EGLE-RRD considers the upper two feet of the vadose zone as surficial soil. Vadose zone soil below two feet is considered subsurface soil (refer to MIRBCA Section 5.1.2). Exposure pathways evaluated in surficial soil include: direct contact (ingestion and dermal contact routes of exposure), ambient air inhalation of volatiles, and ambient air inhalation of particulates. These pathways are evaluated in surficial soil but are not evaluated in subsurface soil. In Tier 1, these four routes of exposure are evaluated as 3 separate pathways. In Tier 2, these four routes of exposure are combined into a single direct contact pathway.
Other pathways, including VIAP, groundwater protection, and surface water protection, are evaluated in vadose zone soil, which consists of both surficial soil and subsurface soil.
No. Direct exposure to chemicals occurs through absorption through skin and incidental ingestion. Many nonpetroleum chemicals that are often direct contact concerns (e.g., arsenic, lead) i) exhibit no visual or olfactory indications of their presence, ii) often result from a release over a large area at the surface (e.g., application of pesticides), iii) are often present in unpaved soil at the surface, iv) are considered an unacceptable risk for direct contact at commonly observed concentrations, and v) do not biodegrade over time.
In contrast to this, gasoline-related chemicals from petroleum releases enter the environment from a point source that is below the surface, resulting in contamination in a limited spatial area below ground that is not accessible, most often at paved sites. The Tier 1 RBSLs for direct contact are not commonly exceeded for gasoline chemicals, with the exception of benzene. In order for there to be a potential direct contact concern, the chemicals would have to be present at such high concentrations that would exhibit obvious visual and olfactory indications and would result in nuisance conditions if left unpaved at the surface. Moreover, if gasoline compounds are at the surface they rapidly biodegrade in the presence of oxygen. Benzene, the gasoline chemical that is most likely to result in an unacceptable direct contact risk, would be the quickest to biodegrade.
Yes. The exposure domain for direct contact will generally be the same as the exposure domain for the ambient air inhalation pathways. In other words, the area of surficial soil to which a receptor could be exposed by direct contact (dermal absorption and incidental ingestion of soil) would be the same area of surficial soil that would contribute vapors and particulates to ambient air. The same representative concentration should be used for all surficial soil pathways in Tier 1. In Tier 2, dermal absorption, incidental ingestion of soil, inhalation of particulates, and inhalation of vapors is combined into a single direct contact pathway.
The exposure domain is the volume of soil within the upper two feet of the vadose zone that contributes to a receptor’s risk. For the surficial soil pathways, the exposure domain is the area of impacted surficial soil. The risk is evaluated in Tier 1 by comparing the recent maximum concentration within the exposure domain to Tier 1 RBSLs and in Tier 2 by comparing the recent average concentration within the exposure domain to the Tier 2 SSTLs.
No. The upper two feet of vadose zone soil is considered surficial soil, where the direct contact of petroleum-contaminated soils could reasonably occur. The representative concentration from the upper two feet of vadose zone soil must be compared to the RBTLs to determine if the risk is acceptable.
No. The direct contact pathway is not considered complete below depths of two feet, and the direct contact risk is thus not evaluated in soil below two feet in depth.
However, highway and road right-of-way (ROW) construction zones (which usually extend to 15 feet in depth) must be evaluated for impact, including soil with respect to Tier 1 residential direct contact RBSLs. This is primarily for awareness of the ROW owner to assist in the management of the impacted soil, rather than for evaluating chronic exposure to the soil. Refer to the MIRBCA guidance and FAQs on evaluating ROWs.
A restrictive covenant can be used to eliminate the surficial soil pathways for future use by requiring the maintenance of an exposure barrier, such as pavement. This may be necessary if there is either: i) impacted soil within the upper two feet of the vadose zone and the representative concentrations for the surficial soil pathways exceed the applicable RBTLs (Tier 1, 2 or 3); or ii) mobile NAPL is present in depths less than two feet. In these situations, the future risk is assumed to be unacceptable and must be addressed by either reducing concentrations and eliminating mobile NAPL, or by eliminating the pathway for future use.
Risk must be evaluated for current use and future use. If the area of impacted soil is paved, there is no current exposure, and the surficial soil pathways are not complete for current use. In the MIRBCA process, it is assumed that the pavement could be removed in the future unless there is a restrictive covenant requiring the maintenance of the pavement. Thus, even if the site is currently paved, the surficial soil pathways are considered complete for future use and must be evaluated by comparing the representative concentrations to applicable RBTLs.
