Here is a question most Americans answer wrong: besides you, who tests your drinking water?
If a utility bills you for water, the answer is a federal law, a set of enforceable limits and a state agency that checks the paperwork. If you draw from a well in your own yard, the answer is nobody. Not EPA, not your state, and quite possibly not your local health department either.
The federal estimates of how many households that covers do not quite agree, which is itself worth knowing. EPA’s basic drinking water page says about 10 percent of people in the United States rely on private wells. A US Geological Survey circular on domestic well quality, published in 2009, put it at more than 43 million people, about 15 percent of the population at the time. CDC’s 2026 page on private drinking water lands between them, estimating roughly one American in eight, about 41.8 million people, using 2015 USGS water-use data and Census population figures.
Call it somewhere between one household in ten and one in seven. Whichever number is closest, the split determines who is responsible for noticing a problem, and that responsibility is the first thing to get straight about your own water.
Two systems, one glass of water
The line between them is a headcount, not a judgment about water quality.
A public water system, in EPA’s definition, delivers water for human consumption through pipes or other constructed conveyances to at least 15 service connections, or serves an average of at least 25 people for at least 60 days a year. It can be publicly or privately owned. Those two thresholds are the entire test.
EPA sorts public systems into three kinds. A community water system serves the same population year-round, which is what most people picture. A non-transient non-community system regularly serves at least 25 of the same people at least six months a year, which covers schools, factories and hospitals running their own supply. A transient non-community system serves people passing through, like a campground or a highway rest stop.
By EPA’s count, there are over 148,000 public water systems in the country, and systems regulated by EPA and its delegated states and tribes supply drinking water to 90 percent of Americans.
Everything below that headcount is a private well, or a small shared system that has not crossed the threshold. A deep, well-built municipal well and a deep, well-built private well can be the same hole in the same aquifer. The paperwork is what differs, and the paperwork turns out to matter enormously.
What the Safe Drinking Water Act requires, and where it stops
Congress passed the Safe Drinking Water Act in 1974 and amended it in 1986 and 1996. Under it, EPA sets legal limits on more than 90 contaminants in drinking water, along with the testing schedules and laboratory methods that systems must follow.
A limit is not the only tool. For some contaminants EPA sets a maximum contaminant level, an enforceable ceiling. For others it requires a treatment technique, a mandated process where measuring a number at the tap is not workable. Article 700 in this library covers that vocabulary and the annual report where you will meet it.
Enforcement usually is not federal. States and tribes can take primary enforcement responsibility, known as primacy, if their standards are at least as strict as EPA’s. In practice that means your water system answers to a state drinking water program, which reviews sampling results, issues violations and decides on deadlines.
Three obligations shape what a customer sees. Systems sample on a published schedule using approved methods. Under the Public Notification Rule, they must tell customers about violations and certain situations, with faster deadlines for problems that threaten health immediately. And EPA’s Unregulated Contaminant Monitoring Program periodically requires systems to test for substances that have no standard yet, which is how national occurrence data gets built for contaminants before they are regulated.
Now the limit of all this. The Safe Drinking Water Act regulates the system. It does not regulate your building. Water that leaves a treatment plant in compliance can pick up metals from the service line and the plumbing between the main and your kitchen tap, which is why a clean utility report and a clean glass of water are not the same claim. Article 698 covers that route in detail.
On a private well, you are the utility
EPA does not soften this. The agency’s private wells page states that the quality and safety of water from private domestic wells are “not regulated by the Federal Government under the Safe Drinking Water Act.” EPA adds that most state governments and laws do not regulate them either. EPA’s overview of the law gives the mechanism: the statute does not reach private wells serving fewer than 25 individuals.
Then EPA says who does hold the job: “Private well owners are responsible for delivering safe drinking water to their households.” The agency frames that as water meeting the same public drinking water standards for chemical contaminants, micro-organisms, heavy metals and radionuclides that a utility must meet. Same target. No sampling schedule, no inspector, no violation letter.
Local government fills some of the gap, unevenly. CDC, citing a 2016 national profile of local health departments, reports that 56 percent of them regulate, inspect or license private drinking water. That leaves close to half that do not, and even where a county program exists it usually governs how a well is drilled, not what comes out of it a decade later.
The practical consequence is a missing feedback loop. In a public system, a bad result triggers a process: resampling, a violation, a notice, a deadline. On a well, a bad result exists only if someone chooses to look for it.
Most people do not. A 2017 survey of Wisconsin private well users, published in Science of the Total Environment, found that about half had tested their well at any point in the previous ten years, and of those, only about one in ten had tested in the past twelve months. The problems they reported most often were iron and hardness, which affect taste and laundry rather than health.
What testing finds when it happens is not reassuring. The 2009 USGS circular sampled roughly 2,100 domestic wells across 48 states and found that about 23 percent had at least one contaminant above a human-health benchmark, with microbial contaminants in as many as a third of the roughly 400 wells tested for them. A 2018 study of 118 Maryland wells in the International Journal of Environmental Research and Public Health found 43.2 percent failed at least one federal health-based standard.
