Sulfidic schist release of As, Cu, and Pb in laboratory experiments and across eleven watersheds in central Massachusetts, USA

Abstract

Sulfidic schists are important rock formations due to their trace metal and metalloid (TMM) content and carry the potential for pyrite and pyrrhotite to hydrate and oxidize leading to acid-enhanced chemical weathering. The objectives of this study were to compare TMMs in sulfidic schists to other co-occurring bedrock, evaluate conditions that optimize TMM rock weathering, and examine streamwater TMMs in relation to bedrock lithology and human development in eleven streams across central Massachusetts. Sulfidic schists samples had the highest As (72 ± 46 mg kg^-1), Cu (63 ± 21 mg kg^-1), and Pb (63 ± 33 mg kg^-1) concentrations. Electron Probe Microanalysis (EPMA) images show As and Pb were widely distributed across silicate and sulfide minerals in both the mica schist and sulfidic schists, not exclusively hosted in sulfide minerals as hypothesized. Batch reactors had TMM dissolution rates an order of magnitude higher for sulfidic schists than granite and mica schists. Furthermore, TMM dissolution was greatest under pH 9 than pH 7 or pH 5 and dissolution rates were two times greater under anoxic conditions compared to oxic conditions. Streamwater concentrations of As (0.01 to 10.3 μg L^-1), Cu (0.2 to 206 μg L^-1), and Pb (0.001 to 8.3 μg L^-1) were below Massachusetts Surface Water Quality Standards. Across the eleven watersheds, % sulfidic schists were positively correlated with mean streamwater S and Cu concentrations and area-normalized annual export. Streamwater As and Pb concentrations were significantly correlated with %Developed land and %Mica schist, which strongly covaried. Our study confirmed the elevated abundance of TMMs in sulfidic schists but laboratory experiments suggest the precipitation of amorphous Fe oxyhydroxide phases decreased dissolved TMMs during oxic weathering. Future work will need to incorporate groundwater and stable isotope systems to separate anthropogenic and geogenic analyses.

Keywords: Batch reactor; Streamwater; Sulfide weathering; Trace metals and metalloids.