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Volume 4: Investigative approach for petroleum VIAP

Attachment A - Addressing acute vapor risks

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 shot-term or acute (i.e., less than chronic) exposure concerns. The potential for these to occur should be evaluated throughout the investigative process for structures within the LIZ and are further described below.

Fire and Explosion

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 groundwater or 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 vapor data may also be used. The monitoring frequency of the explosive conditions should be established on a case-by-case basis and should 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 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 vapor 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.

Table A-1
Lower Explosive Limit (LEL %) at 25o Celsius of Commonly Detected
Hazardous Substances in Gasoline

Hazardous Substance LEL % LEL (ppm) 10% LOL (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 3.9 E+07 3.9 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.

Short Term and Acute Health Effects

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 limited. 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.

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. In response to the TSG report (EGLE 2020), EGLE developed the Appendix D.3 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 vapor.

The following scenarios are site conditions when short term or acute health effects are possible and need to be evaluated:

  • NAPL is less than 5-feet from a structure,
  • Groundwater or soil vapor above Appendix D.3 (EGLE 2020) MSSLs, TS MSSLs, and/or applicable VIAC,
  • Petroleum odors have been identified within a structure due to the direct volatilization to the indoor air, or
  • NAPL or a source of petroleum dissolved groundwater has entered an underground conduit that can transport vapor directly to a structure.

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.

Interim response actions resulting from exceedances of shallow groundwater, groundwater not in contact, and/or soil vapor MSSLs and/or TS MSSLs should be completed consistent with the Appendix D.3 guidance (EGLE 2020). Specifically, interim response actions should be completed as soon as possible when concentrations exceed MSSLs and interim response actions should be completed immediately when concentrations exceed TS MSSLs.

Interim response actions resulting from exceedances of applicable shallow groundwater or groundwater not in contact (GWNIC) VIAC, MSSLs, and/or TS MSSLs should include representative soil gas sampling. Interim response actions resulting from exceedances of applicable soil vapor VIAC, MSSLs, and/or TS MSSLs should include indoor air sampling. Indoor air samples should be paired with a representative number of sub-slab samples or a conduit vapor sample (when appropriate). Though indoor air samples 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 for the number of sampling locations, number of sampling rounds, and sampling frequency.

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