Artesian water is a unique type of groundwater defined by the physical conditions under which it is found and accessed. It is groundwater under enough natural pressure to rise above the level of the aquifer when a well is drilled. This natural upward flow distinguishes it from standard wells, which require mechanical pumps. The term “artesian” originates from the former French province of Artois, where flowing wells were drilled in the Middle Ages. This phenomenon depends entirely on a specific geological structure that creates and maintains this underground pressure.
The Geological Requirements: Confined Aquifers
The source of artesian water is a confined aquifer, a layer of permeable rock or sediment that holds water. This water-bearing layer, often composed of materials like sandstone, gravel, or fractured rock, is sandwiched between two layers of impermeable material. These impermeable layers are known as aquicludes or aquitards, typically made of dense clay or shale, which prevents the water from escaping vertically.
The system relies on topography where the aquifer is exposed at the surface in a higher-elevation area, known as the recharge area. This elevated area allows precipitation to seep down and replenish the water. Once the water enters the aquifer, the overlying impermeable layer traps it as it moves horizontally to lower elevations, resulting in pressure greater than atmospheric pressure.
The confining layers create a sealed system, allowing pressure to build up within the porous aquifer material. Without this confinement, the water would be part of an unconfined aquifer, where the top is the water table, and it would not be under the necessary pressure. The geological structure acts like a sealed plumbing system, where the elevated recharge area provides the height difference needed to generate the pressure.
The Mechanism of Flow: Hydrostatic Pressure
The rising of artesian water is explained by hydrostatic pressure, the pressure exerted by a fluid at equilibrium due to gravity. The weight of the water column in the elevated recharge area exerts a continuous downward force on the water trapped in the lower part of the confined aquifer. This pressure forces the water upward when an opening is created, such as by drilling a well.
The measure of the potential height to which the water will rise is called the potentiometric surface. This imaginary surface represents the level the water would reach if the pressure were released. If the potentiometric surface is above the ground elevation where a well is drilled, the water flows freely out of the well without pumping, creating a flowing artesian well.
If the ground surface is above the potentiometric surface, the water will still rise significantly in the well casing, resulting in a non-flowing artesian well. In this situation, a small amount of pumping is required to bring the water the rest of the way to the surface. The natural pressure has done most of the work, which is the defining characteristic of an artesian source.
Accessing and Utilizing Artesian Sources
Artesian water is accessed either through a natural artesian spring or by drilling an artesian well. Springs occur when a break in the overlying impermeable rock, such as a fault or fracture, allows the pressurized water to escape to the surface. Drilled wells intentionally penetrate the confining layer to tap into the pressurized aquifer below.
This water is sought after for commercial bottling and municipal use due to the protected nature of its source. The impermeable layers above the aquifer often shield the water from surface contaminants, leading to an association with purity and natural filtration. However, the water’s quality still needs testing, as natural filtration does not guarantee the removal of all potential contaminants.
The sustainability of an artesian source depends on balancing the rate of water extraction with the natural recharge rate. Excessive drilling or over-extraction can cause the pressure within the aquifer to drop, lowering the potentiometric surface and potentially reducing a flowing well to a non-flowing one, or even causing it to run dry. Careful management is necessary to maintain the equilibrium of this underground resource.