11 Human Health Risk Assessment Analytical Investigation of Soil Samples Identifies True Source of Mercury Contamination Elemental Mercury Levels Exceeds Inhalation-Based Screening Criteria Our client was in the final phases of corrective action at a 700-acre facility located outside Chicago. Final confirmation sampling conducted in accordance with the regulatory requirements identified that the concentration of elemental mercury exceeded the inhalation-based screening criteria across large portions of the facility. Gauges and meters containing elemental mercury were used at the facility; however, elemental mercury was not observed during demolition or restoration activities. Innovative Extraction Procedure Solves Mystery of Mercury Levels Our team determined that the presence of mercury across large swaths of the facility suggested a potential background source unrelated to historical operations. Mercury generally exists in one of three phases including elemental, inorganic, or organic. The presence of mercury can be from natural sources such as volcanic eruptions and forest fires or from anthropogenic sources such as burning of coal and fossil fuels. We determined that the analytical laboratory that the client had selected was reporting total (elemental, inorganic, and organic) mercury concentrations within the final confirmation samples. Through discussions with the regulatory agency, it was decided that a small, but representation number of soil samples would be collected across the facility to speciate mercury using a six-step selective sequential extraction procedure. This six-step process separated elemental mercury from other forms. Analysis Connects Mercury to Upwind Power Plant, Avoiding Costs and Expediting Redevelopment The analytical results indicated that elemental mercury was not present within any of the representative soil samples and therefore there was not an inhalation risk for future receptors on the facility. It was determined that the source of mercury was related to emissions from a decommissioned power plant located approximately 1-mile upwind of the facility, which explained the ubiquitous presence of total mercury across the facility. Our findings resulted in expedited redevelopment and avoided unnecessary costs associated with additional corrective actions. Project at a Glance • Elemental mercury levels exceed screening criteria across 700-acre facility • Team linked presence of mercury to background source not related to historical operations • Small soil sample collected to speciate mercury using six-step selective sequential extraction procedure • Analysis linked mercury source to decommissioned power plant upwind • Solution expedited redevelopment and avoided unnecessary corrective actions and costs.