The Reynolds Group finished a Vapor Intrusion Mitigation (VIM) Feasibility Test. The objective was to see if TRG could prevent the intrusion of PCE, a chlorinated solvent, into an occupied building by depressurizing the limited space between the building’s concrete slab on grade and the soil immediately beneath it. Typically, the source of chemical vapors that intrude into indoor environments is contaminated soil or contaminated groundwater. VIM is not considered a means of remediation to the source of soil or groundwater contamination. Here is some information on Vapor Intrusion Mitigation to see if TRG can help you.
DTSC and VIM
The Department of Toxis Substances Control (DTSC) developed a Vapor Intrusion Mitigation Advisory (VIMA, April 2009) to assist with selecting, designing, and implementing appropriate response actions for sites where a potential vapor intrusion risk has been identified for occupants of existing or future buildings. Responsible parties may use the VIMA when 1) risk according to vapor intrusion is estimated by modeling or indoor air sampling and 2) mitigation is proposed as part of a response action. According to the DTSC 2005 and 2009 guidance documents, the responsible party may propose vapor intrusion mitigation as a pre-emptive solution for a perceived rather than actual threat.
VIM Methods for Existing Buildings
According to the DTSC 2009 VIMA, sub-slab depressurization systems (SSD) are most applicable for existing slab-on-grade building construction. This guidance is also contained in the Interstate Technology and Regulatory Counsel (ITRC) Technical and Regulatory Guidance document dated January 2007. A SSD system is designed to function by continuously creating a lower pressure directly underneath a building floor relative to the pressure within a building. The resulting sub-slab negative pressure inhibits soil gases from flowing into the building, thus reducing the VOC entry into the building. VOCs caught in this negative pressure field are collected and piped to an ambient air discharge point. The vacuum or depressurization under the slab is typically accomplished with a motorized blower. The blower draws air from the soil beneath a building and discharges it to the atmosphere though a series of collection and discharge pipes. Other remedies in addition to, or as alternatives to, SSD systems are available to address site specific conditions including sealing cracks and openings in the building foundation/floor and adjusting the building HVAC systems or installing a new system to maintain positive pressure indoors relative to the sub-slab area.
VIM Feasibility Testing
For the project recently completed, TRG constructed an extraction point through the concrete slab by coring through the slab and installing a PVC extraction pipe. The pipe was connected to a small blower and the outlet of the blower was piped to two carbon filters in series. Several monitoring points that intercepted the space between the slab and the soil beneath it were installed through the slab. The monitoring points were used to measure the drop in pressure (vacuum) beneath the slab compared to the pressure inside the building.
The vacuum readings taken at the monitoring points showed that a small blower could be piped to two or three extraction points to create a vacuum under the entire building slab. Laboratory analyses of the extracted vapor stream showed the system was, in fact, recovering PCE. The PCE was captured by the carbon filters.
The results of the feasibility test showed that a small mitigation system could be installed and operated for a minimal cost to protect the building occupants from risks associated with vapor intrusion.