The effects of mountaintop mines and valley fills on the physicochemical quality of stream ecosystems in the central Appalachians: A review

This review assesses the state of the science on the effects of mountaintop mines and valley fills (MTM–VF) on the physicochemical characteristics of streams in the central Appalachian coalfields of West Virginia, Kentucky, Virginia and Tennessee, USA. We focus on the impacts of mountaintop removal coal mining, which involves removing all – or some portion – of the top of a mountain or ridge to expose and mine one or more coal seams. Excess overburden is disposed in constructed fills in small valleys adjacent to the mining site. MTM–VF leachate persistently increases the downstream concentrations of major ions. Conductivity is a coarse measure of these ions, which are dominated by a distinct mixture of SO42−, HCO3−, Ca2+ and Mg2+, that reflects their source, the oxidation of pyrite to form acid followed by neutralization of the acidity by carbonate minerals within the valley fills. This results in neutral to alkaline pHs, a range at which many metals are relatively insoluble. Other compounds within coal or overburden are solubilized and occur at elevated albeit lower concentrations, including K+, Na+, Cl−, Se and Mn. In terms of physical characteristics, the valley fills act like headwater aquifers, baseflows increase in streams below valley fills and water temperatures exhibit reduced seasonal variation. Peak discharges may be increased in response to intense precipitation events, because of compaction of base surfaces of the MTM–VF areas, but newer approaches to reclamation reduce this compaction and may ameliorate these peak flows. Although the sedimentation pond is intended to capture fine particles that wash downstream from the valley fill, some studies found increased fine sediments in streams downstream from valley fills. However, a proportion of these fines may be eroded from stream banks rather than the valley fills. This is probably a result of the alterations in stream flows.

► We review the physicochemical effects of mountaintop mines/valley fills on streams.
► Downstream ion concentrations increase, particularly SO42−, HCO3−, Ca2+, Mg2+ and Se.
► These major ions dominate the increased conductivity.
► Stream water pH is neutral to alkaline and most metals are not elevated.
► Stream baseflows generally increase.