Jessica Barros is a civil engineer in TRC's Concord, California office. She has experience in environmental remediation, including: treatment of chlorinated solvent, hydrocarbon, and hexavalent chromium plumes; monitored natural attenuation; soil vapor investigations and vapor intrusion assessments; statistical analyses; groundwater extraction and treatment; and remediation system design and permitting. Her areas of technical expertise include enhanced in situ bioremediation (EISB), in situ chemical reduction (ISCR), in situ biogeochemical transformation, and interpreting chemical, geochemical, and molecular biological data.
ALTERNATE PLATFORM PRESENTER – Oxidative and Reductive Treatments
Sustained Remediation of Chlorinated Solvents Using In Situ Formation and Regeneration of Ferrous Sulfide
Groundwater at a site in Southern California had been impacted with tetrachloroethylene (PCE). Access limitations influenced the remedial approach for the site, where injection wells were installed in a grid-like pattern to target the accessible portion of the plume containing the highest concentrations of PCE. An amendment design was required that would treat constituents of concern (COCs) over an extended period of time and in a cost-effective manner since the PCE plume extended upgradient of the treatment zone.
Injections of dry cheese whey and water were delivered via wells to promote enhanced in situ bioremediation (EISB). Groundwater at the site contained elevated concentrations of nitrate and sulfate, with baseline concentrations in the treatment zone observed as high as 39 and 430 mg/L, respectively. Amendment longevity was adversely affected by competing electron accepting processes, specifically nitrate and sulfate reduction and methanogenesis.
Following the EISB injections, concentrations of ferrous iron in all performance wells influenced by the injections increased by at least two orders of magnitude from baseline levels. Additionally, hydrogen sulfide (H2S) odors were observed following the injection. These observations suggest elevated concentrations of naturally occurring ferric iron and sulfate were reduced as a result of the EISB injections.
Performance monitoring results suggest COC degradation was sustained for an extended period of time after the dry cheese whey amendment had largely been utilized. Sustained degradation of PCE may be attributed to iron sulfide minerals formed in situ from the byproducts of biologically-mediated ferric iron and sulfate reductions (i.e., ferrous iron and sulfide, respectively). Additional lines of evidence supporting the in situ formation of iron sulfide minerals involved the collection and laboratory testing of sediment samples from within the casing of treatment zone wells. The findings from these studies provide insight into using in situ formation and regeneration of ferrous sulfide minerals as an engineered remedial approach.
Intricacies of Bentonite Slurry Design: Geochemical Considerations during Bench Studies and Field Implementation
A pilot scale composite barrier wall was installed at a landfill site where groundwater is impacted by volatile organic compounds (VOCs). The wall consisted of a 100-mil HDPE geomembrane installed within a three-foot wide open trench supported by a bentonite slurry mixture. The proposed slurry design was comprised of water, bentonite, slag cement, and Portland cement, where the Portland cement would be introduced into the slurry-backfilled trench, following installation of the geomembrane, to facilitate curing.
Prior to field installation, bench scale testing was performed to evaluate the performance of the slurry mix. Of particular concern was the compatibility of the wall material with the geochemically-complex site groundwater, which is characterized by high calcium and magnesium concentrations, supersaturated concentrations of ferrous carbonate and to a lesser degree calcium carbonate, and a high degree of carbon dioxide (CO2) supersaturation. When exposed to the atmosphere, dissolved CO2 off-gasses from groundwater, resulting in a decrease in pH and subsequent formation of calcium carbonate and ferrous carbonate precipitates, which could interfere with bentonite swelling.
Due to the complex groundwater geochemistry and the method of sample collection and storage, it was concluded that the groundwater samples initially used to perform bench scale tests were not representative of in-situ conditions. Unconventional groundwater sampling and storage methods and prescriptive slurry preparation protocols were required to model the influence of site groundwater on the slurry at the bench scale level.
Additional challenges were encountered field that were not observed during bench scale testing. During the test wall pilot study, the water, bentonite, and slag cement slurry hardened within the onsite mixing plant. Laboratory tests were methodically performed to eliminate potential causes for the premature curing. Ultimately, this result was attributed to residual portland cement that may have originated in the onsite mixing plant or as an impurity in the slag cement.
To mimic in-situ conditions, it was required that CO2 remain in solution until the bench scale tests were performed. To accomplish this, groundwater was purged into small beer kegs for transport and storage. To prevent groundwater from coming in contact with air, the atmosphere within the keg was displaced using a tank of CO2. When mixing the groundwater with the bentonite slurry in the lab, tubing from the outlet of the keg was placed within the slurry mixture to further minimize contact with the atmosphere.
At this geochemically complex site, replication of in-situ conditions was required in order to produce meaningful bench test results. Despite best efforts to mimic field conditions, the sensitivity of the slurry to external factors, such as residual contaminants or temperature, may cause the slurry to behave differently in the field than at the bench scale level.