ALTERNATIVE EXPOSURE PATHWAY OF VOC VAPORS
Figure 1 - A common site conceptual VOC vapor intrusion mitigation model (modified after others, original from US EPA, 2002), with sub-slab mitigation considered effective.
After nearly two decades of indoor air monitoring in structures above con- taminant plumes, practitioners have developed a term “alternative exposure pathways” to address contaminated vapor intrusion into indoor air from sources other than underlying ground- water or vapor plumes. Many regulatory documents reference trench backfill that contains piping conduits as a poten- tial pathway for contaminated vapor exposure, but few, if any, discuss the implications for the piping conduits (pipe interiors) themselves to serve as vapor exposure pathways. The current term alternative exposure pathways com- monly refers to trenching and piping for sewer-plumbing systems, land drains, storm drains, abandoned pipelines, cable ducts, steam lines, utility lines, other pipes and other conduits. When these alternative exposure pathways, includ- ing the air within the conduits, enter into, or are proximal to, structures, they may serve as unintended convey- ance systems for VOCs into buildings. A land drain beneath a research house in northern Utah owned by Arizona State University was documented to be a VOC conduit for detected indoor air impacts (Johnson, 2014; Guo and oth- ers, 2015). Field investigations at Hill Air Force Base have shown some indoor air contamination to be the result of the connectivity of contaminated sewer air to indoor air (Gorder and Dettenmaier, 2011). Three other studies (McHugh et al., 2017; Pennell et al., 2013; and Riis et al., 2010) documented the presence of tetrachloroethene (PCE) in sanitary sewer gas to be an important source of
www.aipg.org
indoor air contamination at three vapor intrusion study sites. Trichloroethene (TCE) and other VOCs were detected in a sanitary sewer at locations hundreds of feet away from a well-defined VOC groundwater plume, providing evidence that sewers are potentially important when considering the extent of vapor intrusion exposure risks (Roghani et al., 2017).
Based on these reports of alternative pathways, such as the migration of air inside of the sewer-plumbing system, an updated conceptual model to guide vapor intrusion studies is needed. Some of the combinations of factors related to plume location, vapor seal integrity, and possible VOC exposure in indoor air are shown in Figure 2 below. If VOCs exist in the sewer main, houses connected to the sewer main can have VOCs in the laterals, in the cleanouts and in the discharging gases at the roof vent stacks (the yellow emission clouds – although the concentrations will vary). Houses on VOC contaminated sewer systems and not having working vapor traps and properly functioning pipes and seals can have VOCs in the indoor air. Note that in Figure 2, the SSD protects buildings over VOC groundwater plumes from VOCs rising through the vadose zone into indoor air. SSDs cannot protect indoor air quality in buildings when VOCs in sewer air leak into indoor air through failed plumbing seals. The amount of indoor air contamination varies by func- tional space and is directly related to the location of the specific plumbing failure. Not all permutations of foundation, sub- surface depressurization system (SSD) and sewer configuration are shown. The rationale for this updated conceptual model is provided below.
Figure 2: An example of an alternative exposure pathway model showing sewer gases and VOCs entering indoor air through ineffective plumbing vapor seals. Note: VOCs are released to the indoor air and to
outside air through the vent line on the roof. VOC data (Riis et al., 2010; and Pennell et al., 2013) sup- exposure pathways are completed or not completed. A: Intact vapor seals and not over VOC plume (exposure pathway not completed) B: Leaky vapor seals and not over VOC plume (exposure pathway completed) C: Intact vapor seals and working SSD over VOC plume (exposure pathway not completed) D: Leaking vapor seals and working SSDs over VOC plume (exposure pathway completed)
Jul.Aug.Sep 2017 • TPG 29
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64
