STRONTIUM ISOTOPE VARIATIONS AS A PROXY FOR SELENIUM CONTAMINATION FROM MOUNTAINTOP MINING

Mountaintop mining (MTM) is the dominant form of coal mining and the largest landscape changing in the central Appalachian. During MTM, several overburden layers of sedimentary rocks are removed to access coal layers. The residual mined rock is typically placed in valleys adjacent to the surface mine, generating valley fills (VF), which is becoming the source for headwater feeding the local streams. Numerous reports have highlighted the water quality degradation associated with MTM, in particular elevated levels of salinity, sulfate, and trace metals such as selenium, which pose serious toxicity risks to the ecological systems in watersheds affected by MTR. Here we report a direct link between the strontium isotope ratio and selenium concentrations in streams in the Upper Mud River watershed in Boone County, Western Virginia. The data show that Se (up to 35 mg/L) and Sr are highly correlated and associated with relatively low ⁸⁷Sr/⁸⁶Sr ratios (0.7131) in streams draining “active” MTM sites, while streams originated from non-operated MTM have significant lower Se and Sr contents and high ⁸⁷Sr/⁸⁶Sr ratios (0.7174). In contrast, both active and non-active MEM sites generate saline effluents with high sulfate contents (up to 1,200 mg/L). Preliminary data from sequential leaching of local coals suggests differential Sr isotope ratios within the coal as the “reactive” phases (desorption, carbonate) have low ⁸⁷Sr/⁸⁶Sr ratios (~0.7100) while “non-reactive” (silicate, organic) phases have significant radiogenic ⁸⁷Sr/⁸⁶Sr ratios (~0.7250). We propose that leaching of coal in active MTM produces effluents with high concentrations of Se and distinctive Sr isotope ratio of reactive coal. These isotopic fingerprints can be useful to delineate the impact of MTM on water quality in streams, and to verify the differential impacts of active and non-active MTM operations on the quality of affected aquatic systems.