Man-Made Chemicals Found in Drinking Water at Low Levels

Low levels of about 130 of the man-made chemicals were detected in streams and rivers before treatment at the public water facilities (source water). Nearly two-thirds of those chemicals were also detected after treatment. Most of the chemicals found were at levels equivalent to one thimble of water in an Olympic-sized pool.

"Low level detection does not necessarily indicate a concern to human health, but rather indicates what types of chemicals we can expect to find in different areas of the country," said USGS lead scientist, Gregory Delzer. "Recent scientific advances have given USGS scientists the analytical tools to detect a variety of contaminants in the environment at low concentrations; often 100 to 1,000 times lower than drinking-water standards and other human-health benchmarks."

Testing sites include the White River in Indiana; Elm Fork Trinity River in Texas; Potomac River in Maryland; Neuse River in North Carolina; Chattahoochee River in Georgia; Running Gutter Brook in Massachusetts; Clackamas River in Oregon; Truckee River in Nevada; and Cache La Poudre in Colorado. The populations in communities served by these water treatment plants vary from 3,000 to over a million.

This study is among the first by the USGS to report on a wide range of chemicals found before and after treatment. The full source-water quality assessment and listing of chemicals are available online.

Chemicals included in this study serve as indicators of the possible presence of a larger number of commonly used chemicals in rivers, streams, and drinking water. The most commonly detected chemicals in the source water were herbicides, disinfection by-products, and fragrances. Many of these chemicals are among those often found in ambient waters of 186 rivers and streams sampled by USGS since the early 1990s, and are highly correlated with the presence of upstream wastewater sources or upstream agricultural and urban land use. About 120 chemicals were not detected at all.

Measured concentrations of chemicals detected in both source and treated water were generally less than 0.1 part per billion. Although potential human-health effects and risk were not assessed in this study, adverse effects to human health are expected to be negligible based on comparisons of measured concentrations and available human-health benchmarks.

More than 75 percent of source- and treated-water samples in this study contained 5 or more chemicals. The common occurrence of chemical mixtures means that the total combined toxicity may be greater than that of any single contaminant present. The USGS report identifies the need for continued research because the additive or synergistic effects on human health of mixtures of man-made chemicals at low levels are not well understood. The study also did not look at implications to ecosystems or aquatic health.

The USGS National Water-Quality Assessment Program is planning to complete as many as 21 additional surface-water assessments through 2013. A companion study is scheduled for release in 2009 that summarizes the occurrence of the same chemicals in high-production wells and the associated treated water in 13 states.