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Mayor's Update: TTHM


If you live in the City, you have received a recent letter stating that the drinking water being supplied to customers had exceeded the Maximum Contaminant Level (MCL) for total trihalomethanes (TTHM). Please note that you do not need to use an alternative water supply nor is the City under a boil water notice.

All drinking water sources could contain microorganisms such as bacteria and viruses that may cause serious illnesses. Drinking water is disinfected with free chlorine to destroy those microorganisms. Chlorine is the most common disinfectant for treating drinking water. When chlorine is added to water that has organic matter such as decaying plants and algae, disinfection by-products (or side effects) can occur. Trihalomethanes (THMs) are the most common type of by-product and form when free chlorine combines with organic precursors that are present in the source water supplied to the treatment plant. THMs are made up of four compounds: chloroform, bromodichloromethane, dibromochloromethane, and bromoform. THMs will generally form in water with higher organic content which usually includes surface water like lakes, reservoirs, streams, and rivers; groundwater from deep wells usually has lower organic content and are less likely to form THMs. The City of Blanco relies on surface water and is currently relying on purchased water from Canyon Lake Water Service Company (CLWSC). This increased time within the distribution system allows the potential for more byproducts to develop. According to samples collected on Sept. 19, 2022, the yearly TTHM average for the City’s water is 0.082 mg/L and the CLWSC yearly TTHM Average at Stallion Estates is 0.059 mg/L; the MCL for TTHM is 0.08 mg/L. Increased flushing should help to reduce the THMs due to the need to turn over the water source within the distribution system more often and the City has begun flushing twice a month on the North end of town will help pull old water to the furthest reaches of the distribution system.

Please contact the City Administrator Warren Escovy at (830) 833-4525 if you have any questions or concerns. For more information about disinfection byproducts in public water systems, please visit the Texas Commission on Environmental Quality’s website: https://www.tceq.texas.gov/drinkingwater/chemicals/dbp.