Research Results

A pilot study collected tapwater samples from low-income California communities in five regions (Gold Country, San Francisco Bay Area, Central Valley, and southeast Los Angeles) with suspected water quality challenges and elevated breast cancer rates, analyzing 251 organic chemicals and 32 inorganic constituents. The five most frequently detected contaminants were barium (100% of samples), disinfection byproducts including total trihalomethanes (90%), bromodichloromethane (86.7%), and chloroform (85%), and copper (95%), with mixtures of regulated and unregulated contaminants varying by region, water source, and public water system size. Multiple exceedances of health-based Maximum Contaminant Level Goals (MCLGs) were observed—including 54 samples exceeding zero tolerance for total trihalomethanes, 52 for bromodichloromethane, 11 for lead, and 10 each for PFOA and PFOS—along with enforceable Maximum Contaminant Level (MCL) exceedances for total trihalomethanes (3 samples) and PFAS compounds (9-10 samples for PFOA/PFOS). The findings underscore critical water quality concerns in socially disadvantaged communities and provide a foundation for future studies examining potential linkages between tapwater contaminant mixtures and breast cancer rates in vulnerable California populations facing compounded environmental stressors.