Background/Objectives A large scale enhanced in-situ bioremediation/reduction remedy was implemented at a coastal site in Southern California with chlorinated solvent and hexavalent chromium impacts in groundwater.
Beginning with a 100-point injection in 2006 across an area of approximately 1 acre, the treatment area was later expanded to other portions of the site with an additional 850+ injection locations over a total of approximately 4 acres. Since the initial injection in 2006, continuous monthly, followed by quarterly and semiannual sampling has tracked the long-term degradation trends, stability, and effectiveness of the single-injection passive bioremediation remedy. Although the emulsified vegetable oil (EVO) electron donor was likely exhausted in the first 1-2 years following the injection, active and effective biodegradation has continued to be observed throughout the treated area, possibly sustained by carbon contained within the biomass generated during the initial growth period. Approach/Activities. Passive in-situ bioremediation/reduction was used to treat chlorinated solvent impacts in the saturated zone over two areas and hexavalent chromium impacted groundwater in a third area. Injections were performed using direct-push injection rods through which an average of approximately 1,300 gallons of 1% EVO solution and 1-liter of microbial culture were injected into each point for the VOC treatment areas. A similar injection approach was used to reduce the hexavalent chromium (CrVI) impacted areas. Trends in VOC/CrVI concentrations were then monitored over a period of more than 7 years, along with total organic carbon concentrations, ethene/ethane, microbial density, and general chemistry parameters to evaluate the suitability of the environment for supporting active reductive degradation. Results/Lessons Learned. The project was originally designed for an expected 1-2 year period of active degradation following a single-event passive injection of electron donor and microbial culture. Although extremely rapid degradation was observed in the 6-9 months immediately following the initial injection event, concentrations in the majority of wells have either continued to decline in concentration or remained at non-detect for many years following the design lifespan of the donor. In one location where a modest rebound of VOCs was observed six years after the initial injection event, application of a small amount of donor was enough to rapidly stimulate the residual microbial population resulting in reduction of the observed VOC rebound in a matter of weeks. CrVI reduction was initially attempted using Ferrous Sufate (FeSO4), which was successful in bench treatability tests. Although CrVI concentrations were rapidly reduced following the application of FeSO4, concentrations rebounded within approximately six months. Subsequent application of EVO achieved rapid, and lasting reduction of CrVI to non-detect levels. These observations have wide ranging implications for the long term effectiveness of bioremediation remedies at many sites, and significantly strengthen the argument for post-remediation monitored natural attenuation without the need for ongoing re-injection of donor.