Comparing Vapor Intrusion Mitigation System Performance for VOCs and Radon

This article summarizes a long‐term study of vapor intrusion mitigation system performance in a historic, unoccupied residential duplex with an extensive set of temporal variability observations. The experimental design included multiple cycles of subslab depressurization (SSD) system operation and shut‐off during a seven‐month period, followed by a year‐long period of continuous operation. Results showed that the system provided rapid pressure field extension and radon control as much as 100 days of operation before optimum volatile organic compound (VOC) mitigation was achieved. Greater variability in VOC concentrations than in radon concentrations was observed during the initial mitigation system cycling. Subslab VOC concentrations at numerous locations increased during this initial period of SSD operation, and indoor air VOC concentrations were more variable than radon. However, indoor air concentrations were considerably less variable (and lower) during the first year of continuous mitigation system operation. ©2015 Wiley Periodicals, Inc.     

Variables affecting emissions of PCDD/Fs from uncontrolled combustion of household waste in barrels

The uncontrolled burning of household waste in barrels has recently been implicated as a major source of airborne emissions of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs). A detailed, systematic study to understand the variables affecting emissions of PCDD/Fs from burn barrels was performed. The waste composition, fullness of the barrel, and the combustion conditions within the barrel all contribute significantly to determining the emissions of PCDD/Fs from burn barrels. The study found no statistically significant effect on emissions from the Cl content of waste except at high levels, which are not representative of typical household waste. At these elevated Cl concentrations, the impact of Cl on PCDD/F emissions was found to be independent of the form of the Cl (inorganic or organic). For typical burn conditions, most of the PCDD/F emissions appear to be associated with the later stages of the burn when the waste is smoldering. Polychlorinated biphenyls (PCBs) were also measured for a subset of the tests. For the nominal waste composition, the average emissions were 76.8 ng toxic equivalency units (TEQ)WHO98/kg of waste combusted, which suggests that uncontrolled burning of household waste could be a major source of airborne PCDD/Fs in the United States.     

Aquatic ecological risks due to cyanide releases from biomass burning

Aquatic toxicity due to the creation and mobilization of chemical constituents by fire has been little studied, despite reports of post-fire fish kills attributed to unspecified pyrogenic toxicants. We examined releases of cyanides from biomass burning and their effect on surface runoff water. In laboratory test burns, available cyanide concentrations in leachate from residual ash were much higher than in leachate from partially burned and unburned fuel and were similar to or higher than the 96-h median lethal concentration (LC50) for rainbow trout (45 microg/l). Free cyanide concentrations in stormwater runoff collected after a wildfire in North Carolina averaged 49 microg/l, again similar to the rainbow trout LC50 and an order of magnitude higher than in samples from an adjacent unburned area. Pyrogenic cyanide inputs, together with other fire-related stressors, may contribute to post-fire fish mortalities, particularly those affecting salmonids.     

Bench-scale studies on the simultaneous formation of PCBs and PCDD/Fs from combustion systems

The presence of endocrine disrupting chemicals (EDCs) in the environment has wide-ranging potential ecological and health impacts on animals and humans. A significant amount of experimental and theoretical work has been performed the examining formation and control of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs), which account for only part of the EDCs being emitted from combustion devices. Generally accepted mechanistic theories for PCDD/F formation propose heterogeneous reactions in the cooler regions of the combustor involving gas-phase organic precursors (such as chlorobenzenes or chlorophenols), a chlorine donor [such as hydrogen chloride (HCl)], and a flyash-bound metallic catalyst (such as copper chloride). There is evidence that some other proposed EDCs, including polychlorinated biphenyls (PCBs), are formed through a similar mechanistic pathway as PCDD/Fs. In addition, there is evidence that certain important steps in the catalytic reaction between the copper catalyst and the organic precursors may suggest a common rate limiting step for the heterogeneous formation of the previously mentioned EDCs. This paper reports on a bench-scale experimental study to characterize a newly built reactor system that was built to: produce levels and distributions of PCDD/F production similar to those achieved by previous researchers; verify similar responses to changes in independent variables; examine the hypothesis that PCB formation rates exhibit trends similar to PCDD/F formation rates as reactor variables are changed; and begin to explore the dependence of PCB formation on temperature and precursor type. The reactor system has been built, and initial reactor characterization studies have been performed. Initial experiments yielded results that support the hypothesis of a similar formation mechanism of PCBs and PCDD/Fs in combustors. Initial experiments uncovered potential deficiencies with the reactor system and the experimental procedures and have suggested corrective action to improve the experimental system.     

Emissions of PCDD/F from uncontrolled, domestic waste burning

Emissions of polychlorinated dibenzodioxin and dibenzofuran (PCDD/F) result from inefficiencies of combustion processes, most typically waste combustion. Uncontrolled combustion, such as occurs during so-called "backyard burning" of domestic waste, may therefore produce optimal conditions for formation and emission of PCDD/F. However, few assessments of PCDD/F emissions are available from these sources. This work describes the first known comprehensive assessment of PCDD/F emissions from uncontrolled, domestic waste burning. Emissions were copious, but highly variable, ranging over several orders of magnitude. The potential for emissions appears to be related primarily to combustion parameters and concentrations of various gas-phase species, the latter which may be affected by changes in waste composition, waste orientation, and/or combustion conditions.     

Chlorinated vapor intrusion indicators,tracers, and surrogates (ITS): Supplemental measurements for minimizing the number of chemical indoor air samples—Part 1: Vapor intrusion driving forces and related environmental factors

Vapor intrusion (VI) assessment is complicated by spatial and temporal variability, largely due to compounded interactions among the many individual factors that influence the vapor migration pathway from subsurface sources to indoor air. Past research on highly variable indoor air datasets demonstrates that conventional sampling schemes can result in false negative determinations of potential risk corresponding to reasonable maximum exposures (RME). While high‐frequency chemical analysis of individual chlorinated volatile organic compounds (CVOCs) in indoor air is conceptually appealing, it remains largely impractical when numerous buildings are involved and particularly for long‐term monitoring. As more is learned about the challenges with indoor air sampling for VI assessment, it has become clear that alternative approaches are needed to help guide discrete sampling efforts and reduce sampling requirements while maintaining acceptable confidence in exposure characterization. Indicators, tracers, and surrogates (ITS), which include a collection of quantifiable metrics and tools, have been suggested as a potential solution for making VI pathway assessment and long‐term monitoring more informative, efficient, and cost‐effective. This review, compilation, and evaluation of ITS demonstrates how even low numbers of indoor air CVOC samples can provide high levels of confidence for representing the RME levels (e.g., 95th percentile) often sought by regulatory agencies for less than chronic effects. A two‐part compilation of available evidence for select low‐cost ITS is presented, with Part 1 focused on introducing the concepts of ITS, meteorologically based ITS, and the evidence from data‐rich studies to support lower cost CVOC VI assessments. Part 1 includes the results of quantitative analyses on two robust residential building VI datasets, where numerous supplemental metrics were collected concurrently with indoor air concentration data. These are supplemented with additional less‐intensive studies in different circumstances. These analyses show that certain ITS metrics and tools, including differential temperature, differential pressure, and radon (in Part 2), can provide benefits to VI assessment and long‐term monitoring. This includes indicators that narrow the assessment period needed to capture RME conditions, tracers that enhance understanding of the conceptual site model, and aid in the identification of preferential pathways and surrogates that support or substitute for CVOC sampling results. The results of this review provide insight into the scientifically supportable uses of ITS.