Groundwater is a natural resource stored beneath the Earth’s surface within geological formations. Wells are drilled to access this water and bring it to the surface. While all wells tap into these underground reservoirs, standard and artesian wells differ significantly in their construction and function. Understanding these distinctions clarifies how each system interacts with subsurface water.
Understanding a Standard Well
A standard well draws water from an unconfined aquifer. This aquifer type has a water table open to the atmosphere, meaning there is no impermeable layer immediately above it to trap water under pressure. Water levels within an unconfined aquifer can fluctuate significantly, directly responding to factors such as rainfall, drought conditions, and the rate of water extraction. Since the water in an unconfined aquifer is not under natural pressure, a standard well requires a mechanical pump to bring water to the surface. When drilled, the water level inside the well settles at the same elevation as the surrounding water table; installation involves drilling a borehole, inserting casing to prevent collapse, and lowering a pump for use.
Understanding an Artesian Well
An artesian well taps into a confined aquifer, a water-bearing layer situated between two relatively impermeable layers of rock or clay. These confining layers prevent the water from rising freely and subject the groundwater within the aquifer to hydrostatic pressure, which originates from the weight of water in the recharge area, often at a higher elevation than the well itself. When an artesian well is drilled, the water within the well bore rises above the top of the confined aquifer due to this natural pressure. The potentiometric surface is the imaginary level to which water would rise in wells penetrating a confined aquifer. If this surface is above ground level, the well flows naturally without a pump, creating a flowing artesian well; if below ground, a pump may still be necessary, though often requiring less energy than for a standard well.
The Underlying Geology and Key Differences
Geological Distinctions
The differences between standard and artesian wells stem from the geological conditions of the aquifers they access. Standard wells draw from unconfined aquifers, which are open to the surface with water levels at atmospheric pressure, allowing water levels to move freely in these aquifers in response to surface conditions. Artesian wells access confined aquifers, characterized by impermeable rock layers above and below the water-bearing stratum. This geological confinement creates hydrostatic pressure within the aquifer, causing the water to seek a higher level than the aquifer’s top. The presence or absence of these confining layers dictates pressure dynamics and water behavior when a well penetrates the aquifer.
Operational and Recharge Differences
Regarding water extraction, pressure differences lead to distinct operational requirements. Standard wells rely on mechanical pumps to overcome atmospheric pressure and lift water from the water table. Artesian wells can deliver water to the surface without pumping if natural pressure is sufficient to push water above ground level; even if a pump is needed, internal pressure often means less energy is required for extraction compared to a standard well. Recharge areas for these aquifer types also differ. Unconfined aquifers recharge directly from rainfall and surface water percolating through overlying soil. Artesian aquifers, though locally confined, recharge where the water-bearing layer is exposed at a higher elevation, allowing water to infiltrate and build pressure as it flows underground beneath confining layers, and this elevated recharge area maintains the hydrostatic pressure characteristic of artesian systems.