Assessment of Chlorinated Solvents Degradation Pathways in a ZVIPermeable Reactive Barrier Using Compound-Specific Isotope Analysis

摘要

A pilot-scale permeable reactive barrier (PRB) was installed in an industrialfacility. It used an azimuth-controlled hydraulic fracturing method, and the zerovalentiron (ZVI) fillings were emplaced in a guar gel mixture. Groundwater multilevelmonitoring wells, located both upgradient and downgradient, have been continuouslysampled post-PRB installation for VOCs and inorganic compounds to predict the longtermPRB performance. A decrease in VOC concentrations was observed in some of thewells where high levels of TOC were detected. The inorganic data indicated that the localaquifer is under sulfate-reducing conditions in the areas where VOC reduction was observed,suggesting that biotic processes were the main mechanisms of degradation. Compound-specific isotope analysis (CSIA) was included in recent groundwater samplingrounds as a complementary tool for evaluating the role of abiotic and biotic processes oncontaminant degradation within the PRB. The isotope data showed isotopic fractionationconfirming that VOC reduction is occurring along the PRB. A Batch experiment usingsite-specific groundwater collected near the VOC source and ZVI was performed toevaluate the site-specific abiotic isotopic fractionation patterns. Field isotopic trends werequite different to those obtained during the batch experiment. These differences in isotopictrends combined with the VOCs and redox parameters indicated that biotic processesare the predominant pathways involved in degradation of VOCs in the PRB.