Volume 5: Response activity for the VIAP
2. Timeframes for completing response activities for the VIAP

Appropriate timeframes for completing response activities, which include evaluation, will vary depending on current site conditions, hazardous substances released and the relative toxicity, and hazardous substance location relative to receptors. In some cases, immediate response activities may be warranted, such as when the hazardous substance is present in the indoor air. In other cases, such as when the hazardous substance is only measured in groundwater or soil gas, further evaluation may be warranted before completion of response activities.

When it has been determined that the risks to human health from the VIAP are unacceptable, the investigator must determine the appropriate timeframe for completing response activities. To aid in that determination, EGLE, in collaboration with the Michigan Department of Health and Human Services (DHHS), has developed Recommended Interim Action Indoor Air Screening Levels (RIASLs) and Time Sensitive Recommended Interim Action Indoor Air Screening Levels (TS-RIASLs) to assist in determining how quickly response activities should be implemented when concentrations of hazardous substances are detected in indoor air. EGLE has adopted RIASLs as acceptable air concentrations (AACs) and site-specific volatilization to indoor air criteria and VIAP screening levels developed by EGLE are based on AACs with fate, transport, and/or partition modeling to calculate criteria and screening levels for various media. If indoor air data exceeds AACs, mitigation of current inhalation exposures should begin as soon as possible (e.g. weeks to months) and if indoor air data exceeds TS-RIASLs, mitigation of current inhalation exposures via rapid response and ventilation (see Section 3.1) should begin immediately (e.g. day to weeks). After the current inhalation exposures are mitigated, the ongoing exposures or potential exposures should be mitigated in accordance with Table 1.

Note: RIASLs and TS-RIASLs are not meant to define protective levels for all potential indoor air exposures and are not de facto cleanup levels or criteria (EGLE, 2020). 

Generally, the exceedance of a criteria or screening level in soil or groundwater does not indicate that an exposure has occurred or is occurring, but rather it suggests that a sufficient mass of a hazardous substance is present in the media that could result in soil gas concentrations above the soil gas screening levels. Soil gas samples and sub-slab soil gas samples also do not indicate that an exposure has occurred or is occurring, but it does indicate that there is a potential for exposure to occur and the greater the concentration, the greater the potential for exposure. For hazardous substances to create an unacceptable VIAP risk, concentrations of a hazardous substance must be present in the indoor air above an AAC. When a TS-RIASL exceedance is found in indoor air, the immediate priority should be the safety of the occupants and conducting response activity without delay. An exceedance of a TS-RIASL in indoor air will necessitate rapid response and ventilation (see Section 3.1), which may include evacuating building occupants in coordination with the local health department and DHHS.

The response activities and timeframes needed when site data exceeds criteria or screening levels will be based on the media being sampled and the proximity to a receptor. Table 1 is a general outline of EGLE’s expectations for response activities. Additional information on AACs, VIAP Screening Levels, TS-RIASLs, and TS-MSSLs is available in Volume 6 – Volatilization to Indoor Air Criteria.

Table 1. General outline of EGLE’s response activity expectations