AQUIFERS MODIFIED BY SUBSIDENCE ABOVE LONGWALL UNDERGROUND COAL MINES

In many coal-mining settings, the geologic units overlying the mine consist of aquitards such as shales and relatively poor sandstone aquifers that are marginally adequate for small residential and farm supplies. Mining-induced changes can therefore readily either shift an aquifer to an unusable condition, or in some cases turn a low-yielding unit into a better aquifer. Subsidence due to longwall and other high extraction mining causes strata fracturing which produces temporary and permanent changes in the hydraulic properties of aquifers and aquitards, and thus in potentiometric heads, flow patterns and confined-unconfined conditions, and thus in yields, geochemistry, and interaction between surface and ground water. Each of these changes is also controlled by the mining characteristics (depth, width, extraction thickness, etc.) and by the overall geology and physiographic setting and their site-scale variation. Thus, the mechanisms are complex and the results variable. Studies in Illinois have shown both enhancement of aquifer yield due to increases in permeability and storativity (albeit with reduction of quality) and loss of supplies due to unrecovered head drops. Numerous studies in northern Appalachia have similarly shown large variations in impacts, for example due to topographic position (e.g. recovery of water levels in valleys versus permanently drained hilltop aquifers). We now have a reasonably good understanding of these temporal and spatial hydrogeologic changes as well as a large body of case studies, especially of the potentiometric responses during and after mining. This presentation focuses on an integrated picture of the hydraulic property changes that result in permanently altered aquifer characteristics, and the long-term potentiometric and geochemical changes that result from these.