When drilling a well, at some point, saturated conditions will be encountered. This is the location of the water table, which is the top of the groundwater zone and the top of an unconfined aquifer. This groundwater is stored in an unconfined aquifer, and is labeled in Figure 28b as the aquifer right below the land surface. As drilling continues to greater depth, typically, the top of the first confining bed is encountered (Figure 28b). If the well is sealed with a pipe, so that the groundwater in the unconfined aquifer cannot seep into the well, and water is bailed out of the pipe, there will be an empty pipe with a muddy silt bottom. As drilling continues downward through the confining bed water cannot seep into the well fast enough to fill the pipe because the confining bed is made of low permeability silt or clay. At some point, the drill reaches the bottom of the confining bed and enters the sand layer below. Suddenly, water rapidly enters the borehole, and the water level rises to a level that is higher than the top of the underlying sand bed. This sand bed is another aquifer, because it is porous and permeable, but in this case the water level within the aquifer is higher than the top of the aquifer because the water is under pressure, so it is called a confined or artesian aquifer (Figure 28b). It is confined in the sense that the groundwater in this aquifer is being held in by the silt bed above it. Confining beds are not very permeable, and water moves slowly through them, thus an elevated water pressure is maintained in the underlying confined unit. When the well is drilled through the confining bed, the pressurized water within the confined aquifer flows up into the well casing to the level of the hydraulic head in the confined aquifer (Figure 29 provides a close-up of the wells in the confined and unconfined aquifers). In contrast, the water level in the upper aquifer is not confined by a low permeability bed above, and the water level in the well of this unconfined aquifer rises to the level of the water table. The unconfined aquifer is also called a water table aquifer because it contains the rising and falling water table. The higher water level in the well of the shallow aquifer in Figure 29 relative to the lower water level in the well in the aquifer beneath the confining bed indicates that water must be flowing downward through the confining bed at this location.
To investigate the cause of the groundwater pressure in the confined aquifer, zoom out from Figure 28b to its place in the larger picture shown in Figure 28a where it becomes apparent that the stack of aquifers and confining beds is a small part of a larger groundwater system. The alternating beds of sand, silt and clay are dipping gently toward the sea. This geologic structure of interlayered fine silts and clays with coarse materials like sand and gravel is typical of coastal plains throughout the world. The larger picture reveals that, inland, the confined aquifer slopes upward (toward the left in Figure 28a) and eventually intercepts the land surface. At this location, marked by the red arrow in Figure 28a, the aquifer is no longer confined because the overlying silt layer is not present, so it is an unconfined aquifer and recharge from precipitation infiltrates to its water table. Since this recharge point is higher than where the well was drilled, that extra height translates into weight of water (pressure) bearing down on the location where we punctured the confined aquifer. This is somewhat analogous to the water supply system in a small town with an elevated water tower where the hydraulic head of the water is highest in the tower so that water will flow to all the houses in town (Figure 30). Although the water supply pipe is below the street level, when the tap is opened on the second floor of the house, water gushes out, because the water level in the tower is higher than the water level at the faucet in the house.
We say the water tower is “somewhat analogous” to the groundwater system, because, unlike the subsurface groundwater system, the water supply pipe is not filled with porous material. In comparison, water loses substantially more energy as it flows from one point to another in a porous material than in an open pipe. In a groundwater system, even the unconfined aquifer is pressurized at depth in areas where groundwater flows upward toward streams, Furthermore, a well that is open to only the deep portion of the aquifer can have a water level not only higher than the water table but sometimes higher than the ground surface such that the water will flow out of the well without a pump. A well with a water level higher than the surface is called a flowing well. Flowing wells can occur in both unconfined and confined aquifers.
A famous flowing artesian well tapped into a high‑pressure groundwater aquifer in 1888 in Woonsocket, South Dakota, United States, causing a “gusher” (Figure 31a). The pressure was created by water recharging the outcrop area of the Dakota aquifer on the west end of Figure 31b, and flowing through the Dakota aquifer beneath the shale of low permeability such that the pressure was not relieved by leakage out of the aquifer along the flow path. When the Woonsocket well was drilled into the aquifer (represented by the red line on the east side of Figure 31b), water gushed out at a high flow rate (Figure 31a). Over time, the volume of water stored in the aquifer decreased so the pressure declined, and the well was no longer a flowing well (that is, the water level in the well declined to below the ground surface). After that time, a pump was needed to draw water from the well. Flowing artesian wells used to be common in many places throughout the world, but are now less common because pumping has reduced groundwater pressure in the aquifer.
The distinction between the unconfined and confined aquifers is important for many reasons. For example, being closely connected to the land surface, an unconfined aquifer is more vulnerable to contamination, and its water level fluctuates more in response to rain and drought. A confined aquifer is somewhat shielded, the water tends to contain fewer contaminants, and the water levels are not as responsive to short‑term variations in precipitation. The water in a confined aquifer often has a longer residence time in the groundwater system as shown in Figure 28b. While unconfined water can have a very long residence time in some areas, it is typically on the order of days to years. In contrast, water in confined granular aquifers (not fractured rock) typically has much longer residence times, often on the order of 100s to 1000s of years; that is to say, water in discharge areas fell as precipitation 100s to 1000s of years ago.