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This study evaluates the efficiency of a full-scale, 81 m-wide permeable reactive barrier (PRB) configured by injection of dairy whey in the downgradient region of a contaminant source zone to enhance the in situ biodegradation of high concentrations (102 to 103μg/L) of chlorinated ethenes (CEs). Ten biannual whey injections were completed in a 3.5-year pilot phase and 1.5-year operational phase. Improved and sustained dechlorination was observed at extraction/injection and downgradient wells in the fully-operational phase, when dried whey masses were increased from 13.6 kg to 230–360 kg, whey slurry volumes were increased from 2300 L to 307,000–480,000 L, and extraction/injection well loops were employed for the application of whey. At extraction/injection wells, CEs decreased to low (≤10 μg/L) or undetectable levels. At downgradient wells, average trichloroethene concentrations decreased, by as much as 100% (from ≤384.2 during the pilot phase to ≤102.6 μg/L during the operational phase), while average cis-dichloroethene concentrations decreased by as much as 57.5% (from ≤6466.1 to ≤4912.2 μg/L). Downgradient vinyl chloride averages either increased by as much as 63.8% (from ≤859.6 to ≤1407.9 μg/L) or decreased by 64.0% (from 1375.4 to 880 μg/L). Downgradient ethene + ethane averages increased by as much as 73.2% (from ≤1145.3 to ≤1347.1 μg/L). On the basis of the 2008 average market price, the estimated material cost of whey is $1.96/kg organic carbon or, for the configuration of an 81 m PRB by biannual application of 300 kg whey, $325/year. Carbon substrate cost comparisons and implications for efficient in situ treatment design are discussed.