Investigation of dissolved constituent concentrations and assessment of chemical equilibrium controls in the saturated zone of two coal surface mining spoil banks

The objectives of this research were 1) to characterize chemically and hydrologically the subsurface water in two coal surface mining spoil banks, 2) to test the hypothesis of chemical equilibrium between soluble and solid components, and 3) to generalize constituent conditions in the subsurface water in order to evaluate potential impact on the streams. The study spoil banks were located in two small basins contained in the New River watershed, which accounts for the majority of coal produced in Tennessee. Monitoring networks were established which consisted of the pond, drilled and hand-driven wells, and seepages. Samples were routinely collected and analyzed for a period of five years. Intermittent sampling was also carried out at other locations in the Indian Fork and Bills Branch basins.

It was found that the subsurface water quality is poor by routine standards. The major sources of dissolved constituents are probably the dissolution of pyrite and the weathering of clays. Based on equilibrium modeling, it was concluded that aqueous phase concentrations in the saturated zone are not controlled by equilibrium with respect to a family of coexisting solid phases from the list of candidates considered. The listed included a variety of 26 solid phase carbonates, oxides, oxyhydroxides, sulfates, and mixed carbonate-hydroxides of calcium and magnesium. However, partial equilibrium may exist with respect to amorphous ferric hydroxide. In addition, locations and periods of localized equilibrium may develop with respect to siderite, rhodochrosite, calcite, and magnesite during hydrologic conditions of low flow.

Subsurface saturation can occur in the absence of surface ponding on the mining bench. The spoil material contains significant masses of dissolved constituents as compared with the masses estimated to leave the basin annually in the stream. There was no consistent pattern of decrease in dissolved constituent concentrations in the subsurface water over the monitorring period. Hence, there is no evidence that the constituent sources are being exhausted. Evaluation of long term effects on the basin will require continued monitoring, determination of the rate of discharge to the downslope region, and characterization of attenuation processes affecting constituent transport in this region.