline site illustrates the importance of local hydrogeol ogic characteristics, particularly variations in transmissivity and continu ity with recharge sources, in controlling site-specific responses to longwa ll mining.