Two biokinetic models employing the MichaelisMenten equation for anaerobic reductive dechlorination of tetrachloroethylene (PCE) and trichloroethylene (TCE) were developed. The models were compared with results from batch kinetic tests conducted over a wide range of PCE and TCE concentrations with two different dechlorinating cultures. One model applies Michaelis-Menten kinetics with competitive inhibition among chlorinated aliphatic hydrocarbons (CAHs), while the other model includes both competitive inhibition and Haldane inhibition at high CAH concentrations. Model simulations with competitive inhibition simulated the experimental results well for PCE concentrations lower than 300 AM. However, simulations deviated from the experimental observations for PCE or TCE concentrations greater than 300 -400 AM. The kinetic model that incorporated both competitive and Haldane inhibitions better simulated experimental data for PCE concentrations near the solubility limit (1000 AM), and TCE concentrations at half its solubility limit (4000 AM). Based on the modeling analysis of the experimental results, the PM culture (Point Mugu, CA) had very high Haldane inhibition constants for cis-1,2-dichlororethylene (c-DCE) and vinyl chloride (VC) (6000 and 7000 AM, respectively), indicating very weak Haldane inhibition, while the EV culture (the Evanite site in Corvallis, OR) had lower Haldane inhibition constants for TCE, c-DCE, and VC of 900, 750, and 750 AM, respectively. The BM culture (a binary mixed culture of the PM and EV cultures) had transformation abilities that represented the mixture of the EV and PM cultures. Model simulations of the BM culture transformation abilities were well represented by separate rate equations and model parameters for the two independent cultures that were simultaneously solved. Modeling results indicated that a combination of competitive and Haldane inhibition kinetics is required to simulate dechlorination over a broad range of concentrations up to the solubility limit of PCE and half the solubility limit of TCE.

Kinetic studies with two different anaerobic mixed cultures (the PM and the EV cultures) were conducted to evaluate inhibition between chlorinated ethylenes. The more chlorinated ethylenes inhibited the reductive dechlorination of the less chlorinated ethylenes, while the less chlorinated ethylenes weakly inhibited the dechlorination of the more chlorinated ethylenes. Tetrachloroethylene (PCE) inhibited reductive trichloroethylene (TCE) dechlorination but not cis-dichloroethylene (c-DCE) dechlorination, while TCE strongly inhibited c-DCE and VC dechlorination. c-DCE also inhibited vinyl chloride (VC) transformation to ethylene (ETH). When a competitive inhibition model was applied, the inhibition constant (K I ) for the more chlorinated ethylene was comparable to its respective Michaelis-Menten half-velocity coefficient, K S . Model simulations using independently derived kinetic parameters matched the experimental results well. k max and K S values required for model simulations of anaerobic dechlorination reactions were obtained using a multiple equilibration method conducted in a single reactor. The method provided precise kinetic values for each step of the dechlorination process. The greatest difference in kinetic parameters was for the VC transformation step. VC was transformed more slowly by the PM culture (k max and K S values of 2.4 ( 0.4 µmol/mg of protein/day and 602 ( 7 µM, respectively) compared to the EV culture (8.1 ( 0.9 µmol/mg of protein/day and 62.6 ( 2.4 µM). Experimental results and model simulations both illustrate how low K S values corresponded to efficient reductive dechlorination for the more highly chlorinated ethylenes but caused strong inhibition of the transformation of the less chlorinated products. Thus, obtaining accurate K S values is important for modeling both transformation rates of parent compounds and their inhibition on daughter product transformation.

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.

Information on the transport of dissolved gases in ground water is needed to design ways to increase dissolved gas concentrations in ground water for use in in situ bioremediation (e.g., O2 and CH4) and to determine if dissolved gases are conservative tracers of ground‐water flow (e.g., He). A theoretical model was developed to describe the effect of small quantities of trapped gas bubbles on the transport of dissolved gases in otherwise saturated porous media. Dissolved gas transport in porous media can be retarded by gas partitioning between the mobile aqueous phase and a stationary trapped gas phase. The model assumes equilibrium partitioning where the retardation factor is defined as R = 1 + H′(Vg/Vw) where H' is the dimensionless Henry's Law constant for the dissolved gas, and Vg and Vw are the volumes of the trapped gas and water phases, respectively. At 15°C and with Vg/ Vw= 0.05, the predicted retardation factors for He, O2, and CH4 are 5.8, 2.4, and 2.3, respectively. The validity of the model was tested for dissolved oxygen in small‐scale column experiments over a range of trapped gas volumes. Retardation factors of dissolved oxygen increased from 1 to 6.6 as Vg/Vw increased from 0 to 0.123 and are in general agreement with model predictions except for the larger values of Vg/Vw. The theoretical and experimental results suggest that gas partitioning between the aqueous phase and a trapped gas phase can greatly influence rates of dissolved gas transport in ground water.