Domestic (Private) Supply Wells
What’s in Your Well Water?
USGS studies the quality of this key source of drinking water and potential concerns for domestic well users
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More than 43 million people—about 15 percent of the U.S. population—rely on domestic (private) wells as their source of drinking water. The quality and safety of water from domestic wells are not regulated by the Federal Safe Drinking Water Act or, in most cases, by state laws. Instead, individual homeowners are responsible for maintaining their domestic well systems and for monitoring water quality.
The USGS National Water Quality Program investigates the quality of water pumped from domestic wells across the United States. These wells are the sole source of drinking water and water for other household needs for most people in many rural areas. Health risks associated with contaminants in domestic well water include gastrointestinal illness related to bacteria and other pathogens and exposure to elevated concentrations of nitrate, arsenic, radon, lead, and organic compounds. Typically, the water supplied by domestic wells is not routinely tested. As a result, people using domestic-supply wells could be drinking water with elevated concentrations of some contaminants.
The quality and safety of water from privately owned domestic wells is not regulated under Federal or, in most cases, state laws. Homeowners are primarily responsible for maintaining their domestic well systems and for any water-quality monitoring. Federal regulations under the Safe Drinking Water Act (SDWA) for public water supplies, although not directly applicable to the regulation of domestic wells, provide useful concentration benchmarks for evaluating the quality of water from domestic wells in a human-health context.
Water Quality of Domestic Wells
In a study of 2,100 domestic wells, water pumped from about one in five wells contained one or more contaminants at a concentration greater than a human-health benchmark for drinking water.
The contaminants most often found at these elevated concentrations were inorganic chemicals, such as metals, radionuclides, and nitrate; all of these but nitrate are derived primarily from natural sources.
Man-made organic compounds, such as pesticides and solvents, were detected in more than half (60 percent) of the domestic wells sampled, but concentrations were seldom greater than human-health benchmarks (less than 1 percent of wells).
About half of the wells had at least one “nuisance” contaminant—a compound that impairs taste, odor, or other aesthetic considerations—at a level or concentration outside the range of values recommended by the U.S. Environmental Protection Agency.
Microbial contaminants (for example, bacteria) were detected in about one-third of the approximately 400 wells that had their water analyzed for those contaminants.
Contaminants found in domestic wells 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.
More information on the quality of water from domestic wells is provided in USGS publications that summarize the quality of water in Principal Aquifers in nine regions of the United States and specifically groundwater tapped by domestic wells in Principal Aquifers.
Where Are Domestic Wells Used?
USGS maps show where people across the Nation rely on private wells for their drinking water. The maps estimate the number of people using private wells in a given area, but individual wells are not identified. The maps can help determine where focused groundwater-quality testing or further studies are needed to help ensure safe drinking water from private wells. For instance, overlaying information on private well use with a map of potentially corrosive groundwater could be used to focus water-quality testing for lead in areas with a high density of private wells and a high likelihood of potentially corrosive groundwater.
Types of Domestic Wells
There are several types of domestic wells used by homeowners. The oldest wells were dug by hand. These wells typically are shallow and of large diameter (several feet), and may be lined with stones, brick, or tile. They typically extend only a short distance below the water table. Driven wells are constructed by driving small-diameter pipe into saturated unconsolidated sand and gravel. Driven wells typically are deeper than dug wells but are still relatively shallow. Most modern wells are drilled using truck-mounted machines and rotary or percussion methods. Drilled wells may be several hundred to more than a thousand feet deep, and can penetrate consolidated sediment and bedrock. Drilled wells in unconsolidated sediment have a well casing and a well screen to prevent collapse of the surrounding rock and sediment into the well. Drilled wells in bedrock typically are cased through the overlying soil and thin sediment and are open holes below. Domestic wells typically pump water from shallower depths than public-supply wells.
Interested in the water quality of public-supply wells?
Information on USGS studies of the deep wells that supply the water used by most of us for drinking can be found here.
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