Private wells supply drinking water to millions of Americans, but the available statistics combine national estimates, modeled populations, selected samples, and local program results. The figures below identify the population, period, geography, measurement type, and threshold used for each result.
Key Private Well Statistics
The most broadly useful figures are:
- In 2021/current EPA estimates, more than 23 million U.S. households rely on private wells for drinking water.
- On the EPA’s current page, around 15% of the U.S. population, or over 43 million people, rely on private wells.
- In 2015, 42.5 million people provided their own domestic water in the United States, a USGS category that may include cisterns.
- A 2019 study modeled nearly 40 million domestic-well users in the contiguous United States.
- The modeled domestic-well population was 37.29 million people in the contiguous United States in 2010.
- From 1990 to 2000, the modeled domestic-well population rose 1.5%, from 36.70 million to 37.25 million.
- Between 2010 and 2015, self-supplied domestic withdrawals declined 8% in the United States.
- In 2015, self-supplied domestic withdrawals were 3,260 million gallons per day nationally.
- In 2015, about 98% of self-supplied domestic withdrawals came from fresh groundwater.
- In 2015, the national average self-supplied domestic-use coefficient was 77 gallons per capita per day.
- In a selected 1991–2004 study of U.S. principal aquifers, about one in five of 2,100 private wells had a contaminant above a health benchmark.
- In the same 1991–2004 study, arsenic exceeded 10 micrograms per liter in 6.75% of samples.
- In the selected 1991–2004 samples, at least one organic compound was detected in about 60% of wells.
- In a 1995–2007 regional study of New England, New Jersey, and New York, radon was at or above proposed 300 pCi/L in 95% of 943 wells.
- In agricultural study areas, 22% of domestic wells exceeded the nitrate MCL.
- From 2015 to 2018, a Connecticut program collected 700 additional private-well samples.
- In New Jersey’s 2016 study, arsenic was identified by 20% of post-PWTA households versus 4% of pre-PWTA households.
- In Madison County, New York, 39% of more than 400 tested supplies were coliform-positive.
Contents
- How many people use private wells
- Private well participation and withdrawal trends
- Private well water use and groundwater withdrawals
- Contaminants in domestic wells
- Regional private well water-quality patterns
- Testing, regulation, and well management
- Measured outcomes from testing and policy
How Many People Use Private Wells?
The EPA estimates that more than 23 million U.S. households rely on private wells for drinking water, based on its 2021/current presentation. The same EPA page estimates that around 15% of the U.S. population, or over 43 million people, rely on private wells; both figures are rounded estimates. EPA, Private Drinking Water Wells
USGS measures a related but broader category called self-supplied domestic water. In 2015, 42.5 million people provided their own domestic water, although the category may include cisterns and is not identical to private-well use. USGS, Domestic Water Use
The modeled domestic-well series for the contiguous United States gives a narrower historical comparison:
| Period or year | Domestic-well population | Geography and definition |
|---|---|---|
| 1990 | 36.70 million | Modeled domestic-well population, contiguous United States |
| 2000 | 37.25 million | Modeled domestic-well population, contiguous United States |
| 2010 | 37.29 million | Modeled domestic-well population, contiguous United States |
| 2019 study | Nearly 40 million | Modeled domestic-well users, contiguous United States |
From 1990 to 2000, the modeled population rose 1.5%, from 36.70 million to 37.25 million. It then rose from 37.25 million in 2000 to 37.29 million in 2010. USGS, Updated Information on Locations of Domestic Well Use
Private Well Participation and Withdrawal Trends
Population and water-use measures do not move identically. Between 2010 and 2015, USGS estimated that self-supplied domestic withdrawals declined 8%, while total domestic water use, combining public and self-supplied use, declined about 3%.
These are national estimates, and self-supplied withdrawals are often unmetered. USGS, Domestic Water Use
The modeled 2010 domestic-well population was 37.29 million people, or 12.2% of the contiguous-U.S. population. The 2019 study estimate of nearly 40 million is modeled and should not be treated as a direct census or as an independently verified count. USGS, Updated Information on Locations of Domestic Well Use
The available series therefore shows a relatively stable modeled domestic-well population from 2000 to 2010 alongside lower self-supplied withdrawals from 2010 to 2015. Because the measures use different definitions and periods, they should be read as related indicators rather than as a single continuous trend.
