USING A CONCEPTUAL SITE MODEL IN STAKEHOLDER ENGAGEMENT AND DECISION MAKING AT A MERCURY-CONTAMINATED SEDIMENT SITE: ST. LAWRENCE RIVER AREA OF CONCERN
Cornwall, Ontario has long been a center of industrial activities, relying upon the St. Lawrence River as a source of freshwater for manufacturing processes and effluent dilution and dispersion. In 1985, the International Joint Commission designated the Cornwall area of the St. Lawrence River as an Area of Concern (AOC) under the Canada-U.S. Great Lakes Water Quality Agreement. In 2005, monitored natural recovery in combination with administrative controls was selected as the risk management approach for three zones of the AOC proximate to Cornwall. A conceptual site model was prepared as a framework for synthesizing over 40 relevant reports and to support diverse stakeholders evaluation of the effectiveness of monitored natural recovery. The conceptual site model details sources of mercury, migration and transformation pathways for mercury, human and ecological receptors that potentially contact or contacted mercury, and exposure pathways by which those receptors are or were exposed to mercury. The goal of the conceptual site model is to accurately convey current conditions within the three zones, with sufficient characterization of the processes acting on the mercury to support stakeholders development of site-specific objectives, goals, targets, and associated monitoring approaches. By organizing extensive and diverse information into the framework of the conceptual site model, it is possible to identify key factors influencing the success of monitored natural recovery, as well as fundamental data gaps that impede demonstration of that success. Multiple lines of evidence indicate that mercury in the sediment of the three zones likely poses negligible risks to human health and the environment. Detectable decreases in sediment concentrations are difficult to discern, however, due to inconsistent sampling locations over time, limited sediment core data, and continuing inputs of mercury to the river. For example, storm sewers and combined sewer overflows appear to continue to convey mercury from upland sources to the river. Once all sources of mercury have been mitigated to the greatest extent possible, ongoing monitoring will be necessary in order to determine the effectiveness of monitored natural recovery. Monitoring will demonstrate whether surface sediment mercury concentrations have changed over time and whether it is likely that concentrations consistent with upstream can be achieved in the future.