Native Plants in Floodplain in Controlling Hg Flux into East Fork Poplar Creek of Oak Ridge - 18166

Legacy Hg contamination has been identified as the highest priority clean-up risk on the Oak Ridge Reservation (ORR) by the Department of Energy Office of Environmental Management (DOE-EM), Tennessee Department of Environment and Conservation (TDEC), and the US Environmental Protection Agency (EPA). During the 1950's and early 1960's, elemental mercury (Hg) was used to produce enriched Li isotope at the Y-12 National Security Complex (Y-12 Plant) for manufacturing components of various nuclear weapons systems. It is estimated that 350,000 kg of Hg was released to the environment contaminating facilities, soil, sediment, surface water, and groundwater within the boundaries of the Y-12 Plant and the downstream environment (i.e., East Fork Poplar Creek, EFPC). EFPC is a 23-km stream where the headwaters start in the Y-12 Plant, flows through the city of Oak Ridge, and empties into the Clinch River. Although various remediation efforts have reduced Hg fluxes from the Y-12 Plant over the last 25 years, the Hg concentration in water continue to exceed both the regulatory limit (51 ng/L) and the remediation goal (200 μg/L). Perennial grasses as floodplain plants have been successfully employed to control surface runoff and stabilize the floodplain. Hg has been strongly bound to particles suspended with surface runoff. Miscanthus grass is a perennial, tall grass that is adjusted to coarse, fine and medium textured soils. Although it grows best in deep, well-drained floodplain soils it is tolerant of poor to excessive well-drained soils, has an extensive tolerance for acidic to alkaline soils, and tolerates soil textures ranging from sand to clay. These long tangled root systems can reach moisture more deeply than other grasses during drought thus anchoring the plant into the soil. The fine appearance and branching hairs grow to form a dense mat that helps improve soil structure by increasing porosity and adding organic material that helps to bind soil particles together. In compacted or clay soils, spreading roots increase soil porosity by creating fractures in soil aggregates allowing water to penetrate deeper within the subsoil. This study showed that DOE remediation significantly reduced residual Hg to be <400 mg/kg in floodplain. However the residual Hg was mostly present in non-cinnabar mercury which is more bioavailable than cinnabar. The current study also found that Hg transfer flux from floodplain soils through surface soils with trees and grasses under natural rainfall events was an average of 0.28±0.15 μg/L, which was similar to the Hg concentrations in water of EFPC as reported by ORNL. Total Hg concentrations and Hg and speciation in floodplain soils were strongly affected by particle size distribution. Fine particles had higher Hg concentrations rich in cinnabar Hg. Clay particles contained as high as 2000-1600 mg/kg Hg with most Hg as cinnabar Hg, while coarse particles contained less total Hg, dominated by non-cinnabar Hg. Hg speciation in floodplain soil strongly controls its solubility, mobility, and bioavailability in both terrestrial/ aquatic ecosystems and downstream Hg concentrations. (authors)