Performance of DNAPL Source Depletion Technologies at 59 Chlorinated Solvent‐Impacted Sites

Abstract

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.

Mass-removal and mass-flux-reduction behavior for idealized source zones with hydraulically poorly-accessible immiscible liquid

“…For example, Suchomel and Pennell (2006) conducted flow-cell experiments to examine the impact of surfactant-enhanced solubilization on mass-fluxreduction/mass-removal behavior. Several examples of end-point analyses based on field studies have recently been reported (Brooks et al, 2004;Childs et al, 2006;McGuire et al, 2006;Brusseau et al, 2007). The end-point analysis approach provides critical information for evaluating the impact of a source-zone remediation effort on mass flux.…”

Impact of mineral micropores on transport and fate of organic contaminants: A review

“…The few studies that monitored contaminant concentrations for extended periods beyond completion of source depletion activities have observed some levels of concentration rebound (Goldstein et al, 2004;Gomez-Lahoz et al, 1994;McGuire et al, 2006). The prevalent rebound occurrence could result from heterogeneous subsurface permeability, volatilization of contaminants into the vadose zone, intra-aggregate diffusion, and sorption on aquifer materials (Abriola et al, 2004;Gomez-Lahoz et al, 1994;McGuire et al, 2006;Wilkins et al, 1995). The micropore-sorbed contaminants are not readily available for external removal/destruction by physical, chemical, and biological processes during the period of engineered remediation (which typically lasts weeks to months).…”

Enhanced Attenuation Technologies: Passive Soil Vapor Extraction

“…In a survey of 59 chlorinated solvent sites (McGuire et al, 2005) where active source treatment technologies -chemical oxidation, enhanced bioremediation, thermal treatment, and surfactant/co-solvent flushing -were implemented, none of the technologies were able to achieve maximum contaminant levels (MCLs) 1 to 5 years after the treatment. Furthermore, despite the fact that active technologies no longer offer the cost-benefits required to continue operation at many of these sites, discontinuing source treatment is not considered an acceptable option due to the potential for the residual mass to act as a long-term source of low-level groundwater contamination.…”