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Volume 4: Investigative approach for petroleum VIAP
The VIAP may pose unacceptable risks as immediate threats to safety (e.g., fire or explosion potential from petroleum vapors or methane) and adverse health effects from inhalation of hazardous substances that represent short-term or acute (i.e., less than chronic) exposure concerns. While rare, the potential for these to occur is greatest after a new release but should be evaluated initially in the investigative process with old releases for structures within the LIZ to verify that the conditions do not exist. The evaluation is described below and in ASTM E2993-23 Standard Guide for Evaluating Potential Hazard in Buildings as a Result of Methane in the Vadose Zone.
The risk from fire and explosion associated with petroleum is greatest shortly after the release has occurred and when mobile NAPL has been found to be in contact or has entered a structure. These risks should be assessed after a new petroleum release has been discovered, when the NAPL vapor source is either in contact or has entered a current structure, or vapors are found to be either migrating to or into a structure.
The assessment from fire and explosion is done using an appropriately calibrated field meter (e.g., four gas meter) until either the vapor source has been addressed through a response action, or there is sufficient information collected that affirms that the petroleum release does not pose a potential fire or explosion risk. Groundwater and soil gas data may also be used. The monitoring frequency of the explosive conditions should be established on a case-by-case basis and be based on the site conditions present and the potential that concentrations exceed the lower explosive limit (LEL).
Table A-1 provides example LEL readings and concentrations for commonly identified compounds in gasoline. Additional LELs for other hazardous substances are available in R 299.50 Toxicological and Chemical-Physical Properties under Part 201. When concentrations in indoor air or soil gas are found near a structure at a site that are approximately 10% of the LEL, constant monitoring and ongoing assessment of the potential of fire and explosion should begin. The installation of calibrated gas leak detectors and alarms may be appropriate in situations that require frequent monitoring and can reduce the need to complete constant physical monitoring.
| Hazardous Substance | LEL % | LEL (ppm) | 10% LEL (ppm) | LEL (µg/m3) | 10% LEL (µg/m3) |
|---|---|---|---|---|---|
| Gasoline | 1.4 | 14,000 | 1,400 | 4.1 E+07 | 4.1 E+06 |
| Benzene | 1.2 | 12,000 | 1,200 | 3.8 E+07 | 3.8 E+06 |
| Ethanol | 3.3 | 33,000 | 3,300 | 6.2 E+07 | 6.2 E+06 |
| Ethyl Benzene | 0.8 | 8,000 | 800 | 3.5 E+07 | 3.5 E+06 |
| Hexane | 1.1 | 11,000 | 1,100 | 3.9 E+07 | 3.9 E+06 |
| Toluene | 1.1 | 11,000 | 1,100 | 4.1 E+07 | 4.1 E+06 |
Note: Additional hazardous substances are available in R 299.50
Methane is commonly associated with the anaerobic biodegradation of petroleum or other organic material and is likely to be found on a facility where a petroleum release has occurred. However, methane will aerobically degrade, and the presence of methane alone may not indicate a risk from fire and explosion. When methane is present, an evaluation of the degradation, mass, concentration, and pressure should be completed as part of the analysis to determine the potential need for immediate interim response actions. If concentrations of approximately 10% of the LEL are identified near a structure, constant monitoring, and ongoing assessment of the potential of fire and explosion should begin to ensure no unacceptable risk occurs in the future.
Because of rapid biodegradation, potential exposure to petroleum hazardous substances that represent short term or acute adverse health effects (not associated to fire and explosion) for petroleum is very rare. In addition, the screening levels, and criteria for the majority of the hazardous substances associated with petroleum releases are based on 30-year (chronic) exposures.
In 2016, the Toxics Steering Group (TSG) VIAP Workgroup was tasked by EGLE and the Michigan Department of Health and Human Services (MDHHS) to evaluate and develop indoor air screening levels for volatile substances that are protective against human health effects that may result from ongoing VIAP exposures and was revised in 2020. Specific to petroleum releases, EGLE’s Toxics Steering Group (TSG) found that toluene, n-propylbenzene, diisopropyl ether, and ethanol represent short-term or acute exposure risks at concentrations less than levels that present chronic risks (EGLE 2020). In addition, the TSG report also identified concentrations for which exceedances of benzene, ethylbenzene, n-hexane, trimethylbenzenes (inclusive of all three isomers), and/or xylenes pose short-term or acute exposure risk concerns at concentrations above chronic exposure risks. The TSG report (EGLE 2020) includes recommended interim action screening levels (RIASLs) and time sensitive RIASLs (TS RIASLs) for indoor air. The TS RIASLs represent elevated concentrations that pose short-term or acute exposure concerns even for those hazardous substances that are typically considered a chronic exposure concern. Based on those findings, EGLE developed media-specific volatilization to indoor air interim action screening levels (MSSLs) and time-sensitive MSSLs (TS MSSLs) for soil, shallow groundwater, groundwater not in contact, and soil gas and can be found in Appendix D.3 of the May 2013 Guidance Document for the Vapor Intrusion Pathway and will be placed in Volume 6 – Volatilization to the Indoor Air Criteria upon completion, As discussed in the document, soil that does not contain NAPL will not be used for risk evaluations at petroleum release sites and groundwater above applicable TS MSSLs do not represent an acute risk but rather require further evaluation if they do not screen out with vertical separation distances.
The following scenarios are site conditions when short term or acute health effects are possible and need to be evaluated:
If any of these situations have been identified, it is critical to immediately assess the risks and identify if there are response actions that must be immediately implemented to abate the unacceptable risks. The response actions should identify the vapor source causing the potential short term and acute health effects and prevent vapors from continuing to be within the structure.
NOTE: Indoor air Recommended Interim Action Screening Levels (RIASLs) and Time Sensitive Recommended Interim Action Screening Levels (TS-RIASLs) are identified in the TSG Report (EGLE 2020) that may be used as part of a line of evidence for the potential VIAP risks. Compliance decisions and long-term protectiveness for VIAP must be based on applicable VIAC.
Prior to implementing interim response actions resulting from exceedances of applicable shallow groundwater or groundwater not in contact (GWNIC) VIAC or TS MSSLs, representative soil gas sampling or sub-slab soil gas sampling is recommended and may also include indoor air sampling. If an evaluation is made in accordance with Rule 299.14(5) using more representative data such as soil gas, the soil gas data can be used to show compliance with the groundwater VIAC. If indoor air samples are collected, they should be paired with a representative number of sub-slab samples or a conduit vapor sample (when appropriate). Though indoor air samples alone in Michigan do not allow for closure, it does provide a line of evidence and allows for an evaluation of the potential exposure at the time of sampling and/or the effectiveness of any implemented corrective or response actions. See Attachment C.4 for the number of sampling locations, number of sampling rounds, and sampling frequency.
