DIRECT COMPARISON OF CONTAMINANT EXPOSURES AND POTENTIAL RISK IN PUBLIC AND PRIVATE TAPWATER IN A SOLE-SOURCE AQUIFER SETTING

In the United States (US), federal and state agencies regulate and monitor public-supply drinking water while private-supply monitoring is rare. Insufficient comparable information on contaminant-mixture exposures and risks between private- and public-supplies undermines tapwater (TW) consumer decision-making. We compared private- and public-supply residential point-of-use TW at Cape Cod, Massachusetts, where both supplies shared the same groundwater source. TW from 10 private- and 10 public-supply homes was analyzed for 487 organic, 38 inorganic, 8 microbial indicators, and 3 in vitro bioactivities. Concentrations were compared to existing protective health-based benchmarks, and aggregated Hazard Indices (HI) of regulated and unregulated TW contaminants were calculated along with ratios of in vitro exposure-activity cutoffs. Seventy organic and 28 inorganic constituents were detected in TW. Median detection counts were comparable between public- and private-supply samples, but median cumulative concentrations were substantially higher in public supply due to disinfection byproducts in six locations with chlorine‑disinfected TW and corresponding lower heterotrophic plate counts. Concentrations equivalent to Environmental Protection Agency maximum contaminant (nitrate) and treatment action (lead and copper) levels were exceeded only in private-supply TW samples. Exceedances of health-based HI screening levels of concern were common to both public and private TW supplies. Results indicate comparable cumulative human-health risk from diverse contaminant exposures in private- and public-supply TW in a shared source-water setting.