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East Fork Poplar Creek (EFPC) in Oak Ridge, Tennessee, has been heavily contaminated with mercury (also referred to as Hg) since the 1950s as a result of historical activities at the U.S. Department of Energy (DOE) Y-12 National Security Complex (formerly the Oak Ridge Y-12 Plant and hereinafter referred to as Y-12). During the period from 1950 to 1963, spills and leaks of elemental mercury (Hg{sup 0}) contaminated soil, building foundations, and subsurface drainage pathways at the site, while intentional discharges of mercury-laden wastewater added 100 metric tons of mercury directly to the creek (Turner and Southworth 1999). The inventory of mercury estimated to be lost to soil and rock within the facility was 194 metric tons, with another estimated 70 metric tons deposited in floodplain soils along the 25 km length of EFPC (Turner and Southworth 1999). Remedial actions within the facility reduced mercury concentrations in EFPC water at the Y-12 boundary from &gt; 2500 ng/L to about 600 ng/L by 1999 (Southworth et al. 2000). Further actions have reduced average total mercury concentration at that site to {approx}300 ng/L (2009 RER). Additional source control measures planned for future implementation within the facility include sediment/soil removal, storm drain relining, and restriction of rainfall infiltration within mercury-contaminated areas. Recent plans to demolish contaminated buildings within the former mercury-use areas provide an opportunity to reconstruct the storm drain system to prevent the entry of mercury-contaminated water into the flow of EFPC. Such actions have the potential to reduce mercury inputs from the industrial complex by perhaps as much as another 80%. The transformation and bioaccumulation of mercury in the EFPC ecosystem has been a perplexing subject since intensive investigation of the issue began in the mid 1980s. Although EFPC was highly contaminated with mercury (waterborne mercury exceeded background levels by 1000-fold, mercury in sediments by more than 2000-fold) in the 1980s, mercury concentrations in EFPC fish exceeded those in fish from regional reference sites by only a little more than 10-fold. This apparent low bioavailability of mercury in EFPC, coupled with a downstream pattern of mercury in fish in which mercury decreased in proportion to dilution of the upstream source, lead to the assumption that mercury in fish would respond to decreased inputs of dissolved mercury to the stream's headwaters. However, during the past two decades when mercury inputs were decreasing, mercury concentrations in fish in Lower EFPC (LEFPC) downstream of Y-12 increased while those in Upper EFPC (UEFPC) decreased. The key assumption of the ongoing cleanup efforts, and concentration goal for waterborne mercury were both called into question by the long-term monitoring data. The large inventory of mercury within the watershed downstream presents a concern that the successful treatment of sources in the headwaters may not be sufficient to reduce mercury bioaccumulation within the system to desired levels. The relative importance of headwater versus floodplain mercury sources in contributing to mercury bioaccumulation in EFPC is unknown. A mercury transport study conducted by the Tennessee Valley Authority (TVA) in 1984 estimated that floodplain sources contributed about 80% of the total annual mercury export from the EFPC system (ORTF 1985). Most of the floodplain inputs were associated with wet weather, high flow events, while much of the headwater flux occurred under baseflow conditions. Thus, day-to-day exposure of biota to waterborne mercury was assumed to be primarily determined by the Y-12 source. The objective of this study was to evaluate the results of recent studies and monitoring within the EFPC drainage with a focus on discerning the magnitude of floodplain mercury sources and how long these sources might continue to contaminate the system after headwater sources are eliminated or greatly reduced

Sources of Mercury to East Fork Poplar Creek Downstream from the Y-12 National Security Complex: Inventories and Export Rates