Private Well Water Use and Groundwater Withdrawals
USGS estimated 3,260 million gallons per day of self-supplied domestic withdrawals in the United States in 2015. That amount represented about 1% of all U.S. withdrawals, using an all-use denominator. USGS, Domestic Water Use
A separate USGS water-use summary reports the same 2015 total and breaks it into source categories:
| 2015 source category | Withdrawal | Scope |
|---|---|---|
| Groundwater | 3,210 million gallons per day | Self-supplied domestic source category |
| Surface water | 49.1 million gallons per day | Self-supplied domestic source category |
| Total | 3,260 million gallons per day | Self-supplied domestic withdrawals |
Groundwater accounted for about 98% of self-supplied withdrawals in 2015. The USGS cautions that self-supplied domestic water is broader than private wells, so these figures describe a related water-use category rather than a count of private-well withdrawals alone. USGS, Water use across the conterminous United States, water years 2010–20
The national average self-supplied domestic-use coefficient was 77 gallons per capita per day in 2015. This is a USGS per-capita coefficient, not a metered average for individual private-well households; state coefficients ranged from 36 gallons per capita per day in Connecticut to 186 in Nevada. USGS, Water use across the conterminous United States, water years 2010–20
Contaminants in Domestic Wells: National Statistics
National water-quality results come from selected sampled wells and study periods, not a census of all private wells. The USGS 1991–2004 study of domestic wells in principal aquifers found that about one in five of 2,100 private wells had a contaminant above a health benchmark. USGS, Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004
Selected benchmark exceedances in that study were:
| Constituent | Result in 1991–2004 samples | Threshold type or level |
|---|---|---|
| Arsenic | 6.75% exceeded 10 micrograms per liter | Health benchmark |
| Uranium | 1.74% exceeded 30 micrograms per liter | Health benchmark |
| Manganese | 5.22% exceeded 300 micrograms per liter | HBSL |
| Strontium | 7.32% exceeded 4,000 micrograms per liter | HBSL |
| Boron | 1.30% exceeded 1,000 micrograms per liter | Health benchmark |
| Lead | No sampled well exceeded 15 micrograms per liter | Action level |
The same selected sample found iron above 300 micrograms per liter in 19.1% of samples and manganese above 50 micrograms per liter in 21.3%, using secondary standards. These secondary standards concern non-health characteristics such as taste, staining, or appearance and should not be treated as equivalent to health-benchmark exceedances. USGS, Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004
Screening thresholds also show why detection and exceedance are different measures. Uranium exceeded one-tenth of its benchmark in 24.35% of samples, arsenic exceeded one-tenth of its benchmark in 42.7%, and at least one organic compound was detected in about 60% of wells, while 17 wells, or 0.80%, had an organic compound above an MCL or HBSL. USGS, Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991–2004
Regional Private Well Water-Quality Patterns
Regional results can differ sharply from national selected-sample results. In a 1995–2007 study of crystalline-rock aquifers in New England, New Jersey, and New York, arsenic was at or above 10 micrograms per liter in 13.3% of 2,054 wells, uranium was at or above 30 micrograms per liter in 14.2% of 556 wells, and combined radium was at or above 5 pCi/L in 3.2% of 564 wells. USGS, Quality of water from crystalline rock aquifers in New England, New Jersey, and New York, 1995–2007
The same regional study reported these results for fluoride and radon:
- Fluoride was at or above 4 milligrams per liter in 1.4% of 2,167 wells, and at or above 2 milligrams per liter in 7.5% of those wells; the latter used a secondary standard.
- Radon was at or above a proposed 300 pCi/L in 95% of 943 wells and at or above a proposed 4,000 pCi/L in 33% of those wells.
Other local studies illustrate geographic and land-use variation. In U.S. agricultural study areas grouped in USGS Circular 1350, 22% of domestic wells exceeded the nitrate MCL. USGS, Circular 1350: Table 6-1
In Park County, Colorado, during 2001–2004, total coliform was detected in about 11% of domestic wells, while about 91% of groundwater samples had radon at or above a local proposed level of 300 pCi/L. USGS, Ground-Water Quality and Potential Effects of Individual Sewage Disposal System Effluent on Ground-Water Quality in Park County, Colorado, 2001–2004
Private Well Testing, Regulation, and Management
Testing and management programs provide activity and participation measures rather than national coverage rates. A Connecticut effort collected 674 samples in 2016, with 7% exceeding an arsenic or uranium MCL, and collected 700 additional samples from September 2015 to August 2018. CDC, Success Stories
CDC recipients across multiple U.S. jurisdictions identified 210,000 well records, digitized 102,000 quality records, distributed 9,060 free sample kits, and conducted 484 outreach events. In Tacoma-Pierce County, Washington, bacterial-testing compliance rose from below 70% to almost 80% since 2015; the local figures are approximate. CDC, Success Stories
These counts show the scale of information and outreach work, but they do not establish that a particular share of all private wells was tested. The CDC’s private-well testing resources describe program activities and should be interpreted with each jurisdiction’s coverage and eligibility limits. CDC, Private Well Water Testing Program
Measured Outcomes from Private Well Testing and Policy
A New Jersey study published in 2016 examined 12 counties during the Private Well Testing Act era. It found that 21% of area wells were above the 5-micrograms-per-liter arsenic standard, while the survey response rate was 37%. CDC, Arsenic in Private Well Water Part I: Impact of the New Jersey Private Well Testing Act on Household Testing and Mitigation Behaviors
The study reported differences between households surveyed after and before the policy period:
| Reported measure | Post-PWTA households | Pre-PWTA households | Study geography and note |
|---|---|---|---|
| Arsenic identified | 20% | 4% | New Jersey, 12 counties |
| Arsenic treatment reported | 19% | 3% | Self-reported; New Jersey, 12 counties |
These survey comparisons indicate higher reported identification and treatment after PWTA, but they are not a national causal estimate. The 37% response rate and the study’s selected geography limit generalization. CDC, Arsenic in Private Well Water Part I: Impact of the New Jersey Private Well Testing Act on Household Testing and Mitigation Behaviors
In Madison County, New York, 39% of more than 400 tested supplies were coliform-positive. Sanitary-cap drilled wells were 67% less likely to be bacterially contaminated, an observational result rather than a randomized intervention estimate. CDC, Success Stories
After Madison outreach, 40% of participating households disinfected wells, 61% replaced caps, and 35% requested resampling. These percentages apply to participants in that program, not to all private-well households. CDC, Success Stories