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The current study investigates the occurrence and specific conditions that favour the formation of iodo-THMs during the disinfection of source waters with elevated sodium content (over 200 mg/L sodium) and naturally occurring ammonium. Based on the previous data, the current study focused on 16 water treatment plants (WTPs) using source waters with sodium content between 10 mg/L and 760 mg/L and naturally occurring ammonium. Source water, treated water and mid-point distribution water samples were collected under both winter and summer conditions. Samples were stabilized, shipped cold, and analysed within 4 days from collection for the six iodo-THMs, using solid phase micro extraction gas chromatography with electron-capture detection (SPME-GC-ECD). Data for 20 other neutral DBPs (including THMs, haloacetaldehydes, haloacetonitriles, chloropicrin, cyanogen chloride, cyanogen bromide), 7 nitrosamines, as well as general water quality parameters, elemental analysis, free/total chlorine and bromide concentrations, were also collected. Questionnaires were used to collect information about the plant treatment processes. One or more of the iodo-THM congeners were detected in 15/16 WTPs investigated. Maximum total iodo-THM concentrations determined in the survey ranged from 0.11 µg/L to 26.82 µg/L. The maximum total iodo-THM concentrations were > 1 µg/L, in 8/16 WTPs and the highest maximum total iodo-THM concentration of >10 µg/L was seen in 4/16 WTPs. The total concentrations and the speciation of iodo-THMs were strongly influenced by the presence of ammonium in the source water and by the treatment regime (e.g., free chlorine residual). The bromide content of the source water did not have a significant influence on iodo-THM concentrations or speciation. The results of this study show that elevated sodium levels and the presence of naturally occurring ammonium in the source water are good markers for selecting WTP with the potential to form elevated levels of iodo-THMs in the treated water and thus, could provide the basis for future decisions regarding changing secondary disinfectant from chlorine to chloramine. This is important in Canada since WTPs using water sources with elevated salinity typically have difficulty maintaining the total THM levels to within the guideline level of 100 µg/L. These findings also support the need to monitor for naturally occurring ammonia, especially in high saline waters. Because iodo-THMs were found to occur in a large number of the systems investigated, more information about the toxicology of iodo-THMs would be important in assessing the potential risks for human exposure to iodo-THMs.

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