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Isotopic and molecular techniques for determining the efficiency of in-situ bioremediation and chemical oxidation of chlorinated compounds

Final Report Summary - IMOTEC-BOX (Isotopic and molecular techniques for determining the efficiency of in-situ bioremediation and chemical oxidation of chlorinated compounds)

At industrial-related polluted sites chlorinated solvents, as chloroform (CF), carbon tetrachloride (CT), trichloroethylene (TCE) and tetrachloroethylene (PCE) are common groundwater contaminants. IMOTEC-BOX project constituted a step forward from previous studies developed at a fractured bedrock aquifer heavily polluted with these compounds among others due to the activity of a former chemical plant in Òdena, Catalonia, Spain. The early measures for contamination mitigation were the removal of potential sources and the installation of interception trenches filled with construction wastes in 2006. Specific objectives of this research included: (1) evaluation of different oxidation reactions to realize potentiality and limitations at the interception trenches, (2) better understanding of the biodegradation/metabolic processes occurring naturally in the aquifer, (3) evaluation of the impact of the addition of different electron acceptors/donors on the indigenous microbial community composition and the rates of biodegradation for potential in situ biostimulation (4) tracking the efficiency of the selected remediation techniques at field scale by isotopic fractionation and molecular biomarkers.

In the interception trenches, the presence of concrete produced extremely basic pH which induces CF degradation by alkaline hydrolysis. The significant CF carbon isotopic fractionation allowed quantifying the efficacy of this remediation approach at the field up to 30-40% probably depending on water residence time and precipitation regime. In addition, four chemical oxidants were tested with real interception water to remediate the rest of the chlorinated pollutants unaltered by alkaline hydrolysis. The two most effective oxidants for chlorinated ethenes (persulfate and permanganate) were tested again respect to more recalcitrant chlorinated methanes removal. Although none of the treatments was able to degrade CT, a dose of 10 g/L persulfate would be a good choice for enhancing CF and chlorinated ethenes degradation in the alkaline trenches, with high enough carbon isotopic fractionations to follow its efficacy in future in situ remediation and much less toxic than permanganate according to the MICROTOX® values. Moreover, CT degradation and isotopic fractionation observed in the interception trenches might be due to abiotic reductive dechlorination by Fe(II) minerals as relatively high amounts of iron patinas were observed on top of gravel samples extracted by a borehole from the saturated zone of the trenches.

The results from a complete sampling campaign of the aquifer in March 2013 and the efforts to better characterize the hydrogeology and the subsoil by pumping/slug tests and electrical tomography have allowed the comparison and interpretation of all campaigns data (2006-2013) in order to evaluate the effectiveness of the sources removal as well as to shed light on natural attenuation processes happening closer to the former sources (S3, S1). Performed anaerobic microcosm studies showed no chlorinated ethenes degradation, but proved chlorinated methanes biodegradation accelerated and dependent on the addition of vitamin B12, whereas none of the tested electron donors produced any biostimulation effect on the responsible microbial community structure. Dehalobacter sp. and its CfrA genes encoding CF dehalogenase seems to be involved in the CF degradation and might be a good biomarker to track the efficiency of future biostimulation strategies in Òdena field site.