Impact of Salinization and Disinfectant Type on Regulated and Unregulated Disinfection Byproduct Formation, Speciation, and Relative Calculated Toxicity

The salinization of water is increasing from climate change and anthropogenic activity. Seawater intrusion elevates halide concentrations. While chloride and sulfate have aesthetic U.S. secondary maximum contaminant levels of 250 mg/L, halides like bromide and iodide influence disinfection byproduct (DBP) formation. This study examined how disinfection strategy (chlorination, preformed chloramination) affected DBP formation across a 0–30% seawater gradient. During emergencies, many coastal utilities lack alternative sources or blending capacity, making high intrusion levels relevant. In situ chloramination at 0–2% seawater was also investigated. Regulated and unregulated DBP classes were evaluated including, trihalomethanes, haloacetic acids, iodinated trihalomethanes (I-THM) and haloacetic acids, haloacetaldehydes, haloacetonitriles, haloacetamides, and nitrosamines. Relative cytotoxicity was assessed using calculated additive toxicity (CAT). Chlorination produced the highest DBP levels (>400 μg/L at 8% seawater) and CAT. Trihalomethanes exceeded regulatory limits at all seawater levels under chlorination. Preformed chloramination reduced DBPs and CAT, with no regulatory exceedances. However, I-THM (up to 35 μg/L) and N-Nitrosodimethylamine (NDMA) formation occurred (up to 19 ng/L). In situ chloramination reduced regulated DBPs, but not enough to meet trihalomethane regulations. Iodinated DBP formation was controlled during in situ, but NDMA levels increased with increasing seawater, surpassing health advisory levels (29 ng/L at 2% seawater).