No. Complete MIRBCA Form 12 series and refer to Section 5.3 of the MIRBCA guidance to determine if the pathway is complete, if current or future buildings vertically screen out or if they require evaluation. For each current and future building within the LIZ that does not vertically screen out, determine if VIAP Scenario 1, 2 or 3 applies to the building and select the applicable Tier 1 RBSLs and medium from MIRBCA Tables 3, 4, 5 and 6 with which risk will be evaluated.
No. The 2024 VIAP SLs for soil and groundwater should not be used. The 2024 VIAP SLs for soil gas are the same as the soil gas RBSLs in Tables 5 and 6 of the MIRBCA guidance document and are valid for Tier 1 evaluations. Complete MIRBCA Form 12 series and refer to Section 5.3 of the MIRBCA guidance to determine if the pathway is complete, if current or future buildings vertically screen out or if they require evaluation. For each current and future building within the LIZ that does not vertically screen out, determine if VIAP Scenario 1, 2 or 3 applies to the building and select the applicable Tier 1 RBSLs and medium from MIRBCA Tables 3, 4, 5 and 6 with which risk will be evaluated.
In most cases, no. Under Part 213 and the MIRBCA process, the O/O (and consultant) is responsible for conducting a RBCA evaluation, which includes calculating SSTLs. The first step is to evaluate or reevaluate the VIAP in Tier 1 and determine the VIAP Scenario and applicable Tier 1 RBSLs for each building that does not vertically screen out. In many cases, the buildings can be evaluated in Tier 1. If the representative concentration exceeds the applicable Tier 1 RBSL, a Tier 2 evaluation can be conducted.
A Tier 1 evaluation of the VIAP uses conservative assumptions for future use that a future building can be constructed anywhere on the property and would include an 8-foot-deep basement. The basement would have a sump if the depth to groundwater is less than 8 feet. In Tier 1, the recent maximum concentration within the exposure domain for each current and future building is used to make the risk decision. In Tier 2, a future building location and construction can be based on site-specific information. The representative concentration in Tier 2 is the recent average rather than the recent maximum. This average is both spatial and temporal for soil gas and groundwater. In Tier 1, the RBSLs are the values in Tables 3, 4, 5, or 6, as applicable to the site. In Tier 2, SSTLs are calculated using the equations in MIRBCA Appendix A and the parameters in Tables 7 through 12, along with site-specific inputs.
In a Tier 3 evaluation of the VIAP, a consultant or O/O can use any technically defensible approach to evaluate risk. This may be necessary when the fate and transport of vapors entering a building can't be modeled by the J&E model or the sump model, or if a party chooses to use an alternative fate and transport model, such as incorporating biodegradation, to depict vapors entering a building.
First, determine how vapors enter a current or future building (refer to Section 5.3 of MIRBCA). For Scenario 1, the VIAP can be evaluated in Tier 1 using the soil or groundwater RBSLs in Table 3 and 4 or the soil gas RBSLs in Table 5 and 6. For Scenario 2, the VIAP should be evaluated with the soil gas RBSLs in Table 5 and 6. For Scenario 3, the VIAP should be evaluated with the groundwater RBSLs in Table 5 and 6.
Not necessarily. The Tier 1 RBSLs for VIAP are used in a Tier 1 evaluation to determine if the current risk and the future risk are acceptable. Each current building within the lateral inclusion zone must be evaluated to determine if the current risk is acceptable. This is done by comparing the representative concentration within the building footprint to the Tier 1 RBSLs that are applicable for that particular building. The evaluation can be accomplished in any medium (soil gas, groundwater, or soil), but soil gas is preferred, and soil is not recommended. A future building must also be evaluated to determine if the future risk is acceptable. In Tier 1, the assumption is that a future building has an 8-foot deep basement and is located over the area that would result in the greatest risk. If the average depth to groundwater in this area is less than eight feet, the building would have a sump. In Tier 2, the future building construction and location can be determined based on a site-specific evaluation. Also, Tier 2 SSTLs rather than Tier 1 RBSLs can be selected as the corrective action goals that must be met at a site.
Thus, this pathway is not evaluated by comparing all soil, groundwater, and soil gas concentrations to RBSLs, but rather, by identifying all current buildings and a future building within the lateral inclusion zone, and determining if the risk is acceptable for each building by comparing the representative concentrations within each building footprint to the applicable RBSLs in the medium used to evaluate risk.
