Final Guidance Issued For Assessment of Health Risks From Soil Vapor At Contaminated Sites in CaliforniaBy Dustin Till
The California Department of Toxic Substances Control (“DTSC”) recently published final guidance (the “Guidance”) to help developers, consultants, and regulators evaluate and mitigate vapor intrusion into buildings at contaminated sites. The Guidance reflects an increasing focus by federal and state regulators on the potential human health risks posed by vapor intrusion, for which there are now multiple competing standards and methodologies. The new requirements, as well as the uncertainty associated with them, have increased the costs of redeveloping brownfields and other contaminated sites, and have raised concerns that previously closed cleanups may again come under the scrutiny of regulators.
Soil vapor is the gas residing between the particles of dirt in soil. When soil or groundwater is contaminated with volatile materials, such as gasoline or trichloroethylene (“TCE”), the contaminants vaporize into this soil vapor and may migrate through soil vapor more rapidly than they do in groundwater or soil alone. The contaminated soil vapor can penetrate building foundations and basement walls in a manner that presents a health threat to occupants. Soil vapor intrusion can be a significant driver of cleanup requirements where soil and/or groundwater standards would not otherwise require cleanup.
Over the past decade, regulators have increasingly focused on the potential for vapor intrusion at contaminated sites. In 2002, EPA released its Draft Guidance for Evaluating the Vapor Intrusion to Indoor Air Pathway from Groundwater and Soils, which presented technical and policy recommendations of the Office of Solid Waste and Emergency Response for evaluating subsurface soil vapor intrusion. Last year, EPA reviewed its draft vapor intrusion guidance, concluding that there is greater complexity in the processes and number of variables that affect the migration and distribution of VOCs in soil vapor. EPA is currently updating its draft guidance, and is scheduled to release its final vapor intrusion guidance in November 2012.
Vapor intrusion is also the subject of mounting state-level requirements. As of last year, 29 states had developed state-specific vapor intrusion guidance, with the remaining states relying on EPA’s guidance, or guidance prepared by ASTM and the Interstate Technology and Regulatory Council (“ITRC”). At least one state, New York, has begun a process for reevaluating over 400 closed cleanup sites where VOC contamination was present to determine whether additional mitigation is required. While other states do not have necessarily have similar systematic approaches to reevaluating closed sites, the potential for such reevaluation exists – particularly when evidence suggests that residual contamination presents significant vapor intrusion risks.
Evaluating Vapor Intrusion Risk
The risk posed by soil vapor intrusion is measured by the concentrations of contaminants in indoor air and comparing them to specific risk thresholds defined as “safe” by regulatory agencies. In general terms, the “safe” level of a substance known to cause cancer is the level at which the probability of exposure causing cancer is less than one-in-one-million (1x10-6). Similar exposure concentrations are established for toxic chemicals on the basis of data identifying the point at which certain toxic effects are measurable. For indoor air, these risk thresholds translate directly into airborne concentrations of various contaminants.
The concentrations of contaminants that migrate into indoor air will be much lower than the concentrations found in soil or groundwater, because not all of the contamination will migrate into the building. Mitigating factors include soil density, moisture, air pressure, building construction and other physical factors. By making various assumptions about such factors, agencies can calculate “safe” screening levels for soil and groundwater concentrations of contaminants.
The California Office of Environmental Health Hazard Assessment (“OEHHA”) has developed screening level concentrations for many contaminants under a range of different circumstances. Different tables of screening levels exist for shallow and deep soils, for areas where groundwater is a likely source of drinking water and where it is not such a source, and for different land uses (residential versus commercial/industrial). In general, if concentrations of contaminants in different media are below screening levels, no action is required. If concentrations exceed the screening levels, then site-specific risk assessment may be required.
Site-specific risk assessment usually involves the collection of additional data regarding specific on-site conditions that affect the risk, such as actual soil density and permeability, and the type and quality of building construction. Using actual site data, risk assessment computations are performed in which all the relevant contaminants are assessed and a composite risk number may be calculated. If the aggregate risk assessment indicates that there is an unacceptable risk of cancer or toxic effects, then action must be taken to mitigate the risk. In many cases, site-specific risk assessment allows contamination to remain in place without further action even though it exceeds screening levels, because the use of site-specific data generates a lower risk profile than the conservative assumptions of standard models.
Basic Requirements of the Guidance
The Guidance describes actions that can be taken and requirements that can be imposed with respect to contaminated sites in order to assess and reduce the risk of soil vapor intrusion. Some of these are changes to existing practice; others are refinements to existing practices that may change the approach being taken at sites currently under analysis.
The most important aspect of the Guidance is the extensive process required to assess soil vapor intrusion. The Guidance reflects DTSC’s understanding of the complexity of assessing these risks, and provides for a comprehensive 11-step program for such assessments. For simplicity, we break these steps down into 6 basic tasks.
Task 1 (steps 1, 2 and 3): Assess site, identify spills and releases, and determine the pathway for volatile contaminants. When volatile compounds are found at a site, the process of assessment is triggered. All spills and releases must be identified, and potential pathways to building interiors must be investigated. The purpose of this task is to determine whether there is a potential for soil vapor intrusion into buildings. Note that no particular types or levels of contamination are required to commence this assessment requirement. Any site with known volatile contaminants is subject to this process.
Task 2 (step 4): Assess imminent hazards. For existing buildings, the Guidance requires an assessment of imminent hazards presented by vapor intrusion. Although it is rare for such risks to be present, a first step is to check for symptoms in exposed persons. If there are indications of immediate risks, the buildings would be evacuated and emergency responses commenced.
