Mercury Issues and Complexities in Oak Ridge, Tennessee; Redefining the Conceptual Model - 12277

Releases of mercury from an industrial facility in Oak Ridge, Tennessee in the 1950s and early 1960s resulted in contamination of soil and groundwater within the facility, as well as downstream surface waters. Remediation efforts, which began in the 1980s, have decreased waterborne mercury concentrations near the facility, but elevated levels of mercury remain in the soil, sediment, water, and biota. Widespread distribution of mercury sources and complex mercury transport pathways are some of many challenges at the site. For effective environmental management and closure decision making relative to mercury contamination at the facilities, an up-to-date conceptual model of mercury source areas, processes, likely flow paths, and flux was deemed necessary. Recent facility and reconfiguration efforts, site characterizations, remedial actions, and research are facilitating the collection of new mercury data in Oak Ridge. To develop the current model, a multi-organizational team reviewed existing conceptual models from a variety of sources, consolidated historical data and source information, gathered input from local experts with extensive site knowledge, and used recently collected mercury data from a variety of sampling programs. The developed site conceptual model indicates that the nature and extent of mercury concentration and contaminant flux has significantly changed in the ten years since flux-based conceptual models were used for previous remedial action decisions. A new water treatment system has effectively reduced mercury inputs to the creek and is removing substantially greater quantities of mercury from groundwater than was expected. However, fish concentrations in downstream waters have not responded to decreased water concentrations in the stream. Flux from one large outfall at the creek’s headwaters appears to be a greater percentage of the overall flux leaving the site than previous years, albeit year to year variation in flux is large, and the many small sources of mercury identified in the model may also be important if the goal is to reach very low mercury levels in stream water and fish. The conceptual model is a key reference in helping to prioritize future remedial actions, defining future monitoring, conducting numerical modeling efforts, and evaluating research needs.