How a private well works: from the aquifer to your tap, and what your well's design means for testing

The short answer: a private well is a hole in the ground held open by a pipe, the casing, that reaches down into an aquifer, a layer of sand, gravel or rock whose pores and cracks are full of water. A pump pushes that water up the casing, out through a fitting below the frost line and into a pressure tank, usually in the basement or a utility room. Compressed air in the tank pushes water to your faucets, and a pressure switch restarts the pump when the pressure drops. Well water is simply groundwater that you pump yourself.

The part that matters to an owner: how your well was built, and which layer it draws from, decide what can get into the water. A 25-foot driven point in sand and a 300-foot drilled well cased through clay have different weak spots, so they need different tests. This guide covers the geology, the parts and the path to your tap, then turns it into a testing table, using only USGS, EPA, CDC, state well programs and Penn State Extension.

What well water is: groundwater and aquifers

Well water starts as rain and snowmelt. The USGS Water Science School describes it this way: some of the precipitation that lands on the ground "infiltrates into the subsurface," and the part that keeps moving down "until it reaches rock material that is saturated is groundwater recharge" (USGS, What is groundwater?).

Near the surface, in the unsaturated zone, soil and rock "contain air and some water." Below the water table, in the saturated zone, water "fills the tiny spaces (pores) between rock particles and the cracks (fractures) of the rocks." Dig a hole at the beach and it partly fills from below; USGS says you have "essentially dug a 'well' that exposes the water table, with an aquifer beneath it" (USGS, Aquifers and groundwater).

Not every wet layer is useful. USGS reserves the word for a layer that gives water up easily: "When a water-bearing rock readily transmits water to wells and springs, it is called an aquifer." The Minnesota Department of Health (MDH) gives the counterpart: a confining layer is "a layer of sediment or rock that slows or prevents the downward movement of water," and "a thick layer of clay is an example."

Groundwater moves, often slowly: USGS says it "may move several meters in a day" in some materials and "only a few centimeters in a century" in others. The trip filters a lot out. MDH says bacteria, viruses and many chemicals "are removed or filtered from the water as it moves downward through silt, sand, and gravel deposits." It also adds minerals: groundwater quality, per MDH, "depends on the type of soil, sediment, or rock through which the water is moving, the length of time water is in contact with geologic materials, and whether any contaminants are present."

Unconfined vs. confined aquifers

USGS describes a layered system: an unconfined aquifer on top with no confining layer, and in some places confined aquifers below it, held "under pressure from the rock weight" (USGS, Artesian water and artesian wells). A well into a confined aquifer is an artesian well; if the pressure lifts water above the wellhead, it flows. Depth alone does not make a well artesian. Our artesian well guide covers flowing wells, state rules and why artesian water still needs testing.

Dug, driven (sand point) and drilled wells

EPA and CDC both sort private wells into three types: dug or bored, driven, and drilled. Type sets how deep a well can go and whether it has a continuous casing, the two things the sources tie most closely to contamination risk.

MDH's Well Owner's Handbook puts the three shallow types together: "Dug wells, bored wells, and drive-point wells are often less than 50 feet deep, and are more likely to be contaminated by surface water, sewage from septic systems, or chemical spills." The numbers differ from EPA's (30 to 50 feet for driven wells); both describe typical wells, not limits. If yours is a sand-point well, row A of the testing table below applies: coliform bacteria and nitrate at least every year, plus extra samples after heavy rain or snowmelt, because MDH says nitrate in drive-point and other shallow wells "may only be present during the spring, or after a heavy rainfall."

How deep is a water well?

There is no national typical depth; local geology sets it. USGS says water "may be plentiful just 10 feet below the land surface, or a deep well might have to be drilled many hundreds of feet." MDH lists what drives the choice: "1) depth to groundwater, 2) the groundwater quality, and 3) the geologic conditions at the well site."

