Trace Contaminants in Drinking Water Chemicals Objectives: This research represents the most comprehensive national investigation of trace contaminants in water treatment chemicals ever performed. Key objectives were to establish whether there were widespread problems with contaminants in drinking water chemicals, quantify contamination levels in typical water treatment chemicals, identify likely sources of problems, and provide practical recommendations for utilities and manufacturers on how to limit contamination. Highlights: In general, no widespread problems with trace contaminants in U.S. water treatment chemicals were found, but serious isolated contamination incidents continue to occur. These were often transport-related. Problems with incoming chemical deliveries were typically averted when on-site inspection and evaluation programs were used by the utilities. Proper control of the manufacturing process and associated raw materials can improve the quality of treatment chemicals, for example recent improvements in bromate composition of sodium hypochlorite. Trace contaminants contributed by coagulants during full- and pilot-scale studies partitioned primarily into the residuals streams rather than into finished water. Results/Findings: The most commonly reported contamination incidents from the utility survey were transport-related, including improperly cleaned delivery vehicles and transfer hoses. Widespread problems with contaminants in treatment chemicals were rare, though serious isolated contamination incidents continue to occur, even with NSF Standard 60 certified products. Problems with incoming chemical deliveries were typically detected by utilities when on-site inspection and evaluation programs are utilized, including simple activities such as verifying paperwork, visual inspections, observation of unusual odors or color of product, and basic physical tests. Poorer quality raw materials, such as recycled materials, result in relatively poorer quality treatment chemical products. More trace contaminants were contributed by coagulants than other treatment chemicals, but these contaminants predominantly partitioned into residuals streams rather than finished water during treatment. Lowering or adding new regulatory limits, e.g., recent changes in arsenic and bromate MCLs, could affect chemical selection by utilities and/or force manufacturers to lower levels of particular trace contaminants. Even though the NSF Standard 60 approach is conservative with respect to predicting impact of treatment chemicals on finished water quality, limiting doses calculated by this method using data from this research were still higher than those typically required at U.S. treatment facilities.