Drinking-water analysis turns up even more toxic compounds
The chronic presence of genotoxic compounds at low levels in U.S. drinking water presents conflicting goals for some water utilities.
For the first time, researchers have published a study that quantifies the levels of iodo-acid disinfection byproducts (DBPs) in drinking water and that includes a toxicity analysis for each compound. In a collaborative study, analytical chemists, analytical biologists, engineers, and toxicologists analyzed water samples from 22 U.S. cities and 1 Canadian city. The findings could present a conflict for water utilities seeking the best technique for disinfecting drinking water, the authors note.
|Six-year occurrence study finds relatively low levels of highly toxic byproducts in U.S. drinking water.|
Susan Richardson, with the U.S. EPA’s National Exposure Research Laboratory, and Michael Plewa of the College of Agricultural, Consumer, and Environmental Studies at the University of Illinois Urbana−Champaign were interested in growing evidence showing that the formation of iodinated DBPs in drinking water may be higher when utilities use chloramines, rather than chlorine, ozone, or chlorine dioxide, as a disinfectant. Many U.S. utilities have switched from chlorine to chloramines to meet EPA Stage 1 and Stage 2 DBP rules designed to reduce DBP formation, Richardson says.
DBPs are created when the compounds used for disinfecting drinking water react with natural organic matter, bromide, or iodide. Research shows that iodoacetic acid is highly cytotoxic and more genotoxic in mammalian cells than bromoacetic acid, which is the most genotoxic of the haloacetic acids (HAAs) regulated in the U.S. Iodoacetic acid also has been shown to cause developmental abnormalities in mouse embryos.
In new research published in ES&T (10.1021/es801169k), Plewa, Richardson, and colleagues set out to develop an analytical method to quantify five iodo-acids in drinking water, measure the concentrations of iodo-acids in several water samples treated with chloramination as a disinfectant, and investigate the mammalian cell toxicity of seven synthesized iodo-acids and six iodo-trihalomethanes (iodo-THMs). The researchers found that five iodo-acids and two iodo-THMs were present in the water samples. The levels for these same chemicals were highest at treatment plants with relatively short free-chlorine contact times and were lowest at a chlorine-only plant with long free-chlorine contact times. Of the 13 compounds measured, 7 were genotoxic, they report. In general, compounds that contain an iodo-group have enhanced cytotoxicity and genotoxicity as compared with their brominated and chlorinated analogs.
“Iodo-acid DBPs are going to become very important in the near future,” Plewa predicts. “Fifty percent of the U.S. population lives within 50 miles of a coast.” The paper notes that urban areas with high levels of iodide in their source waters, including many coastal cities, had higher levels of the iodo-acids. This is likely due to the intrusion of seawater into natural source waters, Plewa says. “As people draw down water sources to meet population growth, more and more seawater gets into the water source,” he says. Other U.S. regions also have high iodide levels as a result of salt deposits left from ancient seas.
The collaborators say that they built on a nationwide DBP occurrence study (Environ. Sci. Technol. 2006, 40, 7175−7185) in which iodo-acids were identified for the first time as DBPs in drinking water disinfected with chloramines. The results also support research published in 1999 detailing just how iodo-organic compounds form during disinfection of iodide-containing natural waters. Urs von Gunten, a chemist at the Swiss Federal Institute of Aquatic Science and Technology (Eawag) and coauthor of the 1999 ES&T paper (33, 4040−4045), says he marvels at how far the researchers have come. “In our paper we developed the basic knowledge of the kinetics of this process,” he says. “But at that point, no one was talking about other iodo-organic compounds and the toxicity” of these DBPs, von Gunten says.
The paper includes a simple engineering solution for water utilities, Plewa adds. Once you know the relative toxicity and occurrence of these DBPs, “you can sit down and then you can modify your approach, so you can produce very good water . . . that is less toxic and includes fewer DBPs,” he says.
Despite the presence of the genotoxic compounds, the researchers say that they aren’t worried about the safety of any treated drinking water in the U.S. They note that if the plants studied had used only chlorine they would likely have exceeded the EPA standard for THMs and HAAs. Plewa notes that this raises competing issues for water utilities—how can they meet current EPA standards and produce drinking water free of potentially hazardous byproducts while simultaneously keeping it hygienically safe?