Which statement best describes THMs in drinking water?

• A. Trihalomethanes are formed when chlorine reacts with organic matter in drinking water
• B. Trihalomethanes are formed by nitrate reduction
• C. Trihalomethanes are formed by ozone reacting with organics
• D. Trihalomethanes are formed by chloramine reactions with minerals

Answer: (A)

THMs are disinfection byproducts formed when a chlorine-based disinfectant interacts with natural organic matter in source water. When chlorine is added to water containing organic material, it can oxidize and substitute halogen atoms on those organics, producing trihalomethanes such as chloroform and related compounds. The amount of THMs formed depends on how much organic matter is present, how much chlorine is used, how long the water sits with the disinfectant, and conditions like temperature and pH; more organic material and longer contact times tend to increase THM formation. Utilities monitor THM levels and may reduce precursors, adjust disinfection strategies (for example, using alternative methods or chloramination), or optimize treatment to balance effective disinfection with byproduct control. Nitrate reduction, ozone reacting with organics, or chloramine interactions with minerals describe different processes or byproducts and don’t explain THM formation.