Occurrence of chlorinated volatile organic compounds (VOCs) in a sanitary sewer system: Implications for assessing vapor intrusion alternative pathways.

Sewer systems have been recently recognized as potentially important exposure pathways to consider during vapor intrusion assessments; however, this pathway has not been well-characterized and there is need for additional information about the occurrence of volatile organic compounds (VOCs) in sewer systems. This paper reports the results of sewer gas sampling conducted in a sanitary sewer over the years of 2014-2017. Sewer gas samples were collected and analyzed using several different techniques, including TO-15 (grab), TO-17 (passive), Radiello® (passive) and a novel continuous monitoring technique, the Autonomous Rugged Optical Multigas Analyzer (AROMA). The applicability of each of the different approaches used in this study is discussed in the context of investigating sanitary sewers as a vapor intrusion alternative pathway. The data confirmed that trichloroethylene (TCE) concentrations in sewer gas were detected adjacent to and extending hundreds of feet away from a previously defined vapor intrusion area, where TCE was a primary contaminant. TCE concentrations detected in sewer gas ranged from non-detect to 1600µg/m3. Temporal variability was observed in TCE concentrations over timescales that ranged from minutes to months to years at discrete sampling locations. Spatial variability in sewer gas concentrations was also observed throughout the study area. Temporal and spatial variability may be caused by groundwater contamination sources in the study area, as well as sewer gas transport mechanisms.

Air exchange rates and alternative vapor entry pathways to inform vapor intrusion exposure risk assessments.

Vapor intrusion (VI) is a term used to describe indoor air (IA) contamination that occurs due to the migration of chemical vapors in the soil and groundwater. The overall vapor transport process depends on several factors such as contaminant source characteristics, subsurface conditions, building characteristics, and general site conditions. However, the classic VI conceptual model does not adequately account for the physics of airflow around and inside a building and does not account for chemical emissions from alternative "preferential" pathways (e.g. sewers and other utility connections) into IA spaces. This mini-review provides information about recent research related to building air exchange rates (AERs) and alternative pathways to improve the accuracy of VI exposure risk assessment practices. First, results from a recently published AER study for residential homes across the United States (US) are presented and compared to AERs recommended by the US Environmental Protection Agency (USEPA). The comparison shows considerable differences in AERs when season, location, building age, and other factors are considered. These differences could directly impact VI assessments by influencing IA concentration measurements. Second, a conceptual model for sewer gas entry into buildings is presented and a summary of published field studies is reported. The results of the field studies suggest that alternative pathways for vapors to enter indoor spaces warrant consideration. Ultimately, the information presented in this mini-review can be incorporated into a multiple-lines-of-evidence approach for assessing site-specific VI exposure risks.