Vapor intrusion risk evaluation using automated continuous chemical and physical parameter monitoring

Abstract

Vapor intrusion risk characterization efforts are challenging due to complexities associated with background indoor air constituents, preferential subsurface migration pathways, and representativeness limitations associated with traditional randomly timed time‐integrated sampling methods that do not sufficiently account for factors controlling concentration dynamics. The U.S. Environmental Protection Agency recommends basing risk related decisions on the reasonable maximum exposure (RME). However, with very few exceptions, practitioners have not been applying this criterion. The RME will most likely occur during upward advective flux conditions. As such, for RME determinations, it is important to sample when upward advective flux conditions are occurring. The most common vapor intrusion assessment efforts include randomly timed sample collection events, and therefore do not accurately yield RME estimates. More specifically, researchers have demonstrated that randomly timed sampling schemes can result in false negative determinations of potential risk corresponding to RMEs. For sites experiencing trichloroethylene (TCE) vapor intrusion, the potential for acute risks poses additional challenges, as there is a critical need for rapid response to exposure exceedances to minimize health risks and liabilities. To address these challenges, continuous monitoring platforms have been deployed to monitor indoor concentrations of key volatile constituents, atmospheric pressure, and pressure differential conditions that can result in upward toxic vapor transport and entry into overlying buildings. This article demonstrates how vapor intrusion RME‐based risks can be successfully and efficiently determined using continuous monitoring of concentration and parameters indicating upward advective chemical flux. Time series analyses from multiple selected 8‐ and 24‐hr time increments during upward advective TCE flux conditions were performed to simulate results expected from the most commonly employed sampling methods. These analyses indicate that, although most of the selected time increments overlap within the same 24‐hr window, results and conclusions vary. As such, these findings demonstrate that continuous monitoring of concentration and parameters such as differential pressure and determination of a time‐weighted concentration average over a selected duration when upward advective flux is occurring can allow for a realistic RME‐based risk estimate.

Simultaneous monitoring of volatile organic contaminant concentration and controlling factors for vapor intrusion risk evaluations—Two select cases

“…Conversely, at the beginning of a rise in barometric pressure, indoor TCE concentrations decreased and remained relatively low until the next drop in barometric pressure. This inverse correlation between concentration and barometric pressure trend was consistent throughout the monitoring duration and with observations documented by others (Hosangadi et al, 2017; Kram et al, 2019, 2020). As described in Hosangadi et al (2017), it is believed that as the barometric pressure decreases at the earth's surface (e.g., as the land is heated and the overlying air expands and rises), an increase in differential pressure across the building foundation can result.…”

“…Therefore, it behooves practitioners to understand the causes of concentration dynamics and the concentration patterns at each building investigated. Practitioners have demonstrated that changes in temperature, barometric pressure, ventilation, and differential pressure can impact and control dynamic indoor concentration patterns (Hosangadi et al, 2017;Kram et al, 2019Kram et al, , 2020Schuver et al, 2018;USEPA, 2015a). Understanding concentration patterns and controlling factors can help determine when the most appropriate time for sample collection will occur.…”

“…Calculation of time-weighted average values from concentration time series analyses at appropriately selected time ranges (e.g., during advective upward vapor flux conditions) has the potential to improve the probability that the resulting RME will be more accurately calculated. As such, risk assessment conclusions and response decisions can be significantly improved (Kram et al, 2020). This article describes a risk assessment approach using data derived from continuous concentration and controlling factor monitoring from two selected sites.…”

“…Given that concentrations can be dynamic (Holton et al, 2013;Hosangadi et al, 2017;Kram, 2015Kram, , 2016Kram, , 2019USEPA, 2015aUSEPA, , 2015b, sample methods, timing, and time-weighted average results significantly impact how vapor intrusion-related risks are managed. In some cases, concentrations can range by more than two orders of magnitude (Holton et al, 2013;Kram et al, 2019) and can be controlled by fluctuations in barometric pressure that induce differential pressure across a building foundation and vapor intrusion (Hosangadi et al, 2017;Kram et al, 2019Kram et al, , 2020.…”

Indoor Air Background Levels of Volatile Organic Compounds and Air‐Phase Petroleum Hydrocarbons in Office Buildings and Schools

“…Nuanced and iterative strategies such as those developed on behalf of the United States Navy (NAVFAC 2011) may be needed to make determinative vapor intrusion pathway evaluations when contribution from indoor air background is suspected. These may include comparison with site-spe-cific or published background values, review of constituent ratios between media, and methods such as differential pressure monitoring, pressure cycling, tracer compound analyses, and real-time monitoring, including continuous monitoring of spatial and time series concentration data patterns combined with confirmation using discrete sampling and analysis (e.g., Kram et al 2019Kram et al , 2020. If it is determined that these methods are not conclusive, environmental forensic analysis may be needed (e.g., Plantz et al 2008;Beckley et al 2016).…”