To treat groundwater contaminants in situ, enhanced anaerobic bioremediation processes can be stimulated through addition of soluble substrates. At a dry cleaners site located in San Jose, California, the goal was to find a substrate that is long lasting and easily distributed into the saturated soils. After evaluating several alternatives, in situ bioremediation using an emulsified edible oil substrate (EOS®) was selected as the preferred alternative for groundwater remediation.
At this site, the impact of injecting substrate into the upper aquifer was observed in an unconfined groundwater aquifer. Tetrachloroethene (PCE) breakdown was monitored at three locations across the site. The highest PCE and trichloroethene (TCE) concentrations in the January 2005 pre-EOS injection-sampling event were detected in well MW-1A at concentrations of 8,500 μg/L and 200 g/L, respectively. The highest cis-1,2- dichloroethene (cis-DCE) was detected in well MW-1A at concentration of 160 μg/L. Trans-1,2-DCE (trans-DCE) was also detected and only small amounts of VC were detected in the groundwater prior to treatment.
After 2.5 months post-injection (July 2005), the PCE concentration in MW-1A was reduced to 18 μg/L and the TCE concentration was reported to be 100 μg/L. The concentration of cis-DCE had increased in MW-1A to 1,200 μg/L, suggesting the presence of enhanced bioremediation. No PCE, TCE, or 1,1-DCE was detected in the shallow wells during the October 2005 sampling event (6-months post-injection). Conversely, the concentration cis-DCE continued to increase and was detected in well MW-1A at 2,300 μg/L. By six months after treatment, VC was readily detected in each of the monitor wells at concentrations of 39, 200, and 35 μg/L in MW-1A, MW-2, and MW-3, respectively.
Sub-reportable levels of PCE, TCE, and 1,1-DCE were detected again in the shallow wells during the January 2006 sampling event (9-months post-injection) The concentration of cis-DCE also began to decrease and was detected in well MW-1A at 630 μg/L. By nine months after treatment, VC was readily detected in each of the monitor wells at concentrations of 300, 40, and 88 μg/L in MW-1A, MW-2, and MW-3, respectively.
The results of the pre- and post-injection sampling of three wells in the treatment zone showed the rapid conversion of the aquifer to anaerobic reducing conditions favorable for reductive dechlorination to occur. The enhanced conditions resulted in rapid disappearance of PCE from 8,500 μg/L to below the MDL, reductions in TCE, and a measurable increase of cis-DCE and VC at all the shallow zone wells. Some methane is being produced, but ethane or ethene production has yet to be detected. The emulsified oil substrate (EOS®) is expected to continue to sustain favorable conditions for an extended duration. Continued monitoring is expected to eventually document to complete remediation of the site.