Borehole

A borehole is the cylindrical hole drilled into the earth to create a water well. It's the foundational structure that everything else — the casing, the pump, the drop pipe — sits inside. The terms 'borehole' and 'well' are often used interchangeably in everyday language, but technically the borehole is the physical hole in the ground, while the well is the complete installed system including the casing and equipment inside it.

How a Borehole Is Drilled

Modern residential water wells are drilled using rotary drilling equipment — a drill rig mounted on a truck that uses a rotating bit and a combination of downward pressure and drilling fluid to break through soil and rock. As drilling progresses, the driller monitors the geological formations being penetrated and the water-bearing zones encountered. Once a productive water-bearing zone is reached and the required depth is achieved, the drill string is removed and the well casing is installed inside the borehole.

The annular space between the outside of the casing and the borehole wall is grouted — sealed with a bentonite or cement mixture — from the surface down to a specified depth. This grout seal is critical: it prevents surface water, shallow groundwater, and contaminants from traveling down the outside of the casing and into the well water supply. A properly grouted well is one of the most important structural elements of a safe private water system.

Borehole Diameter and Its Effect on Equipment

The diameter of the borehole determines the diameter of the casing that can be installed, which in turn determines the pump size that will fit inside it. Most residential drilled wells use 4-inch or 6-inch diameter casing. A 4-inch casing requires a 4-inch submersible pump — the industry standard for residential applications. A 6-inch casing accommodates larger pump configurations and is used when higher yield capacity or specific pump configurations are required.

Borehole Depth and What Determines It

Borehole depth is determined by the depth at which productive water-bearing rock or sediment is encountered and by local well regulations that may specify minimum casing depths and grouting requirements. In crystalline rock terrain — granite, gneiss, schist — the driller may need to penetrate hundreds of feet to find adequate fracturing for water flow. In productive alluvial aquifer areas, a well may reach water at 50 to 100 feet. The required depth is not predictable in advance; it's determined by the geology encountered during drilling.