Disinfection by-products are substances that form when chlorine interacts with impurities in ground water. Chlorine is added to control microorganisms such as bacteria and viruses that may be present in source water. Samples are collected from locations within the distribution system that have high water age, usually large reservoirs and water towers, and are expected to have the highest levels of four trihalomethanes and five haloacetic acids. Since the levels of organic matter in the source water are low and the utility adds relatively small amounts of chlorine, disinfection by-product levels are also very low in Madison drinking water.
Results of testing for Disinfection By-Products
Federal regulations require that some water utilities test drinking water for specific unregulated contaminants, substances for which the EPA has not set a maximum contaminant level (MCL) or MCL Goal. Monitoring for these substances helps the EPA determine where certain contaminants occur and whether the agency needs to regulate these contaminants to protect public health.
Between 2013 and 2015, water utilities around the country monitored a group of 30 contaminants, including hormones, volatile organic compounds, perfluorinated compounds (including PFOA and PFOS), metals, chromium-6, and 1,4-dioxane. All Madison wells were tested twice for 28 chemical contaminants. Some samples were also collected from the distribution system.
Results of testing for Unregulated Contaminants
More information on the regulation (UCMR3) and the contaminants can be found on the EPA website.