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.
Why is drinking water chlorinated/chloraminated?
Drinking water is often chlorinated or chloraminated to kill microorganisms that could cause serious illnesses. Overall, chlorination of drinking water has benefited the public health enormously. There are other methods of disinfecting public drinking water, but they are often more expensive, and the potential health effects of using these other methods are generally less well understood. Additionally, some alternative disinfectants do not remain effective as the water is transported from the source to the tap through the distribution system.
Is there a regulatory standard for these chemicals in drinking water?
Drinking water utilities that use chlorination or chloramination are required by law to sample water throughout their distribution system, average the total THM measurements, and report the results to the New Mexico Environment Department. The Maximum Contaminant Level (MCL) permissible for Total THMs in drinking water by New Mexico and federal law is currently an average of 80 parts per billion (ppb) over four consecutive quarters. In other words, a utility has not violated the standard unless the average system-wide level over the past year is more than 80 ppb. There are no standards currently for the individual trihalomethanes.
What are the New Mexico Environment Department and EPA doing to look at the issue of THMs in drinking water?
In 2006 the MCL for THMs and HAAs was revised to clarify that the MCL must be met at every location, and not just in the system as a whole.