Well exposed: exploring the chemical and microbial composition in well and municipal drinking waters in Iowa†

The Safe Drinking Water Act (SDWA) regulates water quality in public drinking water systems, leaving most individuals who obtain their drinking water from private wells unprotected by this legislation. Given that 15% of the U.S. population relies on unregulated, privately owned wells for their drinking water (well drinking water; WDW), there is an urgent need to assess whether WDW contains elevated levels of water quality constituents that could detrimentally affect human health. Additionally, the SDWA does not regulate many emerging microbial contaminants, including drinking water-associated pathogens that can infect immunocompromised individuals (DWPIs), which are part of the broader group of microbial contaminants estimated by the CDC to cause 7.15 million waterborne illnesses annually in the U.S. This study compared concentrations of 33 chemical parameters and the absolute abundance of two DWPIs in 20 well and 20 municipal drinking water (MDW) samples in northeast Iowa. Differences in microbial community structure were also assessed using 16S rRNA amplicon sequencing. Samples were collected from 11 municipal systems, and WDW samples were selected based on proximity to municipal service areas. WDW samples, on average, contained higher concentrations of most chemical contaminants and DWPIs, and exhibited twice the species richness of MDW samples. Among regulated chemicals, only nitrate exceeded the SDWA limit, and only in one WDW sample. At the microbiome level, WDW and MDW samples had distinct community compositions, with the specific aquifer supplying the water explaining the greatest variance in structure. These findings provide new insights into potential exposures among private well users.