Tracking disinfection by-products in northern Canada: impacts of dissolved organic matter and regional variation

Dissolved organic matter (DOM) concentrations are increasing globally in surface waters—a phenomenon known as browning—especially in northern ecosystems. Browning affects lake mixing, light penetration, oxygen availability, and microbial communities, posing challenges for drinking water safety. In northern communities, where surface water is generally treated without DOM removal, chlorination can produce disinfection by-products (DBPs), such as trihalomethanes (THMs) and haloacetic acids (HAAs), which are regulated for potential health risks. We assessed DBP concentrations in drinking water from eight northern communities in the Northwest Territories, Nunavut, and Nunavik, and examined relationships with DOM quantity and composition. Despite oligotrophic source waters with low dissolved organic carbon (DOC < 6 mg L-1), DBPs were detected in all treated samples, occasionally exceeding Canadian guidelines (up to 109 µg L 1 for THMs and 100 µg L 1 for HAAs). DBP formation correlated primarily with DOC and, secondarily, with pH and DOM optical properties. Regional DOM quality variations—characterized by optical and fluorescence indicators—were linked to DBP composition differences. These findings underscore the vulnerability of northern drinking water systems to DBP formation even under low-DOM conditions, and the importance of DOM quality as a driver of DBP variability. As climate change intensifies browning through precipitation, permafrost thaw, and runoff, DBP risks will rise, requiring enhanced monitoring, improved treatment, and long-term planning.