SRK designed and implemented an Enhanced In-Situ Bioremediation strategy to mitigate the impacts of a historical chlorinated hydrocarbons (CHC) contamination to both soil and groundwater and a site in South Africa. The EISB utilises a biobarrier generated through the addition of electron donor (emulsified vegetable oil (EVO)) into the aquifer. The site has a complex hydrogeological setting comprising both primary and fractured rock aquifers.
The performance evaluation of the EISB system based on the interpretation of concentration data alone is ill-advised as contaminant concentrations may be influenced by various non-degradation processes (including dilution, sorption, volatilisation). Despite of the complexity of the site, the application of Compound Specific Isotope Analysis (CSIA) along with groundwater concentration data and historical information was important in providing a direct and unequivocal line of evidence that degradation has been stimulated by the microbial consortia established in the biobarrier.
CSIA is the measurement of the relative abundance of the two stable isotopes of carbon (13C and 12C) for a specific contaminant of concern (e.g. TCE or PCE). The CSIA evaluation is sensitive enough to detect isotopic differences in the sources of contamination, as well as shifts in isotopic signatures caused by abiotic or biotic degradation where bacteria preferential utilise the contaminant molecules containing the lighter C isotopes. The isotopic fractionation (shifts in isotopic composition) results in an enrichment in the heavy isotope in the remaining undegraded parent compound, with a corresponding enrichment of the lighter isotope in the intermediary or daughter products.
The monitoring of the geochemical environment, and concentration monitoring data following the completion of a 5-year EVO injection programme in 2020 showed some evidence of CHC attenuation in the monitoring wells downgradient of the biobarrier. However, the constant dissolution and recharge of contaminants from the source area made it difficult to determine if the attenuation was biotically mediated. The CSIA and the groundwater monitoring data for from monitoring wells downgradient of the biobarrier show a strong correlation between enriched δ13C signatures and lower PCE and TCE concentration, with an estimated extent of biodegradation >90% at some locations. This evaluation was critical in proving that the enhancement of the naturally occurring microbial biomass was successfully stimulated by the EVO to result in the observed order of magnitude decrease mass flux downgradient of the biobarrier.
