water. This permitted the development of a biodégradation zone with water circulating between the two wells to clean the aquifer. Successful evaluation was completed in March 1997. With pumping at 25 liters per min at each well and the introduction of 9 mg of toluene liter-1, 30 mg of dissolved oxygen liter-1, and 41 mg of hydrogen peroxide liter-1 for fouling control and additional oxygen, 83 to 85% TCE biodégradation was achieved with each pass through a treatment well. An estimated 60-m width of the TCE-contaminated plume was treated with this system, reducing its upgradient TCE by about 98% from 1,200 to 25 |Jtg liter-1. The toluene concentration was reduced to 1.4 + 0.6 p,g liter-1 at the 22- by 22-m boundary of the steady zone. Potential clogging was successfully controlled.
While aerobic cometabolic transformations of TCE may be a suitable approach in some cases for the bioremediative cleanup of TCE-contaminated aquifers, anaerobic dehalogenation may also be a useful approach (122). Palumbo et al. (240), for example, found that the presence of perchloromethene inhibited the methylotrophs, suggesting that anaerobic perchloromethene removal would be necessary prior to stimulation of methylotrophs to remove TCE. Several anaerobic methods for removal of chlorinated solvents from aquifers have been investigated (329). Sem-prini et al. (283) found that carbon tetrachloride-, trichloroethane-, and Freon-contaminated sites could be bioremediated by stimulating indigenous denitrifying populations through the addition of acetate. Anaerobic TCE degradation occurs by reductive dechlorination with TCE being sequentially reduced
to DCE, VC, and ethene. Because VC is a potent human carcinogen, it is critical to ensure that it does not accumulate in this process. Fumarate injection has been tested as a suitable substrate for anaerobic dehalogenation in TCE-contaminated sites (25). Rapid fumarate reduction to succinate was observed in wells where TCE reductive dechlorination was occurring. It appears that fumarate amendment has the potential to stimulate reductive dechlorination, even in aquifers for which no reductive dechlorination activity has been reported previously.
Injection of lactate has been tested for use in a deep, fractured rock aquifer contaminated with a TCE plume at the Idaho National Engineering and Environmental Laboratory. After 8 months of lactate addition, complete dechlorination was occurring at all monitoring points from 200 to 400 ft below ground within 100 ft of the injection well, and ethene was present in higher concentrations than any of the chlorinated ethene compounds (298). In situ anaerobic biodégradation enhanced with the injection of lactate has also been used to treat a large TCE plume that was due to historical injection of sludge waste into a basalt aquifer at the Pinellas Northeast Site, Largo, Fla. (105). The innovative remedy is known as reductive anaerobic biological in situ treatment technologies. For 8 months, lactate was injected 200 to 300 ft below ground surface. The success of the project in a complex fractured basalt aquifer may be a milestone both for fractured rock remediation and for in situ bioremediation of chlorinated solvent source areas.
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