Short definition
An artesian well is a well drilled into a confined aquifer — a water layer trapped between impermeable strata — where natural pressure pushes water up the casing on its own. If the pressure is high enough, water flows out at the surface without a pump (a “flowing artesian”); otherwise it rises partway and is pumped from there.
What it is
Confined aquifers are recharged at higher elevation and bounded above and below by clay or solid rock. That builds up hydrostatic pressure along the aquifer. When you drill into one, the trapped water has somewhere to go — up your borehole.
Two outcomes:
- Flowing artesian. Pressure brings water above ground level. The well discharges continuously unless capped or valved. The well cap and casing must be pressure-rated, because they’re holding back live system pressure 24/7.
- Non-flowing artesian. Water rises above the regional water table but stays below ground surface. You still need a pump to lift it the rest of the way, but the starting point is much shallower than the natural water table — a real efficiency advantage over an ordinary drilled well.
By contrast, an ordinary drilled well taps an unconfined aquifer, where water sits at the natural water table and pumping is always required.
Why it matters to a homeowner
If your well log calls the well “artesian,” you’ll want to know which kind you have. Flowing artesians come with extras: the well-head must be sealed against the constant pressure, and continuous discharge has to go somewhere — often a controlled overflow that may need a discharge plan under WA Department of Ecology rules. The upside is reliable supply that pumps very little or not at all, which means lower electric costs and fewer pump replacements over decades.
Non-flowing artesians behave like ordinary wells from a pump-and-pressure-tank standpoint, but the static water level can be dramatically higher than the surrounding water table — useful information when sizing a replacement pump or troubleshooting yield.
In Western Washington’s lowland glacial geology, artesian conditions are common in confined gravel layers below clay caps; in Eastern Washington, they show up in deep basalt aquifers.