FIELD TESTING AND MODELING OF IN SITU GROUNDWATER ARSENIC REMEDIATION THROUGH IRON SULFIDE PRECIPITATION NEAR TACOMA, WASHINGTON

Field pilot testing of the remediation product EHC-M was conducted to evaluate in situ treatment of groundwater arsenic by incorporation into iron sulfides under reducing conditions. Equilibrium and reaction path modeling with Geochemist’s Workbench were completed to identify stable solid phases under test conditions and assess their precipitation rates and stability under varied conditions. The source of the arsenic is copper smelter slag mixed with wood waste in an unlined landfill in the Puget Sound lowland region of Washington. The focus of the study is remediation of a multi-acre plume of elevated arsenic, which is present primarily as arsenite at concentrations up to 3 mg/L, and occurs with elevated ferrous iron (up to 100 mg/L). The shallow, alluvial sand aquifer setting provides for a case study with broad application for remediation of groundwater arsenic using in situ reductive precipitation.

Treatment slurry containing sulfate, organic carbon, and microscale zerovalent iron was injected in transects of closely-spaced borings to evaluate a permeable reactive barrier application designed to intercept the plume near the leading edge. Groundwater quality trends in total and dissolved arsenic and iron, sulfate and sulfide, redox potential and pH were evaluated for a five-month period following injection of a high dosage treatment (0.6% of soil mass) and a low dosage treatment (0.2% of soil mass) in separate test locations. The results were used to assess arsenic removal performance and provide model inputs.

Geochemist’s Workbench was used to model equilibrium solid phases under test conditions as a basis to interpret key arsenic removal mechanisms: coprecipitation with amorphous iron monosulfide and incorporation into pyrite/arsenian pyrite and arsenopyrite. Precipitation and oxidative dissolution reactions were modeled to assess the rate and effectiveness of the in situ remediation in response to theoretical changes in groundwater redox potential.