Hydrologic and hydrochemical processes on mine spoil fills

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Virginia Tech


Appalachian surface coal mining operations fracture rocks (termed mine spoils), resulting in the weathering of minerals and release of water-soluble ions to streams. Collectively, the concentration of water-soluble ions in streams is called total dissolved solids (TDS) and streams with elevated TDS often have altered biota. The surficial, subsurface, and discharge properties of mine spoils influence TDS discharge concentrations. This study aimed to improve understanding of how hydrologic and hydrochemical processes occur and function in coal mining areas. These processes were characterized by infiltration and dye staining tests, mine spoil leaching experiments and modeling, and mining-influenced stream discharge monitoring. Results indicate that many factors influence hydrologic and hydrochemical processes in Appalachian coal mining areas, but these processes evolve over time as subsurface flow paths develop, mine rocks weather, and TDS is released from mine spoils. Fourteen years after placement, mean infiltration rates of mine soils reclaimed with trees were statistically greater than areas reclaimed with grasses, and different subsurface flow types were evident, indicating vegetation type influenced hydrologic processes. Specific conductance (SC) leaching patterns from mine spoils conformed to an exponential decay and linear segmented regression model. Maximum SC values (1108 ± 161 µS cm⁻¹) occurred initially during leaching, exponentially decayed, then exhibited linear SC releases (276 ± 25 µS cm⁻¹) that were elevated relative to natural background levels at the end of leaching. Major element (S, Ca, Mg, K, Na) leaching patterns resembled those of SC, whereas trace elements (As, Cd, Cu, Ni, Pb, Se) transitioned to linear release earlier in the leaching period. Mining-influenced stream SC discharge patterns varied by season and by precipitation amounts during storm events. Storm responses were characterized by either infiltration-excess overland flow or delayed SC releases due to internal flow through the VF. Given these results, mining companies wishing to control TDS discharges may be selective and pre-test mine spoils for total S and paste SC to determine TDS-generation potential. Isolation of spoils with high-TDS release potentials (i.e. unweathered sandstones and mudstones) from water-rock contact may help improve TDS discharges.



total dissolved solids, coal mining, disturbance hydrology, reclamation