The Behavior of Mercury in Coal Combustion Residue-Contaminated Sediments

Abstract:

Coal combustion residues (CCRs), the products of coal combustion, contain high concentrations of heavy metals such as mercury. Recent structural failures of on-site containment ponds and leaching of CCRs has potentially endangered the health of adjacent water bodies. This study examines the influence of CCR enrichment of river sediments through the study of mercury, an abundant constituent of CCRs, and the concomitant production of methylmercury. We hypothesized that CCR contamination increases the exposure to mercury for aquatic life through leaching and resuspension mechanisms. Resuspension experiments were conducted using CCR-contaminated sediments from the Dan River and uncontaminated sediments enriched with 0%, 10%, and 30% CCRs in the laboratory. Sediments were sieved to obtain the silt-clay fraction, which was then resuspended in solution with a dispersant to obtain the separate silt and clay fractions and then analyzed for total mercury concentrations. We found that CCR particles and the mercury they contain are present primarily in the silt and clay fractions of sediment and there is a direct relationship between CCR concentrations and total mercury concentrations. These findings have implications for both the bioavailability of mercury to methylating bacteria, higher organisms prone to direct ingestion of fine particles, CCR spill event remediation, current industrial waste disposal practices, and further research required in this field. Our seven day incubations of river sediment cores enriched with CCRs did not increase methylmercury in porewater above controls, suggesting that there is no immediate risk of increased methylmercury bioaccumulation, however this does not necessarily reflect the long-term effects of CCRs on river ecology, which requires further research.