From the Source to Tap: Exploring the Nationwide Occurrence and Calculated Cytotoxicity of Regulated and Unregulated DBPs in U.S. Water Systems

Disinfection is essential for safe drinking water, yet it generates hundreds of disinfection byproducts (DBPs); many unregulated DBPs are more toxic than those currently regulated. This study provides a nationwide assessment of multiple DBP classes across 24 U.S. water utilities, linking their occurrence with distribution system dynamics and calculated cytotoxicity. Among 61 measured species, dichloroacetonitrile (DCAN), dibromoacetonitrile (DBAN), bromochloroacetonitrile (BCAN), dibromoacetamide (DBAM), monobromoacetic acid (MBAA), and dichloroacetamide (DCAM) emerged as the dominant calculated additive toxicity (CAT) drivers. CAT increased by 40-50% in the distribution systems of chlorination plants, indicating elevated CAT at distal locations relative to the point of entry within the measured DBP framework. In contrast, chloraminated plants exhibited more variable CAT behavior, including net decreases or delayed downstream peaks. Halogen-specific analyses revealed total organic bromine (TOBr) as a robust screening-level indicator of CAT, while total organic iodine (TOI) showed situational predictive value in iodide-rich, chloraminated waters. High CAT values were consistently associated with elevated halides, wastewater, and algal contributions, as well as select operational practices such as preozonation and short chlorine contact times before ammonia addition to form chloramines. Source-water dissolved organic carbon (DOC) and the Br/DOC ratio emerged as central, actionable controls linking precursor availability to halogen substitution pathways. These findings suggest a holistic approach of bromide control, organic precursor control, optimized treatment, and adoption of DOC, Br/DOC ratio, and TOBr-based monitoring is needed to produce better water for public consumption.