Yes. The Part 201 Rules require three meters depth to groundwater for the RBSLs to be applicable; however, the Part 201 Rules do not apply to Part 213. In order for the J&E model to be applicable, the fate and transport of vapors at the site must occur by diffusion through the vadose zone and advection into cracks in the building. This can happen with any amount of separation between the water table and the building floor. The Tier 1 RBSLs were calculated based on a distance of two feet between the water table and the building floor. These Tier 1 RBSLs can be used when the distance to groundwater is between two and five feet from the building floor (if groundwater is greater than five feet the building vertically screens out and does not require quantitative risk evaluation). If the distance between the building floor and the groundwater is less than two feet but the means by which vapors enter a building is advection through cracks, then the J&E model can be used to calculate SSTLs in Tier 2 with site-specific information.
The Johnson and Ettinger (J&E) model is a fate and transport model that calculates a soil gas RBSL based on an acceptable indoor air concentration. The model depicts vapor transport through the vadose zone by diffusion and vapor transport into a building by advection. The model assumes a perimeter crack around the edge of the building floor. The soil gas RBSLs that are calculated by the J&E model can be converted to groundwater or soil RBSLs through a 3-phase equilibrium conversation that assumes the presence of an adsorbed phase, a dissolved phase, and a gaseous phase in equilibrium with each other. Free phase (NAPL) is absent in the equilibrium conversion model. Therefore, if NAPL is present within the exposure domain of a current or future building, the VIAP risk for the building cannot be evaluated using soil or groundwater RBSLs because the equilibrium conversion equations are not valid when NAPL is present.
The sump model is used to predict the indoor air concentration in a building when vapors enter by direct volatilization from water in a sump and foundation drain. Essentially, it is assumed that the house is placed in the plume of contaminated groundwater and the foundation drain and sump are dewatering the foundation to keep it dry. The sump model should be used in Tier 1 when current buildings with sumps are potentially in contact with contaminated groundwater or to evaluate future buildings when the depth to groundwater is less than eight feet. In Tier 2, the sump model should be used when it is reasonable to assume that a future building would have a sump and foundation drain that could be in contact with contaminated groundwater. Tier 1 groundwater RBSLs calculated using the sump model are presented in Table 5 and 6 of the MIRBCA guidance. The equations for calculating Tier 2 SSTLs based on the sump model are presented in Appendix A. Sump model RBSLs and SSTLs can be applied to water samples from a sump or to groundwater samples.
If NAPL is present within the exposure domain of a current building or a future building, the building should be evaluated based on soil gas concentrations, if possible. The existing soil and groundwater Tier 1 RBSLs for VIAP are back-calculated from soil gas RBSLs by assuming equilibrium conversion in a three-phase system (dissolved phase, adsorbed phase, and gaseous phase). Free-phase liquid (i.e., NAPL), is assumed to be absent and is not accounted for in the equilibrium conversion equations. Thus, the Tier 1 soil and groundwater RBSLs for VIAP are not valid within an exposure domain if NAPL is present in the exposure domain. Tier 2 VIAP SSTLs for soil and groundwater are also not valid because of the same rationale.
Any conclusions or audit decisions about the VIAP should be reevaluated using the MIRBCA guidance. First, determine if there is a vapor source and apply a lateral inclusion zone. Determine if current and future buildings within the LIZ vertically screen out. For each building that does not vertically screen out, determine how vapors would enter the building (VIAP Scenario 1, 2, or 3) and select the applicable Tier 1 RBSLs. Select the representative concentration for each exposure domain and determine if the risk is acceptable under a Tier 1 evaluation.
Delineation is part of site assessment (not risk assessment) and should be to the Delineation Criteria listed in Table 2 of the MIRBCA guidance document.
As long as the system is needed to address an unacceptable risk related to the LUST release, the release cannot close under Part 213. To determine if the system is needed, the release should be evaluated under the MIRBCA process. For example, if a mitigation system was presumptively installed to address the VIAP risk due to soil or groundwater concentrations exceeding the 2024 VIAP Screening Levels or other EGLE-generated SSTLs for the VIAP, the site should be reevaluated using the MIRBCA process (refer to MIRBCA Section 5.3) and the Tier 1 RBSLs listed in Tables 3-6 of the MIRBCA guidance document.
Each current and future building within the LIZ should be evaluated to determine if it vertically screens out. If the future building vertically screens out on a property, then no RC is required for that property. If the future building does not screen out but the representative concentrations within the exposure domain are below applicable RBSLs, then no RC is required. If the representative concentrations within the exposure domain for a future building exceed Tier 1 RBSLs, options to address it include: an RC to eliminate the pathway, remediation to meet Tier 1 RBSLs, or a Tier 2 evaluation. If the representative concentrations within the exposure domain for a future building meets Tier 2 RBSLs, an RC is not required.
