USGS Study: Water Quality A Potential Concern in Private Wells

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).

About 43 million people, 15 percent of the nation's population, use private wells, which are not regulated by the Federal Safe Drinking Water Act.

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.

Nitrate was the most common inorganic contaminant derived from man-made sources-such as from fertilizer and septic-tanks-that was found at concentrations greater than the federal drinking-water standard for public-water supplies (10 parts per million).  Nitrate was greater than the standard in about 4 percent of sampled wells.

The study shows that the occurrence of selected contaminants varies across the country, often following distinct geographic patterns related to geology, geochemical conditions and land use.

For example, elevated concentrations of nitrate were largely associated with intensively farmed land, such as in parts of the midwest corn belt and California’s central valley. Radon was found at relatively high concentrations in crystalline-rock aquifers in the Northeast, in the central and southern Appalachians, and in central Colorado.

Other contaminants found in private wells included man-made organics, including herbicides, insecticides, solvents, disinfection by-products, and gasoline chemicals. Few organic contaminants (seven 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).

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 it, if necessary, can contact local and state health agencies for guidance and information about well maintenance and siting, 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 related links to sources of information and recommendations for private well owners.

(Editor’s Note: To access this article in its entirety, go to http://www.usgs.gov/newsroom/article.asp?ID=2173).