New Waves March 2009

Breaking news about water resources research and education in Texas

Water quality of private wells: A potential concern

More than 20 percent of private domestic wells sampled nationwide contain at least one contaminant at levels of potential health concern, according to a study by the U.S. Geological Survey (USGS).

USGS scientists sampled about 2,100 private wells in 48 states and found that the contaminants most frequently measured at concentrations of potential health concern were inorganic contaminants, including radon and arsenic. These contaminants are mostly derived from the natural geologic materials that make up the aquifers from which well water is drawn. In about four percent of the sampled wells, nitrate was the most common inorganic contaminant at concentrations greater than the federal drinking-water standard for public-water supplies (10 parts per million).

"The results of this study are important because it shows that a large number of people may be unknowingly affected," said Matt Larsen, USGS associate director for water. "Greater attention to the quality of drinking water from private wells and continued public education are important steps toward the goal of protecting public health."

Other contaminants found in the private wells were man-made organics, including herbicides, insecticides, solvents, disinfection byproducts, and gasoline chemicals. Few organic contaminants (7 out of 168) exceeded health benchmarks, and were found above health benchmarks in less than one percent of sampled wells.

Contaminants found in private wells usually co-occurred with other contaminants as mixtures rather than alone, which can be a concern because the total combined toxicity of contaminant mixtures can be greater than that of any single contaminant. Mixtures of contaminants at relatively low concentrations were found in the majority of wells, but mixtures with multiple contaminants above health benchmarks were uncommon (about four percent). The USGS report identifies the need for continued research because relatively little is known about the potential health effects of most mixtures of contaminants, and the additive or synergistic effects on human health of man-made chemical mixtures at low levels are not well understood.

Bacteria, including total coliform bacteria and Escherichia coli, were found in as many as one third of a subset of 400 wells. These bacteria are typically not harmful but can be an indicator of fecal contamination. About half of the 2,100 sampled wells had at least one property or contaminant outside recommended ranges for cosmetic or aesthetic purposes, such as total dissolved solids, pH, iron, and manganese.

Private well owners, who generally are responsible for testing the quality of their well water and treating, if necessary, can contact local and state health agencies for guidance and information about well maintenance, water quality and testing options, and in-home water treatment devices. Access the Quality of Water from Domestic Wells in the United States Web site for complete study findings, related links, and recommendations for private well owners.

Portions of this story are from a USGS Newsroom release. Read the complete news release.

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