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 ground water 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 1 year of posttreatment 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 posttreatment monitoring (i.e., 1 to 5 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 posttreatment monitoring period. Minimal concentration change was observed over the posttreatment monitoring period at surfactant/cosolvent sites (n= 2) and thermal treatment sites (n= 1) evaluated for rebound. Based on current data, none of the 59 source depletion projects was able to meet maximum contaminant levels throughout the treatment zone for all CVOCs.

Long-term ͑e.g., 5-15 years͒ groundwater concentration versus time records were compiled from 47 near-source zone monitoring wells at 23 chlorinated solvent sites ͑52 total records͒. Chlorinated volatile organic compound ͑CVOC͒ concentrations decreased significantly in most of the 52 temporal records, with a median reduction in concentration of 74%. A statistical method based on a Mann-Kendall analysis also showed that most sites had statistically significant decreasing concentration trends over time. Median point decay rate constants ͑k point ͒ values were calculated for nine sites containing tetrachloroethene ͑PCE͒; 13 sites containing trichloroethene ͑TCE͒; two sites containing cis-1,2-dichloroethene ͑DCE͒; and six sites containing 1,1,1-trichloroethane ͑TCA͒. The TCA sites had the highest k point values ͑0.34/year͒ followed by PCE, DCE, and TCE ͑0.23/year, 0.16/year, and 0.11/year, respectively͒ ͑equal to decay half-lives of 2.0, 3.0, 4.3, and 6.1 years, respectively͒. If the median point decay rates from these sites are maintained over a 20 year period, the resulting reduction in concentration will be similar to the reported reduction in source zone concentrations achieved by active in situ source remediation technologies ͑typical project length: 1-2 years͒.

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A survey of experts in the application of natural attenuation was conducted to better understand how monitored natural attenuation (MNA) is being applied at chlorinated solvent sites. Thirty-four

A recent U.S. Environmental Protection Agency (US EPA) expert panel on dense nonaqueous phase liquid (DNAPL) remediation concluded that uncertainty in the costs and benefit of applying source-depletion technologies (i.e., active remediation in source zones) is one key factor that discourages widespread use of these technologies at DNAPL sites (Kavanaugh et al., 2003). To reduce this uncertainty, a detailed evaluation of remediation costs for four active source-depletion technologies was conducted. The source-depletion technologies evaluated were enhanced bioremediation, chemical oxidation, surfactant/cosolvent flushing, and thermal treatments. An extensive review of peer-reviewed literature, conference proceedings, state and federal government agency reports, Internet databases, and technical surveys yielded cost and performance data at 36 full-scale and pilot-scale source-depletion sites. The data indicated that enhanced bioremediation has the lowest median cost per treatment volume of $29/yd 3 (n = 11), followed by thermal, chemical oxidation, and surfactant/cosolvent at $88/yd 3 (n = 13), $125/yd 3 (n = 6), and $385/yd 3 (n = 6), respectively. Only a slight correlation was observed between treatment size and total treatment cost; however, longer treatment durations correlated to lower treatment costs per volume. Treatment performance appeared to be independent of unit treatment costs. The resulting cost statistics and unit costs can be used to compare the cost of source-depletion projects against the life-cycle cost of long-term plume management techniques such as monitored natural attenuation or plume containment.