Topic Area: 8a. Advances in Tools and Techniques for Assessing MNA

Formatted Title:

Groundwater Remedy Optimization Study to Evaluate Transition from Active Treatment to Monitored Natural Attenuation

Background/Objectives:

This groundwater remedy optimization study was conducted using a multiple lines of evidence approach to evaluate whether a transition to MNA from active treatment was appropriate in order to achieve site remedial goals. The project site is located on the eastern side of San Francisco Bay and south of Oakland Inner Harbor in California. Potential risks are posed by the discharge of VOCs in groundwater to surface waters of San Francisco Bay. This study established a conceptual site model (CSM) for the ongoing attenuation of: 1) chlorinated ethenes (CEs) through complete reductive dechlorination to ethene and ethane; and 2) benzene likely through aerobic and anaerobic degradation. In addition to developing the CSM, this study sought to develop performance indicators to monitor for potential impacts to adjacent surface water. Three rounds of ISCO were completed at the site between 2012 and 2014 to reduce concentrations of trichloroethene (TCE), 1,1-dichloroethene (DCE), vinyl chloride (VC), and benzene. LNAPL is also present at the site, which poses potential impacts to the reductive dechlorination process.

Approach/Activities:

The groundwater remedy optimization study was a multi-step approach to develop a conceptual site model that included: (1) review of historical documents for the VOC plume area; (2) review of chemical, geochemical, and microbial results of monitoring wells in the ISCO treatment area; (3) evaluation of post-ISCO treatment effectiveness; (4) evaluation of post-ISCO treatment geochemical and microbial data; and, (5) evaluation of the presence of LNAPL and dissolved-phase petroleum hydrocarbons on the VOC remedy. The evaluation looked at the target VOC concentrations on a molar basis to develop several tools to evaluate remedial progress. These tools included concentration-time trend analysis, restoration time frame estimations, concentration-distance analysis, geochemical conditions, and microbial species and populations. Total CEs, including cis-1,2-DCE and trans-1,2-DCE which are not site COCs, as well as ethene and ethane concentrations, were evaluated to gain a more complete picture of the CE dechlorination process occurring at the site. Once the CSM was established, predictive limits were developed to use as performance indicators for the ongoing attenuation of CEs and benzene.

Neil Hey

Affiliation

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(Group 2, Poster Board #31) Groundwater Remedy Optimization Study to Evaluate Transition from Active Treatment to Monitored Natural Attenuation