Coupled chemical-isotope assessment of potential metal releases to the water column from river sediments impacted by coal ash spill

Abstract

In December 2008, the Kingston coal ash spill led to contamination of the Emory, Clinch, and Tennessee rivers with toxic metals and metalloids (e.g., arsenic, cadmium, chromium, mercury, vanadium). At present, it is unclear whether the surface waters are impacted by releases from the coal ash residing in the bottom river sediments. Therefore, in April 2016 we collected new samples from several depths of the water column (0, 4, 6, 10 m) from seven impacted and three non-impacted sites along these river systems. The sampled water was analyzed for trace metal and metalloid concentrations and carbon-sulfur-hydrogen-oxygen (C-S-H-O) isotope compositions. The obtained results show that most trace metals and metalloids were below detection limit in the water column (<0.002 mg/L) with only manganese (0.11–0.29 mg/L) exceeding the Environmental Protection Agency drinking water guideline (0.05 mg/L) in two locations. The studied river waters are of local meteoric origin (δ18O of −6.4 to −5.6‰ and δ2H of −37.4 to −32.4‰) and classified as mainly calcium-bicarbonate (Ca-HCO3) type originating from dissolution of local carbonate bedrock. The latter likely increases the buffering capacity of the studied rivers resulting in pH at neutral to moderately alkaline levels (6.5–8.5) where trace metals and metalloids are most likely to be found adsorbed onto sediments. At the sediment-water interface, however, the Emory River water had the lowest pH (6.6), DO (5.0 mg/L), alkalinity (15–60 mg/L), and δ13C of DIC (−13.7‰), suggesting some acidification occurring due to organic matter decomposition in the river sediments. Given that elevated acid-leachable metals and metalloids from coal ash spill are still present at elevated concentrations in the shallow sediments of Emory River, the decreasing pH might be a concern as it could lead to their releases into the water column. Moreover, S isotopes appear to be useful tracers of coal ash and bioremediation of arsenic via microbial sulfide reduction. In the most impacted sediments, the highest δ34S of +4 to +12‰ at depth of ∼5–15 cm points to the raw coal ash enriched in heavier 34S isotopes. In less impacted sediments, the negative δ34S of sulfides (−8 to −6‰) were accompanied by higher leachable arsenic concentrations suggesting that microbial sulfate reduction might be involved in formation of arsenic-rich biogenic sulfides.