One focus of approaches to developing cleanup levels for soils has been to protect groundwater from infiltrating that transports contaminants from overlying soils. During redevelopment of an industrial site along a Puget Sound shoreline, a hydrogeologic model was used in a risk-based, site-specific approach to set cleanup levels for subsurface soil chemicals that are protective of exposures of marine organisms to groundwater that has surfaced in waters of the Sound. Remedial goals-for polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), and total petroleum hydrocarbon (TPH) in subsurface soils were identified to protect marine receptors and their consumers. State and federal marine water quality criteria, or surrogate values, were the starting points in the model, and corresponding concentrations of chemicals in groundwater beneath the site were back-calculated through a hydrogeologic model. The model included a mixing zone component in the receiving bay and dilution/attenuation factors along the groundwater transport pathway that were determined from onsite groundwater and surface water chemical ion concentrations. A rearranged Summers equation was then applied to groundwater concentrations in a second back-calculation to determine corresponding chemical concentrations in subsurface soils. The Summers equation was based on calculated aquifer tow rates for the local watershed and rates of infiltration, calculated by the HELP model, for Various surface cover materials. Concrete, asphalt, ballast, and a landfill geomembrane were modeled as soil surface cover material, which were planned to prevent direct contact with surface soils and attenuate infiltrating surface water. Results of the risk-based hydrogeologic model indicate that, depending on type of soil cover after site remediation, concentrations in subsurface soils of PCBs ranging from 2 to 1000 mg/kg, PAHs from 448 mg/kg for chrysene, to saturation levels for less water soluble compounds, and TPH from 28 000 mg/kg to saturation concentrations would not result in risks to marine organisms or their consumers in the receiving water of Puget Sound.
