Theme: 09. Vapor Intrusion

Topic Area: 9b. Vapor Intrusion Mitigation and Effectiveness

Formatted Title:

SSDS Enables Safe Implementation of ERD Beneath Buildings

Background/Objectives:

The process of enhanced reductive dechlorination (ERD) generates dissolved methane in groundwater that can off-gas and present a vapor intrusion (VI) risk. Although methane is not considered to be toxic, it can be flammable or explosive if present in sufficient concentrations with the necessary amount of oxygen. Historically, ERD injection wells have either been placed within buildings, with no measures in place to address the methane risks, resulting in potentially unsafe conditions, or placed outdoors and away from buildings to manage this risk. Since ERD is an effective technology for accessing mass present beneath buildings, it is desirable to have a means of implementing it safely beneath buildings. When ERD is being considered for remediation of chlorinated volatile organic compounds (CVOCs) under existing buildings, a sub-slab depressurization system (SSDS) may be utilized to mitigate the risk of methane gas entering the building. In some cases, a SSDS may have already been installed to mitigate VI from the CVOCs that are present under the building. However, at most buildings where a SSDS has been installed, it was not designed to safely handle methane gas within the extracted soil vapor and would need to be modified to handle the methane gas safely.

Approach/Activities:

The properties of methane gas will be discussed along with the considerations for designing ERD programs to minimize methane generation and designing SSDSs to safely handle potentially flammable methane vapors. Additionally, safety measures for preventing buildup of pressure within injection wells will also be discussed. Methane gas is lighter than air and therefore is very mobile. It also attenuates in the presence of oxygen. These properties make methane unique from other compounds that are typically mitigated and play a role in the performance of an SSDS. How to choose the correct intrinsically safe equipment, including determining the proper National Fire Protection Association (NFPA) hazardous area classifications, will be discussed along with the design and placement of electrical panels and components to comply with hazardous area classification requirements.

Results/Lessons Learned:

Case studies will be shared which demonstrate the potential for methane to build up within injection wells and the sub-slab soil gas environment and the safe implementation of ERD beneath existing buildings where the SSDSs have been successfully upgraded to intrinsically safe standards. Data will be presented demonstrating that in addition to maintaining depressurization beneath the slab, the concentration of methane was kept much lower than in an unmitigated building undergoing ERD. A comparison will also be provided of the cost to upgrade an existing system versus proactively implementing an intrinsically safe system at sites where ERD may be considered in the future.

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Kevin Erickson