ALTERNATIVE EXPOSURE PATHWAY OF VOC VAPORS
which are immediately dangerous to life and health (IDLH) for an instantaneous exposure of PCE. However, exposure to a low-concentration carcinogen (such as trichloroethylene (TCE) has been identified as a clear health risk; the hypothesis of sewer air exposure is being tested by others in order to document the incidence of VOC exposure through vapor seal failures (see Figure 7 on page 32 for examples of seal failures) and to minimize the VOC exposure to unsus- pecting occupants.
Summary Figure 4: Conceptual diagram showing wastewater flow components (modified after SASM, 2010).
and the (above ground) plumbing pipes within each structure.
Sewer Air Considered with Respect to Indoor Air Quality Investigations
Indoor air quality degradation caused by vapor intrusion of VOCs into struc- tures has been a health concern inves- tigated by US EPA (2011) and other agencies for decades. However, until recently (US EPA, 2015), the roles of public sewer and private plumbing sys- tems as vapor conduits have not been considered explicitly in the standard site conceptual models for indoor air quality developed by US EPA (2002) and others.
In the past decade PCE-specific vapor intrusion studies which document PCE indoor air concentrations resulting from failed plumbing-sewer systems that intersect mapped PCE groundwa- ter plumes have been undertaken in Skuldelev, Denmark (Riis et al., 2010), in Boston, Massachusetts (Pennell et al., 2013) and in Indianapolis, Indiana (McHugh et al., 2017). In these stud- ies, iterative testing of indoor air (after PCE was identified in indoor air) led to direct sewer air testing. The find- ings established that the air in a sewer that intersected a PCE groundwater plume contained PCE. The sampling also established that the sewer air was contributory to the presence of VOCs in the indoor air.
In the Denmark study (Riis et al., 2010), PCE was reported in indoor air
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in the cabinet under a kitchen sink at levels as high as 810 μg/m³. In the Boston study (Pennell, et al., 2013), the concen- tration of PCE detected in bathroom air was 37 μg/m³. A faulty plumbing con- nection at the toilet was presumed to be the source of PCE; the concentration of PCE detected in the sewer gas (sampled directly from the sewer pipe connected to the toilet) was 58 μg/m³. When the toilet connection was sealed, the bathroom air PCE concentration decreased to 2.6 μg/ m³. Variability between sampling events in VOC-sewer air concentrations was documented in the Boston case and has since been shown to depend on many factors in addition to the integrity of the sewer seals. In both cases, the concentra- tions of PCE detected inside the build- ings were orders of magnitude higher than the levels generally considered safe for long-term indoor air exposure. In both cases, mitigation with sewer air vapor seal installation was successful.
At the US EPA Duplex Site, (McHugh et al. 2017), tracer gas demonstrated the complexity of VOC transport from a combined sanitary/storm sewer and reported the detection of PCE in sewer gas near dry cleaner sites. Interestingly, in Denmark, 20% of contaminated dry cleaner sites with vapor intrusion in the Central Denmark Region are com- plicated with sewer gas intrusion issues (Nielsen and Hivdberg, 2017).
Regulatory Levels
The levels of PCE detected in indoor air (Riis et al., 2010; and Pennell et al., 2013) are small compared to the levels
Many urban areas in North America are underlain by VOC groundwater plumes that are intersected by failed sewer systems. The referenced studies document elevated PCE concentrations in indoor air from breached sewer- plumbing systems in areas with PCE groundwater plumes combined with failed vapor seals. Although much of
Figure 5: Examples of vapor leak locations (modi- 1. Cracked waste stack 2. Dry P-trap 3. Cracked main vent 5. Faulty wax ring seal 6 Leaking joints
Jul.Aug.Sep 2017 • TPG 31
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