Well Screen — Aquifer Intake Filter at the Bottom of a Well

A well screen is a perforated or slotted intake section installed at the bottom of a well casing to allow groundwater to enter the well from the surrounding aquifer while filtering out sand, gravel, and fine sediment particles.

What It Is

A well screen is essentially a filter at the bottom of the well. It is a cylindrical section of pipe with precisely sized openings — slots or perforations — that are large enough to allow water to flow freely from the aquifer into the casing, but small enough to retain the formation material (sand, gravel, or fine sediment) outside the well. The slot size is selected by the well driller based on the grain size analysis of the aquifer material — typically the screen retains 40 to 60 percent of the formation material, a balance that maximizes flow while minimizing sand infiltration.

Above the screen, a gravel pack — coarser gravel than the natural formation material — is placed in the annular space around the screen to stabilize the formation, provide additional filtration, and improve the hydraulic connection between the aquifer and the well. In bedrock wells, where the borehole penetrates solid rock, a screen is not always necessary because the rock walls provide their own stability. Screens are primarily used in unconsolidated aquifers composed of sand, gravel, or silt.

Types

Continuous-slot wire-wrap screens are constructed from V-shaped wire wrapped around a skeleton of vertical rods, creating continuous horizontal slots. They offer high open area and are the industry standard for high-yield production wells. Slotted PVC screens are the most common in residential wells — the casing is factory-slotted with rows of narrow cuts and is economical and easy to install. Perforated pipe screens use round holes rather than slots and are used in coarser gravel formations where larger openings are acceptable. Louvered screens and bridge-slot screens are used in specific applications requiring high collapse resistance.

Where It Is Used

Well screens are installed at the bottom of the well, positioned in the most productive portion of the aquifer — the zone that yields the most water. In deep drilled wells, this may be at depths of 100 to 400 feet or more below ground surface. The screen length is determined by the thickness of the productive aquifer zone. In shallow driven wells, the screen is at the tip of the drive point.

How to Identify One

The well screen is not visible during normal operation — it is fully submerged below the water table at the bottom of the casing. Signs of screen problems include increasing sand or sediment in the water, declining well yield, or the pump pulling sand. A downhole camera can visually inspect the screen for damage, incrustation, or collapsed sections.

Replacement

A plugged or incrusted well screen can sometimes be rehabilitated by acidizing or mechanical brushing performed by a well driller. When the screen is collapsed, corroded, or irreparably plugged, the entire well section must be pulled and the screen replaced, or the well must be re-drilled. Screen replacement is specialized work requiring a licensed well contractor and a permit in virtually every jurisdiction. Regular water testing that includes turbidity and sand content monitoring helps detect screen deterioration before pump damage occurs.