Public Supply Wells

Updates to Groundwater-Quality Trends Tool

Updates to Groundwater-Quality Trends Tool

The trends mapper now has water-quality trend information for additional well networks. Try the mapper to learn about trends in concentrations of pesticides, nutrients, metals, and organic contaminants in U.S. domestic and public-supply wells.

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Are you among the more than 100 million people in the U.S. who relies on a public-supply well for your drinking water? Although the quality of finished drinking water from public water systems is regulated by the EPA, long-term protection and management of the raw groundwater tapped by public-supply wells requires an understanding of the occurrence of contaminants in this invisible, vital resource.

The USGS National Water Quality Program investigates the quality of water pumped from public-supply wells across the United States. These wells are the source of drinking water and water for other household needs for more than one-third of the U.S. population. There are about 140,000 public water systems that use groundwater as their source.

Population relying on public wells for drinking water

Population relying on public wells for drinking water (image is from USGS Circular 1346).

Although the quality of finished drinking water (after treatment and before distribution) from these public water systems is regulated by the EPA under the Safe Drinking Water Act (SDWA), long-term protection and management of groundwater, a vital source of drinking water, requires an understanding of the occurrence of contaminants in untreated source water. Sources of drinking water are potentially vulnerable to a wide range of manmade and naturally occurring contaminants, including many that are not regulated in drinking water under the SDWA.

 

Water Quality of Public Supply Wells

In a study of 932 U.S. public-supply wells, water pumped from about one in five source-water samples (that is, before treatment) contained one or more contaminants at a concentration greater than a human-health benchmark for drinking water. Supporting information and summary data for the study can be found here.

  • Naturally occurring trace elements and radionuclides accounted for about three-quarters of contaminant concentrations greater than human-health benchmarks in source-water samples.

  • Manmade organic compounds, such as pesticides and solvents, were detected in nearly two-thirds of samples, but typically at a concentration that did not exceed a human-health benchmark.

  • Many organic (manmade) contaminants detected in source water also were detected in treated water at similar concentrations.

  • Human-health benchmarks are not yet available for many manmade contaminants, including some that were frequently detected in source water.

  • Contaminants found in groundwater used as a source of public supply usually co-occurred with other contaminants as mixtures, rather than alone, which is a potential concern because the total toxicity of a mixture can be greater than that of any single contaminant.

Public wells where water samples were collected and sampled for chemical contaminants

Public wells where water samples were collected and sampled for chemical contaminants (image is from USGS Circular 1346).

Read informative fact sheets about current water-quality conditions in public supply wells that pump water from Principal Aquifers across the country. More detailed information on the quality of water from public-supply wells is provided in USGS publications that summarize the quality of water in Principal Aquifers in nine regions of the United States.

 

What causes public-supply wells to be vulnerable to contamination?

Public-supply-well vulnerability to contamination starts with groundwater vulnerability to contamination. The vulnerability of the water from public-supply wells to contamination depends on contaminant input in the area that contributes water to a well, the mobility and persistence of a contaminant once released to the groundwater, and the ease of groundwater and contaminant movement from the point of recharge to the well. Wells within a single aquifer, however, may not be equally vulnerable to contaminants in the aquifer because individual wells produce unique mixtures of the groundwater from different depths in the aquifer and with different ages (time since recharge).

A study done from 2001 to 2011 sheds light on factors that affect the vulnerability of water from public-supply wells to contamination. The study also identified measures that can be used to determine which factor (or factors) plays a dominant role at an individual public-supply well. These measures are particularly useful for indicating which contaminants in an aquifer might reach an individual public-supply well and when, how, and at what concentration they might arrive. Case-study examples show how such information can be used to improve water quality.

 

Interested in the water quality of domestic (private) wells?

Find information on USGS studies of domestic water-supply wells used for drinking water by thousands of people in rural areas.

 

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