For Environmental Professionals (EPs) specializing in Vapor Intrusion (VI) investigations, it is common knowledge that indoor air is a highly dynamic medium due to the many interacting forces present within, beneath, and outside a structure. Yet most EPs continue to utilize conventional outdated methods that are slow and poorly suited for addressing temporal variability, resulting in uncertainty, incorrect conclusions, and increased exposure liabilities. Lacking temporal resolution, these methods typically cannot distinguish between intrusion and indoor (foreign) sources or identify vapor entry points. Furthermore, conventional VI sampling methods cannot provide decision-quality acute exposure guidance before a duration of concern has transpired. As such, traditional methods are not preventative.
Recent technology innovations were developed to address these shortcomings by integrating remote telemetry communication with automated analytical platforms that offer near real-time temporal and spatial monitoring capabilities. Why is this important? For sites with chemical VI conditions, indoor contaminant concentrations have been shown to vary dramatically over periods that cycle anywhere from minutes to days. Vapor migration into a structure by advective and diffusive transport is partly determined by fixed factors like geology, building design, and structural defects. However, VI investigators employing methods that collect temporally dense data sets are observing that dynamic acting forces may be of even greater importance for understanding the indoor exposure risks, since these factors can significantly influence both air exchange rates and diffusive/advective flow rates.
Key dynamic factors include source strength, mechanized systems (operation of HVAC), design elements (windows & doors), and natural elements (temperature, pressure, humidity, soil moisture, and wind). The many forces acting on the system are best imagined by picturing a storm front. As a cold weather front passes through an area with structures, dropping barometric pressures, changing wind effects, and temperature changes can create pressure differentials between the structure and the subsurface environment. These differentials can drive vapor migration and concentration changes that are heterogeneous throughout and beneath the structure. Dynamic conditions can be further complicated by potential contributions from indoor (foreign) contaminant sources, which can even migrate from indoors into the subsurface when the building is over-pressurized relative to the underlying soil.
If the goal of VI risk assessment is to evaluate and respond to indoor air exposures by sensitive receptors, then indoor air quality data should be collected using methods that consider the spatial and temporal dynamics and resolve risks in a way that minimizes receptor exposure times. This need is particularly apparent at sites experiencing trichloroethylene (TCE) vapor intrusion, since the EPA has concluded that short-term low-dose inhalation exposures by first-trimester pregnant women can result in heart defects to the fetus. While most consider 24 hours to be a duration of concern for residential TCE exposures (and 8 hours for commercial settings), this implies that risk prevention must include measurement capabilities that allow for responses within 24 (or 8) hours. According to some industry experts, “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” (Kram et al., 2016). To address some of these shortcomings, the VI investigation community has been evolving their methodologies to incorporate “multiple lines of evidence” to meet current VI regulatory guidance. One effective approach has been to deploy analytical sensing platforms capable of onsite continuous monitoring of TCE, other analytes of concern and pressure conditions conducive to advective flux.
In my last blog article, I introduced the VaporSafeTM continuous VI monitoring platform which some have described as a “surgical” tool for attacking difficult VI challenges. I will review some of the lessons learned from actual TCE VI site challenges and share some of the benefits realized from temporal-spatial monitoring. Given the sensitivity of this topic, the responsible party sites were kept anonymous.
Undisclosed East Coast Manufacturing Facility
Challenge: A large manufacturing building overlies groundwater with elevated TCE impacts. Years of traditional VI sampling could not discern VI pathways into the structure.
Investigation Approach: Continuous monitoring stations were established at 15 locations within the building based on potential VI exposure pathways. The sample conveyance lines were deployed and connected to the analytical platform staged in a centrally located room. Telemetry was activated to allow real-time reporting of the measurements and to automatically deliver threshold exceedance alerts.
Findings and Benefits: Within 24 hours of deployment, temporal-spatial data revealed several likely TCE VI entry points requiring further investigation. The VaporSafeTM monitoring system was re-activated for continuous monitoring while two suspect VI entry point locations were covered with plastic sheeting. Indoor TCE concentrations were observed to decrease below risk thresholds over a few hours of monitoring, which aided the consultant’s long-term mitigation strategy. This demonstrates that by simple building manipulations (e.g., covering suspected entry points), one can rapidly derive immediate and long term mitigation strategies.
