THE STRONTIUM ISOTOPIC FINGERPRINT OF COAL COMBUSTION RESIDUALS FROM THE UNITED STATES: A DISTINCTION BETWEEN THE ISOTOPE RATIOS OF THE BULK AND WATER-SOLUBLE STRONTIUM

Coal combustion residuals (CCRs), also known as coal ash, are one of the largest industrial waste streams in the United States, and generally contain elevated concentrations of hazardous elements such as As, Se, Cd, and Cr that could pose risks to the environment and human health. In order to delineate the environmental impacts of CCRs, previous studies have explored the isotopic fingerprints (e.g., B, Sr, and Ra) as geochemical tools to identify the presence and pathways of coal ash contamination in soils and water systems. The relatively high concentrations of Sr in CCRs, combined with its high solubility, provides a unique tracer to detect coal ash contaminants in water resources. While previous studies have reported the Sr isotope ratios for the water-soluble phase of CCRs, the relationships between the Sr isotopic characteristics of the bulk CCR and the water-soluble phase have not been fully established. Here we characterize the ⁸⁷Sr/⁸⁶Sr ratios of bulk (fully digested) CCRs originated from the three major coal basins in the U.S., relative to the isotope ratios of corresponding water leachates. Preliminary data show systematic higher ⁸⁷Sr/⁸⁶Sr ratios in bulk CCRs as compared to those in water leachates, indicating that the soluble fraction of Sr in CCRs is less radiogenic than the bulk. The data show a wide range of ⁸⁷Sr/⁸⁶Sr ratios in bulk CCRs, with high radiogenic ⁸⁷Sr/⁸⁶Sr ratio in CCRs originated from Illinois coals (0.718029±0.00326), followed by CCRs from the Appalachian coals (0.712641±0.00083), and those from the Powder River Basin coals (0.711715±0.00044). In contrast, the ⁸⁷Sr/⁸⁶Sr ratios in the water leachates are generally lower (0.71137±0.00103, 0.71049±0.00035, 0.71133±0.00075, respectively). Our study provides a comprehensive dataset for the Sr isotopic variations in both bulk CCRs and their water leachates from the major coal sources in the U.S. The distinction between the strontium isotope ratios of the bulk coal ash and water leachates is important to facilitate the identification of CCRs contamination derived from both coal ash solids and mobilization of CCR-contaminants into water resources.