Trichloroethene (TCE) is a ubiquitous soil and groundwater contaminant. The most common bioremediation approach for TCE relies on the process of reductive dechlorination by Dehalococcoides mccartyi. D. mccartyi uses TCE, dichloroethene, and vinyl chloride as electron acceptors and hydrogen as an electron donor. At contaminated sites, reductive dechlorination is typically promoted by adding a fermentable substrate, which is broken down to short chain fatty acids, simple alcohols, and hydrogen. This study explored microbial chain elongation (MCE), instead of fermentation, to promote TCE reductive dechlorination. In MCE, microbes use simple substrates (e.g., acetate, ethanol) to build medium chain fatty acids and also produce hydrogen during this process. Soil microcosm using TCE and acetate and ethanol as MCE substrates were established under anaerobic conditions. In soil microcosms with synthetic groundwater and natural groundwater, ethene was the main product from TCE reductive dechlorination and butyrate and hydrogen were the main products from MCE. Transfer microcosms using TCE and either acetate and ethanol, ethanol, or acetate were also established. The transfers with TCE and ethanol showed the faster rates of reductive dechlorination and produced more elongated products (i.e., hexanoate). The microbial groups enriched in the soil microcosms likely responsible for chain elongation were most similar to Clostridium genus. These investigations showed the potential for synergistic microbial chain elongation and reductive dechlorination of chlorinated ethenes.

This research focused on the enhanced reductive dechlorination of trichloroethene (TCE) and its surrogate, trichlorofluoroethene (TCFE), using two bioremediation methods in anaerobic conditions. Two anaerobic bioremediation studies were conducted to investigate the effects of microbial communities in the presence of different electron acceptors and donors during anaerobic reductive dechlorination of TCE and TCFE. The first study was conducted in the groundwater microcosm bottles, filled with groundwater and sediments collected from Richmond site, CA. Parallel reductive dechlorination of TCE and TCFE was evaluated in the presence of fumarate and its product, succinate, while active reduction of high background concentrations of sulfate (2.5 mM) occurred. Because sulfate was assumed as a favorable electron acceptor during reductive dechlorination of chlorinated aliphatic hydrocarbons (CAHs), all microcosms receiving TCE and TCFE with substrates showed enhanced reductive dechlorination activity and even no substrate addition microcosms generated biotransformation products. From the electron mass balance calculations, more than 87.5% of electrons went to sulfate reduction and less than 10% of available electrons involved in dechlorination after sulfate reductions. After amending varying concentrations of sulfate (0 2.5 mM), no inhibition was found between reductive dechlorination of TCE and sulfate reduction. The result indicated that reductive dechlorination could be directly competed with sulfate reduction for available electrons. The second study investigated the effectiveness of in situ push-pull tests to evaluate bioaugmentation in physical aquifer models (PAMs) using dehalogenating strains to reductively dechlorinate TCE to ethene and TCFE to FE in the TCE contaminated sediments. Complete reduction of TCE to ethene occurred in less than 14 days with repeated additions of TCE (13.0 to 46.0 mg/L) and TCFE (15.0 mg/L) was completely transformed to FE in under 24 days. Increased rate and extent of dechlorination in the bioaugmented PAM compared to the nonaugmented control PAM indicated successful transport of the bioaugmented culture through the PAM. Similar transformation rates and time course of TCE and TCFE also indicated that TCFE was a bioprobe for reductive dechlorination of TCE. TCE and TCFE were transformed to cisdichloroethene (c-DCE) and cis-dichlorofluoroethene (c-DCFE) respectively at two of the three sampling ports after 50 days of incubation in the nonaugmented PAM indicating reductive dechlorination activity of indigenous microorganisms. The results showed that it is possible to increase the rate and extent of reductive dechlorination of TCE and TCFE by bioaugmentation and that push-pull tests are effective tools for detecting and quantifying these processes in situ. The third study focused on numerical modeling of the second study. The objectives of this study were (1) to evaluate a simplified method for estimating retardation factors for injected solutes and bioaugmented microorganisms using "pushpull" test injection phase breakthrough curves, (2) to identify whether bioaugmented microorganisms have kept the same transformation capacity of Evanite culture using Michaelis-Menten