Undisclosed Military Facility
Challenge: A large operational support building overlies shallow groundwater with elevated TCE, PCE and Stoddard Solvent impacts. Years of traditional VI sampling revealed dynamic risk results that challenged the stakeholder’s understanding of VI conditions and stalled development of long term mitigation strategies.
Investigation Approach: Continuous monitoring stations were established at 6 locations based on previous VI data and sample conveyance lines were deployed and connected to the analytical platform located in a central room. At one of the sampling points, a digital micromanometer was also deployed to monitor the differential pressures between the sub-slab and indoors. Telemetry was activated to allow real-time reporting of the measurements including threshold exceedance alerts.
Findings and Benefits: Time-series plots of the data revealed fluctuating TCE concentration patterns with peaks up to 400+ ug/m3. The high frequency spatial data sets allowed probable discernment of a cyclical inverse correlation between TCE concentration and barometric pressure. The findings also documented temporal exposure patterns that could be used to align TCE exposures with occupant schedules in support of risk management. The implications are significant, as it suggests that barometric pressure cycling can control indoor TCE concentrations, which can impact exposure expectations and optimal monitoring time-frames.
Undisclosed West Coast Thermal Remediation Site
Challenge: A release from an industrial facility resulted in elevated volatile organic compound (VOC) impacts that extended beneath multiple commercial and residential structures. Thermal remediation combined with soil vapor extraction (SVE) for VOC recovery and treatment were to be employed. However, concerns about potential fugitive emissions and intrusion into overlying commercial and residential buildings were significant due to the shallow groundwater.
Investigation Approach: Continuous monitoring of TCE, PCE, and VC levels was established in multiple structures and the vapor recovery/treatment emissions stack using 13 continuous monitoring stations. Reportedly, this effort represents the first time automated continuous VC monitoring has ever been accomplished. The sample conveyance lines were deployed and connected to an analytical platform in a centrally located trailer. Telemetry was activated to enable real-time cause-and-effect analysis during system adjustments and for documenting safe conditions.
Findings and Benefits: Continuous automated monitoring revealed the temporal-spatial dynamic ranges for indoor and outdoor exposure risks during system operation for more than four consecutive months. Automated continuous monitoring allowed for constant system adjustment, optimization and immediate response to concentration increases without having to wait for laboratory results and well before an exposure duration of concern had transpired.
The environmental compliance industry has only marginally tapped into the potential benefits realized from combining spatial data with temporally-relevant data. The three applications described above were cost-effective and revealed insights previously not available due to temporal resolution limitations associated with traditional methods. To quickly summarize the key benefits observed at the above sites, stakeholders gained:
- Real-time understanding of temporal-spatial dynamics including TCE exposure patterns for improved risk assessment and validation;
- VI entry point location and confirmation;
- Valuable decision support through “cause & effect analysis”;
- Insights into diurnal pressure dynamics effecting TCE flux;
- Real-time liability management support though automated threshold alerts;
- Valuable engineering support in real-time to optimize remediation systems;
- Legal protection through automated, proactive risk data archiving.
The cost of continuous monitoring has been shown to be competitive with traditional VI testing methods when five or more monitoring locations are being investigated (Kram et al, 2016). Adding the ability for rapid decision-support, improved risk certainty, and immediate mitigation optimization offers incalculable value to stakeholders. Consultants that have embraced temporal-spatial monitoring methods are upstaging the less innovative investigators by rapidly resolving VI challenges that have plagued sites for years. With these tools, consultants can offer their client conclusive evidence of the full range of VI exposures matched to occupant exposure times and deliver a higher level of certainty regarding potential risks and mitigation options. In addition, temporal-spatial risk conclusions are backed up by a high-density sample data archive for legally defensible liability management. Furthermore, real-time indoor air quality data allows practitioners to confirm exposure risks and then proceed with rapid mitigation measures such as entry point sealing, blower activation, or similar responses. This last advantage can be offered by consultants as additional services to RPs concerned about discovering potential indoor exposures.