Trihalomethanes (chloroform, bromoform, bromodichloromethane, and chlorodibromomethane) are commonly found in drinking water that has been chlorinated or chloraminated. Trihalomethanes (THMs) form when chlorine reacts with organic matter in the water. THMs are found mainly in water that originally came from surface sources, such as rivers and lakes. THM levels are typically low in groundwater (produced by wells). THMs have been associated with increased cancer risk, at least in animals, and the EPA has for many years regulated the amount of THMs allowable in drinking water.