kinetics by the values provided by Yu et al. (2005) and to verify in situ rates of TCFE reductive dechlorination rates of push-pull tests by numerical modeling, and (3) to investigate a reasonable answer for the nonuniform recovery of ethene and FE during the activity test and the push-pull test. The bioaugmented microorganisms were effectively transported through Hanford sediment. The estimated retardation factor was 1.33. A numerical simulation predicted cell transport in the PAM as far as port 5. This was qualitatively confirmed by cell counts obtained during bioaugmentation but, cells were distributed nonuniformly. The transport test indicated that TCE and TCFE transport was relatively retarded compared to coinjected bromide tracer (retardation factors ranged from 1.33-1.62 for TCE and from 1.44-1.70 for TCFE). The modeling simulation of Michaelis-Menten kinetics for the activity test was well matched for reductive dechlorination rates for TCE and less dechlorinated ethenes using the previous published values of kmax and Ks of chlorinated ethenes by Yu et al. (2005); the model match indicated that the bioaugmented microorganisms kept the same transformation capacity as the original source, Evanite culture (Yu et al., 2005) over 4 months in the PAM. A numerical simulation resulted in the simple first order FE production rate of 1 day' using STOMP code (2002) and the value of FE production rate was in the range of the transformation rates of TCFE during the activity test. The bioaugmented PAM has caused slow loss of injected CAHs during the activity test and the push-pull test.

Halogenated organic compounds have had widespread and massive applications in industry, agriculture, and private households, for example, as degreasing solvents, flame retardants and in polymer production. They are released to the environment through both anthropogenic and natural sources. The most common chlorinated solvents present as contaminants include tetrachloroethene (PCE, perchloroethene) and trichloroethene (TCE). These chlorinated solvents are problematic because of their health hazards and persistence in the environment, threatening human and environmental health. Microbial reductive dechlorination is emerging as a promising approach for the remediation of chlorinated solvents in aquifers. In microbial reductive dechlorination, specialized bacteria obtain energy for growth from metabolic dechlorination reactions that convert PCE to TCE, cis-1,2-dichloroethene (cDCE), vinyl chloride (VC), and finally to benign ethene. Field studies show incomplete dechlorination of PCE to ethene due to lack of electron donors or other populations competing for the electron donor. Mathematical models are good tools to integrate the processes affecting the fate and transport of chlorinated solvents in the subsurface. This thesis explores the use of modeling to provide a better understanding of the reductive dehalogenation process of chlorinated solvents and their competition with other microorganisms for available electron donors in continuous flow systems such as a continuous stirred tank reactor (CSTR) and a continuous flow column. The model is a coupled thermodynamic and kinetic model that includes inhibition kinetics for the dechlorination reactions, thermodynamic constraints on organic acids fermentation and has incorporated hydrogen competition among microorganisms such as homoacetogenesis, sulfate reducers and ferric iron reducers. The set of equations are coupled to those required for modeling a CSTR. The system of model equations was solved numerically using COMSOL 3.5 a, which employs finite-element methods. The kinetic model was verified by simulation results compared to previously published models and by electron balances. The simulation process progressed by simulating the anaerobic reductive dechlorination, coupled with thermodynamic limitation of electron donor fermentation in batch systems to the modeling of CSTR, and finally to simulate anaerobic reductive dechlorination in continuous flow column, aquifer column including the processes of advection, dispersion and sorption along with the microbial processes of dehalogenation, fermentation, iron and sulfate reduction. The simulations using the developed model captured the general trends of the chemical species, and a good job predicting the dynamics of microbial population responses either the CSTRs or continuous flow column. Although, the kinetic of anaerobic dechlorination processes of chlorinated solvents in those systems have been researched in the past, little progress has been made towards understanding the combined effects of the dechlorination and thermodynamic constraints in continuous flow systems. This work provides a rigorous mathematical model for describing the coupled effects of these processes.