Groundwater contaminated with 500−1200 μg/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 μg/L in new water entering the 480 m2 monitored treatment zone to an average of 18−24 μg/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 μg/L at the 22 m × 22 m boundaries of the study zone, well below the goal of 20 μg/L maximum.

Results are presented from a field study that document the in‐situ biotransformation of trichloroethylene (TCE), cis‐dichLoroethylene (cis‐DCE), trans‐dichloroethylene (trans‐DCE), and vinyl chloride (VC) in a saturated, semiconfined aquifer. The enhanced biotransformation was accomplished by stimulating the growth of indigenous methane‐oxidizing bacteria (methanotrophs), which transform chlorinated aliphatic compounds by a cometabolic process to stable, nontoxic end products. Experiments were performed in the presence and absence of biostimulation by means of controlled chemical addition, frequent sampling, and quantitative analysis. The degree of biotransformation was assessed using mass balances and comparisons with bromide as a conservative tracer. Biostimulation of the test zone was successfully achieved by injecting methane‐ and oxygen‐containing ground water in alternating pulses under induced gradient conditions. After a few weeks of stimulation, methane concentrations gradually decreased below the detection limit within two meters of travel. Under active biostimulation conditions, 20 to 30% of the TCE was biotransformed during the first season of testing. Direct evidence for biotransformation of VC, trans‐DCE, cis‐DCE, and TCE was obtained in the second and third seasons of field testing. In the absence of biostimulation, the organic compounds concentrations at observation wells reached 95% of the injection concentration, demonstrating negligible losses due to abiotic processes. Biostimulation of the test zone resulted in a concurrent decrease in concentration of methane and the halogenated aliphatic compounds. The organic compounds were transformed within two meters of travel as follows: TCE, 20–30%; cis‐DCE, 45–55%; trans‐DCE, 80–90%; and VC, 90–95%. These results are in qualitative agreement with methane‐utilizing, mixed‐culture laboratory studies which indicate that the rate of biotransformation is more rapid when the molecules are less halogenated. A biotransformation intermediate was observed which was identified by GC‐MS analysis as trans‐dichloroethylene oxide (trans‐DCE epoxide), an expected intermediate based on laboratory studies. When methane addition was stopped, the concentration of the intermediate rapidly decreased, while halogenated compound concentrations slowly increased, indicating that active methane utilization was required for biotransformation to occur.

The Moffettfield site was used for further evaluation of in situ biotransformation of chlorinated aliphatic hydrocarbons with phenol and toluene as primary substrates. Within the 4 m test zone, representing a groundwater travel time of less than 2 days, removal efficiencies for 250 pg/L TCE and 125 pg/L ~is-1~2dichloroethylene were greaterthan 90%, and that of 125 pglL trans-l,2-dichloroethylene was -74%, when either 9 mg/L toluene or 12.5 mgIL phenol was used. Phenol and toluene were removed to below 1 p g l L. Vinyl chloride removals greater than 90% were also noted. However, only 50% of the 65 pglL 1,ldichloroethylene was transformed with phenol addition, and significant product toxicity was evident as concomitant TCE transformation was here reduced to -50%. Hydrogen peroxide addition performed as well as pure oxygen addition to serve as a required electron acceptor.

Enhancement of in situ anaerobic biodegradation of BTEX compounds was demonstrated at a petroleum-contaminated aquifer in Seal Beach, CA. Specifically, combined injection of nitrate and sulfate into the contaminated aquifer was used to accelerate BTEX removal as compared to remediation by natural attenuation. An array of multi-level sampling wells was used to monitor the evolution of the in situ spatial distributions of the electron acceptors and the BTEX compounds. Nitrate was utilized preferentially over sulfate and was completely consumed within a horizontal distance of 4-6 m from the injection well; sulfate reduction occurred in the region outside the denitrifying zone. By combining injection of both nitrate and sulfate, the total electron acceptor capacity was enhanced without violating practical considerations that limit the amount of nitrate or sulfate that can be added individually. Degradation of total xylene appears linked to sulfate utilization, indicating another advantage of combined injection versus injection of nitrate alone. Benzene degradation also appears to have been stimulated by the nitrate and sulfate injection close to the injection well but only toward the end of the 15-month demonstration. The results are consistent with the hypothesis that benzene can be biodegraded anaerobically after other preferentially degraded hydrocarbons have been removed.