Theme: 09. Vapor Intrusion

Topic Area: 9d. Vapor Intrusion Risk Assessment and Site Management

Formatted Title:

Optimizing Long-Term Vapor Intrusion Monitoring during Site Management

Background/Objectives:

Long-term monitoring (LTM) for vapor intrusion (VI) at buildings where VI has been documented, future VI cannot be ruled out, or VI monitoring is conducted at the request of regulators, can be expensive, intrusive, and potentially unnecessary. As monitoring results become more predictable and/or subsurface contaminant concentrations decrease, available data may be effective at demonstrating VI LTM can be discontinued or optimized. For a United States Navy installation in the southeast, VI LTM has been conducted since 2009 at multiple buildings overlying groundwater impacted by chlorinated volatile organic compounds (cVOCs). This presentation will discuss effective VI LTM optimization strategies considered during long-term site management that are supported by an evaluation of the available data.

Approach/Activities:

A data compilation was prepared that included VI data collected as part of the Remedial Investigation (RI) and the Remedial Action-Operation (RA-O). The VI LTM program was implemented to determine if changing site subsurface conditions during RA (e.g., increases in cVOC daughter products) or changes in building characteristics created a potential for increased VI or explosive hazards (e.g., methane and hydrogen sulfide) at the buildings located in the land-use control (LUC) boundary. The selected remedy implemented at the site included in situ chemical reduction via zero-valent iron injections, enhanced reductive dechlorination via emulsified vegetable oil (EVO) injections, and LUCs. Site-related chemicals of concern (COCs) in groundwater included trichloroethene (TCE), cis-1,2-dichloroethene (DCE), 1,1-DCE, and vinyl chloride (VC). Data were available from three EVO injection events, 16 RA-O phase groundwater, stormwater, and VI LTM events, and two additional VI investigations at buildings proposed for re-occupancy. Additional data included site and LUC inspections and five-year reviews. Sufficient data were available to conduct trend analyses of COCs in groundwater, subslab, and indoor/outdoor air, and assess building characteristics.

Results/Lessons Learned:

Discontinuing VI LTM of cVOCs in the subslab, indoor, and outdoor air was recommended based on the results of the VI LTM optimization evaluation. The optimization evaluation found that the concentrations and size of the TCE and cis-1,2-DCE groundwater plumes had decreased significantly since the initiation of the groundwater remedy. VC concentrations in groundwater were found to fluctuate, with an increase in plume size; however, no significant VI impacts to indoor air or increases in subslab soil gas VC concentrations were observed. The groundwater plumes are expected to continue to move away from the occupiable buildings. Continued biodegradation associated with the groundwater RA is expected to result in COC concentration reductions, indicating that future VI impacts are unlikely. Continued monitoring of potential explosive hazards was recommended until three consecutive events show subslab concentrations below criteria. Site and building inspections will continue under the established LUC inspection program. These recommendations provided decision-makers with the information needed to refine the VI conceptual site model, demonstrate future VI impacts are not expected, optimize VI LTM data, and significantly reduce LTM costs during site management.

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Adrienne Jones