How a Water Well Actually WorksDrilling, Casing, Seals & How Bacteria Gets In
Most people with a private well have never seen what is under the steel pipe in their yard. Here is how the hole gets drilled, how it is sealed and cased, how the water actually reaches your tap, and why a broken seal is the reason a well starts failing bacteria tests.
12 min read · Waterlab Corp Laboratory Staff
Anatomy of a Drilled Water Well
Cross-section, not to scale
Where well water actually comes from
There is no underground lake beneath your property. Groundwater sits in the pore spaces between grains of sand and gravel, or in the cracks and fractures of solid rock. Picture a bucket of gravel filled with water — the gravel is the aquifer, and the water in the gaps is what your well pulls from.
The water table is simply the depth at which those pore spaces become completely full. Above it is the unsaturated zone, where the gaps hold a mix of air and moisture. Below it, everything is saturated. A well has to reach far enough below the water table that seasonal fluctuation never leaves the screen dry.
That water got there by falling as rain or snow and soaking downward, sometimes over years or decades. As it filters through soil and sediment, most bacteria are physically strained out and consumed. This is why deep groundwater is usually biologically clean when it enters a properly built well — and why bacteria showing up in your results points to a shortcut rather than a contaminated aquifer.
In the Willamette Valley, wells commonly draw from alluvial sand and gravel deposits. Elsewhere in Oregon, wells tap fractured basalt, where water moves through cracks instead of pore spaces. Fractured rock is far less forgiving: cracks can carry contamination a long way with almost no natural filtration.
How the hole gets drilled
Most modern Oregon wells are drilled with a rotary rig. Older wells were often made with a cable tool rig that repeatedly dropped a heavy chisel bit to pound through soil and rock — slower, but effective, and many of those wells are still in service today.
Siting and permitting
Before drilling, the site is chosen for setback distance from septic systems, livestock areas, and surface water. Oregon requires a start card and a well report for every new well.
Boring the hole
A rotary rig turns a bit at the end of a lengthening string of pipe. Drilling fluid or compressed air circulates down the pipe and back up, carrying cuttings to the surface and keeping the hole from collapsing.
Setting the casing
Steel or PVC casing is lowered into the finished borehole. It lines the hole, holds back loose soil, and becomes the sealed pipe that separates your water from everything above the aquifer.
Sealing and completing
Grout is pumped into the annular space around the casing, a filter pack of clean gravel is placed around the screen, and the pump, drop pipe, and sanitary cap are installed.
Casing and grout: how the well is encased
Once the borehole is finished, it is still just an open hole — unstable and wide open to contamination. Two things turn it into a well.
The casing is the pipe that lines the hole. In Oregon it is usually steel, sometimes PVC in shallower wells. It keeps loose soil from caving in, gives the pump a protected shaft, and forms a continuous wall between your water and every layer the hole passed through.
But drilling a hole means the hole is wider than the pipe you put in it. That leaves a ring-shaped gap around the casing called the annular space, and an open annular space is a straight pipe from the surface down to your water. Sealing it is the most important step in well construction.
That seal is the annular grout seal — bentonite clay or cement grout pumped into the gap so it fills from the bottom up and bonds against both the casing and the borehole wall. Oregon requires a minimum seal depth for new wells, and it is the single feature most often missing or degraded in older ones.
Intact seal
Grout fills the annular space completely. Surface runoff hits the seal and is forced to filter down through soil instead, the way nature intended.
Failed or missing seal
With the annular space open or channeled, runoff carrying bacteria travels down the outside of the casing and enters near the screen, skipping the soil entirely.
How the water gets out of the hole
Water enters the well through the screen — a slotted or perforated section of casing set in the aquifer. The slots are sized to let water in while holding back sand. Around the screen, the driller places a filter pack of washed gravel that stabilizes the formation and keeps fine sediment out.
Because the aquifer is saturated and under pressure, water rises inside the casing on its own and settles at the static water level. In most wells that level is well above the screen, which is why a 300-foot well might hold standing water starting at 80 feet.
Nearly all modern wells use a submersible pump — a narrow motor-and-impeller assembly hanging below the water level, pushing water up rather than sucking it. Water travels up the drop pipe, exits sideways through a pitless adapter below the frost line, and runs underground to a pressure tank in the house. The pitless adapter matters for sanitation too: it lets the line leave the well without ever breaking the seal at the top of the casing.
When the pump runs, the level inside the well drops. That drop is called drawdown, and how far it falls and how quickly it recovers is what tells a driller whether a well can keep up with a household.
Problems that wells can develop over time
A well is a piece of buried infrastructure with a service life. Steel corrodes, grout cracks, screens plug, and water levels shift. Most Oregon wells were drilled decades ago, and the failures below are all common.
Failed or missing annular seal
The grout ring around the casing is the single most important sanitary barrier in the well. If it was never installed to depth, or it has cracked and channeled over decades, rainwater and surface runoff can travel down the outside of the casing and enter near the screen.
Cracked or corroded casing
Older steel casing rusts from the inside out and can perforate at the water line. PVC can crack from ground movement or a dropped tool. Either way, shallow water that never passed through the soil column gets in.
Damaged or unsealed well cap
A cracked, loose, or homemade cap lets insects, rodents, and standing water enter directly at the top. Earwigs and mice in the casing are a common and badly underestimated source of coliform bacteria.
