State and Regional Vapor Intrusion Sampling Guidance: Differences in Sampling Frequency, Timing, and Decision-Making

Abstract Description: Diverse methods are used across the country for vapor intrusion (VI) site assessment and risk management decision-making. The U.S. Environmental Protection Agency (USEPA) 2015 VI guidance defines key concepts related to VI assessment, such as reasonable maximum exposure (RME) and concordance between multiple lines of evidence. In addition, indicators, tracers, and surrogates (ITS) have been proposed as an approach to better time sampling and reduce the number of samples required to describe RME indoor air concentrations. While USEPA’s VI guidance is not prescriptive and, therefore, open to innovative strategies, many state and regional guidance documents offer less flexibility.
The current state of practice for soil gas sampling in the United States was evaluated by reviewing VI guidance documents published by state regulatory agencies (most states have guidance either in standalone form or as part of cleanup program guidance) and national-level guidance published by U.S. federal agencies or departments, such as the USEPA (or USEPA regions), Interstate Technology and Regulatory Council, Agency for Toxic Substances and Disease Registry (ASTDR), and Department of Defense (DoD). Information on regulatory requirements for each guidance documents was summarized as follows:
• Expected number of indoor air and/or soil gas sampling events and timing during the calendar year;
• Sampling guidelines and recommendations as appropriate (e.g., differential temperature guidelines, references to radon and ITS);
• Information related to heating, ventilation, and air conditioning (HVAC) operation and related recommendations during sampling, if any; and
• Risk management criteria (e.g., indoor air/soil gas decision matrix approach, soil gas decision-based only, indoor air decision-based only, risk levels, variations in exposure duration or toxicity).
Three key issues were identified:
1) Reliance by many states on soil gas data as the primary mode of decision-making, which may lead to overly conservative risk management decisions.
2) Decision-making based on a very small number of seasonally-timed indoor air sampling events that may not yield representative exposure estimates; and
3) High sampling costs and lack of decision-making certainty that may lead to avoiding VI assessments altogether, especially at sites with numerous buildings.
Suggested strategies to incorporate current knowledge in VI assessments will be proposed that can allow confident decisions to be reached while controlling cost.