Short definition
A well cap is the sanitary seal at the top of a well casing. It’s vermin-proof, watertight to surface water, and includes a screened vent for pressure equalization. Most modern caps are aluminum or plastic with a rubber gasket and bolted flanges. A failed or missing well cap is the most common cause of a positive coliform test on a private well.
What it is
Walk up to a well head and you’ll see the steel or PVC casing rising about 6 inches above grade, with a domed or flat cap bolted on top. Inside, the cap has a rubber compression gasket against the casing, a screened vent fitting that lets the well “breathe” without admitting insects, and pass-throughs for pump wiring and an observation line.
The cap’s job is sanitary: keep insects, rodents, surface debris, and runoff out of the casing. Modern caps are mechanically secured or welded to the casing, designed for that specific purpose, and rated to keep small vermin out while allowing pressure equalization.
Why it matters to a homeowner
If your annual coliform test fails, the well cap is the first thing to inspect. The aquifer itself rarely contaminates overnight; what changes is usually a seal at the surface — a cap that’s loose, cracked, or missing entirely, or a vent screen torn open. Surface water carrying bacteria from rodent or insect activity finds the path of least resistance straight into the casing.
Other warning signs:
- Bucket inverted over a casing instead of a real cap on legacy wells. Not vermin-proof, not watertight.
- Cracked or missing rubber gasket.
- Vent screen torn or clogged — may admit insects or restrict pressure equalization.
- Loose flange bolts from corrosion or frost heave.
Replacement is straightforward and homeowner-doable on modern caps: shut off the pump, unbolt the cap, install a code-compliant replacement with a fresh gasket, restore power. If the casing is non-standard or the wiring is old, a Pro should handle it.
Washington note
Under WAC 173-160-291 (Department of Ecology well construction rules), all drilled wells in Washington must be capped with a device “designed for that purpose and consist of metal or plastic material that is mechanically secured or welded to the casing.” Vent and observation lines must extend from the upper end of the well through watertight piping to at least 6 inches above land surface, with terminals shielded or sealed to prevent foreign-matter entry. The same section requires the casing itself to extend at least 6 inches above grade and at least 2 feet above 100-year flood elevation in flood-prone sites.
Failing any of these on inspection is a cure-before-close item on rural Washington property transactions.
Common failure modes
- Loose, cracked, or missing cap — leads to coliform contamination.
- Gasket failure — same outcome.
- Vent screen torn or clogged — insect ingress or pressure issues.
- Frost heave displacement on shallow installations.