Declining yield or dropping water level
Long-term aquifer decline, seasonal drawdown, nearby heavy pumping, or a screen plugged with mineral scale and bacterial slime all reduce how much water the well can deliver.
Pump and pressure problems
Short cycling, air spitting from the tap, or a pump that runs constantly usually points to the pressure tank, the pressure switch, a leaking drop pipe, or a pump set too high in a well whose level has dropped.
Silt, sand, or sudden turbidity
Sand in the fixtures means the screen or filter pack is failing. Water that suddenly turns cloudy after heavy rain is a serious warning sign that surface water is reaching the well.
What a licensed well constructor can do
When results come back bad or a well starts behaving strangely, a licensed well constructor is who physically diagnoses and repairs it. In Oregon, well construction, alteration, and abandonment must be done by a constructor licensed and bonded through the Oregon Water Resources Department.
Inspect and camera-survey
A downhole video camera shows the actual condition of the casing, the joints, the screen, and the water level. This is the only way to see a hairline casing crack or a corroded seam.
Flow and drawdown testing
The driller pumps the well at a measured rate and tracks how far the water level falls and how fast it recovers. That determines sustainable yield and whether the pump is set correctly.
Repair the sanitary barriers
Re-grouting, installing a liner or new casing, replacing a failed cap, correcting a well pit, or extending casing above the flood line all restore the seal that keeps contamination out.
Disinfect and redevelop
Shock chlorination clears bacteria from the well and plumbing. Surging, brushing, or acid treatment can reopen a screen plugged with scale or bacterial slime.
Collect samples for the lab
After a repair, the driller often draws the follow-up samples and brings them to an accredited lab. Third-party collection is also what Oregon real estate transactions require.
Deepen or decommission
When a well cannot be salvaged, it has to be properly abandoned and sealed. An open, unsealed well is a direct conduit into the aquifer that everyone nearby shares.
“Well testing” means two different things
People searching for well testing are usually after one of two completely different services. It helps to know which one you actually need.
Flow or yield testing
Measures how much water the well produces — gallons per minute, drawdown, and recovery rate. This is mechanical work performed on site.
Who does it: a licensed well constructor or pump installer.
Water quality testing
Measures what is in the water — coliform bacteria, E. coli, nitrate, arsenic, lead, hardness, and more. This is laboratory analysis of a collected sample.
Who does it: an accredited lab. That is us.
Waterlab Corp performs water quality analysis only — we do not drill, repair, or flow-test wells. If you need that side of the work, the Oregon Water Resources Department maintains the licensing records for well constructors and the public well log database, which also lets you look up the original construction report for your own well. oregon.gov/owrd
How bacteria and E. coli gets into wells
Deep groundwater in an intact aquifer is usually free of coliform bacteria. Soil is an extremely effective biological filter. So when a lab finds coliform or E. coli in a well, the question is not “why is the aquifer contaminated” — it is “where is the shortcut?”
Down the outside of the casing
With no intact grout seal, the annular space becomes a pipe of its own. Rain, snowmelt, irrigation, and barnyard runoff follow the casing down and bypass the soil that would normally filter them.
Through a break in the casing wall
A corrosion hole or crack lets shallow groundwater — the water most likely to carry bacteria from septic drainfields and animal waste — enter directly.
In through the top
A missing vent screen, a cracked cap, or a casing cut off at or below grade allows insects, rodents, and pooled surface water into the well head. Flooding over the casing top contaminates a well almost immediately.
Backwards from the plumbing
A hose left submerged in a stock tank or a chemical sprayer can back-siphon into the system when pressure drops, pushing contamination into the well from the house side.
Timing tells you a lot
Bacteria that appear right after heavy rain, spring runoff, flooding, or nearby manure application point strongly to a surface-water pathway rather than the aquifer. Note what the weather was doing when you collected the sample — it is genuinely useful diagnostic information.
Why it matters, and why you retest after a repair
Total coliform is the alarm, not the fire
Coliform bacteria are common in soil and vegetation and are mostly harmless themselves. Their presence in a sealed well means one thing: something from the surface is getting in. That is why labs test for them.
E. coli means fecal contamination
E. coli lives in the intestines of people and animals. Finding it in well water indicates human or animal waste has reached the water, and with it the possibility of pathogens like Salmonella, Campylobacter, Giardia, Cryptosporidium, and rotavirus.
The risk is acute, not gradual
Unlike arsenic or nitrate, where harm accumulates over years, bacterial contamination can make someone sick from a single glass. Infants, older adults, pregnant people, and anyone immunocompromised are at highest risk.
Fixing without testing is guessing
Chlorinating a well with a broken seal only clears the bacteria already inside. Without repairing the pathway, contamination returns. Retest after any repair or disinfection, and look for two consecutive clean samples before you trust it.
A reasonable testing rhythm
- Coliform bacteria and nitrate: once a year, and any time the taste, smell, or clarity changes.
- After any well work: pump replacement, casing or seal repair, or shock chlorination — then retest to confirm.
- After flooding: if surface water reached the well head, treat the water as unsafe until a lab result says otherwise.
- Arsenic and lead: at least once, since both vary by geology and plumbing rather than by season.
- Selling a home on a well: Oregon requires coliform, nitrate, and arsenic testing under ORS 448.271.
Related reading: Oregon well water testing requirements (ORS 448.271) and how to test your drinking water in Oregon.
Suspect your well seal has failed?
Start with a coliform and E. coli test. ORELAP-accredited results, and staff who will walk you through what they mean.