According to the USGS, 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. 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. However, the USGS feels that the study of the four different aquifer systems can be applied to similar aquifers and wells throughout the nation.
Examples of specific chemical findings from the USGS study:
* In the Central Valley Aquifer 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.
* In the Glacial Aquifer in Woodbury, Conn., it was 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 near Tampa, Fla., it was 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.
The study of public-supply well vulnerability to contamination is one of five national priority topics being addressed by the USGS with their National Water-Quality Assessment (NAWQA) Program. The study began in 2001 with the following general objectives:
1. Identify the dominant contaminants and sources of those contaminants in public-supply wells in representative water-supply aquifers across the Nation;
2. Assess the effects of natural processes (such as degradation) and human activities (such as irrigation) on the occurrence of contaminants in public-supply wells in representative aquifers;
3. Identify the factors that are most important to incorporate into public-supply well vulnerability assessments in different settings and at different spatial scales;
4. Develop simple methods and models for screening public-supply wells for vulnerability to contamination in unstudied areas and from newly emerging contaminants; and
5. Increase understanding of the potential effects of water-resource development and management decisions on the quality of water from public-supply wells.
Approximately 35% of the U.S. population receives their drinking water from public groundwater systems. Public drinking water systems are considered public when 25 or more people are connected to the well or there are at least 15 service connections for a minimum 60 days per year.
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