This article from the How To Talk Ground Water series looks at just some of the below-ground aspects of well design that can easily be explained to well owners.
The well's diameter must be large enough to take the pump equipment necessary to move the water to the surface. Most home wells use 4-inch diameter pumps. The well casing lining the drilled hole must extend down far enough to reduce the risk of any surface or near-surface contamination. The well may need a screen to allow for the efficient flow of water from the aquifer into the well. In some cases, it may be necessary to place additional casing to seal off parts of the drilled hole where, for example, the water may have high iron content or some other unwanted chemical attribute.