Trihalomethane Concentrations Measured at the Tap are a Poor Predictor of Haloacetonitrile Concentrations in Small Rural Drinking Water Systems

Haloacetonitriles are drivers of disinfection byproduct (DBP) toxicity with in vitro assays. Regulated trihalomethanes have historically been an indicator of haloacetonitriles; however, differences in precursors, disinfectants used, and network variability reduce the reliability of trihalomethanes as an indicator for toxicity. The spatial–temporal variability in haloacetonitriles and trihalomethanes in rural drinking water systems was analyzed using monthly tap water collected in two systems for two years (nhomes = 34; nsamples = 623). Median concentrations were 69.8 μg L–1 (Interquartile Range (IQR) 43.2–98.5) for total trihalomethanes and 5.1 μg L–1 (IQR 4.5–5.9) for total haloacetonitriles. Concentrations were influenced by temperatures and changes in precursors which were exacerbated by treatment operations (e.g., prechlorination, greater source water DBP forming potential). The ratio of haloacetonitriles to trihalomethanes was greater during the spring and in homes with decreased water ages. Brominated species contributed a larger portion of the mass concentration of haloacetonitriles than the trihalomethanes. Trihalomethanes and haloacetonitrile concentrations were poorly correlated (rPearson = 0.41). The cytotoxicity of trihalomethanes and haloacetonitriles were more strongly correlated (rPearson = 0.72) in the presence of increased Br–. Variation in haloacetonitriles with respect to surface water quality and system operation conditions differ significantly from trihalomethanes, making trihalomethanes a poor surrogate. In systems with elevated Br–, trihalomethanes may serve as an indicator of the occurrence of brominated-haloacetonitriles.