Geochemical Assessment of Leaching from Coal Ash Disposal Sites

Abstract

The disposal of coal combustion residues in landfills and impoundments is a long-term environmental safety concern, resulting from the potential leaching of oxyanions (As, B, Cr, Mo, Se, and V). Laboratory leaching characterization, field case studies, and geochemical speciation modeling were used to provide new insights and improved models allowing for better understanding and estimation of the leaching behavior of oxyanions. Geochemical speciation modeling was carried out on coal fly ashes from a wide range of sources to identify mechanisms controlling leaching as a function of material composition. Uncertainties of the geochemical speciation model were characterized to improve the fidelity and applicability of the model for estimation of leachate compositions. In addition, oxyanion partitioning under field conditions, wherein effects of leaching-controlling factors (i.e., pH, oxidation-reduction potential, liquid-to-solid ratio, and site configurations) were considered, was evaluated based on field studies for a closed ash impoundment and an active ash management unit. Field geochemical conditions were characterized through analysis of porewater parameters and monitoring of subsurface gas over approximately one year at each site. The results of laboratory leaching tests conducted on samples from each site were compared to porewater measurements, to understand the implications of laboratory and field geochemical conditions on leaching. The findings suggest that leaching test results should be evaluated in the context of site-specific conditions to improve the understanding of field leaching behavior. Finally, percolation column tests were carried out on a range of coal ashes to determine the impacts of different chemical processes, field weathering reactions, and the redox conditions on the elution of oxyanions. Column test results were used in conjunction with water discharge from a hydrologic model for a conceptual field disposal scenario to estimate the long-term release of constituents as a function of time.