An artesian well taps into a confined aquifer where the water is naturally pressurized by the surrounding geology. This pressure causes the water to rise in the well casing above the top of the water-bearing layer. While many assume the term suggests a free-flowing source, a significant number of these wells are non-flowing. This means the water rises but stops short of reaching the ground level. The difference depends on the geological pressure and the elevation of the wellhead.
How Artesian Wells Develop Pressure
Artesian pressure relies on three geological conditions that create a natural underground plumbing system. The primary requirement is a confined aquifer, a layer of permeable rock (such as sandstone or fractured limestone) saturated with water. This layer must be sandwiched between two layers of impermeable material, known as aquitards or aquicludes (like dense clay or shale). These confining layers prevent the water from escaping, trapping it under pressure.
The third condition is the recharge area, where the aquifer is exposed to the surface, often at a higher elevation. Water from precipitation infiltrates this elevated area and travels downhill through the confined aquifer. Gravity acts on this column of water, creating a driving force called the hydraulic head.
This hydraulic head is the source of the artesian pressure. The difference in elevation between the recharge area and the well determines the magnitude of this pressure. This force attempts to push the water back up the well casing to the elevation of its source.
The Role of Ground Elevation and the Potentiometric Surface
The reason some artesian wells do not flow relates to the potentiometric surface, the imaginary level representing the height to which the pressurized water would naturally rise. This surface slopes downward from the elevated recharge area, reflecting the loss of energy as water flows through the aquifer. The potentiometric surface maps the elevation corresponding to the pressure at every point in the confined aquifer.
A well is a flowing artesian well only if the ground elevation at the wellhead is situated below the potentiometric surface. Here, the hydraulic pressure is strong enough to overcome gravity and push the water out onto the land surface without mechanical assistance.
Conversely, a well is still classified as artesian if the ground elevation is above the potentiometric surface, but it is a non-flowing artesian well. The water still rises significantly in the well casing due to underground pressure. However, because the ground level is higher than the pressure head’s maximum limit, the water column stops before reaching the surface. This necessitates using a pump to lift the water the remaining distance.
Why Aquifer Conditions Reduce Flow
While ground elevation is the primary reason for a non-flowing well, dynamic changes within the aquifer system can cause the potentiometric surface to drop over time. These changes can cause a once-flowing well to stop or require deeper pumping.
One significant factor is aquifer depletion, which occurs when water withdrawal from numerous wells exceeds the natural recharge rate. Over-pumping lowers the overall hydraulic head, causing the potentiometric surface to recede across a wide area. A well previously below this surface may find itself above the new, lower level, ceasing to flow naturally. Reduced recharge, often caused by prolonged drought or extensive paving in the recharge area, also contributes to this decline by limiting the water entering the system.
Water moving through the aquifer and the well casing experiences friction loss, which diminishes the final pressure available at the wellhead. This natural energy loss reduces the water’s ability to reach its full theoretical height. Finally, if the impermeable confining layer is naturally breached or damaged by poor well construction, the water pressure can dissipate into unconfined layers above, leading to a permanent reduction in the artesian flow.