Lithium Isotope Fingerprints in Coal and Coal Combustion Residuals from the United States

Coal is a major energy source worldwide and is a large fraction of the total energy production in the US. The disposal of coal combustion residuals (CCRs) into landfills and surface impoundments present an environmental challenge given the high levels of toxic metals in CCRs that are highly mobile into the aquatic phase. Here we present the lithium isotope composition of coals and CCRs originated from several U.S. basins. δ7Li values in CCR leachates from the Appalachian Basin, the Illinois Basin and the Powder River Basin vary from -7‰ to +12.8‰, with an average of ±1.6±5.0‰. CCR's from the Powder River Basin had a higher δ7Li of 7.6±5.4‰ compared to the Appalachian and Illinois Basins (+0.3±6.6‰ and -0.1±1.3‰, respectively). Effluents from six coal ash ponds in North Carolina show a similar range in lithium isotopes (δ7Li = -6.2 to +8.7‰) suggesting limited lithium isotope fractionation during storage in surface impoundment. We show that lithium isotopes, particularly when coupled with boron isotopes, could distinguish CCR contaminants from oil and gas wastewater, including conventional produced waters and hydraulic fracturing fluids.