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How Well Drilling Works

What actually happens on site, from the first site visit to the moment your tap runs clear, and what each step is for.

A residential water well isn't just a hole in the ground. It's a built structure with a casing, a seal, a pump, and a series of construction steps that each protect the water supply in a specific way. Here's what those steps are, in the order a licensed contractor typically performs them, and why each one matters.

1. Site Assessment and Water Needs

Before any drilling starts, a contractor works out whether the site can support a well and how big that well needs to be. According to the National Ground Water Association (NGWA), most homes need a well that can produce about 7 gallons of water per minute; if a site can't meet that, the alternative is usually a storage system holding at least one day's water supply for the household.

To judge whether a given site can deliver that, contractors and homeowners review existing records of nearby wells and groundwater studies on file with local and state water departments and natural resource agencies. NGWA notes these records typically include depths, rock types, water levels, and expected yields for the area, which is far more reliable than guessing. Because low-yielding or dry wells aren't uncommon in some parts of the country, NGWA specifically recommends drilling the well before building the house on a vacant lot, since a poor water supply undermines the value of a home built on top of it.

2. Choosing a Drilling Method

Per the EPA, drilled wells (as opposed to shallower dug or driven wells) are constructed using percussion or rotary-drilling machines. The choice between them, and between variants like mud rotary or air rotary, comes down to the geology the crew expects to pass through. Softer ground drills faster; rock and bedrock take longer and require different bit types and drilling fluids, which is a major reason drilling costs vary so much by region. See our cost guide for how geology and depth translate into dollars.

Depth also depends heavily on local geology. As one example of how variable this is, the Rhode Island Department of Environmental Management (whose guidance the EPA republishes on its private wells site) describes drilled wells in that state as typically penetrating 100 to 400 feet into bedrock, since the well has to intersect a water-bearing fracture in the rock to produce water. Other regions look completely different: some markets have groundwater within a few dozen feet of the surface, others require going down several hundred feet or more. This is exactly why a contractor's on-site assessment and local well records matter more than any single national figure.

3. Drilling the Borehole

With a method selected, the rig drills the borehole to the target depth, cutting through soil and rock while circulating water, air, or drilling mud to carry cuttings up and out of the hole. The crew monitors what's coming out of the hole (rock type, water shows) as they go, which is part of why an experienced local driller who knows the area's geology is valuable, unexpected conditions are common enough that NGWA advises homeowners to trust the contractor's on-the-spot judgment when the ground doesn't match expectations.

4. Installing the Casing

Once the borehole reaches a usable depth, the driller installs casing, a pipe that lines the hole to keep the borehole open and keep shallow, less protected groundwater and surface contaminants out of the finished well. Per the EPA's construction guidance, casing is typically metal or plastic pipe, commonly around six inches in diameter for a residential well, extending from above the ground surface down into the water-bearing formation.

States set their own minimum casing requirements, which is one more reason local rules matter more than generic ones. Rhode Island's rule, cited by the EPA as an example, requires casing to extend at least 18 feet into the ground with at least 5 feet of that in bedrock, and to rise a foot or two above ground level. After the casing is set, a sealant, typically cement grout or bentonite clay, is poured into the space between the casing and the borehole wall up to the surface. That grout seal is what actually stops surface water and shallow contamination from tracking down alongside the casing into the aquifer; without it, the casing alone doesn't fully protect the well.

5. Well Development (Flushing the Well)

Drilling stirs up fine sediment and, depending on the method, drilling fluid, and none of that belongs in a finished well. Development is the step where the crew clears it out and gets the surrounding formation flowing cleanly into the well. Standard techniques, described in EPA well-development guidance and echoed in state well-construction rules, include surging (moving a plunger up and down in the well to agitate the formation and pull fines into the well where they can be removed), bailing, high-pressure jetting, and airlift or overpumping, where air or a pump lifts sediment-laden water out until it runs clear.

Penn State Extension notes that high-velocity jetting is generally regarded as one of the most effective agitation methods for this purpose, and that adequate development stabilizes the material around the well so that normal future pumping doesn't keep pulling in sediment. In practice, a driller keeps developing the well until the discharge runs visibly clear and, ideally, until basic water parameters like pH and conductivity stop fluctuating between checks, that stability is the practical signal that development is done.

6. Installing the Pump

With the well developed, the driller or a pump installer sizes and installs the pump. Per the EPA, submersible pumps, positioned down in the well near the bottom, are the most common choice for drilled wells, while a shallower water table can sometimes support a jet pump located inside the home instead. Pumps need dedicated wiring and electrical service, and the EPA specifically recommends having pumps installed and serviced by a qualified, state-registered professional rather than as a DIY job.

Most modern drilled wells also use a pitless adapter, a fitting that attaches to the casing below the frost line and lets the discharge line leave the well underground through a sanitary, watertight connection. That design keeps the water line below the frost line (preventing freezing) while sealing out surface contamination at the point where the pipe exits the casing, a real improvement over older well designs that lacked this feature. See our cost guide for what pump installation typically adds to the total project cost.

7. Disinfection

Drilling equipment, casing, and tools inevitably introduce some bacteria into a new well, so disinfection is a standard step before the well goes into service. Penn State Extension's guidance on well construction and rehabilitation recommends a general disinfection dose of 50 milligrams per liter of free chlorine following routine well and piping construction, pump installation, or repair. The well and plumbing are dosed, then flushed, before the system is put into regular use.

8. Final Testing and Well Completion

The last step is confirming the well actually performs, and documenting what was built. NGWA recommends that a well-drilling contract spell out several specifics up front so there's no ambiguity at completion: the diameter of the borehole, the size and type of casing used, the type of well screen (where one is used), how long the well will be test-pumped to confirm its yield, and when the driller will furnish a well log, a record of what was found and built at each depth.

Water quality testing is separate from that yield test, and it matters just as much. The EPA recommends testing a private well annually for total coliform bacteria, nitrates, total dissolved solids, and pH, with more frequent testing warranted in households with small children, elderly residents, or pregnant or nursing women. Penn State Extension likewise recommends annual coliform and E. coli testing by a state-accredited lab. A newly completed well should be tested before anyone relies on it as a primary water source, and then re-tested on that annual cadence going forward, since a clean initial result doesn't guarantee the well stays that way.

What This Means for Hiring a Driller

Every step above is a place where quality varies between contractors, adequate casing depth and grouting, thorough development, correct pump sizing, and proper disinfection all depend on the crew doing the job right rather than rushing to the next site. See our guide to hiring a licensed well driller for the questions to ask and the credentials to verify before you sign a contract, and our cost guide for how each of these components typically shows up in a quote.

Sourced from: US EPA, Learn About Private Water Wells; US EPA, Construction Features of Drilled Wells (content from the Rhode Island Department of Environmental Management); US EPA, Protect Your Home's Water; US EPA Ground Water Forum, Monitoring Well Development; National Ground Water Association, Having a Well Drilled: What You Need to Know; and Penn State Extension, Water Well Maintenance and Rehabilitation. Well construction requirements, including minimum casing depth, vary by state; confirm current local rules with your state's water-well licensing agency.