Abstract The industrial solvents tetrachloroethylene (PCE) and trichloroethylene (TCE) are among the most ubiquitous chlorinated compounds found in groundwater contamination. One potential method for managing PCE/TCE contaminated sites is the intrinsic bioremediation. Recent regulations adopted by U.S. Environmental Protection Agency allow intrinsic bioremediation to be considered as an alternative during development of corrective action plans. In some remediation cases, enhanced bioremediation are performed to accelerate the contaminant biodegradation rate. The main objective of this study was to evaluate the potential of using intrinsic and enhanced bioremediation technologies to clean up PCE/TCE contaminated aquifers. PCE/TCE bioavailability was evaluated by laboratory microcosms under four reduction/oxidation (redox) conditions including aerobic cometabolism, methanogenesis, iron reduction, and reductive dechlorination. Acclimated bacteria, activated sludge, and aquifer sediments from a pentachlorophenol contaminated site were used as the inocula in this study. Methane, toluene, phenol, sludge cake, and cane molasses were used as the primary substrates (carbon sources) in the cometabolism and reductive dechlorination microcosms. Results from this study show that PCE and TCE can be significantly biodegraded under reductive dechlorination and aerobic cometabolism conditions, respectively. All five carbon sources evaluated in this study can be applied as the primary substrates by microbial consortia to enhance the aerobic cometabolism of TCE. The highest TCE degradation rate [Up to 100% of TCE removal (with an initial concentration of 3.6µM)] was observed in the microcosms with toluene enrichment bacteria as the microbial inocula and toluene as the primary substrate. Under reductive dechlorination conditions, both sludge cake and cane molasses could be used as the primary substrates by microbial consortia (from activated sludge and aquifer sediments) and enhanced the biodegradation of PCE. The highest PCE degradation rate [Up to 100% of PCE removal (with an initial concentration of 17µM)] was observed in the microcosms with anaerobic activated sludge as the microbial inocula and sludge cake as the primary substrate. Except for reductive dechlorination microcosms, no significant PCE removal was observed in the microcosms prepared under iron reduction conditions. Results from this feasibility study would be useful in designing a scale-up in situ (e.g., in situ biobarrier system) or on-site bioremediation system (e.g., bioslurry reactor) for field application. Moreover, the application of non-toxic organic waste to enhance PCE/TCE biodegradation has the potential to become an environmentally and economically acceptable technology for the bioremediation of chlorinated-solvent contaminated groundwater.

Methyl tert-butyl ether (MTBE) has been used as a gasoline additive to improve the combustion efficiency and to replace lead since 1978. It is the most commonly used oxygenate now due to its low cost, convenience of transfer, and ease of blending and production. MTBE has become a prevalent groundwater contaminant because it is widely used and it has been disposed inappropriately. MTBE has been demonstrated an animal carcinogen. The US Environmental Protection Agency (US EPA) has temporarily classified MTBE as a possible human carcinogen and has set its advisory level for drinking water at 20-40 µg/L based on taste and odor concerns. The Taiwan Environmental Protection Administration (TEPA) also classifies it as the Class IV toxic chemical substances. Currently, natural attenuation (NA) as well as natural bioremediation or enhanced bioremediation are attractive remediation options for contaminated sites due to their economic benefit and environmental friendly. In general, in situ microorganisms at the contaminated site play a very important role in site restoration. Although early studies suggested that the biodegradability of MTBE was not significant, recent laboratory and field reports reveal that MTBE can be biodegraded under aerobic and anaerobic conditions. In addition, evidences and some successful cases of MTBE attenuation have been reported that make natural attenuation a considerable remedial strategy. However, the biodegrading rate might decrease if the nutritional and physiological requirements are not met. Thus, it is important to assess the biodegradability of natural microorganisms under various site conditions to obtain optimal remedial conditions. Contributions of intrinsic biodegradation and other abiotic mechanisms to the removal and control of contaminants should also be evaluated to provide sufficient information for remedial option determination. Moreover, isolation and identification of the dominant native microorganisms will be helpful to following remediation