Eight years after the coal ash spill – Fate of trace metals in the contaminated river sediments near Kingston, eastern Tennessee

Abstract

In December 2008, the failure of a coal ash retention pond at the Kingston Fossil Plant in eastern Tennessee, United States contaminated the Emory River and Clinch River located upstream of the Watts Bar Reservoir on the Tennessee River. Despite an extensive cleanup effort, further leaching of metals to river water from residual coal ash remains a cause of concern. Also, it is unknown whether coal ash has undergone substantial burial by younger river sediments. In order to address these uncertainties, in spring 2016 we collected six 30 cm long sediment cores from five contaminated and one uncontaminated portions of the river systems. The cores were subdivided into ∼2–3 cm sediment layers, which were exposed to water and acid leaching for metal and metalloid analyses. The measured concentrations of arsenic are 10 times higher in the river sediments near the Kingston Plant compared to downstream locations. Using ratios of various metals (e.g., cadmium, chromium, selenium, vanadium) to arsenic, we have established that only arsenic and selenium in downstream sediments are predominantly sourced by coal ash (80–100%). For other metals (e.g., cadmium, chromium, vanadium) coal ash-derived contamination comprises <20% of the total metal concentrations, which are likely of other anthropogenic origin. Under current conditions, the ash from the 2008 spill appears to be buried by 13–18 cm of natural sediment in downstream locations. However, elevated ash and metal and metalloid concentrations are still present in the shallow sediments (0–3 cm) in a close proximity to the Kingston Plant, suggesting redistribution of coal ash from further upstream. Overall, much higher concentrations of metals and metalloids were observed in the acid leachates compared to smaller, if any, concentrations in water-soluble fractions, suggesting that they may not be easily released to the water column. Through comparisons of metal and metalloid concentrations in raw coal ash and river sediments that had been in the river for 2 years, it appears that their quantities are similar. This implies that the majority of coal ash associated metals and metalloids are not mobilized by river waters.