Site-Specific variation in the potentiometric response to subsidence above active longwall mining

Longwall underground coal mining produces major changes in the hydraulic properties and groundwater levels of overlying shallow aquifers because of the 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 strains and hydraulic responses at sites in Illinois address this and other aspects of the conceptual model. At a study site in Saline County, inconsistent permeability changes and large rapid head drops, without significant recovery, were observed in a low transmissivity sandstone over an active longwall panel. Corresponding head changes in overlying glacial drift units reflected localized drainage from the drift to the bedrock. In comparison to our earlier study at a site in Jefferson County, Illinois, in which full recovery occured in a moderately transmissive sandstone aquifer affected by longwall mining, the Saline site illustrates the importance of local hydrogeologic characteristics, particularly variations in transmissivity and continuity with recharge sources, in controlling site-specific responses to longwall mining.