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INDOOR AIR SAMPLING, SUB-SLAB VAPOR SAMPLING, AND SLAB AIR-PERMEABILITY TESTING AT THE RAYMARK SITE
Citation:
DiGiulio*, D C. INDOOR AIR SAMPLING, SUB-SLAB VAPOR SAMPLING, AND SLAB AIR-PERMEABILITY TESTING AT THE RAYMARK SITE. Presented at Seminar on Indoor Air Vapor Intrusion, Dallas, TX, January 14 - 15, 2003.
Impact/Purpose:
To inform the public.
Description:
By 2005, U.S. EPA and its state, tribal and local partners must assess, reduce, and/or control the risk to human health and the environment at more than 374,000 contaminated Superfund, RCRA, underground storage tank (UST), brownfield and oil sites. U.S. EPA is currently developing guidance to specifically address one potential pathway of exposure - vapor intrusion. Vapor intrusion is defined as the migration of volatile organic compounds (VOCs) into occupied buildings from contaminated soil or ground water. The intent of the guidance is to: (1) offer a methodology to screen out sites where vapor intrusion is unlikely, thereby (2) identify sites where there is a potential for vapor intrusion, thereby requiring further investigation. The guidance employs a three-tiered approach to be followed sequentially to achieve these objectives. These tiers are summarized as follows:
Tier 1 (Primary Screening) - Involves general knowledge of a site and the chemicals known or reasonably suspected to be present in the subsurface. It does not require ground-water, soil-gas, or indoor air measurements.
Tier 2 (Secondary Screening) - Involves the use of ground-water and/or soil-gas data and empirical attenuation factors to screen out sites where vapor intrusion is presumed unlikely, thereby avoiding unnecessary additional investigation costs. If a site cannot be screened out through the use of these empirical factors, the user can consider implementation of the steady-state version of the Johnson Ettinger model employing constrained variables. This leads to computation of "scenario specific" attenuation factors to screen out sites where vapor intrusion is presumed unlikely.
Tier 3 (Site-Specific Pathway Assessment) - If a site cannot be screened out in Tier 1 or 2, the user is prompted to conduct a site-specific pathway assessment to declare the vapor intrusion pathway to be incomplete. This involves collection of indoor air samples which may be supplemented by sub-slab soil-gas collection.
A complicating factor in implementation of Tier 3 is the presence of VOCs in indoor air due to ambient atmospheric contamination and use of common household solvents. When VOCs from these two sources are the same as VOCs detected in subsurface media, it difficult to assess the origin of these VOCs in indoor air. It will be demonstrated in this presentation that sub-slab vapor sampling provides a definitive mechanism to distinguish the origin of VOCs in indoor air. In addition, the installation of sub-slab probes during vapor sampling offers a means to directly measure gas permeability of sub-slab soils and continuity of gas flow underneath a building. After installation of a sub-slab depressurization system, sub-slab vapor probes can be used to verify system performance (i.e., reduction of gas concentration in the sub-slab below health-based standards).