A drinking water DBP is formed when the chemical used for disinfecting the drinking water reacts with natural organic matter and/or bromide/iodide in the source water. Popular disinfectants include chlorine, ozone, chlorine dioxide and chloramine. Source waters include rivers, lakes, streams, ground water and sometimes seawater. We have only known about DBPs since 1974, when chloroform was identified as a DBP resulting from the chlorination of tap water. Since then, hundreds of DBPs have been identified in drinking water.
Millions of people in the United States are exposed to these drinking water DBPs every day. While it is vitally important to disinfect drinking water, as thousands of people died from waterborne illnesses before we started disinfection practices in the early 1900s, it also is important to minimize the chemical DBPs formed. Several DBPs have been linked to cancer in laboratory animals, and as a result, the EPA has some of these DBPs regulated. However, there are many more DBPs that have still not been identified and tested for toxicity or cancer effects. Currently, we have only identified less than 50 percent of the total organic halide that is measured in chlorinated drinking water.
There is much less known about DBPs from the newer alternative disinfectants, such as ozone, chlorine dioxide and chloramine, which are gaining in popularity. Are these alternative disinfectants safer than chlorine? Or do they produce more harmful by-products? And what about the unidentified chlorine DBPs that people are exposed to through their drinking water - both from drinking and showering/bathing? The objective of EPA's research is to find out what these DBPs are - to thoroughly characterize the chemicals formed in drinking water treatment - and to ultimately minimize any harmful ones that are formed.
EPA recently completed a major nationwide DBP occurrence study in which it sampled drinking water across the United States (disinfected with the different disinfectants and with different water quality, including elevated levels of bromide in the source water). In addition to obtaining important quantitative information on these new DBPs (to help in prioritizing health effects testing), important new discoveries were made regarding the use of alternative disinfectants. While the use of alternative disinfectants lowered the levels of the four regulated trihalomethanes and five haloacetic acids (as compared to chlorine), many of the DBPs were formed at higher levels with these alternative disinfectants. For example, the highest levels of iodinated DBPs were found in chloraminated drinking water, the highest levels of trihalonitromethanes were found in pre-ozonated drinking water, MX and brominated MX analogs (BMXs) were highest at a plant using chlorine dioxide (followed by chlorine-chloramines) and dihaloaldehydes were highest at a plant using chloramines and ozone.
EPA's new work includes obtaining quantitative occurrence information on the iodo-acids that were identified for the first time in the Nationwide DBP Occurrence Study. Chloraminated waters (where levels are expected to be highest) will be targeted for this work. In addition, a toxicity-based identification approach (using mammalian cell and medaka fish assays) will be used to ensure toxicologically important DBPs are not being missed.