What is a bedrock well?

Figure 1 is a diagram of a drill bedrock well. For a well finished in bedrock, the casing typically extends from just above the land surface to a short distance into the bedrock, where the casing is seated into the rock to keep it in place.

The borehole continues down into the bedrock aquifer for some distance below the casing, usually until sufficient water is available to the well. This could be 50 feet or it could be a 1000 feet. Because bedrock is solid, it acts as the well casing below the sitting of the surface casing. The borehole intercepts fractures in the bedrock, which provide water to the well when it is pumped. In some instances, the casing may extend the entire length of the well; where the rock isn't solid enough to ensure it won't crack or cave in, or where the water quality of some geologic units requires that they'll be sealed off with the casing.

Depending on where you live, the location of the fractures near a bedrock aquifer may be well-understood or almost unknown. In some areas, land owners even try to line up their well with neighboring wells, where they get a lot of water from a certain large fracture. If they do manage to hit that fracture, the well will produce plenty of water. But if they miss it, they could end up with a well that doesn't produce much water at all. In general, there are more fractures near the top of the geologic formation, so as you drill deeper in the same formation, you would expect fewer fractures to be available.

The Private Well Class is a collaboration between the Rural Community Assistance Partnership and the University of Illinois through the Illinois State Water Survey and the Illinois Water Resource Center and funded by the United States Environmental Protection Agency. The views expressed here solely from the class authors and not endorsed or reviewed by US EPA for more information on private wells, sign up for our free 10-week email course at PrivateWellClass.org.