How a private well works

The USGS estimates that more than 43 million people in the United States get their drinking water from private wells. Unlike city water, nobody else tests, treats, or maintains that supply. Knowing how your well is built helps you spot problems and talk to a well contractor with confidence.

Three kinds of wells

EPA, drawing on USGS descriptions, sorts private wells into three types.

• Dug or bored wells are dug by shovel or backhoe and lined with stone, brick, tile, or similar material. They're wide and shallow, about 10 to 30 feet deep, and not cased continuously, so surface water can get in easily.
• Driven wells are made by driving pipe into the ground. They're cased continuously but still shallow, about 30 to 50 feet, and draw from aquifers near the surface, so they can be contaminated easily.
• Drilled wells are made with percussion or rotary drilling machines and can be thousands of feet deep. EPA says their depth and continuous casing give them a lower risk of contamination.

Most wells built in recent decades are drilled. Older farmhouses sometimes still have a dug well, which deserves extra attention to testing and to keeping surface water away.

The parts you should know

• Casing: the pipe that lines the well from the water-bearing layer to the surface. Along with grout around it, it keeps dirt and shallow water out. Steel and plastic are the most common materials. Some states set minimum casing lengths.
• Well cap: the cover on top of the casing. It keeps out debris, insects, and small animals, and has a screened vent. A cracked or loose cap is a common way bacteria get in.
• Well screen: at the bottom of the casing, it stops too much sediment from entering.
• Pitless adapter: a fitting that lets the water line leave the casing underground, below the frost line, while keeping a sanitary seal.
• Pump: EPA notes that jet pumps, mounted above ground, are common on shallow wells of about 25 feet or less. Submersible pumps, which sit down in the casing, are the norm for deeper wells.
• Pressure tank: in the house or a well pit. It stores some water under pressure so the pump doesn't start every time you open a tap.

Where the well should be

EPA says a well should be sited so rainwater flows away from it, because runoff can carry bacteria and chemicals into the well. It should also be at the required distance from septic systems, both yours and your neighbors'. States and local governments set these setback distances. See protecting your well from your septic system.

Who to call

Your state's water-well contractor licensing agency or your local health department can tell you about construction rules. EPA points to the National Ground Water Association for finding certified contractors. Use a licensed, insured contractor for drilling, pump work, and repairs. After any work on the well, test the water for bacteria. See well water testing.

Your yearly routine

Test at least once a year for total coliform bacteria, nitrates, total dissolved solids, and pH, as EPA recommends. Check the cap and the ground around the casing. Keep chemicals, fuel, and fertilizer away from the wellhead. Our checklists put this on one page.

Keeping water on in an outage

No power means no pump. See how to size a generator for a well pump if you want water during outages.