Temporal variability of chlorinated volatile organic compound vapor concentrations in a residential sewer and land drain system overlying a dilute groundwater plume

Some subsurface sewer and land drain networks will facilitate the migration of chlorinated volatile organic compounds (CVOCs) from dissolved contaminant groundwater plumes to indoor air. As this vapor intrusion (VI) pathway has only recently been documented, guidance for evaluating it, including recommendations for timing, frequency, duration and location for vapor sampling in subsurface piping networks is non-existent. To address this gap, a three-year investigation of CVOC concentrations from land drains, storm drains, and sanitary sewers was undertaken in a neighborhood overlying a large-scale dissolved chlorinated VOC (CVOC) groundwater plume. Vapor sampling included the collection of grab (time-discrete) samples from up to 277 manholes, hourly grab sampling from three manhole locations, and 24-h duration collection during week-long sampling from 13 land drain and sewer manholes. The spatial distribution of vapor and water concentrations and the temporal variations in the vapor values observed in this study suggest that week-long vapor sampling conducted at different times of the year and with samples collected at manhole locations overlying and outside a dissolved plume might be needed to ensure robust VI pathway assessment at other sites. These findings are expected to be of relevance to regulatory agencies involved in the development of current or future VI pathway assessment guidance.