Isotopic imprints of mountaintop mining contaminants

Mountaintop mining (MTM) is the primary procedure for surface coal exploration within the central Appalachian region of the eastern United States, and it is known to contaminate streams in local watersheds. In this study, we measured the chemical and isotopic compositions of water samples from MTM-impacted tributaries and streams in the Mud River watershed in West Virginia. We systematically document the isotopic compositions of three major constituents: sulfur isotopes in sulfate (δ³⁴S_SO4), carbon isotopes in dissolved inorganic carbon (δ¹³C _DIC), and strontium isotopes (⁸⁷Sr/⁸⁶Sr). The data show that δ³⁴S_SO4, δ¹³C _DIC, Sr/Ca, and ⁸⁷Sr/⁸⁶Sr measured in saline- and selenium-rich MTM impacted tributaries are distinguishable from those of the surface water upstream of mining impacts. These tracers can therefore be used to delineate and quantify the impact of MTM in watersheds. High Sr/Ca and low ⁸⁷Sr/⁸⁶Sr characterize tributaries that originated from active MTM areas, while tributaries from reclaimed MTM areas had low Sr/Ca and high ⁸⁷Sr/⁸⁶Sr. Leaching experiments of rocks from the watershed show that pyrite oxidation and carbonate dissolution control the solute chemistry with distinct ⁸⁷Sr/⁸⁶Sr ratios characterizing different rock sources. We propose that MTM operations that access the deeper Kanawha Formation generate residual mined rocks in valley fills from which effluents with distinctive ⁸⁷Sr/⁸⁶Sr and Sr/Ca imprints affect the quality of the Appalachian watersheds.