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A subsurface plume of non-aqueous phase liquid (NAPL) in contact with the side wall of a large combined sanitary and storm water trunk sewer was suspected as a source of toxic and odorous vapors in ambient air in the vicinity of the sewer manholes and catch basins and in residences serviced by the sewer. An evaluation involving vapor transport modeling was conducted to assess the potential for vapor release through untrapped storm water catch basin openings to outdoor air and through faulty residential traps to indoor air. The results were used to assess the potential for human exposure and associated health risks.

The NAPL, characterized as a light petroleum naphtha with traces of gasoline, was discovered floating on the groundwater surface, and chemical components of the NAPL were detected in the underlying groundwater. Chemicals of potential concern for the evaluation included benzene, ethylbenzene, naphthalene, toluene, xylenes.

Two time frames were evaluated. In the first, the pre-remediation phase, the potential for human exposure due to NAPL infiltrating the sewer and floating on the wastewater surface was predicted. An oil film model was used to estimate emissions from the NAPL layer on top of the in-sewer wastewater surface. In the second, a post-remediation phase (i.e., following recovery of the NAPL), the residual contaminated groundwater infiltrating the sewer reach was assumed to have mixed with the wastewater, resulting in volatilization from the water surface into the sewer air. Emissions to the air from the contaminated wastewater were estimated using the ToxChem+ model (Enviromega, Inc.). In both time frames, the estimated chemical-specific emission rates were then partitioned between sewer air and the service connections (i.e., a catch basin and sewer lateral/faulty residential sewer trap). The estimated emission rates from the service connections were input to a computerized atmospheric dispersion model and a household air-exchange model to estimate chemical concentrations in ambient outdoor and indoor air, respectively.

Two potential human exposure scenarios were developed for each time frame based on likely indoor and outdoor activity patterns and representative exposure parameters. In the first, contaminated air from the sewer was assumed to escape by way of catch basin openings, dispersing downwind in the direction of nearby residences. In the second, contaminated air from the sewer was assumed to enter a residence by way of a faulty sewer trap. The estimated exposure levels were coupled with appropriate toxicological criteria to characterize pre-remediation health risks and predict post-remediation health risks associated with post- NAPL removal residuals.

This paper presents the conceptual models for the chemical migration and exposure scenarios, the development of emission rates and dispersion model inputs and outputs, and the potential risks estimated using these techniques.

Document Type: Research Article


Publication date: January 1, 2000

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  • Proceedings of the Water Environment Federation is an archive of papers published in the proceedings of the annual Water Environment Federation® Technical Exhibition and Conference (WEFTEC® ) and specialty conferences held since the year 2000. These proceedings are not peer reviewed.

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