A borehole is a narrow hole drilled into the ground to access resources below the surface, most commonly water. Boreholes range from shallow holes just a few feet deep to engineered shafts extending hundreds or even thousands of feet underground. While the term is often used interchangeably with “well,” a borehole technically refers to the drilled hole itself, before any casing, pump, or other equipment is installed.
How Boreholes Work
A drilling rig bores through layers of soil, clay, and rock until it reaches an underground water source called an aquifer. An aquifer is a layer of permeable rock or sediment that holds and transmits groundwater. Once the drill reaches this zone, water naturally fills the borehole from the surrounding material.
After drilling, a steel or PVC casing is inserted to prevent the hole from collapsing and to keep surface contaminants from seeping in. A screen or filter is placed at the bottom section where water enters, and a pump is installed to bring water to the surface. The space between the casing and the surrounding earth is sealed with grout near the top to protect the water supply.
Common Uses
Water supply is by far the most common reason people drill boreholes, whether for drinking water, irrigation, or livestock. But boreholes serve several other purposes:
- Geothermal heating and cooling. Vertical boreholes drilled 100 to 400 feet deep allow heat pump systems to exchange temperature with the earth, which stays a constant 40°F to 70°F year-round just below the surface. Plastic tubing is inserted into these holes in a U-shaped loop, and a water-based solution circulates through it to absorb heat in winter or shed heat in summer.
- Environmental monitoring. Shallow boreholes are drilled near industrial sites, landfills, or contaminated land to sample groundwater and track pollution levels over time.
- Mineral and oil exploration. Exploratory boreholes help geologists assess what resources lie underground before committing to full-scale mining or drilling operations.
- Construction and engineering. Geotechnical boreholes collect soil and rock samples so engineers can evaluate ground conditions before building foundations, bridges, or tunnels.
How Deep Are They?
Depth depends entirely on what the borehole is for and the local geology. A residential water borehole might be anywhere from 50 to 800 feet deep, depending on how far down the aquifer sits. In areas with high water tables, a usable well might be less than 100 feet. In arid regions or places with deep bedrock aquifers, drilling past 500 feet is not unusual.
Geothermal boreholes for home heating systems typically fall in the 100 to 400 foot range. The holes are roughly four inches in diameter and spaced about 20 feet apart when multiple loops are needed. Environmental monitoring wells tend to be much shallower, often under 100 feet, since they target the uppermost groundwater layer.
Drilling Costs
As of 2025, residential water borehole installation typically costs between $25 and $65 per foot, including drilling and casing. A relatively shallow borehole of 150 feet might run $4,000 to $10,000 in total. Deeper wells in the 600 to 800 foot range often exceed $15,000 to $50,000 before adding pumps, water storage tanks, or treatment systems.
Several factors push costs higher: hard rock formations slow down drilling and wear out equipment faster, remote locations add mobilization fees for the rig, and local geology that requires special casing or grouting adds material costs. The pump itself is a separate expense, and deeper boreholes need more powerful (and more expensive) pumps to lift water to the surface.
Permits and Regulations
Most jurisdictions require a permit before drilling a borehole, and the work must be done by a licensed contractor. Regulations vary widely by state and country. In Florida, for example, both the contractor and the well itself must be permitted through the Department of Environmental Protection, and specific construction standards govern how the well is cased, sealed, and completed. Many states have minimum setback distances from septic systems, property lines, and potential contamination sources.
Even in areas where small domestic wells are exempt from permitting, there are typically rules about construction standards and reporting. If you’re considering drilling, your state or county water resources agency can tell you exactly what’s required in your area.
Testing the Water Supply
Once a borehole is drilled, a pump test determines how much water it can reliably produce. The driller runs the pump at a steady rate while measuring how far the water level drops inside the hole. This drawdown is tracked over hours, often a full 24-hour cycle, to see whether the aquifer can keep up with demand. If the water level stabilizes, the borehole has a sustainable yield at that pumping rate. If it keeps dropping, the aquifer may not recharge fast enough for the intended use.
Water quality testing is equally important. Borehole water should be tested for bacteria, nitrates, pH, hardness, and any contaminants common to your region before it’s used for drinking. Iron and manganese are frequent issues that affect taste and stain fixtures, while bacterial contamination poses a direct health risk. Many homeowners install filtration or treatment systems based on what the initial tests reveal.
Lifespan and Maintenance
A professionally installed borehole typically lasts 20 to 50 years, with some lasting even longer when properly maintained. The borehole itself, meaning the drilled hole and casing, tends to outlast the mechanical components. Pumps generally need replacing every 8 to 15 years depending on usage, water chemistry, and sediment levels.
Annual servicing is the single most important thing you can do to extend the life of a borehole. A yearly check covers pump performance, water clarity, pressure systems, and contamination testing. Over time, mineral buildup can clog the screen at the bottom of the well, reducing flow. Bacterial growth on the casing walls can also restrict water movement. Both problems are treatable if caught early, but neglecting them can lead to expensive rehabilitation or even the need for a new borehole entirely.
Signs that something is wrong include a gradual drop in water pressure, sand or sediment in the water, changes in taste or color, or a pump that cycles on and off more frequently than usual. Any of these warrant an inspection before a minor issue becomes a major repair.