Characterizing TDS Risk in Appalachian Landscapes: Techniques to Identify Mine Spoil TDS Generation Potentials

Surface mining for coal in the Central Appalachians contributes total dissolved solids (TDS) to headwater streams, especially below larger mines and associated valley fills. The objective of this study was to characterize the geochemical properties of a range of surface soils and associated geologic strata from the Central Appalachian coalfields and to relate those properties to simple field indicators, such as color or rock type. We hypothesized that these indicators can accurately predict certain geochemical properties. Thirty‐three vertical weathering sequences were sampled from eight surface mines throughout the Central Appalachian coalfields, for a total of 204 individual samples. No differences were found among sites in overall saturated paste specific conductance (SC; used as a proxy for TDS) levels, but significant geochemical differences existed among samples. Sulfate release dominated SC levels, followed closely by Ca and Mg. Surficial soils and sandstones were yellowish‐brown in color and low in SC, compared to the underlying grayish to black sandstones, shales, and mudstones. Saturated paste extractable As and P levels were higher in A horizons, whereas Se was significantly higher in unweathered bedrock than in soil or weathered bedrock. Samples generating exothermic reactions with 30% H2O2 produced higher SC levels, sulfate, Mg, and Se. In the field, weathered surface materials were frequently abruptly separated from underlying non‐weathered strata by thin shale layers or coal seams. In conclusion, the mine spoils studied varied widely in geochemical properties. The simple field indicators presented here, such as color, weathering status, rock type, and H2O2 reaction can provide valuable guidance for identifying TDS risk which would greatly improve operator’s ability to actively minimize TDS release. We recommend using soil and weathered, yellowish‐brown sandstone layers as a source of low TDS spoil material whenever possible. Underlying unweathered bedrock layers should be treated as "potentially high TDS spoils". The H2O2 field test is useful for identification of both TDS and Se risk in these layers. Particularly high risk spoils include gray to black mudstones and shales, coals, and coal associated shales, mudstones, and clays directly associated with coal seams. We recommend hydrologically isolating these spoils using techniques similar to those used historically for acid‐forming materials.