Usually vinyl chloride is considered to be the product of reductive dechlorination of chlorinated ethenes. However, at some sites vinyl chloride may also be present as primary contaminant. The goal of the study was to evaluate if stable carbon and chlorine isotopes can be used to identify vinyl chloride as primary contaminant and to trace the fate of the compound. The main study consists of a site where residues from PVC production were disposed and a plume of VC was detected in a sandy aquifer with absence of other chlorinated ethenes. The VC plume was delineated using monitoring wells and profiling with a direct-push rig. VC concentrations of up to 11 mg/L were found. Carbon isotope analysis of the VC revealed very depleted 13C values between -61 and -73‰ in wells with the highest concentration. The depleted 13C value could be linked to the VC production processes or due to isotope fractionation associated with the polymerization process. The very depleted isotope values may serve as an indication of VC as primary contaminant for other sites with similar wastes. Downgradient of the source, a substantial shift of the carbon isotope ratios was observed suggesting transformation of the compound. Currently, combined carbon and chlorine isotope analysis of VC is carried out. The dual isotope approach in conjunction with characterization of redox conditions should provide insight into the pathway of VC degradation, as oxidative and reductive transformation are expected to lead to different relative shifts for the two isotopes. Once the pathway of VC degradation is identified, biodegradation rates will be estimated based on shifts in isotope ratios and groundwater flow velocities, which help to predict the future evolution of the VC plume.
Origin and Fate of Vinyl Chloride as Primary Contaminant
GARGINI, Alessandro;PASINI, Monica
2006
Abstract
Usually vinyl chloride is considered to be the product of reductive dechlorination of chlorinated ethenes. However, at some sites vinyl chloride may also be present as primary contaminant. The goal of the study was to evaluate if stable carbon and chlorine isotopes can be used to identify vinyl chloride as primary contaminant and to trace the fate of the compound. The main study consists of a site where residues from PVC production were disposed and a plume of VC was detected in a sandy aquifer with absence of other chlorinated ethenes. The VC plume was delineated using monitoring wells and profiling with a direct-push rig. VC concentrations of up to 11 mg/L were found. Carbon isotope analysis of the VC revealed very depleted 13C values between -61 and -73‰ in wells with the highest concentration. The depleted 13C value could be linked to the VC production processes or due to isotope fractionation associated with the polymerization process. The very depleted isotope values may serve as an indication of VC as primary contaminant for other sites with similar wastes. Downgradient of the source, a substantial shift of the carbon isotope ratios was observed suggesting transformation of the compound. Currently, combined carbon and chlorine isotope analysis of VC is carried out. The dual isotope approach in conjunction with characterization of redox conditions should provide insight into the pathway of VC degradation, as oxidative and reductive transformation are expected to lead to different relative shifts for the two isotopes. Once the pathway of VC degradation is identified, biodegradation rates will be estimated based on shifts in isotope ratios and groundwater flow velocities, which help to predict the future evolution of the VC plume.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
