Performance of DNAPL source depletion technologies 2 at 59 Chlorinated solvent-impacted sites
Performance and rebound of intensive source depletion technologies were evaluated at 59 chlorinated solvent sites where remediation targeted dense non-aqueous phase liquid (DNAPL) source zones. The four technologies included in the study are chemical oxidation, enhanced bioremediation, thermal treatment, and surfactant/cosolvent flushing. Performance was evaluated by examining temporal groundwater concentration data before and after source remediation was performed. Results indicated that all four technologies have median concentration reductions of 88% or greater for the parent chlorinated volatile organic compound (CVOC). Approximately 75% of the source depletion projects were able to achieve a 70% reduction in parent compound concentrations. A median reduction in total CVOC concentrations (parent plus daughter compounds) of 72% was observed at 12 chemical oxidation sites and 62% at 21 enhanced bioremediation sites. Rebound was assessed at sites having at least one year of post-treatment data. Rebound occurrence was most prevalent at sites implementing chemical oxidation. At chemical oxidation sites where rebound was evaluated (n=7), the median parent CVOC concentration reduction was 90% immediately following treatment compared to 78% at the end of post-treatment monitoring (i.e., one to five years after treatment). For enhanced bioremediation sites where rebound was evaluated (n=10), median parent CVOC concentration reduction changed from 77% to 96% over the post-treatment monitoring period. Minimal concentration change was observed over the post-treatment monitoring period at surfactant/cosolvent sites (n=2) and thermal treatment sites (n=l) evaluated for rebound. Based on current data, none of the 59 source depletion projects was able to meet Maximum Contaminant Levels (MCLs) throughout the treatment zone for all CVOCs.