Theme: 04. Addressing Challenging Site Conditions

Topic Area: 4e. DNAPL Source Zone Remediation: Lessons Learned

Formatted Title:

TCE Removal from MGP DNAPL via Enhanced In Situ Bioremediation of Groundwater

Background/Objectives:

Trichloroethylene (TCE) and/or TCE containing wastewater was released in the early 1980s at a manufacturing facility located along the Willamette River in Portland, Oregon. Prior to construction of the manufacturing facility, the site was used for historical disposal of manufactured gas plant (MGP) waste produced by the gasification of oil, resulting in the presence of significant volumes of MGP dense non-aqueous phase liquid (DNAPL) present on the site, which has commingled with TCE-impacted groundwater. Prior to remedy implementation, groundwater concentrations of TCE in the release area ranged as high as 592,000 µg/L. In the TCE source area, MGP DNAPL also contained a significant fraction of TCE. Remediation of MGP impacts is being conducted by a separate party, and no significant removal or remediation of MGP DNAPL has occurred in the TCE source area. Remediation of TCE and degradation products via enhanced in situ bioremediation (EISB) began in January 2009 and consisted of injections of the proprietary in situ chemical reduction (ISCR) reagent EHC (a combination of zero-valent iron and hydrophilic organic carbon) and KB-1 (dechlorinating bacterial consortium). Following EISB injections dissolved phase groundwater concentrations of TCE fell rapidly meeting the initial remedial action objective at most wells within 9 months. Now, more than 10 years from the implementation of bioremediation, the mass of TCE in source area groundwater has declined by over 99%, and active microbial dechlorination of TCE and degradation products is ongoing. MGP DNAPL remains a reservoir of TCE and associated degradation products.

Approach/Activities:

Chemical partitioning is likely to be the primary mechanism controlling release of TCE from MGP DNAPL to groundwater at the site. As TCE concentrations in groundwater decrease, the concentration gradient between the DNAPL and aqueous phases grows larger and TCE is released from DNAPL into the groundwater. This process is well established for source zones that contain chlorinated solvent DNAPL, but has not previously been evaluated in the context of TCE commingled in MGP waste DNAPL. Over more than 10 years of site monitoring activities, both groundwater and MGP DNAPL analytical data were collected from multiple locations. These data are used to evaluate TCE partitioning from MGP DNAPL before and after implementation of the EISB system, and evaluate site specific effective partitioning coefficients and effective solubility limits for TCE partitioning in an environment of active biologically mediated reductive dechlorination. Monitoring data are also used to evaluate TCE trends in MGP DNAPL and groundwater in contact with MGP DNAPL to determine if the ongoing active bioremediation in the aqueous phase is accelerating TCE removal from the MGP DNAPL.

Results/Lessons Learned:

Effective partitioning coefficients for TCE increased substantially following implementation of EISB remedy, indicating that the EISB system removes TCE from the aqueous phase more quickly than TCE partitions out of the MGP DNAPL. MGP waste DNAPL composition and TCE trends varied notably in different areas of the site. In most areas, TCE concentrations in MGP DNAPL have declined, beginning with the EISB implementation, and are now below detection limits. In areas with the highest initial concentrations of TCE in MGP DNAPL (up to 8.5%), TCE concentrations have decreased substantially since EISB implementation, and continue to decline. These results indicate that TCE is being depleted from MGP waste DNAPL by the ongoing EISB occurring in groundwater.

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Christian Sifford