Hydrologic conditions in the coal mining district of Indiana and implications for reclamation of abandoned mine lands

Bedrock strata of the mining district of Indiana (Indiana Coal Mining District, ICMD) include numerous coalbeds of economic importance, together with underclays, roof shales, limestones, and sandstones of Pennsylvanian age. These are typically poor aquifers with low hydraulic conductivities and specific yields. Surficial materials include loess, till, alluvium, and other deposits of pleistocene age. The loess and till also have low hydraulic conductivities, so that very few shallow aquifers exist in the vicinities of abandoned mine land (AML) sites, except where they are close to the alluvial fill of large bedrock valleys. The hydrologic cascade at AML sites in Indiana is strongly conditioned by the existence of elevated deposits of coarse-grained coal-preparation refuse and flooded underground mine workings. Flooded mines are the principal conduits of groundwater flow in the area, but their boundaries, flowpaths, and mechanisms of recharge and discharge are very different from those of natural aquifers and are poorly understood. Acidic mine drainage often emerges as seepages and springs on the edges of the elevated refuse deposits, but the low permeability of the natural surficial materials and bedrock inhibits the development of off-site groundwater contaminant plumes. The water balance across the surface of the refuse deposits is criticalmore » to reclamation planning and success. Enhancing runoff through reduction of infiltration capacity has the beneficial effect of reducing recharge through the acid-generating refuse, but the excess runoff may be accompanied by soil erosion that can lead to reclamation failure. Furthermore, during cool seasons and stormy periods, a well vegetated surface promotes recharge through increased infiltration, resulting in greater rates of acidic baseflow seepage. Passive Anoxic Limestone Drains (PALDs) have been successfully coupled with wetland treatment systems to improve surface waters that discharge from AML sites. Storm runoff from reclaimed refuse deposits is often alkaline, and techniques could be devised to utilize this alkalinity in the on-site treatment of both groundwater and surface water. In some instances, the flooded underground mines also contain alkalinity that could be used to treat acidic baseflow, but to date, no efforts have been made in Indiana to explore the possibility of beneficially incorporating underground mines in reclamation planning.« less