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Background/Objectives. In 2003, 1,300 drums and over 3,000 tons of soil were excavated from a drum disposal area in New England. Residual DNAPL created a 2,500 foot long plume that includes chlorobenzenes, toluene, and chlorinated ethenes. The plume discharges to a wetland and has led to vapor intrusion concerns at downgradient properties. Full-scale source zone remediation has been implemented to mitigate potential risks to ecological and human receptors.
Approach/Activities. Following extensive site characterization, bench-scale testing, and a successful field pilot test, the full-scale source zone remedy began in November 2010. The combined remedy approach uses physical, chemical, and biological treatment mechanisms to destroy the residual DNAPL in the vadose and saturated zones. Soil vapor extraction (SVE) and air sparging target the more volatile compounds, while in situ ozone injection (IOI) targets the less volatile compounds such as chlorobenzenes. Aerobic biological activity is also likely enhanced as a result of oxygen injection from IOI and air sparging. Three ozone injection systems deliver a total of 100 lbs of ozone per day to the subsurface; the SVE system extracts soil vapor at a rate of 650 scfm; the air sparge system continuously sparges air at 50 scfm at approximately 30 psi.
Results/Lessons Learned. Performance monitoring includes analysis of VOC concentrations in soil, soil gas, and groundwater. Quarterly low-flow groundwater sampling has shown substantial decreases in groundwater VOC concentrations relative to baseline. In the first six months of operation, 18 of the 20 monitoring wells sampled showed decreases in total VOC concentrations ranging from 14 to 97% with an average decrease of 57%. Furthermore, the mass discharge of total VOCs in groundwater from the source area has decreased from approximately 105 g/day before pilot-test start-up to less than 4 g/day. Concentrations of VOCs in soil gas are monitored in real-time by an automated soil gas monitoring system equipped with a photoionization detector (PID), and on a periodic basis with Waterloo Membrane Samplers™ that provide a speciated analysis of VOCs in soil gas. Of the 47 soil gas monitoring points that were sampled during the first six months of operation, 36 show decreasing trends in soil gas TVOC concentrations. Minimal rebound was observed in soil gas concentrations after treatment was temporarily suspended. Performance monitoring data have informed periodic system optimization to increase the efficiency of the remedy. These results indicate that the combined remedial technologies are effectively reducing source mass and mass discharge, and that the aggressive two year remedial time-frame will likely be met.