The objectives of this study were to investigate the current status of mercury distribution, speciation and bioavailability in the floodplain soils of Lower East Fork Poplar Creek (LEFPC) after decades of US Department of Energy’s remediation. Historically as part of its national security mission, the U.S. Department of Energy’s Y-12 National Security Facility in Oak Ridge, TN, USA acquired a significant fraction of the world’s supply of elemental mercury. During the 1950s and 1960s, a large amount of elemental mercury escaped confinement and is still present in the watershed surrounding the Y-12 facility. A series of remediation efforts have been deployed in the watersheds around the Oak Ridge site during the following years. The sampling fields were located in a floodplain of LEFPC of Oak Ridge, TN, USA. A series of surface soils (10–20 cm) were sampled from both wooded areas and wetland/grass land. Two 8×8 m fields were selected in the woodland. Five profiles each consisting of three layers were randomly taken from each field. The three layers were the surface layer at 0–10cm, subsurface layer at 50–60 cm, and bottom layer at 100–110 cm. Soil in both wood and wetland areas was well developed with a clear B horizon. The present study clearly shows that the total mercury in floodplain soils of LEFPC significantly decreased after the series of remediation. This study confirmed the long-term effectiveness of these remediation actions, especially after excavation of highly contaminated floodplain soils. However, the average total mercury level of all soil samples collected are in the range of 50–80 mg/kg, still significantly above toxic level (> 5mg/kg). Furthermore, contrary to conventional believing, the major mercury form in current soils of this particular area of floodplain of LEFPC is mainly in non-cinnabar mercury bound in clay minerals (after decades of remediation). The floodplains can act both as a medium-term sink and as long-term sources. Native North American earthworms (Diplocardia spp.) and adjacent soils were taken from each spot in each field. Our results show strong linear relationships between mercury concentrations in earthworms (both mature and immature groups) and non-cinnabar mercury form, while cinnabar mercury is less bioavailable to native earthworms. Earthworms may be used as a potential mercury ecological bio-indicator (bio-marker) for demonstrating mercury bioavailability and ecotoxicity in the ecosystem. The long-term stability, mobility and bioavailability of mercury contaminants in these floodplains still needs to be monitored continuously and closely.

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