Occurrence, Seasonal Dynamics, and Operational Drivers of Elevated Chlorate Concentrations in Drinking Water: A National Case Study from the Czech Republic

The chemical disinfection of drinking water produces a diverse mixture of substances, referred to as disinfection by-products. Owing to their toxicity, the main components of this mixture are being gradually regulated. Therefore, the EU directive 2020/2184 requires member states to start regulating the content of HAAs, chlorite, and chlorate in addition to the existing THMs and bromate as of 2023. In the Czech Republic, chlorite has been regulated since 2004 and chlorate since 2018 (both with a limit value of 200 µg/L, from 2024 then 250 µg/L), so there is already some experience with their monitoring, occurrence, and addressing non-compliance. The non-compliance rate for chlorite decreased from the initial 3% to the current < 0.03%, whereas that for chlorate decreased only from 4% to the current 2.1%. The main source of chlorate in drinking water is disinfection using sodium or calcium hypochlorite. The investigation of problematic supplies revealed that the main causes of the above-limit findings of chlorate are the use of hypochlorite after the expiration date, storage in unsatisfactory conditions, or unjustified over-chlorination of water. Problems are confined to small areas with non-professional operations. Corrective measures are not complicated or costly; however, operators must be instructed in good operating practices.