Task 3 (step 5): Perform screening analysis. The screening analysis applies the CHHSLs to determine whether the site contamination is below levels of concern. There are many factors considered in this step, including the extent to which the site is adequately characterized, and the extent to which the site matches the criteria and assumptions used in formulating the screening criteria. This step requires an assessment of the construction existing on the site, if any, and the applicability of different screening levels depending on the type of construction, land use and other factors. In this step, all known contamination is assessed to develop a cumulative analysis of all risk factors. This means that cumulative health effects from all known contamination would be factored into the analysis. Other health-based screening guidelines may be applicable to factors not covered by the CHHSLs, such as inhalation risk factors used by the US EPA or environmental protection factors established by California EPA or regional water boards.
Task 4 (steps 6 and 7): Collect additional data and perform site-specific vapor intrusion evaluation. If the screening analysis indicates that the screening levels are exceeded or the cumulative risk assessment based on available data is above applicable standards, then a site-specific risk analysis is required. This risk assessment includes the collection of additional data to fill in missing data points, and the analysis of additional site parameters (like soil permeability and gas diffusion rates). The idea is to improve the statistical reliability of the data and provide site-specific characteristics that would affect exposure.
Task 5 (steps 8, 9 and 10): Interior building assessment. If the site-specific analysis shows that the building is subject to intrusion, an analysis of specific building parameters is required, including indoor air sampling. The initial phases of this process involve an inventory of cracks, utility openings and other pathways into the building. An inventory of indoor sources of air contaminants would also be undertaken. Then, sampling of interior air conditions would be required to complete the process. Data collected would be subjected to calculations to determine the cumulative health risk presented by all contaminants in the air (not just those migrating into the building).
Task 6 (step 11): If contamination of indoor air exceeds appropriate risk levels, mitigate vapor intrusion. Such mitigation may include soil or groundwater remediation, soil vapor collection and venting or destruction, soil vapor barriers and other methods of removing contaminants from indoor air (such as slightly pressurizing the building or increasing the replacement of indoor air with outside air). Two rounds of sampling will be required to verify results. The requirements would typically be recorded in documentation filed with the recorder of deeds, including any restrictions on land use that may be imposed.
Key Issues in Guidance
There are several issues raised in the Guidance that affect standard practice in connection with these types of risk assessments.
Public notification. Any activity at a site being investigated under the Guidance – including at Step 1 – should be disclosed to the public. Public disclosure and notification are required throughout the process, especially at the point where interior building risks are identified.
Cumulative impacts. When the Guidance specifies the assessment of cumulative impacts, it requires a cumulative assessment of all exposure risks. For example, the exposure to ambient air contamination, regardless of source (i.e., outside the building), is required in conjunction with indoor air analysis. Assessment of interior exposures to ordinary consumer products and all other air contaminants is required in connection with vapor intrusion analysis.
Applicability to unimproved properties. The Guidance differentiates between improved sites and vacant sites. Vacant sites with contamination issues presenting the potential for soil vapor intrusion must be remediated in advance of construction to prevent soil vapor intrusion. A risk assessment would be required to determine whether any residual risks need to be addressed by active or passive soil vapor remediation systems. Applicants for construction will be required to present data that the contaminant plumes are well-characterized and stable.
Data collection. Soil samples alone will not be sufficient to perform screening analyses. Groundwater sampling and/or soil vapor sampling will be required. Specific techniques for sampling collection and analysis are also required. For example, groundwater data can only be used for soil vapor risk assessment if the wells are screened at or near the surface of the groundwater. In addition, soil vapor data can only be used if collected from permanent soil vapor collection wells, instead of temporary sample points. The locations of soil sampling points are required to be at the areas of highest potential soil concentrations, near the source of contamination.
Buffer zone. A buffer zone of 100 feet is required around buildings. Accordingly, the requirements for sampling and risk analysis will be applied to any contamination within 100 feet of an existing or proposed building location in order to determine the need for further assessment or mitigation.
Status and Use of Guidance
The Guidance is not considered a regulation by DTSC, which means it is not “enforceable” against third parties in California. At the same time, DTSC will apply the Guidance to determine when to require additional sampling and what type to require. Accordingly, the Guidance has the force of law similar to regulations and can be applied by DTSC in its administrative functions.
The Guidance is also not exclusive. Various other soil vapor and risk assessment guidance documents will continue to apply, including DTSC’s Preliminary Endangerment Assessment Guidance Manual and the U.S. EPA’s Risk Assessment Guidance for Superfund. Importantly, Regional Water Quality Control Boards in California, which have jurisdiction over groundwater contamination and threats to groundwater, use the Environmental Screening Levels (“ESLs”). The ESLs apply to a wider range of contaminants than the CHHSLs, and are established to address threats not only to human health, but to other environmental resources, such as surface water and aquatic species. Other specific guidance documents, such as that applicable to school risk assessments or to the assessment of methane, continue to apply.
The Guidance is intended to assist regulated entities and agencies in assessing risks, and outlines risk mitigation measures that may be employed. As further described in DTSC’s Vapor Intrusion Mitigation Advisory, there are a range of permissible technologies for mitigating risks. Some involve passive or active soil venting; others involve construction measures, such as vapor barriers or interior building ventilation requirements. The acceptable mitigation methods are also determined based on risk assessment, with the objective to reduce exposures to acceptable levels as identified by site-specific risk assessment and confirmatory sampling.
While seemingly very technical, the Guidance substantially revises the approach to soil vapor in California, and complicates the process of assessment and mitigation. At the same time, the Guidance does not simplify or streamline the process, but instead adds yet another set of requirements to an already crowded field. Owners and operators of contaminated property in California, and those who intend to purchase or build upon such property, need to become thoroughly familiar with the Guidance and its related guidance documents.
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