EPA’s own guidance gives well owners a schedule to work from: test annually for total coliform bacteria, nitrates, total dissolved solids and pH, more often when young children, older adults or a pregnant or nursing person live in the house, and immediately after flooding, nearby land disturbance or construction, any repair to the well system, or any change in taste, smell or color. Article 695 covers how to do that testing and which laboratory to use.
How a well is built decides much of the risk
Not all wells are equivalent, and the differences are physical rather than regulatory.
Dug and bored wells are the shallowest, roughly 10 to 30 feet deep by EPA’s description, wide in diameter, lined with stone, brick or tile, and not cased continuously. They tap water close to the surface, and they are the oldest wells on most properties.
Driven wells are built by driving pipe into the ground, typically 30 to 50 feet deep, and they are cased the whole way. EPA still flags them as easy to contaminate, because the aquifer they reach sits near the surface.
Drilled wells are made with percussion or rotary rigs, can reach thousands of feet, and require continuous casing. EPA attributes their lower contamination risk to exactly those two features, depth and unbroken casing.
The casing is the point. It is the tube that holds the hole open from the water-bearing layer to the surface, and with grout packed around it, EPA says it keeps dirt and excess water out of the well. Carbon steel, plastic and stainless are the common materials, local geology usually dictates which, and some states set a minimum casing length. A well cap with a vent keeps out insects, debris and small animals. A pitless adapter carries the pipe out below the frost line without breaking the seal.
Siting does the rest. EPA’s instruction is simple and widely ignored: the ground should slope so rainwater drains away from the well, because water that pools there can carry surface bacteria and chemicals down alongside the casing.
Septic systems deserve specific attention, since EPA lists leach field effluent among the sources of microorganisms in well water. Required separation distances are set locally, so the number that applies to your property comes from your county health department rather than from any article. What does generalize is EPA’s maintenance advice: keep solvents, petroleum products and pesticides out of a septic system entirely, and pump and inspect it as often as your local health department recommends.
One concept worth carrying away is the idea of groundwater under the direct influence of surface water. EPA notes this is decided site by site under state criteria. A well can be legally a groundwater source and behave, hydraulically, like a stream. If yours does, bacteria arrive on a timescale of days.
Groundwater is not a sealed container
The most common belief about well water is also the most dangerous one: this well has been fine for thirty years, so it is fine. Thirty uneventful years is not thirty years of evidence. If nobody sampled, the record is empty.
Water quality in a single well moves, and researchers who sample repeatedly keep finding it. A 2025 study of private wells in southern Alabama, published in the Journal of Water and Health, found pH outside the acceptable range in 92 percent of wells and total coliform in 25 percent, with iron, lead, nitrate and arsenic exceedances behind them. The finding that matters here is that 68 percent of the wells changed between sampling rounds, frequently testing positive for total coliform in one campaign and negative in another. One clean test is a photograph, not a guarantee.
Season moves it too. A 2019 study of unregulated domestic wells in Arizona, in the International Journal of Environmental Research and Public Health, measured contaminants across two wet and two dry seasons. Arsenic, PFOA, PFOS and sucralose ran significantly higher in the dry seasons, while E. coli ran higher in the wet ones. Which month you sample changes what you find.
Conditions above ground move as well. A 2018 study in Environmental Science and Technology traced road salt into private wells in New York, with the highest chloride levels in wells downgradient of a salt storage facility or within 30 meters of a major roadway. In bench tests, rising chloride accelerated galvanic corrosion and dezincification in plumbing materials, leaching more metal into the water. The contaminant that entered the aquifer was salt. The problem it produced was metals from the household pipes, which no groundwater test was looking for.
A new septic system next door, a new irrigation well drawing the water table down, a fuel tank, a change in land use upgradient: all of these can alter a well that has never changed before. Nothing about groundwater is static, and nothing about a well guarantees it stays the way you found it.
Floods, and the weeks that follow
A flooded well is the one situation where the right sequence genuinely matters, and the instinct to run the tap and see is wrong.
EPA’s guidance starts with electricity: stay away from the well pump while it is flooded, to avoid shock. Do not drink or wash with the water. Have a well or pump contractor clean and disinfect the well before the pump goes back on, then keep pumping until the water runs clear, and call your county or state health department if it does not.
CDC adds the detail most people get wrong. Disinfection should be done by a well or pump contractor for safety reasons. Drink bottled water until the water is confirmed safe. And if the water smells of fuel or chemicals, neither boiling nor disinfection will fix it, which turns the situation from a cleanup into a call to the health department.
After disinfection, the timing is counterintuitive. CDC says to wait 7 to 10 days before sampling, because chlorine has to flush completely out of the system before a result means anything. Test for total coliform plus either E. coli or fecal coliform. Then retest in two to four weeks, and again in three to four months, and monitor bacterial quality at least twice a year afterward.
