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Vapor Intrusion Assessment – An Overview

Introduction

Redevelopment of contaminated land and land adjacent to contaminated sites can be a relatively straightforward process when you work with a good environmental consultant that understands the special risks and concerns a developer faces.  However, there is one issue that can get any consultant tied up in knots because of the increasing attention given to it by regulators: vapor intrusion.

What’s the Concern?

Vapor intrusion occurs when certain types of contaminants in the soil or groundwater evaporate (volatilize) and migrate through spaces in the soil into occupied spaces within a building on the property.  The type of contaminants that tend to evaporate are called volatile organic compounds (VOCs) and include substances such as benzene, toluene, xylene, acetone, and perchloroethylene (dry cleaning solvent). The scientific studies that regulators rely on suggest that the presence of such VOCs in indoor air can adversely affect the health of building occupants. Because of potential health risks, regulators, especially the EPA, are increasingly focusing their attention on vapor intrusion issues. Therefore, purchasers and redevelopers can be exposed to liability (both regulatory and for personal injuries) if they acquire property with vapor intrusion issues that are not abated. A good consultant should identify such potential risks during the pre-acquisition due diligence process and, depending on site location, the consultant might use one or a combination of the following screening methodologies.

ASTM Assessment

The American Society for Testing and Materials (ASTM) has developed a recommended vapor intrusion standard (E 2600: “Assessment of Vapor Intrusion into Structures on Property Involved in Real Estate Transactions”) which is beginning to be followed by many consultants.  ASTM E2600 prescribes a tiered approach to the vapor intrusion risk assessment.  The first tier (Tier 1) of the analysis relies solely on documentary evidence.  If information shows that there is a contaminated plume or potential plume within 100 feet of a proposed or existing building or property boundary (or within 30 feet for dissolved petroleum hydrocarbons), then a potential vapor intrusion concern (pVIC) is presumed to exist. (The consultant can modify these distances based on site specific conditions such as groundwater flow, depth to groundwater, vapor conduits, etc.). When a pVIC is identified, the consultant moves on to Tier 2 of the analysis, which can include sampling. If sampling is performed as part of Tier 2, the consultant compares the results to determine if any of the contaminants (within the 100 or 30 foot radius) exceed risk-based concentrations (RBCs) established by federal or state policy or site-specific RBCs established by the consultant.  If there are any RBC exceedances, then further testing is undertaken during the Tier 3 analysis to confirm the presence of a VIC. If the presence of a VIC is confirmed in Tier 3, the consultant can propose a mitigation strategy using the Standard’s Tier 4 process.

EPA Assessment (Proposed)

On March 17, 2011 the EPA issued draft guidance for evaluating vapor intrusion entitled “Evaluating the Vapor Intrusion to Indoor Air Pathway from Groundwater and Soils” (which can be accessed here).  EPA’s goal is to finalize the guidance by November 30, 2012. The guidance will be used at RCRA Corrective Action, CERCLA and Brownfield sites but it will not supersede state guidance.  Like the ASTM assessment, the EPA guidance is to be used to determine if there is a potential for an unacceptable risk.  While this risk assessment approach is not appropriate at sites where employees are working with hazardous substances similar to those that are contaminating the site, the guidance is intended for use in other situations.

Like the ASTM standard, EPA’s VI guidance relies on a tiered analysis and the first step (Tier 1) is very similar to the first tier of the ASTM standard. In this step, the consultant determines if existing data indicates that VOCs are near (within 100 feet) of occupied buildings. If they are, then the consultant proceeds to the next step (Tier 2) which can include comparison of available indoor air concentrations to generic criteria established by the EPA or the collection of soil gas samples and comparison of that data to the generic criteria.  Site specific factors such as depth of the contamination and soil type are also considered. Based on the results of Tier 2, the consultant will then move to Tier 3 to further refine the assessment by collecting site specific data such as collecting indoor air and/or sub-slab air samples. The consultant can also use modeling techniques to factor in variables such as soil type, depth to groundwater, and various building characteristics that can impact indoor air such as type of ventilation, air exchange rates, etc.

Michigan DEQ Assessment

Michigan, under Part 201 of its Natural Resources and Environmental Protection Act (NREPA), has promulgated specific numerical screening criteria for many hazardous substances.  The generic criteria that are used to assess vapor intrusion to indoor air for residential and non-residential properties are the groundwater volatilization to indoor air inhalation criteria (GVIIC) and soil volatilization to indoor air inhalation criteria (SVIIC).  (These generic criteria would be the RBCs relied on if the consultant uses ASTM E 2600).  If soil and groundwater testing identifies an exceedance of the GVIIC or SVIIC criteria on property where buildings are or will be located, then there is a potential risk to indoor air quality. If site-specific factors such as lack of cement or block foundation, shallow groundwater plume, or preferential pathways, the rules preclude reliance on the generic criteria and the consultant must perform a site specific analysis to determine if a vapor intrusion (indoor inhalation) risk is present that must be abated.

To clarify the process set forth in its rules, the Michigan Department of Environmental Quality has prepared draft vapor intrusion guidance (“Guidance Document for the Vapor Intrusion Pathway“) that sets forth the steps that can be taken to evaluate the potential for a vapor intrusion risk.  In its current form, the guidance is structured in a 4-step process like ASTM E 2600, however, it uses a 100 foot receptor radius for the preliminary screening area. Step 2 involves the collection of soil-gas data and Step 3 involves the refinement of that data through sampling of indoor air and sub-slab soil gas as well as an evaluation of conditions such as the presence of cracks, utility lines and operational uses of hazardous substances.  If a vapor intrusion risk is confirmed, a remediation strategy is developed in Step 4.

As you can see, when consultants encounter a site that may have a vapor intrusion risk there are several ways of approaching the assessment that are similar but at the same time have subtle differences. Although each of the approaches allows consultants to consider site specific factors and the consultant’s professional judgment, a consultant’s ultimate conclusion could be different depending upon the assessment methodology used.

About Brian J. Considine

Brian J. Considine is a Senior Attorney with Dawda, Mann, Mulcahy & Sadler, PLC. He concentrates his practice in the areas of corporate environmental counseling, commercial real estate due diligence and environmental/toxics litigation. His practice also includes counseling clients on Federal Motor Vehicle Safety Standards.

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