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GeoInsighter Fall 1999 Newsletter
Volume 4 Number 4

Critical Exposure Pathways

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While not extensive, the October 29, 1999 revisions to the MCP included several important new definitions and changes. Foremost of these is the development of the term and approach of "Critical Exposure Pathways" (CEPs) for conducting risk reduction measures at certain disposal sites. CEPs are defined in the MCP revisions as being the transport routes of oil and hazardous materials to human receptors via: (i) vapor-phase emissions to homes, schools, and daycare/pre-school facilities, and (ii) ground water contaminant migration to drinking water wells located at and servicing homes, schools, and daycare/pre-school facilities. 

For 2- and 72-hour notification conditions, the MCP specifies that Immediate Response Actions (IRAs) be conducted. For most IRAs, the MCP presumes that containment or removal actions will be implemented, unless data are obtained to demonstrate that conditions do not pose an Imminent Hazard or that impacts are not likely to get substantially worse. The development of the CEP concept was in response to MADEP's desire to apply a more protective standard under certain conditions. At sites where conditions meet the definition of CEP, the elimination or mitigation of these transport routes may be required. 

MADEP developed CEPs to provide an added level of protection for schools, homes, and daycare/pre-school facilities. This added level of protection was considered appropriate due to: 1) variability in the level of exposures to such receptors that are difficult to predict or monitor, 2) the continuous and direct nature of such exposures to the most sensitive human receptors (children and pregnant women), and 3) concerns over uncertainties inherent in our understanding of the health effects of low-level exposures, especially to sensitive populations.

During recent training seminars, MADEP indicated that the presence of a CEP is not itself a notifiable condition and that the presence of a CEP will not require accelerated cleanup of the entire disposal site. The focus of IRA activities associated with a CEP will be to prevent, interrupt, eliminate, or mitigate targeted, specific pathways of concern, if feasible. In this context, feasibility considers technological and economic factors. Actions to address potential indoor air exposure scenarios include source removal/mitigation, installation of a vapor barrier, and the installation and operation of sub-slab ventilation systems. Actions to address potential drinking water exposure scenarios include source removal/mitigation, connection to the public system, or point-of-entry/use drinking water treatment systems.

CEP requirements apply only to active/ongoing IRAs taken to address a release that does or is likely to impact the indoor air or drinking water of a home, school, or daycare/pre-school facilities. Information obtained during the training seminars clarified that the requirement to eliminate or mitigate a CEP (to the extent feasible) is irrespective of risk (the concentrations of compounds detected does not impact the need to perform the evaluation and eliminate or mitigate the pathway). In addition, MADEP indicated that the sensitive receptors that are targeted by the regulation are young children, with young children being defined as high school aged and younger.


CEP issues are likely to be most prevalent for sites where releases of petroleum occur within close proximity to residential areas. The evaluation of such conditions may require accelerated characterization of the magnitude and extent of the release, or sampling focused upon evaluating the potential for VOCs to be present or migrate into indoor air. This issue will most likely be difficult to evaluate in situations where a plume of dissolved-phase VOCs has been identified in a residential setting, but the plume has yet to impact indoor air quality. The CEP assessment will require that Licensed Site Professionals (LSPs) evaluate whether VOCs are likely to be transported to sensitive receptors and result in measurable concentrations in the living space of a home. MADEP has not offered guidance on how to conduct this evaluation or the level of technical data that is appropriate to support an opinion.

Typical of previous revisions to the MCP, it is evident that the regulatory community has not yet sorted out the specifics associated with implementing the new CEP regulations. We expect that the regulations will be clarified and refined as they are applied by LSPs to sites. For those who are interested in exploring these issues further, we would be glad to provide copies of handouts and supporting information obtained from the MADEP training seminar. Feel free to contact GeoInsight with questions regarding CEPs or Paul Locke of MADEP's Bureau of Strategic Policy & Technology at paul.locke@state.ma.us.

Michael J. Webster
mjwebster@geoinc.com

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