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Borden R.C., S.D. Richardson, and A.A.Bodour.
Journal of Environmental Management 237:617-628(2019)

A pilot test used an emulsified vegetable oil (EVO) and colloidal magnesium hydroxide [Mg(OH)2] formulation to enhance reductive dechlorination of TCE DNAPL in an acidic (pH≤4), heterogeneous aquifer. The test consisted of a single well injection to evaluate Mg(OH)2 distribution and installation of two EVO- Mg(OH)2 permeable reactive barriers (PRB-1 and PRB-2) at varying distances downgradient of the DNAPL source area. Distribution of Mg(OH)2 was observed up to 2.3 m away from the injection point within a permeable coarse sand layer. Mg(OH)2 transport in the overlying clayey-silty sand was minimal. Downgradient of the PRBs, colloidal Mg(OH)2 increased the pH of the coarse sand to levels appropriate for biological reductive dechlorination (pH >~5). However, some settling of Mg(OH)2 in the injection well generated persistent high pH (~9-10) within the PRBs. A redesigned suspension of colloidal Mg(OH)2 was tested and proved to be more effective at raising aquifer pH without an excessive rise in pH within the PRBs. At PRB-1 (closest to the DNAPL source area), limited TCE biodegradation was observed due to the influx of high TCE concentrations (up to 400 mg/L) and inhibition of dechlorinating bacteria. At PRB-2 (25 m downgradient of the DNAPL source area), TCE concentrations were much lower (13-26 mg/L), and production of cDCE and some VC was observed. Subsequent bioaugmentation with a commercial dechlorinating culture at PRB-2 improved conversion of cDCE to VC and ethene at downgradient monitoring wells throughout the study. Results emphasized the importance of PRB location (relative to the DNAPL source), base selection for pH adjustment, source strength, and local heterogeneities for the design and long-term performance of ERD in acidic DNAPL-impacted aquifers.

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