A restrictive covenant can be used to address unacceptable risk in several different ways. First, if risk for future use is being evaluated using nonresidential RBTLs, either a restrictive covenant that restricts the land use to nonresidential or a Notice of Corrective Action recorded on the property is required by Part 213 to ensure the future land use remains nonresidential. Second, a restrictive covenant may be used to eliminate future exposure by either eliminating the pathway or by requiring a VIAP evaluation prior to any new construction. This can be used when a future building does not vertically screen out and the representative concentrations within an exposure domain exceed the applicable RBTLs, which could be Tier 1 RBSLs or Tier 2 or 3 SSTLs, depending on the type of evaluation conducted. The evaluation need not be conducted in all impacted media. Third, if a mitigation system is needed to address an unacceptable risk, a restrictive covenant can be used to ensure the continued operation of the mitigation system, however, Part 213 does not allow a release to be closed is ongoing operation of a mitigation system is needed to address an unacceptable risk.
Yes. In most scenarios, changes in property, such as adding or removing a building, will have little if any effect on soil gas concentrations.
Indoor air should be evaluated if there is potential for the indoor air of a building to be impacted. Examples of when indoor air should be evaluated include: NAPL or contaminated groundwater is present inside a building; subslab soil gas concentrations exceed applicable RBSLs; NAPL is in contact with a building; petroleum odors are observed in a building. It is recommended to contact the Technical Support Section for assistance.
Part 213 is a risk-based program and closure can be achieved when an O/O demonstrates that the current and future risk is at or below acceptable levels. The Tier 1 RBSLs and any calculated Tier 2 or 3 SSTLs are a means to demonstrate that the risk is acceptable. These RBSLs and SSTLs are based on exposure concentrations in indoor air. Direct exposure RBTLs apply at the point of exposure (POE), whereas indirect exposure RBTLs apply at the point of compliance and/or source area as an indirect means to evaluate the POE concentrations. Sampling the actual POE provides the most direct line of evidence to determine the risk (i.e., to determine the receptor’s chance of developing an adverse health effect). While EGLE-RRD does not regulate all potential sources of indoor air contamination, the O/O must demonstrate that the risk for the VIAP is acceptable. Whether evaluated directly or indirectly, the VIAP risk is based on exposure to chemicals in indoor air that have volatilized from the subsurface from a LUST release.
Yes, the Tier 1 RBSLs for VIAP are conservative. Petroleum vapors rapidly biodegrade in the presence of oxygen. Vapor intrusion from petroleum generally happens when NAPL is in contact or close to a building, when mobile NAPL enters a utility that can transport it to a building, or when dissolved groundwater or NAPL enters a building (e.g. through a sump). Petroleum vapor intrusion rarely occurs from a dissolved or soil source outside of these conditions. The Tier 1 RBSLs are reflective of this.
Each current building partially or completely within the LIZ must be evaluated. If a building vertically screens out, no additional evaluation is required. Each current and future building that does not vertically screen out requires a quantitative risk evaluation.
In Tier 1, a future building is assumed on each property within the lateral inclusion zone. In Tier 1, the location of the future building is over the area on each property that would result in the greatest risk. In Tier 1, the building is assumed to have an 8-foot-deep basement, and the basement is assumed to have a sump if the average depth to groundwater in the area of the building is less than eight feet. In Tier 2, the future building location and construction can be determined based on site-specific information. For example, local setback requirements can be used to limit areas where a future building can be constructed.
An exposure domain is the area or volume of media that contributes to risk. For the VIAP, exposure occurs by a receptor breathing impacted air within a building. The exposure domain is the volume of soil, groundwater, NAPL, or soil gas that contributes to the indoor air of a building. It is generally considered the media below the building footprint. This area can be expanded as needed if samples are not available within the building footprint. Refer to the MIRBCA guidance document for details.
Regardless of the vapor source (e.g., NAPL, groundwater), a 30-foot LIZ should be used if the extent of contamination is estimated (e.g., interpolated between two points or extrapolated beyond a point). The 30-foot LIZ would be drawn outward from the estimated extent of contamination.
A 5-foot LIZ should be used with a groundwater or soil source. The 5-foot LIZ would be drawn outward from the extent of the vapor source that is drawn from points below the applicable Tier 1 RBSLs (refer to Table 3, 4, 5, and 6) and not from an interpolated or extrapolated extent of the vapor source.