The state figures we found, each from its own source:

• Minnesota: wells "range in depth from 15 feet to over 1,000 feet." In glacial deposits, the depth of wells "can vary greatly — even if they are located next door to each other" (MDH handbook, updated January 2014).
• Pennsylvania: wells "can range from less than 10 feet to more than 1,000 feet deep. The average well is usually between 100 and 200 feet" (Penn State Extension, Private Water Systems FAQs, updated October 13, 2025).

Depth doesn't predict yield either. USGS says an adequate domestic supply is "usually 5 gallons per minute," and getting it "depends upon the permeability of the rock": dense granite at the surface may yield "little or no water," while porous sandstone far down may yield hundreds of gallons per minute. USGS also notes that "Domestic wells typically pump water from shallower depths than public-supply wells" (USGS, Domestic (private) supply wells).

Your own well's depth is on its well log, if one was filed; finding well and septic records shows where to look it up.

The parts of a private well and what each one does

EPA and USGS adapt their part descriptions from the National Ground Water Association; MDH's handbook adds sizes. A drilled well with a submersible pump, the common modern setup, has these parts:

Whether those parts are required depends on the state. Penn State says Pennsylvania "is one of only a few states that do not have mandatory statewide construction standards for private water wells," grout "is usually not used on private wells in Pennsylvania unless it is required by local ordinances or requested by the homeowner," and "Less than 20% of wells in Pennsylvania have all five" of its recommended sanitary features (Penn State, Sanitary Water Well Construction, updated June 15, 2026; Five Parts of Sanitary Water Well Construction, updated August 24, 2026). Minnesota, by contrast, has had well construction rules since July 1974, per MDH.

It matters: Penn State reports that a 2002 USGS study of more than 100 Pennsylvania private wells found "ungrouted wells were three times more likely to be contaminated with E. coli bacteria compared to grouted wells." Penn State also notes that, in general, "it is not possible to grout an existing well."

How water gets from the aquifer to your tap, step by step

1. Recharge. Rain and snowmelt soak down to the saturated zone, nearby for an unconfined aquifer, from a distance for a confined one.
2. Into the well. Water enters through the screen, or the open hole in bedrock. Pumping lowers the water level around the well into a cone of depression, and USGS explains: "This pressure differential causes water to flow to the well."
3. Up the casing. A submersible pump below the water level pushes the water up. On a shallow well, a jet pump at the surface pulls it up by suction instead. A flowing artesian well needs no pump at all.
4. Out below the frost line. The water leaves the casing through the pitless adapter, usually 6 or 7 feet down, and runs to the house in a buried service pipe. "Because the water service pipe is buried deeper than the frost line, the water will not freeze" (MDH).
5. Into the pressure tank. As water is pumped in, it "compresses the air in the tank" until the pressure reaches the cut-off, typically 40 to 60 psi, and the pump stops (MDH).
6. Out the faucet. When you open a tap, "the air pressure in the tank forces water through the pipes" until pressure falls to the cut-in point, usually 20 to 40 psi, and the pressure switch starts the pump again.

This water is rarely metered: USGS says self-supplied domestic withdrawals "are rarely metered or reported." And it comes out of a shared aquifer. USGS warns that pumping too much too fast can make a well "yield less and less water and even run dry," and that it "can even cause your neighbor's well to run dry if you both are pumping from the same aquifer."

If the pump short-cycles or pressure surges, our well water maintenance guide has a symptom table and says who to call.

How to tell what kind of well you have

CDC says "Looking at the cover and outside (casing) of your well may help you determine the type of well it is." These clues come from MDH's and USGS's descriptions. Treat them as hints, not proof:

The well log, the construction record the driller files, is the real answer: depth, casing and the layers drilled through. Where to look:

• Minnesota: the Minnesota Well Index shows "location, depth, geology, construction, and static water level." MDH began requiring well records in 1974, so the "majority of wells constructed prior to 1974 will not be on file."
• Wisconsin: the DNR's well records search: reports show "drillhole depth and the casing depth"; for wells built before 1988, search by township, range and section.
• Pennsylvania: the Pennsylvania Groundwater Information System "holds hundreds of thousands of water well records."
• Washington: Ecology's well report viewer; locations are accurate only "to the nearest township, section, and range."

