Transport of Contaminants to Public Supply Wells
In 2001, the National Water-Quality Assessment (NAWQA) Program began an intensive study to assess the vulnerability of public supply wells to contamination from a variety of natural and manmade compounds. More than 900 wells within eight major water-supply aquifers across the United States are included in the study, Transport of Anthropogenic and Natural Contaminants to Public Supply Wells (TANC). The major objectives of the TANC study are
- Identify the dominant sources of contaminants in public supply wells in representative water-supply aquifers.
- Assess the effects of natural processes (such as dilution) and human activities (such as well-field management) on the occurrence of contaminants in public-supply wells in representative aquifers.
- Identify the major factors in different settings and at different spatial scales and incorporate them into public-supply well vulnerability assessments; develop simple methods and models for vulnerability to contamination in areas not studied and from newly emerging contaminants.
- Increase understanding of the potential effects of water-resource-development and management practices.
The national TANC study design includes two components—large-area studies and small-area studies. In 2003, large-area studies were begun in eight urban areas where ground water is the major source of public supply.
Dayton, Ohio, in the southeastern part of the WHMI Study Unit, was one of the sites chosen for a large-area study. In the Dayton area, more than 80 million gallons of water per day from the glacial aquifer system are produced for public supply.
A ground-water-flow model of the Dayton area (Dumouchelle, 1998) was updated and refined for this study. Particle tracking and travel-time analysis were used to delineate contributing areas to major public-supply wells. Water samples from 15 public supply wells were analyzed for a comprehensive list of manmade and natural contaminants. In addition, retrospective water-quality data were compiled from multiple sources — the USGS (including the NAWQA Program), Miami Conservancy District, Ohio Environmental Protection Agency, and City of Dayton. These data were used to identify the predominant anthropogenic and natural contaminants, and to estimate the distribution of redox conditions within the aquifer.
Similar large-area studies were conducted in the seven other areas—Pomperang River Basin, Connecticut; Central-Northern Tampa Bay Region, Florida; Salt Lake Valley, Utah; Eastern High Plains, Nebraska; Eagle Valley and Spanish Springs Valley, Nevada; Northeastern San Joaquin Valley, California; and Central Denver Basin, Colorado.
At four of these sites, more intensive studies (small-area investigations) were begun in 2004. Small-area studies involve drilling and sampling wells along a ground-water flowpath, analyzing of solid-phase data, and modeling transport of selected contaminants from land surface to a public supply well. Small-area studies are in the Pomperang River Basin, Connecticut; Central-Northern Tampa Bay Region, Florida; Salt Lake Valley, Utah; and Eagle Valley and Spanish Springs Valley, Nevada.
To download a USGS FactSheet XXX-05 PDF that describes the TANC study in detail, click here.