The groundwater protection pathway is complete for current use when there are water supply wells that are potentially impacted from the confirmed release.  This is true for sites in municipal areas, for rural areas, and areas where groundwater is not in an aquifer.  A well survey is necessary to determine if there are any wells in the area.  Any potentially impacted well must be evaluated.  The level of effort in conducting the well survey is site-specific and depends on the likelihood of impacted water supply wells being present.

The groundwater protection pathway is complete for future use at most sites, regardless of whether it is complete for current use, whether the area is on municipal water, or whether the area properties are required by an ordinance to connect to municipal water.  With limited exceptions, the pathway is complete for future use if groundwater is in an aquifer.  Refer to MIRBCA Forms 13(2) and 13(3) and MIRBCA Appendix B for conditions to conclude that the groundwater protection pathway is not complete for future use.  In Tier 1, the future use risk is evaluated by assuming that exposure would occur from a future well installed at any point in the affected aquifer onsite and on impacted offsite properties.  In Tier 2, the future risk is evaluated at the source area and points of compliance based on a site-specific point of exposure.

A restrictive covenant or other institutional control can be used for future use to i) restrict a property’s land use to nonresidential to allow for comparison with nonresidential RBTLs for future use, or ii) eliminate the future exposure in an area.  If the future risk is unacceptable, the future risk must be addressed by either reducing the concentrations or by eliminating the pathway.  The future risk is unacceptable in a Tier 1 evaluation when the recent maximum groundwater concentrations on a property are above Tier 1 RBSLs.  The future risk is unacceptable in a Tier 2 evaluation when the recent average concentrations at specific points are above Tier 2 SSTLs.  The difference between a Tier 1 and Tier 2 evaluation is that a Tier 1 evaluation assumes that a future well could be installed at any point in the affected aquifer, and therefore risk is evaluated by the maximum concentration on each impacted property.  In Tier 2, however, a site-specific point of exposure is determined for future use.  The risk is evaluated by comparing representative concentrations at locations that would contribute to the future risk (e.g., source area, points of compliance located between the source area and the point of exposure).  If the representative concentrations in the source area and points of compliance are below SSTLs for these locations, then the future risk (at the point of exposure) is acceptable and a restrictive covenant or other alternative mechanism is not needed, even though concentrations may be above Tier 1 RBSLs.  If representative concentrations exceed applicable Tier 2 SSTLs, this indicates that the future risk is not acceptable and additional evaluation or corrective action is necessary.

Determining a future use point of exposure (POE) for the groundwater protection exposure pathway is discussed in MIRBCA Section 5.4.  In Scenario 1, there are no current or reasonable future water supply wells within 300 feet of the site property boundary.  The POE in Scenario 1 is 300 feet downgradient from the edge of the source area.  In Scenario 2, there are no current or reasonable future water supply wells on the site property, but there is either a current or reasonable future water supply well within 300 feet of the site property boundary.  The POE in Scenario 2 is at the nearest downgradient property boundary, excluding rights-of-way or restricted areas.  In Scenario 3, there is a current water supply well or a reasonable future well on the site property.  The POE for Scenario 3 is any point in the affected property onsite and on all impacted offsite properties.  The applicable RBTLs at the POE for Tier 1 and Tier 2 are given in Table 13 of the MIRBCA guidance document.  Tier 2 SSTLs for the source area and points of compliance must be calculated for each site.

In a Tier 2 evaluation, a point of exposure for future use is determined based on site-specific information.  The future risk is evaluated based on exposure occurring at the POE and not at other locations.  Although a site-specific POE is established for future use, this point is usually not directly evaluated.  Certain locations, including the source area and points of compliance located between the source and the POE, are selected as points where future risk will be evaluated.  These points are located along the transport pathway and are used to predict the concentrations at the POE and thus evaluate future risk at the POE.  Concentrations at other locations are not considered because these locations are not along the transport pathway, and therefore concentrations at these locations are not useful in evaluating future risk.

Regardless of the presence of an ordinance, the groundwater protection pathway must be evaluated for current and future use.  A well survey must be conducted to determine existing water supply wells.  If there are potentially impacted wells, the groundwater protection pathway is complete for current use and the wells must be evaluated to determine if the current risk is acceptable.  In a Tier 1 evaluation, it is assumed that a future well could be installed at any point in the affected aquifer, regardless of an existing ordinance.  A restrictive covenant or an alternative mechanism, including a groundwater use ordinance, can be used to eliminate the pathway for future use.  This is usually implemented in a Tier 1 evaluation.  In a Tier 2 evaluation, a site-specific location for a future well is established.  This site-specific point is based on multiple lines of evidence, one of which could be the presence of an ordinance.  In a Tier 2 evaluation, representative concentrations that exceed SSTLs at the source area or points of compliance indicate that the future risk is not acceptable and additional evaluation or corrective actions are necessary.

When relying on an ordinance as an alternative mechanism to eliminate the groundwater protection pathway for future use, the ordinance must comply with Section 10a of Part 213, including but not limited to: the ordinance must include a requirement for the local unit of government to notify EGLE within 30 days of any change to the ordinance; the ordinance must be recorded on properties affected by the ordinance or recorded as an instrument that affects multiple properties.  Other requirements to ensure that the ordinance is reliable, enforceable, and protective include a purpose statement that the ordinance is prohibiting groundwater use to prevent exposure to environmental contamination, a reference to the existing release, and an identified restricted area that is connected to the release.  Refer to EGLE’s Institutional Controls Technical Assistance and Program Support (TAPS) team for assistance. 

The groundwater protection pathway is considered complete for future use when groundwater is in an aquifer.  Regardless of the presence of municipal water, the groundwater protection pathway must be evaluated for current and future use.  A well survey must be conducted to determine existing water supply wells.  If there are potentially impacted wells, the groundwater protection pathway is complete for current use and the wells must be evaluated to determine if the current risk is acceptable.  In a Tier 1 evaluation, it is assumed that a future well could be installed at any point in the affected aquifer, regardless of an existing municipal water system.  A restrictive covenant or an alternative mechanism can be used to eliminate the pathway for future use.  This is usually implemented in a Tier 1 evaluation.  In a Tier 2 evaluation, a site-specific location for a future well is established.  This site-specific point is based on multiple lines of evidence, one of which could be the presence of a municipal water system.  In a Tier 2 evaluation, representative concentrations that exceed SSTLs at the source area or points of compliance indicate that the future risk is not acceptable and additional evaluation or corrective actions are necessary.

The risk for both current use and future use must be at acceptable levels.  A well survey must be conducted to identify any water supply wells.  Any potentially impacted wells must be evaluated to determine if the risk for current use is acceptable.  Assuming that current risk is acceptable, there are several options to evaluate and address future risk.  If the results of a Tier 1 evaluation determine that the future risk is unacceptable, options to address it include: i) monitoring or remediation until the representative concentrations meets applicable Tier 1 RBSLs, ii) eliminating the pathway for future use by restricting the applicable area with either a restrictive covenant or alternative mechanism (health department alternative mechanism, relying on an ordinance as an alternative mechanism, etc.), or iii) conducting a Tier 2 or 3 evaluation.

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.

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 can be 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.