Background/Objective. Enhanced anaerobic biodegradation (EAB) has been selected as the remedial approach to treat a residual chlorinated solvent source area and dissolved phase plume at the Well 12A Superfund Site in Tacoma, Washington. A pilot study utilizing shear-thinning fluids to deliver oil-based amendments for EAB in targeted lithologies has been completed. The primary remedial action objective for the Site is to reduce contaminant mass discharge from the source area by 90 percent. Detailed vertical profiling of the geology and contaminant extent within the plume determined that the majority of mass discharge occurs over a relatively narrow vertical interval within the aquifer, which was associated with a low conductivity silt unit which contains significant diffused contaminant mass. Targeting the interfaces of this silt unit presented an opportunity to optimize the EAB treatment strategy. The EAB treatment zone was defined as the area within the groundwater plume containing greater than 300 parts per billion (ppb) TCE or cis-DCE, and containing soil concentrations in the silt unit greater than 5,000 micrograms per kilogram (μg/kg) TCE or cis-DCE. EAB treatment is targeted in the interval immediately adjacent above and below, as well as within, the silt unit to reduce mass flux from diffusion from the silt unit to the more permeable layers above and below. In order to increase the injection sweep efficiency across the heterogeneous layers of the EAB injection interval, the amendment was modified to exhibit shear-thinning properties to more effectively deliver EAB amendment to low conductivity zones. The shear-thinning fluid amendment was delivered much more uniformly above, below, and within, the silt unit.
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