From where are disinfection by-products samples collected?

Disinfection by-products (DBPs) are typically formed when disinfectants used in the treatment of drinking water react with organic and inorganic materials present in the water. To accurately assess the levels of these by-products, sampling is most commonly conducted in the distribution system. This is because the distribution system represents the final stage where treated water is transported to consumers and is where DBPs can accumulate and vary due to factors such as residence time and water quality changes.

Sampling at the point of entry or point of use may not provide a complete picture of the DBPs that consumers may be exposed to, as these points do not account for the conditions within the distribution system. Groundwater sources, while important for assessing overall water quality and contamination, are not the primary concern when quantifying DBPs formed after disinfection processes. Thus, the distribution system is the most relevant sampling point for detecting and analyzing disinfection by-products effectively.