Evaluation of Electron Donor Materials Used to Create Subsurface Permeable Reactive Barriers for Enhanced Reductive Dechlorination of Chlorinated Ethenes

Chlorinated ethenes (CEs) are widespread ground water contaminants, apparently due to their extensive migration from numerous contamination source points. In this study, a long-term field investigation and aquifer microcosm experiments were combined to evaluate the effectiveness of electron donor materials used to create and maintain subsurface permeable reactive barriers (PRBs) for the enhanced in-situ biodegradation of CEs. The laboratory component includes the first side-by-side comparison of electron donor materials dairy whey, lactate syrup, and Hydrogen Release Compound [Registered] (HRC), in which dechlorination rates, fermentation product distributions, and H[subscript 2] production were monitored as measures of substrate efficiency. Field study allowed the first investigation of the long-term efficacy of a full-scale (~300 ft.) when PRB designed to dechlorinate high concentrations of CEs (10[superscript 2]-10[superscript 3] [micro]g/L) migrating from a source zone. The effects of altering substrate loading volume, loading frequency, and injection method on CE distributions are examined. In donor-amended aquifer microcosms, substrate fermentation to slow-degrading organic acids maintained low H[subscript 2] partial pressures that, as previous studies suggest, may give competitive advantage to dechlorinators over hydrogenotrophic methanogens. Whey-amended and lactate-amended microcosms exhibited faster complete dechlorination. Whey-amended microcosms contained the highest percentage of organic acid carbon upon complete dechlorination. Whey's efficiency improved in microcosms prepared with whey-treatment-zone sediment and ground water, due apparently to the growth of native dechlorinators (e.g. Dehalococcoides) and microbial adaptation to whey within the PRB. In the field study, CEs decreased to low ([less than or equal to]10 [micro]g/L) or undetected levels within the PRB, while trichloroethene and cis-dichloroethene treatment-period average concentrations decreased significantly at downgradient points. Improved and sustained dechlorination was observed following injection of 300 kg whey, as a 750 mg/L slurry, with the use of extraction-injection loops. Results indicate whey loading values of 0.2 kg/m[superscript 3] are appropriate under sufficiently reducing conditions. Comparison of whey fermentation product, chemical oxygen demand (COD), and CE trends indicate whey's lifetime was 4-5 months. Based on the longevity of whey and its 2006 average market price, the estimated material cost of a 300-ft whey PRB is $700/year. Cost comparison, based on determinations of carbon flow in donor-amended microcosms, suggests whey is by far the more cost-efficient PRB material.