tasks. In the first part of this study, microcosm study and microbial identification technologies (denaturing gradient gel electrophoresis, DGGE) were applied to assess the biodegradability of MTBE by indigenous microbial consortia and to identify the dominant microorganisms at a MTBE-contaminated site (Site A). In the second part of this study, thorough field investigations were performed to evaluate the occurrence of natural attenuation of MTBE at two MTBE-contaminated sites (Site A and Site B). In addition, a natural attenuation model, BIOSCREEN, was performed to assess the effectiveness of natural attenuation on MTBE containment. The main objectives of this study contained the following: (1)Evaluate MTBE biodegradability under different redox conditions by the indigenous microorganisms. (2)Determine the dominant native microorganisms in MTBE biodegradation for further application. (3)Assess the feasibility of using natural attenuation to control the MTBE plume. (4)Evaluate the contributions of intrinsic biodegradation patterns on natural attenuation processes by BIOSCREEN. Results from the microcosm study reveal that MTBE could be biodegraded by aquifer sediments without the addition of extra carbon sources under aerobic conditions. The production of tert-butyl alcohol (TBA), a degradation byproduct of MTBE, was detected. Complete removal of TBA was also observed by the end of the experiment. Results from aerobic microcosms study indicate that oxygen might be the major limiting factor of MTBE biodegradation at Site A. Thus, MTBE at this site could be removed via natural biodegradation processes with the supplement of sufficient oxygen. Microcosm study with extracted supernatant of aquifer sediments as the inocula show that the indigenous microorganisms were capable of using MTBE as the sole carbon and energy source. The calculated MTBE degradation rate was 0.597 mg/g cells/h or 0.194 nmole/mg cells/h. No MTBE removal was observed under various anaerobic conditions. Results suggest that aerobic biodegradation was the dominant degradation process and aerobic bioremediation might be a more appropriate option for the site remediation. According to the results of DGGE analysis, aerobic MTBE-biodegrading bacteria, Pseudomonas sp. and Xanthomonas sp., might exist at this site. Although results of microcosm study show that MTBE could not be degraded under anaerobic conditions, the microbial identification indicates that some novel anaerobic microbes, which could degraded MTBE, might be present at this site. In addition, anaerobic microbes caused the consumption of electron acceptors (e.g., nitrate, ferric iron) and removal of benzene, toluene, ethylbenzene, xylenes (BTEX), 1,2,4-trimethyl benzene (1,2,4-TMB), and 1,3,5-trimethyl benzene (1,3,5-TMB) (TMBs) in the anaerobic microcosms. These results also indicate that the potential of anaerobes activities was high at Site A. Based on the results from the field investigation, natural attenuation of MTBE was occurring at both sites. MTBE plume at Site B could be effectively controlled via natural attenuation processes. Nevertheless, MTBE plume at Site A has migrated to a farther downgradient area and passed the boundary of the site. Field investigation results indicate that the natural attenuation mechanisms of MTBE at both sites were occurring with the first-order attenuation rates of 0.0021 and 0.0048 1/day at Sites A and B, respectively. According to BIOSCREEN simulation, biodegradation was responsible for 78% and 59% of MTBE mass reduction at Sites A and B, respectively. The intrinsic biodegradation had significant contributions on the control of MTBE plumes. Moreover, the dilution and dispersion processes might be the major mechanisms for the attenuation of MTBE in the downgradient areas. However, results also reveal that intrinsic biological processes might still fail to contain the plume if the selected point of compliance is not appropriate. Results of this study suggest that natural attenuation might be feasible to be used as a remedial option for the remediation of MTBE-contaminated site on the premise that (1) detailed site characterization has been conducted, and (2) the occurrence and effectiveness of natural attenuation processes have been confirmed. Based on the results from the field investigation and laboratory microcosm studies, MTBE could be biodegraded by natural microbial populations at the studied sites under both aerobic and anaerobic conditions and natural attenuation would be applied as a remedial option at MTBE-contaminated sites. Results from this study would be useful in determining the favorable bioremediation conditions and designing an efficient and cost-effective bioremediation system such as monitored natural attenuation (MNA) or in situ or on-site MTBE bioremediation system for field application.