Identification of Multiple Mercury Sources to Stream Sediments near Oak Ridge, TN, USA

Sediments were analyzed for total Hg concentration (THg) and isotopic composition from streams and rivers in the vicinity of the Y-12 National Security Complex (Y12) in Oak Ridge, TN (USA). In the stream directly draining Y12, where industrial releases of mercury (Hg) have been documented, high THg (3.26 to 60.1 mu g/g) sediments had a distinct Hg isotopic composition (delta Hg-202 of 0.02 +/- 0.15 parts per thousand and Delta Hg-199 of -0.07 +/- 0.03 parts per thousand; mean +/- 1SD, n = 12) compared to sediments from relatively uncontaminated streams in the region (delta Hg-202 = -1.40 +/- 0.06 parts per thousand and Delta Hg-199 of -0.26 +/- 0.03 parts per thousand; mean +/- 1SD, n = 6). Additionally, several streams that are nearby but do not drain Y12 had sediments with intermediate THg (0.06 to 0.21 mu g/g) and anomalous delta Hg-202 (as low as -5.07 parts per thousand). We suggest that the low delta Hg-202 values in these sediments provide evidence for the contribution of an additional Hg source to sediments, possibly derived from atmospheric deposition. In sediments directly downstream of Y12 this third Hg source is not discernible, and the Hg isotopic composition can be largely explained by the mixing of low THg sediments with high THg sediments contaminated by Y12 discharges.