Artesian Flows

Groundwater may flow in unconfined or confined aquifers. In unconfined aquifers, the water travels through beds that extend with more or less uniform permeability to the surface, in both discharge and recharge areas. The level of the reservoir in an unconfined aquifer is the same as the height of the water table.

Many permeable aquifers, typically sandstones, are bounded above and below by shale beds of low permeability. These relatively impermeable beds are aquicludes, and groundwater either cannot flow through them or flows through them very slowly. When aquicludes lie both over and under an aquifer, they form a confined aquifer, in which water flow is contained under pressure.

-22-

The impermeable beds above a confined aquifer prevent rainwater from infiltrating downward into the aquifer. Instead, the confined aquifer is recharged by precipitation over the outcrop area; here rainwater enters the ground and travels down the aquifer (Figure 11.4). Water in confined aquifer – known as an artesian flow – is under pressure. At any point in the aquifer, the pressure is equivalent to the weight of all the water in the aquifer above that point.

If a well is drilled into a confined aquifer at a point where the elevation of the ground surface is lower than that of the water table in the recharge area, the water will flow out of the well spontaneously. Such wells are called artesian wells, and they are extremely desirable because no energy is required to pump the water to the surface. The water is brought up by its own pressure.

Figure 11.4 A confined aquifer is created when an aquifer is situated between two aquicludes (beds of low permeability). The artesian well flows in response to the difference in natural pressure (before the well was drilled) between the height of the water table in the recharge area and the bottom of the well. The actual pressure difference that governs the flow from the top of the well is the difference between the elevation of the water table and that of the top of the well. If the wellhead were as high as the water table in the recharge area, there would be no pressure difference and thus no flow.

-23-

In more complex geological environments, the water table may be more complicated. If there is a low-permeability clay layer – an aquiclude – in a high-permeability san formation, the aquiclude may lie below the water table in a shallower aquifer and above the water table in a deeper aquifer (Figure 11.5). The water table in the shallower aquifer is called a perched water table because it is above the main water table in the lower aquifer. Many perched water tables are small lenses, but some extend for hundreds of square kilometers.

Figure 11.5 A perched water table is formed in geologically complex situations, in this case by a shale aquiclude located above the main water table in a sandstone aquifer. The dynamics of the perched water table’s recharge and discharge may be different from those of the main water table. The main water table in this example can be recharged only from its lower outcrop slopes.

-24-