Buying a house? The well inspection guide covers seller records and what an inspector checks.

Private well vs. city water: who is responsible

Most Americans don't have a well. USGS estimates that 283 million people, "about 87 percent of the total United States population," got household water from public supply in 2015, and that "An estimated 42.5 million people in the United States, or 13 percent of the population, provided their own water for domestic use in 2015." Nearly all of that self-supplied water, 98 percent, came from fresh groundwater (USGS Circular 1441, Dieter and others, 2018). City systems differ: 61 percent of public-supply withdrawals came from surface sources such as lakes and streams.

You'll see other counts too. A USGS data release by Ayotte and others (2017) says "Approximately 44.1 M people in the U.S. use water from domestic wells" (USGS data release). EPA says "Around 15 percent of the U.S. population (over 43 million) rely on private wells" and estimates "more than 23 million households" do (EPA, Private drinking water wells, updated August 25, 2026). The figures come from different studies and years, so they don't match exactly.

Shares in our ten states run from nearly one-half of Maine to 13 percent of New York. In head count, USGS says Pennsylvania had the most self-supplied people in the country, then Michigan and New York.

The legal difference is about the water, not the hole. EPA says the Safe Drinking Water Act exists to regulate "the nation's public drinking water supply" and "does not regulate private wells which serve fewer than 25 individuals" (EPA, SDWA overview). Wisconsin DNR puts the line plainly: private wells "have fewer than 15 connections and serve fewer than 25 people," and their protection and maintenance "is largely the responsibility of homeowners" (Wisconsin DNR, Information for private well owners).

CDC: "Tap water from private wells is not regulated, treated, or monitored by officials." A state may regulate how a well is built, as Minnesota does, and still leave the water to you. For a side-by-side of public and private duties, see Is well water safe to drink?

How your well's design changes what you should test for

Here the geology pays off. Contaminants reach a well from the surface (down the outside of the casing, past a loose cap, or through a shallow aquifer) or from the aquifer itself (its rock, or pollution already spread through it). Well type and construction mostly change the first path, not the second.

Penn State describes the surface path: surface contamination of individual wells "is usually caused by surface water flowing down the outside of a well casing through the annular space and/or from a loose-fitting or absent well cap." But construction has limits: "Wells drilled into aquifers that are contaminated by animal wastes, septic systems, or surface water can contain coliform bacteria regardless of well-construction practices." In Penn State's own study of new wells built with a sanitary cap and grout, 29 percent still had coliform bacteria.

Minnesota writes this into its rules: a well with less than 50 feet of watertight casing that isn't cased through a confining layer such as clay at least 10 feet thick counts as a "sensitive" water-supply well (MDH handbook; the legal definition is in Minnesota Rules 4725.0100, subpart 43a), and under Minnesota Rules 4725.4450 its isolation distances from sources that drain into the soil "are doubled."

For a full checklist by situation, use our which-tests tool; for sampling and timing, the how-to-test guide; for your health department's list, your page under states.

Worked example 1 (illustrative). You pull the well log for a lake cabin and it says: drive-point, 28 feet, 1¼-inch casing, sand. That's row A. MDH recommends testing it more often than once a year, and since nitrate in a shallow well may appear only in spring or after heavy rain, it makes sense to time one sample after snowmelt. If corn fields surround the lot, row B adds a pesticide question for your health department.

Worked example 2 (illustrative). A farmhouse well log shows 240 feet drilled, casing to 85 feet through 30 feet of clay, open hole in sandstone below. That's row E: the clay and long casing make surface contamination less likely, and in Minnesota it wouldn't be "sensitive." But MDH ties groundwater quality to the rock the water moves through. Keep the yearly test CDC recommends, and if your state flags arsenic, test for it at least once.

Questions people ask