ASSESSMENT OF TRACE METAL CONTAMINATION IN RIVERINE SEDIMENTS FROM THE COAL ASH SPILL IN KINGSTON, TENNESSEE AND POTENTIAL BIO-IMMOBILIZATION THROUGH MICROBIAL SULFATE REDUCTION

The Tennessee Valley Authority’s (TVA) Kingston Fossil Plant coal ash pond ruptured in December 2008 leading to contamination of the Emory River, Clinch River and Watts Bar Reservoir. Despite extensive cleanup efforts, further leaching of toxic metals from river sediments remains a cause of concern. In spring 2016, six ~30 cm long sediment cores were collected and analyzed for water- and acid-leachable trace metals and S isotopic compositions. Concentrations of As and Cd leached from the river sediments (3.5-10.9 and 0.7-2.2 mg/Kg, respectively) exceeded sediment quality guidelines (8.2 mg/kg and 1.2 mg/kg, respectively). However, only concentrations of As appear to be sourced by remaining coal ash contaminants. No other trace metals (Cr, Cu, Ni, Pb, Zn) surpassed toxicity guidelines. Furthermore, the highest concentrations of these leachable metals were measured further away and upstream of the coal ash spill location. This suggests that metal fluxes are also coming from other sources, most likely anthropogenic industry, mining, and urban runoff. Changes of metal concentrations with sediment depth were not useful for determining the current burial depth of coal ash contaminants. The δ³⁴S of bulk river sediments, elemental S, and chromium-reducible sulfides were better tracers. In the most contaminated layers, δ³⁴S was the highest (+4 to +11‰) reflecting the δ³⁴S of combusted coal (+9 to +12‰). From these profiles, it can be inferred that the remaining coal ash contaminants are currently buried by ~5-10 cm of younger sediment, in agreement with sedimentation rates measured in this river system. The low δ³⁴S values of elemental S (-3.5 to +1.1‰), acid-volatile sulfides (-2.7 to +1.1‰) and chromium-reducible sulfides (-6.9 to +1.3‰) in the sediment cores suggest that microbial sulfate reduction (MSR) is occurring in the studied rivers, thus it could aid in metal bio-immobilization through the formation of insoluble sulfides. This process, however, appears to be insufficient, as the majority of metals were found in the water-soluble fraction. The very low S content in these sediments could account for this lack of immobilization. We suggest that other natural processes such as dilution and sedimentation are more important in decreasing trace metal fluxes through burial of coal ash contaminants over time.