Seasonal variation of volatilized tetrachloroethene and trichloroethene concentrations in sewer systems in contaminated coastal landscapes.

Volatile organic compounds (VOCs) such as trichloroethene (TCE) and tetrachloroethene (PCE) are commonly detected in urban environments with legacy contamination. Pathways of indoor VOC exposure through sewer infrastructure remain underexplored, particularly in the context of rising groundwater driven by seasonal rainfall and climate change in coastal settings. This study investigates how seasonal groundwater fluctuations influence VOC concentrations in sewers in the San Francisco Bay Area at a site characterized by shallow, unconfined groundwater and vulnerable sewer infrastructure in a setting with soil known to be contaminated by TCE/PCE. Passive air sampling was conducted across three time periods: one in the dry season and two during the wet season, defined by precipitation totals and differences in depth to groundwater. 8 samples were analyzed using Wilcoxon rank-sum tests and results indicate significantly elevated concentrations of TCE/PCE in sewer air during wetter conditions, with PCE showing a marginally significant wet season increase (p = 0.057). No remarkable detections were observed in corresponding indoor or ambient air samples, suggesting that well-maintained plumbing seals in older buildings are critical for limiting indoor exposure to VOCs from contaminated sewer systems. These findings demonstrate that seasonal hydrological dynamics can influence VOC transport in sewers in coastal settings. With sea-level rise and extreme precipitation events intensifying nationally, similar risks will emerge in other coastal cities with legacy contaminants, aging underground infrastructure, and aging buildings. This study highlights the need for increased investigations of sewer systems as preferential pathways for vapor intrusion where groundwater levels are changing and underscores the importance of integrating hydrological and climatic variables into risk assessments for contaminated coastal environments.