The historical operation of industrial activities in Southern California including industrial processing, metal plating, aerospace manufacturing, and dry cleaning have resulted in groundwater contamination in the region. Many aquifers have contamination issues from historic overlying land activities. Several remedial technologies for cleanup of chlorinated volatile organic compounds (CVOCs) in groundwater are available, including, but not limited to, Enhanced In situ Bioremediation (EISB), chemical oxidation, groundwater pump and treat, air-sparging with vapor extraction, high-vacuum dual-phase extraction, thermal injection, electrical resistance heating, permeable reactive barriers using zero-valent iron, and monitored natural attenuation. In evaluating these technologies, several factors are considered, including system effectiveness, implementability, impact on business/facility operations, regulatory acceptance, permitting requirements, safety, and cost.
One cost-effective and feasible solution for remediation of CVOCs in groundwater is EISB, also known as engineered bioremediation. EISB is the acceleration of microbial activities using technology to enhance the degradation or detoxification of environmental pollutants. The purpose of EISB is to increase the rate of microbial activity so the rate of reductive dechlorination is increased. Chlorinated solvents generally biodegrade under anaerobic conditions where the chlorinated solvent (and/or another carbon source) is consumed by microbes present in the aquifer. During reductive dechlorination, anaerobic microbes substitute hydrogen (electron donor) for chlorine on the CVOC molecule, thus converting the CVOC to a degradation product with one less chlorine atom. This is the process whereby tetrachloroethene (PCE) is degraded to trichlorothene (TCE), then to cis-1,2-dichlorothene (DCE), to vinyl chloride, and then finally to ethene (which is benign). The degradation pathway (courtesy of REGENESIS®) is depicted below:
EISB is implemented by the injection of bioamendments into the groundwater, which include an organic carbon source, nutrients, and microbial cultures such as dehalococcoides (DHC). Injection of bioamendments is usually conducted through pre-installed wells screened in the aquifer, or by use of a direct-push drilling rig.
Alta Environmental recently conducted an EISB pilot test at a client project site where the underlying groundwater was heavily impacted by CVOCs, including TCE. The EISB test was implemented by the injection of the following bioamendments manufactured by REGENESIS.
All amendments, including DHC, are approved on the 2014 Los Angeles Regional Water Quality Control Board (LARWQCB), Revised General Waste Discharge Requirements (WDR), dated September 11, 2014. Note that prior to September 2014, DHC was not covered by the general WDR permit, and a site-specific permit to include DHC was required by the LARWQCB. This site-specific permit would have taken about a year for the LARWQCB to review and approve. Injection of the bioamendments, including DHC, were approved in a General WDR permit issued by the LARWQCB during October 2015.
The pilot test had a successful outcome. Laboratory analytical results from groundwater samples collected from the observation test well immediately before the injection and 90 days after injection show significant reductions of TCE concentrations (average of 26%) in post-injection samples. Significant increases of DCE (the first degradation product of TCE) and dramatic increases of DHC were also observed. These results, along with other secondary indicators, show that reductive dechlorination and bio-activity processes are being accelerated as a result of the 3DMe/CRS/DHC (in particular DHC) injection, and that EISB appears to be a viable technology for remediation of the dissolved-phase chlorinated solvent plume at the subject site.
The advantage of EISB over other groundwater remedial technologies is that because it does not require ongoing operation and maintenance after injection, EISB is considered less expensive than other groundwater remedial technologies which incur high electrical and operational costs. EISB also does not require trenching and installation of piping, costs of which are associated with other groundwater remedial technologies such as groundwater pump and treat, air-sparging, and thermal injection. Furthermore, because EISB does not involve high-impact or major installation, excavation, or electrical activities, EISB is considered significantly safer than other groundwater remedial systems.