Hg isotopes reveal in-stream processing and legacy inputs in East Fork Poplar Creek, Oak Ridge, Tennessee, USA

Natural abundance stable Hg isotope measurements were used to place new constraints on sources, transport, and transformations of Hg along the flow path of East Fork Poplar Creek (EFPC), a point-source contaminated headwater stream in Oak Ridge, Tennessee. Particulate-bound Hg in the water column of EFPC within the Y-12 National Security Complex, was isotopically similar to average metallic Hg(0) used in industry, having a mean δ²⁰²Hg value of -0.42 ± 0.09‰ (1SD) and near-zero Δ¹⁹⁹Hg. On average, particulate fraction δ²⁰²Hg values increased downstream by 0.53‰, while Δ¹⁹⁹Hg decreased by -0.10‰, converging with the Hg isotopic composition of the fine fraction of streambed sediment along the 26 km flow path. The dissolved fraction behaved differently. Although initial Δ¹⁹⁹Hg values of the dissolved fraction were also near-zero, these values increased transiently along the flow path. Initial δ²⁰²Hg values of the dissolved fraction were more variable than in the particulate fraction, ranging from -0.44 to 0.18‰ among three seasonal sampling campaigns, but converged to an average δ²⁰²Hg value of 0.01 ± 0.10‰ (1SD) downstream. Dissolved Hg in the hyporheic and riparian pore water had higher and lower δ²⁰²Hg values, respectively, compared to dissolved Hg in stream water. Variations in Hg isotopic composition of the dissolved and suspended fractions along the flow path suggest that: (1) physical processes such as dilution and sedimentation do not fully explain decreases in total mercury concentrations along the flow path; (2) in-stream processes include photochemical reduction, but microbial reduction is likely more dominant; and (3) additional sources of dissolved mercury inputs to EFPC at baseflow during this study predominantly arise from the hyporheic zone.