A 15-foot LIZ should be used with a mobile or residual NAPL source. The 15-foot LIZ would be drawn outward from the extent of the NAPL vapor source that is drawn from points where NAPL is demonstrated to be absent based on multiple lines of evidence (refer to the MLE table in the NAPL guidance document) and not from an interpolated or extrapolated extent of the NAPL vapor source.
It is recommended to use the most conservative applicable Tier 1 residential RBSLs to determine the extent of the vapor source. Different RBTLs may be applicable for different buildings within the lateral inclusion zone for current and future use depending on the VIAP Scenario for each building (refer to MIRBCA Section 5.3). For example, a current building may be considered VIAP Scenario 1 (Groundwater (J&E) RBSLs are applicable) but a future building may be considered VIAP Scenario 3 (Groundwater (Sump) RBSLs are applicable). In this situation, the vapor source should be defined by the Groundwater (Sump) RBTLs, since they are the more conservative applicable RBSLs. However, within the lateral inclusion zone, the risk evaluation must be on a current and future building-specific basis. Thus, even though the vapor source would be determined by the Groundwater (Sump) RBSLs, these RBSLs may not be applicable to all buildings within the lateral inclusion zone.
A unit conversion error was recently discovered by RRD’s Toxicology Unit. The soil gas RBSLs for sec-butylbenzene and tert-butylbenzene have been revised in Tables 5 and 6 of the MIRBCA guidance. These compounds will likely not be risk-drivers for the VIAP.
The VIAP is complete for current use if there is a vapor source of NAPL, groundwater, or soil, and there are existing buildings partially or completely within the lateral inclusion zone. If there is either no vapor source or there are no existing buildings partially or completely within the lateral inclusion zone, then the VIAP is not complete for current use. Both onsite and offsite properties must be considered.
The VIAP is complete for future use if there is a vapor source and a building could be constructed partially or completely within the lateral inclusion zone. In Tier 1, a future building would be located over the area on each impacted property within the lateral inclusion zone that results in the greatest risk. In Tier 2, the future building would be in a location based on site-specific conditions.
No. The MIRBCA process requires a demonstration that the risk is acceptable. Exposure in this pathway occurs by receptors breathing indoor air in buildings. The VIAP evaluation is to demonstrate that indoor air concentrations in current and reasonable future buildings are below levels that would result in an unacceptable risk. Soil gas is a better predictor of indoor air concentrations than groundwater or soil because it is the medium by which receptors are exposed to chemicals. Groundwater and soil RBSLs are derived from soil gas RBSLs and involve additional equations and assumptions to calculate the RBSLs.
In some cases (e.g., VIAP Scenario 3), it may not be possible to evaluate the indoor air inhalation risk using soil gas samples. Groundwater samples can be used to evaluate the indoor air inhalation risk. If conducting the risk evaluation using groundwater samples, it must first be determined how vapors enter a current or future building (i.e., Scenario 1 or Scenario 3) to determine the applicable groundwater RBSLs (Table 3 and 4, or Table 5 and 6).
The risk for current use should be evaluated based on actual site conditions. If a building is a convenience store at an operating gas station, for example, the current use should be evaluated using nonresidential RBTLs. Current use for a house on a residential property should be evaluated using residential RBTLs.
The risk for future use must be evaluated using residential RBTLs regardless of the zoning, reasonably anticipated future use, Tier 1, 2 or 3 evaluation. Nonresidential RBTLs for future use can be used only for properties that have a restrictive covenant that restricts the future land use to nonresidential.
Soil gas sampling is generally the preferred means to evaluate current and future buildings that are within vertical screening distances from a vapor source. If there is sufficient vertical separation distance (i.e., less than five feet from a groundwater or soil source, less than fifteen feet from a NAPL source) for a current or future building, that building does not require quantitative risk evaluation by comparing representative concentrations to RBTLs. The property would not require a restriction if a future building would vertically screen out.
If a current or future building is within vertical separation distances and requires quantitative risk evaluation, refer to the Guidance Document for the Volatilization to the Indoor Air Pathway (VIAP) – Volume 4 – Investigative Approach for Petroleum Volatilization to the Indoor Air Pathway (VIAP). There is no required number of sampling events prior to a FAR. The sampling recommendations listed in the PVI guidance document are to demonstrate that the risk is acceptable for a current or future building, i.e. for a Closure Report.
The average depth to groundwater near a building should be used as it is most representative of long-term, chronic exposure. The average depth of groundwater must be determined by each building (current and future) that is evaluated to determine the vertical separation distance and the application of Tier 1 RBSLs.
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.
