At around 8:00 p.m. on March 16, 2025, thallium (Tl) concentration at the Dahetan cross-city water monitoring section on the Leishui River (between Chenzhou and Leiyang) was detected at 0.13 μg/L—exceeding the national surface water quality standard limit of 0.1 μg/L for centralized drinking water sources. This posed a potential threat to downstream drinking water safety. Following the incident, both Chenzhou and Leiyang cities launched emergency environmental responses. Comprehensive actions were taken, including full-basin water sampling, source tracing, pollution interception, thallium removal measures, enhanced flow regulation, and real-time water quality monitoring along the river to safeguard public drinking water. According to public reports, thallium concentrations at downstream water plants remained below the national drinking water limit of 0.1 μg/L, and water supply safety was not compromised. Notably, the automated monitoring system played a key role by issuing an early warning when thallium exceeded limits at the Dahetan section, buying time for emergency response and supporting rapid environmental monitoring. This incident highlights the critical importance of robust water quality monitoring and pollution source management systems. In regions with high thallium pollution risks, further improvements to these systems remain essential.