ALTERNATIVE EXPOSURE PATHWAY OF VOC VAPORS
Figure 6: Anatomy of a P-trap vapor seal in cross-section. If water evapo- rates or is siphoned to below the trap dip (upper dip) sewer air can be released into indoor air. (modified after U.S. Department of the Army, 2001). Water in P-trap seals VOCs from entering indoor air.
the recent studies have focused on PCE, VOCs other than PCE are likely also of interest. On the basis of current understanding of this exposure pathway occupants in buildings, even outside of the VOC plume area, which have plumb- ing vapor seal leaks, and which are con- nected to failed sewer lines that intersect VOC plumes, are at risk for possible exposure to toxic vapors in indoor air.
Recommendations
Alternative vapor exposure pathways should be considered in environmental indoor air assessments. The scien- tific and regulatory community needs to update vapor intrusion models and consider alternate exposure pathways in health risk evaluations and regula- tory decision making. To establish the risk of exposure of individual building inhabitants to VOCs, we recommend screening (sorbent tubes), or labora- tory testing (passive sorbents and air samples) of nearby sewer manholes and building sewer system cleanouts or vent stacks. Further evaluation of inside vapor seals and plumbing connections can be performed with smoke testing or tracer gas tests.
To clear high VOCs in sewer pipe air, active venting in manholes has been demonstrated by others. The remedy for the vapor seal failure may be as simple as filling a dry P-trap with water or replacing toilet wax seals. For leaking underground sewer system repair, the capital costs including leak inspection and testing, pipe engineering design, and installation is millions of dollars for municipalities or sewer agencies. Ultimately, the breached pipe network should be repaired in order to prevent VOC intrusions into indoor air.
Nationwide, U.S. EPA has been focused on I&I issues of sewer systems as a major source of sewer overflows of
32 TPG •
Jul.Aug.Sep 2017
Figure 7: Migration pathway of sewer gas and VOCs into indoor air (modified after U.S. Department of the Army, 2001).
untreated wastes onto land, as well as into water bodies. With this information about VOC exposure, prioritizing sewer replacement projects in areas where known shallow VOC groundwater plumes co-exist with breached sewer systems lowers the chance for VOCs to enter sewer air and reduces the chance for indoor air exposures.
More research is
needed into assessment and mitigation methods with regards to the presence of VOCs in the sewer-plumbing system and their migration into indoor air.
References Cited
Gorder, K. and Dettenmaier, E. 2011. Portable GC/MS Methods to Evaluate Sources of VOC Contamination in Indoor Air. Groundwater Monitoring & Remediation, National Groundwater Association, Fall, Vol. 31, Issue 4, p. 113-119.
Guo, Yuanming, Holton, C., Luo, H., Dahlen, P., Gorder, K., Dettenmaier, E., and Johnson, P., 2015. Identification of Alternative Vapor Intrusion Pathways using Controlled Pressure Testing, Soil Gas Monitoring and Screening Model Calculations, Env. Sci. Technol., vol.49 (22), pp. 13472-13482.
Jacobs, J.A., Jacobs, O.P., and K.G. Pennell. 2014. Geologists and Site Conceptual Models: VOCs and Sewer Gas in Indoor Air Resulting from Migration from Breached Sewer Conveyance Systems, American Institute of Professional Geologists National Meeting, Abstracts, p. 73-74.
Jacobs, J.A., Jacobs, O.P. and K.G. Pennell, 2015. One Alternate Exposure Pathway of VOC Vapors from Contaminated Subsurface Environments into Indoor Air - Legacy Sewer-Plumbing Systems. Hydrovisions, Groundwater Association of California, Spring 2015 issue, p.20-24.
Johnson, P. C, and R. A. Ettinger. 1991. Heuristic model for predicting the intrusion rate of contaminant vapors in buildings. Environ. Sci. Technol. 25: 1445-1452.
www.aipg.org
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