In situ bioremediation of a cis-dichloroethylene-contaminated aquifer utilizing methane-rich groundwater from an uncontaminated aquifer
Introduction
Geo-pollution by volatile organic halocarbons is one of the major environmental issues worldwide. As an alternative to conventional remedial technique, bioremediation has emerged as a potentially efficient technology to degrade the contaminants directly at a low cost. A methane-oxidizing bacterium (methanotroph) is one of the key organisms for the degradation of most of volatile organic halocarbons (Little et al., 1988). Field tests of bioremediation utilizing methanotrophs have been conducted in trichloroethylene (TCE)-contaminated sites (Semprini et al., 1990; Pfiffner et al., 1997; Iwamoto et al., 2000). Recently, we reported that groundwater pumped and transported from a shallow aquifer in the center of a natural gas field area containing high concentration of methane and potentially high activity of methanotrophs could be effectively utilized for a bioremediation of a TCE-contaminated site (Takeuchi et al., 2004).
In a contaminated site in Chikura, Chiba, Japan, TCE was first found in 1992 in groundwater. The details of the site description are elsewhere (Takeuchi et al., 2000; Chikura geo-pollution research team, 1995). Shortly, the hydrostratigraphic units of Holocene to Pleistocene sediments above the Tertiary rock consist of three aquifers, and two aquitards (Fig. 1, Takeuchi et al., 2000). First, a plume of TCE spread over in the first aquifer and then in the second aquifer. Pumping and treatment had decreased the contaminant, and in 2000, the contaminant remained only in the second aquifer as cis-dichloroethylene (c-DCE) probably transformed by anaerobic dechlorinating microbes (Parsons et al., 1985). Preliminary investigation of this area revealed a unique feature that the groundwater of the third aquifer contained high concentrations of phosphate as well as methane probably because the gas field area is nearby (Table 1). On the other hand, the groundwater of the contaminated second aquifer contained more oxygen and nitrate than those in the third aquifer (Table 1). Therefore, mixing the waters from these two aquifers was considered to form favorable conditions for the growth of methanotrophs, hence the cost-effective removal of c-DCE was expected.
In this paper, we report that methane-rich groundwater existed in the uncontaminated aquifer of the contaminated site was successfully utilized to enhance the degradation of the contaminant. Population of methanotrophs and kinetic parameters for methane uptake during the process were also evaluated.
Section snippets
Chemical analyses
Groundwater was collected in triplicate vials (120 ml) for laboratory measurement of dissolved methane and c-DCE. The vials were sealed with Teflon septa and aluminum caps and stored at 4 °C until analysis. Six milliliter of water was removed and replaced with air, and vials were put into a water bath at 50 °C for 40 min. The headspace was analyzed on a GC-14B gas chromatograph (Shimadzu, Kyoto, Japan) equipped with a flame ionization detector and a 2 mm ID×2 m packed column (Chromosorb WAW DMCS,
In vitro experiment
c-DCE concentrations decreased to 24% and 65% of the initial level after 12 days in the mixture of waters of second and third aquifers (2+3) and the mixture of waters of first and second aquifers (1+2), respectively (Fig. 2a). After 12 days, the dissolved methane concentrations decreased to 0.2% and 20% of the initial level in 2+3 and 1+2, respectively (Fig. 2a). In 2+3, the total methanotrophs increased from 2.9×103 to 9.8×104 MPN ml−1 after 12 days of incubation (Fig. 2b). The sMMO
Discussion
The in vitro experiments showed that an introduction of the methane-containing groundwater from the third aquifer enhanced the growth of methanotrophs and the degradation of c-DCE in the second aquifer groundwater compared to the mixing with the first aquifer groundwater (control water) (Fig. 2). A similar degradation of c-DCE was also observed in situ. Enhanced methanotrophic activity was maintained in the injection well throughout the experiment with methane-rich water. In the monitoring
Conclusions
Degradation of c-DCE in a contaminated aquifer was successfully enhanced by utilizing natural methane-rich water from another aquifer on site. Presently, pumping and treatment is the most common technique for remediation of TCE (or its derivatives, such as DCE(s))-contaminated sites. When methane-rich groundwater is available though such geological settings may be limited to certain areas, an additional injection of this water can increase the efficacy of treatment and shorten the treatment
Acknowledgements
We appreciate the cooperation of the Chikura Town Welfare Department in this study. We also thank the Tateyama Marine Laboratory, Ochanomizu University for providing facilities.
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