NGWA Event and Education Recordings Archive

Enhanced anaerobic biodegradation is being implemented to treat a residual chlorinated solvent source and dissolved phase plume at the Well 12A Superfund Site in Tacoma, Washington. The remedial objective is to reduce contaminant mass discharge from the source by 90 percent. The aquifer comprises a heterogeneous mixture of coarse-grained glacial outwash overlying fine and coarse-grained sedimentary deposits. Consequently, the hydraulic conductivity of the aquifer is highly variable. A thin silt unit containing high concentrations of trichloroethene and cis-1,2-dichloroethene is present over much of the site, acting as a continuing source of contamination. The treatment zone was defined as the area containing greater than 300 parts per billion TCE or cis-DCE, and soil concentrations in the silt greater than 5000 micrograms per kilogram TCE or cis-DCE. Treatment is targeted immediately adjacent and within the silt to reduce mass flux to the more permeable layers above and below. Shear-thinning fluid tends to increase flow through low permeability zones relative to high permeability zones, because the viscosity of the fluid is lower in the fine-grained units, allowing for amendment to remain in close proximity, and potentially penetrate, the silt.

The pilot study consisted of injection of shear-thinning amendment containing xanthan gum, waste vegetable oil, and a tracer blended using a high-shear mixer. Injection was completed into three wells, including one screened across the silt. An additional injection was completed using shear-thinning fluid containing a lipophilic dye, followed by soil core sampling for dye and indicators of oil amendment above and below the silt.

Results indicate that the shear-thinning amendment can be distributed as desired, and amendment was not lost to high-conductivity units. Areas where the amendment was delivered quickly exhibited reductive dechlorination.