In parts of France, the temperature of tap water has risen to as high as 28 degrees Celsius due to prolonged heatwaves. This unusual warmth is being reported in certain communes, with the Lyon metropolitan area experiencing tap water temperatures reaching 21 degrees Celsius. Experts say it will take until the end of October for water temperatures to return to normal levels. The rise in tap water temperature is linked to the warming of underground water sources known as aquifers, which supply drinking water to many regions. In the Rhône River's alluvial aquifer, water is treated at a facility in Villeurbanne. Temperatures at this facility have been measured daily since May, and a steady increase has been recorded. In the winter, temperatures typically hover around 13 or 14 degrees Celsius, but by May, they had already risen to 18 degrees. By July and August, the temperature at the factory outlet reached 21 degrees. According to Marion, a production manager, the ground absorbs and retains heat, which then warms the water networks. To address the issue, the Grand Lyon region plans to prioritize the coolest wells in its aquifer and adjust the chlorine dosage used to disinfect water. Chlorine is the primary treatment method at the factory outlet. However, during heatwaves, chlorine dissipates more quickly in the pipelines. This means that the concentration of chlorine in the water can increase from 0.20 milligrams per liter in winter to 0.30 milligrams per liter during hot periods. While this might affect the taste of the water slightly, the change is temporary and does not pose any health risks, according to the production manager. The large volume of water in the system means that it will take several weeks for the temperature to decrease once the heatwave subsides. Authorities are monitoring the situation closely and adjusting their treatment processes to ensure water quality remains safe and acceptable for consumers.