Full scale evaluation of in situ cometabolic degradation of trichloroethylene in groundwater through toluene injection

Groundwater contaminated with 500-1200 mu g/L trichloroethylene (TCE) was treated in situ over a 410-day period by cometabolic biodegradation through injection of 7-13.4 mg/L toluene, oxygen, and hydrogen peroxide in groundwater circulated between two contaminated aquifers through two treatment wells located 10 m apart. One well pumped contaminated groundwater from the 8 m thick upper aquifer to the 5 m thick lower aquifer, while the other pumped contaminated water from the lower to the upper aquifers using flow rates of 25-38 L/min, effecting groundwater circulation between them. Following 18 days of periodic toluene injection to develop an active biological population, continuous pulses of toluene were added. Over 312 days, an average 87 +/- 8% TCE removal was obtained in the upper aquifer with each pass through the treatment well. In the lower aquifer, removals were 83 +/- 16% over the last 79 days when peroxide addition was reduced. Treatment reduced TCE in the regional groundwater plumes from about 1000 mu g/L in new water entering the 480 m(2) monitored treatment zone to an average of 18-24 mu g/L in groundwater leaving the treatment zone, indicating total TCE removal of 97-98%. Pumping heads for groundwater recirculation were less than 6 m. Toluene was removed by 99.98% through biodegradation to an average of 1.1 +/- 1.6 mu g/L at the 22 m x 22 m boundaries of the study zone, well below the goal of 20 mu g/L maximum.