A cylinder of steel or plastic material used to allow water to enter a well while preventing sediment or rock particles from entering the well. A screen operates something like a sieve. Well screens may be wire wrapped, louver or perforated, and can be made from different materials and at different opening sizes. The selection of well screen design and opening size may depend on characteristics of the geologic formation, required yield and the thickness of the aquifer.
See also Grain Size.