Groundwater contaminated with chlorinated solvents - Air force center for environmental excellence

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Courtesy of Terra Systems, Inc. (TSI)

LOCATION:  Dover, Delaware
PROJECT DATE:  October 1999 to present
CLIENT:  Air Force Center for Environmental Excellence (AFCEE)

BACKGROUND: Groundwater contaminated with chlorinated solvents is a common problem to many US Air Force Bases. TSI in conjunction with Solutions Industrial & Environmental Services (Solutions-IES) of Raleigh, NC approached AFCEE with an innovative method utilizing insoluble substrates such as vegetable oils to address the groundwater contamination in a cost effective manner. The vegetable oil and an emulsion of the vegetable oil and water would be injected into treatment cells to intercept the groundwater plume and provide a long-lasting substrate for dechlorination of the chlorinated solvents. Injection of the inexpensive, long-lasting substrate would avoid the operation and maintenance costs associated with a recirculation system using a soluble substrate such as sodium lactate. Previous lab studies conducted by TSI personnel had shown the effectiveness of the insoluble substrate in enhancing anaerobic bioactivity of indigenous or introduced dechlorinating microorganisms. AFCEE funded TSI and Solutions-IES to test the insoluble substrate addition at three Air Force Bases including Dover.

ACTION TAKEN: Lab studies were performed by TSI to assess the impact of introduction of the insoluble substrate on the hydraulic conductivity of subsurface materials and to predict the expected mobility of the substrate in the aquifer. Two barrier configurations were designed: (1) closely spaced injection points for the emplacement of the insoluble food grade substrate and (2) more widely spaced injection points for the emplacement of emulsified substrate. The substrate and emulsifying agents were food-grade products. This allowed for prompt approval from the Delaware Department of Natural resources and Environmental Control (DNREC).

The barriers were installed side-by-side, perpendicular to the groundwater flow direction. A Geoprobeâ was used to install both the injection points and several closely spaced monitoring wells placed on either side of both barriers. Insoluble substrate or emulsified substrate was injected under pressure into the aquifer formation.

EFFECTIVENESS: The radius of migration of each substrate away from the injection points, the changes in hydraulic conductivity of the aquifer, and degradation of the contaminants are being monitored over time. Initial results suggest rapid removal of chlorinated solvents from the groundwater. Some of the removal was a result of adsorption of contaminants into the oil. Reductive dechlorination of TCE to cDCE has been promoted, however further transformation onto vinyl chloride and ethene or ethane has been limited. Relatively little transformation of 1,1,1-trichloroethane and its daughter products has been observed other than production of chloroethane in some wells. Based on the results to date and our experience with a pilot in another area of the base, we anticipate that bioaugmentation will be required to achieve complete dechlorination of these compounds. The specific capacity of the barrier receiving the emulsion was reduced by approximately two orders of magnitude. TOC levels were elevated up to 15 feet away from the emulsion barrier for over eighteen months.

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