That sequence exists because the least informative test in the whole process is the one taken right after a well is chlorinated. A single clean sample in a freshly disinfected well tells you the chlorine worked, not that the source is safe.
Shared wells, community wells, and the house that changes hands
Shared wells are the middle case, and their legal status is decided by the same headcount as everything else. Two to a dozen houses on one well is a private arrangement. Cross 15 service connections, or supply an average of 25 people at least 60 days a year, and that well becomes a public water system under EPA’s definition, with sampling, reporting and an annual report to its users.
Below the line, the hard part of a shared well is rarely hydrology. It is governance. Who pays when the pump fails at 11 p.m., who schedules testing, who decides to install treatment and who pays for it, and what happens when one household sells. A written shared-well agreement answers those questions in advance. Many older arrangements have nothing in writing at all.
A property sale is the one moment when a well usually gets real scrutiny, and no federal rule requires any of it. EPA’s own page on state programs notes that most states have zero or limited regulations for private drinking water wells. So a water test at closing happens because a buyer asks for it, a lender requires it, or a local ordinance says so.
If you are buying, ask for these before you sign:
- The well log or construction report: total depth, casing length and material, year drilled, and the driller’s license
- Which type it is, since a 25-foot dug well and a 400-foot drilled well carry different risks
- The last three test results, with the laboratory named and the dates visible
- The location of the septic tank and leach field, on this property and on the neighboring ones
- What treatment equipment is installed, what each unit is certified to reduce, and when it was last serviced
- Whether the well is shared, and if so, what the written agreement says
- Whether the property has ever flooded, and what was done afterward
If the seller cannot produce a well log, that is useful information rather than a dealbreaker. It tells you that you are buying a well nobody has records for, and you should price a full water test and a well inspection into the deal.
What to do in the next month
If you are on a utility, find out which system serves you and read its annual report, remembering that it describes the system and not the plumbing in your walls.
If you are on a well, write down four things: the well type, its depth, the date of the last water test, and where the nearest septic system sits. If you cannot fill in three of the four, that is your answer, and the next step is your county health department or a state-certified laboratory.
Call sooner than that in five situations: a pregnancy or a new baby in the house, a household member with a weakened immune system, flooding anywhere on the property, any digging or construction nearby, and any change in how your water looks, smells or tastes. Those are not wait-and-see events.
Nobody is going to mail you a notice about your well. The notice is the test you order.
Sources
- U.S. Environmental Protection Agency. “Private Drinking Water Wells.” epa.gov
- U.S. Environmental Protection Agency. “Learn about Private Water Wells.” epa.gov
- U.S. Environmental Protection Agency. “Protect Your Home's Water.” epa.gov
- U.S. Environmental Protection Agency. “What to Do With Your Private Well After a Flood.” epa.gov
- U.S. Environmental Protection Agency. “Private Drinking Water Well Programs in Your State.” epa.gov
- U.S. Environmental Protection Agency. “Potential Well Water Contaminants and Their Impacts.” epa.gov
- U.S. Environmental Protection Agency. “Information about Public Water Systems.” epa.gov
- U.S. Environmental Protection Agency. “Overview of the Safe Drinking Water Act.” epa.gov
- U.S. Environmental Protection Agency. “Drinking Water Regulations.” epa.gov
- U.S. Environmental Protection Agency. “Basic Information about Your Drinking Water.” epa.gov
- Centers for Disease Control and Prevention. “Private Drinking Water and Public Health.” cdc.gov
- Centers for Disease Control and Prevention. “How to Disinfect Wells After an Emergency.” cdc.gov
- U.S. Geological Survey. “Quality of Water from Domestic Wells in Principal Aquifers of the United States, 1991-2004 (Circular 1332).” pubs.usgs.gov
- PubMed. “Millner S, et al. "Drinking private well water: Groundwater quality and management of wells in southern Alabama." Journal of Water and Health, 2025. DOI: 10.2166/wh.2025.380.” doi.org
- PubMed. “Ornelas Van Horne Y, et al. "Seasonal Variation of Water Quality in Unregulated Domestic Wells." International Journal of Environmental Research and Public Health, 2019. DOI: 10.3390/ijerph16091569.” doi.org
- PubMed. “Malecki KMC, et al. "Private-well stewardship among a general population based sample of private well-owners." Science of the Total Environment, 2017. DOI: 10.1016/j.scitotenv.2017.05.284.” doi.org
- PubMed. “Murray RT, et al. "Prevalence of Microbiological and Chemical Contaminants in Private Drinking Water Wells in Maryland, USA." International Journal of Environmental Research and Public Health, 2018. DOI: 10.3390/ijerph15081686.” doi.org
- PubMed. “Pieper KJ, et al. "Impact of Road Salt on Drinking Water Quality and Infrastructure Corrosion in Private Wells." Environmental Science & Technology, 2018. DOI: 10.1021/acs.est.8b04709.” doi.org