TRACING MULTIPLE MERCURY SOURCES IN RIVER SEDIMENTS NEAR THE TVA KINGSTON SITE USING HG STABLE ISOTOPE RATIOS

Following the December 22, 2008 ash spill at the Tennessee Valley Authority's (TVA) Kingston coal-burning power plant, elevated levels of mercury (Hg) were detected in the Emory and Clinch Rivers’ sediments. However, there are other sources of Hg in the system, including known releases into the Clinch River from the U.S. Dept. of Energy Y-12 plant near Oak Ridge, regional atmospheric Hg, and natural background. Variations in Hg stable isotope abundances may provide a means of tracing the Hg from various sources as it moves through the system.

²⁰²Hg/¹⁹⁸Hg abundance ratios are expressed as δ²⁰²Hg values, which are per mil deviations from NIST SRM-3133. Odd isotope anomalies were also measured and are reported as Δ¹⁹⁹Hg and Δ²⁰¹Hg.

Our results suggest three distinct δ²⁰²Hg “signatures” for the three likely sources influencing the system. Average δ²⁰²Hg values for the Clinch River, Kingston fly-ash, and the natural background (represented by Emory River Mile Marker 12 samples) are -0.26 ± 0.16‰, -1.71 ± 0.42‰, and -1.17 ± 0.13‰, respectively. The data suggest that the upstream Clinch River is dominated by near-zero δ²⁰²Hg values, possibly due to the Y-12 plant near Oak Ridge. Sediment samples around the Kingston TVA plant, on both the Emory and Clinch rivers, are fully consistent with a three end-member mixing model. This model enables us to estimate the contributions of the three regional Hg sources to the total Hg inventory in each sediment sample.

The data also show that the Clinch River has Δ¹⁹⁹Hg and Δ²⁰¹Hg values distinct from the fly-ash and Emory River. The average ¹⁹⁹Hg and ²⁰¹Hg anomalies in the Clinch River samples are -0.10 ± 0.04‰ and -0.08 ± 0.02‰, respectively. The fly-ash and Emory River are indistinguishable from each other (Δ¹⁹⁹Hg values of -0.22 ± 0.01‰ and -0.21 ± 0.06‰, respectively; Δ²⁰¹Hg values of -0.16 ± 0.02‰ and -0.15 ± 0.03‰, respectively).

We infer from the data that Hg isotope ratios in river sediments really are semi-permanent “signatures” that do not change much with time. The range of values of Clinch River Mile 5.5 is 0.40‰ over an 11 month timescale, with no trend. The narrow overall range for these upstream samples suggests a lack of strong shifts as the Hg ages in the river system. However, it would be risky to simply assume that the Hg is completely chemically inert in the system.