Artesian water is groundwater sourced from a deep underground layer that is naturally under pressure. This pressure causes the water to rise without the need for a mechanical pump. This geological phenomenon relies on a specific set of underground structures to trap and pressurize the water, distinguishing it from typical groundwater which requires pumping to reach the surface.
The Required Geological Structure
The existence of artesian water depends on a specific arrangement of rock layers beneath the Earth’s surface. At the center of this formation is a confined aquifer, which is the porous layer of rock, sand, or gravel that holds the water.
The aquifer must be sealed both above and below by impermeable confining layers, often made of materials like clay or dense shale. These layers, sometimes called aquitards, prevent the water from escaping, effectively trapping it and allowing pressure to build. This geological structure is typically angled or tilted, which is necessary for the system to function.
The crucial third component is the recharge area, the exposed part of the confined aquifer where it meets the surface. This area is typically located at a higher elevation than the well site, often in hills or mountains. Rain and melted snow seep into the permeable aquifer in this elevated zone, replenishing the water supply and initiating the underground flow.
How Pressure Creates the Flow
The force that drives artesian water upward is a concept in hydrogeology called hydraulic head. This term represents the total mechanical energy of the water at a given point within the aquifer. The flow occurs because water naturally moves from areas of higher energy to areas of lower energy.
The elevated recharge area provides the water column with a high elevation head, which translates into pressure as the water travels downward and is confined by the impermeable layers. This pressure is similar to the force created by the height of water in a municipal water tower.
The theoretical level to which the water would rise if it were unconfined is known as the potentiometric surface. When a well is drilled, the water level will rise within the well casing until it reaches this surface. The pressure is sufficient to push the water up the pipe because the water’s source in the elevated recharge area is higher than the opening of the well.
Types of Artesian Wells
Artesian wells are classified into two types based on the observable result at the surface. A non-flowing artesian well is one where the potentiometric surface is above the top of the aquifer but remains below the ground level. Water in this type of well rises significantly higher than it would in a standard well, but a pump is still required to lift the water the final distance to the surface.
In contrast, a flowing artesian well occurs when the potentiometric surface is naturally situated above the ground level. When a well taps into this aquifer, the pressure is strong enough to force the water out of the well casing and onto the surface without any mechanical assistance. All flowing wells are artesian, but not all artesian wells flow.
Artesian wells differ from standard wells, which draw from unconfined aquifers where the water table is at equilibrium with atmospheric pressure. In a standard well, water does not rise on its own, and a pump is required to lift the water. Artesian wells, even the non-flowing variety, benefit from the natural underground pressure, reducing the amount of energy needed for extraction.