AbstractsChemistry

Groundwater contamination by petroleum hydrocarbon (PHC) compounds including the high impact and persistent aromatic compounds such as benzene, toluene, ethyl benzene and xylene (BTEX) poses serious risk to human health and the environment. The coupling or sequential use of different remediation technologies, also referred to as a “treatment train”, has become an emerging strategy for the treatment of PHC-contaminated sites. Minimizing clean-up cost and time as well as maximizing the overall treatment efficiency are the primary goals of combined remedies. Coupling in situ chemical oxidation (ISCO) and enhanced bioremediation (EBR) is an example of a plausible treatment train. The general concept behind an integrated ISCO/EBR system is the use of chemical oxidation to target the bulk of the contaminant mass near the source, followed by the enhancement of biological processes to “polish” the remaining mass in the source and the downgradient plume. Persulfate (S2O82-) is a persistent but yet aggressive oxidant which can rapidly destroy a wide variety of PHC compounds. Persulfate degrades complex organic compounds into simpler and more bioavailable organic substrates and produces sulfate, an electron acceptor. The anaerobic environment that is created is ideal for sulfate reduction to be enhanced. Therefore, enhanced bioremediation under sulfate reducing conditions is expected to dominate the mass removal processes following the consumption of persulfate. To assess the viability and performance of a persulfate/EBR treatment train, the role of the intertwined mass removal processes (e.g., persulfate oxidation vs sulfate reduction) and the impact of persulfate on indigenous microbial processes need to be quantified. Hence, a pilot-scale trial was conducted in a 24 m long experimental gate at the University of Waterloo Groundwater Research Facility at CFB Borden over a period of 13 months. After a quasi steady-state plume of dissolved benzene, toluene and xylene (BTX) was established in the gate, two persulfate injection episodes were conducted to create a chemical oxidation zone. As this chemical oxidation zone migrated downgradient it was extensively monitored as it transitioned into an enhanced bioremediation zone. Mass loss estimates and geochemical indicators were used to identify the distinct transition between the chemical oxidation and enhanced biological reactive zones. Compound specific isotope analysis (CSIA) was used to delineate the dominant mass removal process, and to track the fate of the sulfate. Molecular biology tools, including specific metabolite detection and quantitative polymerase chain reaction analysis were used to understand the effect of persulfate on the population and activity of the indigenous microorganisms with a focus on the SRB community. A modelling tool was developed to simulate the coupled processes involved in a persulfate/EBR treatment train, and to quantify the impact of various parameters on the performance of this treatment system. The existing BIONAPL/3D model was enhanced…