6.3 Confined Aquifers

In contrast to unconfined aquifers, confined aquifers typically occur at depth (Figure 46). A confined aquifer is an aquifer that occurs beneath an aquitard which is often referred to as a confining bed. In a confined aquifer, the water level in tightly cased wells rises above the top elevation of the aquifer. Groundwater recharge entering the aquifer typically occurs at an outcrop area where hydraulic heads are higher than the elevation of the confined portion of the groundwater system as shown in Figure 46. When water flows into and through the confined aquifer, heads exceed the top elevation of the fully saturated confined aquifer zone. The overlying and underlying confining units constrain water from leaking out of the aquifer and direct flow parallel to the aquifer boundaries. The surface to which the water rises in wells and piezometers penetrating a confined aquifer is referred to as the potentiometric surface (Figure 46). Water movement is from areas where the potentiometric surface is at a high elevation to where it is at a lower elevation. The potentiometric surface can also be used to describe head distributions in unconfined aquifers as it represents hydraulic head. When a potentiometric surface is presented it should include descriptors that state whether it is describing conditions in an unconfined or confined groundwater system.

Schematic of a confined aquifer
Figure 46 – Schematic of a confined aquifer. The aquifer is located between confining units (aquitards) of low hydraulic conductivity (gray). The recharge area is elevated, and groundwater flow moves into a geologic setting where the confining beds restrict the upward migration of water. Blue arrows indicate the general direction of groundwater flow. The water levels in tightly cased wells rise above the top of the confined aquifer. The surface representing total head in a confined aquifer is referred to as the potentiometric surface (dashed red line). Confined aquifer wells will flow at the land surface without pumping if the potentiometric surface is higher than the top of the well casing as shown by the rightmost confined aquifer well. Confined aquifers can be overlain by other confined aquifers, and, near the surface by an unconfined aquifer.

A confined aquifer can be simulated with a length of garden hose that is filled with water and sediment, then capped on one end. The saturated sediment represents the confined aquifer and the hose walls represent the “confining beds”. When the open end of the hose (aquifer recharge area) is raised, the potential energy of the system increases. Conditions in a confined aquifer are simulated by opening the capped end of the hose and feeding water into the upper end of the hose. If a hole is drilled in the hose (i.e., a “well” is installed) at some distance from the upper end water from the saturated sand system squirts out. By installing tall piezometers (hollow tubes) in the hose, the elevation of water in the piezometers measured from a horizontal datum represents hydraulic head data points of the potentiometric surface. The total head exceeds the height of the confining bed (hose wall) as shown in Figure 47.

Figure showing a sediment-filled garden hose simulating a confined aquifer
Figure 47 – Example of sediment-filled garden hose simulating a confined aquifer. The dashed red line represents the head distribution along the potentiometric surface.

Wells penetrating a confined aquifer are also called confined wells or artesian wells and the aquifer is said to exist under confined or artesian conditions. In some cases, the head in a confined aquifer may rise above the ground surface and the water level in the well casing will be higher than the elevation of the ground surface as shown for the two confined well furthest to the right in Figure 46. When this occurs, if the elevation of the top of the well casing is lower than the elevation of the potentiometric surface, water will flow freely from the well casing without pumping. This is referred to as a flowing well (Figure 46) or often as an artesian well. The term, artesian well, is associated with flowing wells constructed in the old Roman City of Artesium (currently the town of Artios in France) during the Middle Ages. The wells penetrated a confined aquifer and freely flowed at the surface. Based on a purely scientific definition, an artesian well is any well that penetrates a confined aquifer, whether it flows at the land surface or does not. However, artesian is commonly used for any flowing well whether or not it penetrates a confined aquifer. Flowing wells can also occur in discharge areas of unconfined aquifers when large upward gradients are present. Most commonly, wells penetrating a confined aquifer are described as confined wells.

License

Hydrogeologic Properties of Earth Materials and Principles of Groundwater Flow Copyright © 2020 by The Authors. All Rights Reserved.