No. Pathway evaluation language differs in Part 201 from Part 213. In Part 201, if a chemical can be transported from a source to a point of exposure where a receptor can be exposed to the chemical through a route of exposure, then the pathway is considered relevant and requires evaluation with applicable criteria. This is true for current use and future use. In Part 201, if a chemical can be transported from a source to a point of exposure where a receptor can be exposed to the chemical through a route of exposure, then the pathway is considered complete. This is true for current use and future use. In general, the term “relevant” should not be used for Part 213 evaluations, and the term “complete” should not be used for Part 201 evaluations. Note that due care evaluations adopt the commonly accepted pathway evaluation language of “complete or not complete”, regardless if the site is regulated under Part 201 or Part 213.
No. The ambient air inhalation pathways are evaluated through soil within the upper two feet of the vadose zone and not through subsurface soil (below two feet in depth). The VIAP can be evaluated through vadose zone soil (including surface soil and subsurface soil), groundwater, or soil gas. The Tier 1 soil RBSLs for VIAP are applicable for a current or future building if NAPL is not present within the exposure domain for that building. RRD recommends against using soil concentrations to evaluate the VIAP because soil concentrations are a poor indicator of indoor air concentrations and thus a poor indicator of VIAP risk.
Csat criteria should not be used in the MIRBCA process for petroleum releases. The applicable Tier 1 RBSLs for direct contact are listed in Tables 3 and 4 of the MIRBCA guidance document. When completing Form 16 series of the Tier 1 MIRBCA forms, for each pathway that is marked complete, a representative concentration must be entered for each chemical of concern that is evaluated, and when the representative concentration is entered, the RBSL field will be auto-populated with the applicable RBSL.
Mobile NAPL within the upper two feet is considered an unacceptable risk for direct contact. The unacceptable risk can be addressed by remediating the mobile NAPL or by a physical barrier, such as pavement. If pavement is relied upon to address future risk, a restrictive covenant that requires the maintenance of the pavement is necessary.
Mobile NAPL below two feet in depth is not evaluated for the surficial soil pathways, including direct contact. If residual NAPL is present in the upper two feet, the surficial soil pathways should be evaluated by comparing the representative concentration in the exposure domain to the applicable RBSL. Surficial soil pathways are not evaluated at depths greater than two feet except within road or highway rights-of-way.
The MIRBCA process requires the evaluation of exposure to petroleum chemicals to determine if risk for developing chronic adverse health effects is at or below acceptable levels. Direct contact exposure is reasonable for surficial soil and not for soil at depth. Direct contact risk is evaluated by determining the soil within the upper two feet that will contribute to risk and comparing the representative concentration to the applicable RBTL.
Post-remediation samples, including excavation verification samples, are useful for evaluating remediation performance goals, such as determining if remedial objectives have been met, as opposed to making risk-based decisions.
The surficial soil pathways are considered complete when there is impact related to the confirmed release within the upper two feet of unpaved vadose zone soil. There is no prescribed PID level to determine if soil is impacted or not, since PID is a field-screening tool, and since various factors (PID screening technique, gasoline vs. diesel, old vs. new release, etc.) can affect the PID level.
The surficial soil pathways should first be evaluated in Tier 1 by comparing the representative concentration within the exposure domain for current use and future use to the applicable Tier 1 RBSLs. If the result of the Tier 1 evaluation is that the risk is unacceptable for any of the surficial soil pathways for current or future use, either actions to address the unacceptable risk must be implemented or a Tier 2 evaluation can be conducted.
In Tier 2, all routes of exposure for surficial soil (dermal absorption, incidental ingestion of soil, inhalation of volatiles and particulates) are evaluated as a single pathway called direct contact.
MIRBCA is Michigan’s customization of the ASTM International (ASTM) Risk-Based Corrective Action (RBCA) process. MIRBCA refers to the technical guidance document, Tier 1 and Tier 2 report forms and instructions for completing the forms, and an optional computational software package to calculate site-specific target levels (SSTLs). These documents represent a technically defensible procedure for conducting corrective actions and submitting the required reports under Part 213 according to the ASTM RBCA process.
