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EXH AR019613: WAC 173-340-747 Deriving Soil Concentrations Groundwater Protection MTCA

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Sea-Tac Airport
Third Runway
PCHB
Box 09

This Washington State regulation (WAC 173-340-747) establishes methods for determining safe soil contamination levels that protect groundwater quality at cleanup sites. It outlines several scientific approaches, including three-phase and four-phase partitioning models, leaching tests, and empirical demonstrations, to calculate the maximum allowable concentration of hazardous substances in soil before they risk contaminating groundwater. The regulation requires that soil concentrations not cause groundwater cleanup level exceedances and not result in the buildup of nonaqueous phase liquids in or on groundwater.

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Notes

WAC 173-340-747 from the MTCA (Model Toxics Control Act) Cleanup Regulation, effective February 12, 2001, establishing methods for deriving soil concentrations protective of ground water under WAC 173-340-720. Covers fixed parameter three-phase partitioning model (Equation 747-1), variable parameter three-phase partitioning model, four-phase partitioning model for NAPL/petroleum hydrocarbons, leaching tests for metals, alternative fate and transport models, and empirical demonstration methods. Specifies distribution coefficient (Kd), Henry’s law constant, soil organic carbon (foc), soil bulk density (1.5 kg/L), water-filled and air-filled soil porosity, dilution factor (DF=20 unsaturated zone, DF=1 saturated zone), and equations 747-1 and 747-2. Applicable to Method B and Method C soil cleanup levels under WAC 173-340-740 and WAC 173-340-745. Filed as Administrative Record AR019613 in PCHB Third Runway EIS proceedings.

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