Vapor intrusion characterization efforts can be challenging due to complexities associated with background indoor air constituents, preferential subsurface migration pathways, and response time and representativeness limitations associated with conventional low‐frequency monitoring methods. For sites experiencing trichloroethylene (TCE) vapor intrusion, the potential for acute risks poses additional challenges, as the need for rapid response to exposure exceedances becomes critical in order to minimize health risks and associated liabilities. Continuous monitoring platforms have been deployed to monitor indoor and subsurface concentrations of key volatile constituents, atmospheric pressure, and pressure differential conditions that can result in advective transport. These systems can be comprised of multiplexed laboratory‐grade analytical components integrated with telemetry and geographical information systems for automatically generating time‐stamped renderings of observations and time‐weighted averages through a cloud‐based data management platform. Integrated automatic alerting and responses can also be engaged within one minute of risk exceedance detection. The objectives at a site selected for testing included continuous monitoring of vapor concentrations and related surface and subsurface physical parameters to understand exposure risks over space and time and to evaluate potential mechanisms controlling risk dynamics which could then be used to design a long‐term risk reduction strategy. High‐frequency data collection, processing, and automated visualization efforts have resulted in greater understanding of natural processes such as dynamic contaminant vapor intrusion risk conditions potentially influenced by localized barometric pumping induced by temperature changes. For the selected site, temporal correlation was observed between dynamic indoor TCE vapor concentration, barometric pressure, and pressure differential. This correlation was observed with a predictable daily frequency even for very slight diurnal changes in barometric pressure and associated pressure differentials measured between subslab and indoor regimes and suggests that advective vapor transport and intrusion can result in elevated indoor TCE concentrations well above risk levels even with low‐to‐modest pressure differentials. This indicates that vapor intrusion can occur in response to diurnal pressure dynamics in coastal regions and suggests that similar natural phenomenon may control vapor intrusion dynamics in other regions, exhibiting similar pressure, geochemical, hydrogeologic, and climatic conditions. While dynamic indoor TCE concentrations have been observed in this coastal environment, questions remain regarding whether this hydrogeologic and climatic setting represent a special case, and how best to determine when continuous monitoring should be required to most appropriately minimize exposure durations as early as possible. ©2017 Wiley Periodicals, Inc.

Vapor intrusion characterization efforts are challenging due to complexities associated with indoor background sources, preferential subsurface migration pathways, indoor and shallow subsurface concentration dynamics, and representativeness limitations associated with manual monitoring and characterization methods. For sites experiencing trichloroethylene (TCE) vapor intrusion, the potential for acute risks poses additional challenges, as the need for rapid response to acute toxicity threshold exceedances is critical in order to minimize health risks and associated liabilities. Currently accepted discrete time‐integrated vapor intrusion monitoring methods that employ passive diffusion–adsorption and canister samplers often do not result in sufficient temporal or spatial sampling resolution in dynamic settings, have a propensity to yield false negative and false positive results, and are not able to prevent receptors from acute exposure risks, as sample processing times exceed exposure durations of concern. Multiple lines of evidence have been advocated for in an attempt to reduce some of these uncertainties. However, implementation of multiple lines of evidence do not afford rapid response capabilities and typically rely on discrete time‐integrated sample collection methods prone to nonrepresentative results due to concentration dynamics. Recent technology innovations have resulted in the deployment of continuous monitoring platforms composed of multiplexed laboratory grade analytical components integrated with quality control features, telemetry, geographical information systems, and interpolation algorithms for automatically generating geospatial time stamped renderings and time‐weighted averages through a cloud‐based data management platform. Automated alerts and responses can be engaged within 1 minute of a threshold exceedance detection. Superior temporal and spatial resolution also results in optimized remediation design and mitigation system performance confirmation. While continuous monitoring has been acknowledged by the regulatory community as a viable option for providing superior results when addressing spatial and temporal dynamics, until very recently, these approaches have been considered impractical due to cost constraints and instrumentation limitations. Recent instrumentation advancements via automation and multiplexing allow for rapid and continuous assessment and response from multiple locations using a single instrument. These advancements have reduced costs to the point where they are now competitive with discrete time‐integrated methods. In order to gain more regulatory and industry support for these viable options, there is an immediate need to perform a realistic cost comparison between currently approved discrete time‐integrated methods and newly fielded continuous monitoring platforms. Regulatory support for continuous monitoring platforms will result in more effectively protecting the public, provide property owners with information sufficient to more accurately address potential liabil...

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.

The US Environmental Protection Agency (USEPA) recommends basing vapor intrusion risk-related decisions on reasonable maximum exposure (RME). The RME can occur during conditions and factors that control advective vapor transport. The most common vapor intrusion assessment approaches consist of randomly timed sample collection efforts without regard to vapor transport controlling factors. As such, they often do not accurately yield RME estimates and are, therefore, inconsistent with USEPA recommended risk decision criteria. To address these challenges, continuous high-frequency monitoring platforms have been deployed to concurrently track indoor concentrations of key volatile constituents, climatic conditions, and nominal pressure differential conditions that can result in toxic vapor transport and entry into buildings. The objective of this article is to demonstrate how vapor intrusion RME-based risks can be successfully and efficiently characterized by documenting concentrations during advective chemical transport into the building. Time series analyses of data from selected sites and time increments were performed and compared to results expected from the most commonly employed sampling methods. These analyses indicate that time-weighted analyses and resulting conclusions and risk-based decisions can vary depending upon the sample timing. More specifically, these findings demonstrate that RME estimates will only be representative with a sufficient level of confidence when samples are collected at appropriate times. Highfrequency monitoring of dynamic concentration and controlling factors, and determination of a time-weighted concentration average over a selected duration concurrent with advective flux conditions allows for the derivation of a representative RME-based risk estimate. Furthermore, these variable temporal data patterns can prove insightful regarding cause-and-effect relationships.