USGS Groundwater Studies Help Explain Contamination in Water Wells
New U.S. Geological Survey groundwater studies explain what, when, and how contaminants may reach public-supply wells.
All wells are not equally vulnerable to contamination because of differences in three factors: the general chemistry of the aquifer, groundwater age, and direct paths within aquifer systems that allow water and contaminants to reach a well.
More than 100 million people in the United States receive their drinking water from public groundwater systems, which can be vulnerable to naturally occurring contaminants such as radon, uranium, arsenic, and man-made compounds, including fertilizers, septic-tank leachate, solvents and gasoline hydrocarbons.
The USGS tracked the movement of contaminants in groundwater and in public-supply wells in four aquifers in California, Connecticut, Nebraska and Florida. The importance of each factor differs among the various aquifer settings, depending upon natural geology and local aquifer conditions, as well as human activities related to land use and well construction and operation. Findings in the four different aquifer systems can be applied to similar aquifer settings and wells throughout the nation.
“Our findings can help public-supply well managers protect drinking water sources by prioritizing their monitoring programs and improving decisions related to land use planning, well modifications, or changes in pumping scenarios that might help to reduce movement of contaminants to wells,” said Sandra Eberts, USGS groundwater study team leader.
Research on the vulnerability of public-supply wells began in 2001. The USGS also has been working since 1991 to study the occurrence of more than 600 naturally occurring and man-made chemicals from more than 1,100 wells used for public supply across the nation. Scientists found that chemicals are frequently detected, often in mixtures, but seldom at concentrations likely to affect human health.
The quality of drinking water from the nation’s public-water systems is regulated by the U.S. Environmental Protection Agency under the Safe Drinking Water Act. USGS studies are intended to complement drinking water monitoring required by federal, state, and local programs, which focus primarily on post-treatment compliance monitoring.
In the Central Valley aquifer system near Modesto, Calif., the USGS found that agricultural and urban development have enabled uranium to move from sediments to water in the upper part of the aquifer. This water can drain down the well when it is not pumping and enter the lower aquifer. When pumping resumes, contaminant concentrations can be temporarily elevated in water pumped from the well. As a result of these findings, public-supply well managers have changed their pumping schedule, which has reduced the amount of contaminated water pumped from the well.
In the glacial aquifer system in Woodbury, Conn., the USGS found that the young age of the water throughout the aquifer makes it vulnerable to contamination from man-made compounds. The USGS also found that dry wells used in Woodbury to capture stormwater runoff reroute the potentially contaminated water directly into the aquifer used as a drinking water source. This direct transfer prevents soil and unsaturated sediments near the land surface from filtering out some of the contaminants.
In the High Plains aquifer near York, Neb., the USGS found some contaminants in a public-supply well that seems protected by overlying clay. Nearby irrigation wells have allowed water containing nitrate and volatile organic compounds to leak down from an overlying shallow aquifer into the aquifer that serves as the drinking water source for the public-supply well.
In the Floridan aquifer system near Tampa, Fla., the USGS found that a large percentage of young water and contaminants from a shallow sand aquifer travels quickly along natural conduits until it reaches a supply well in a lower rock aquifer that serves as a drinking water source. Because of these natural conduits, the supply well is vulnerable to the man-made contaminants in the upper aquifer, and the mixing of waters from the two aquifers has caused arsenic concentrations to increase in water reaching the supply well.