Groundwater Supply

N4TTHa Water is not always available in the quantities and in the locations where it is needed and might be compromised by pollution.

Real-World Reading Link If you have a bank account, can you withdraw as much money as you want? Of course not. Like a bank account, groundwater can be withdrawn, but only in the amount that has been deposited there.

Wells ilfSf Wells are holes dug or drilled into the ground to reach an aquifer. There are two main types of wells: ordinary wells and artesian wells.

Cone Depression Geology

Figure 10.12 Overpumping from one well or multiple wells can result in a cone of depression and a general lowering of the water table.

Ordinary wells The simplest wells are those that are dug or drilled below the water table, into what is called a water-table aquifer, as shown in Figure 10.12. In a water-table aquifer, the level of the water in the well is the same as the level of the surrounding water-table. As water is drawn out of a well, it is replaced by surrounding water in the aquifer.

Overpumping occurs when water is drawn out of the well at a rate that is faster than that at which it is replaced. Overpumping of the well lowers the local water level and results in a cone of depression around the well, as shown in Figure 10.12. The difference between the original water-table level and the water level in the pumped well is called the drawdown. If many wells withdraw water from a water-table aquifer, the cones of depression can overlap and cause an overall lowering of the water table, causing shallow wells to become dry. Water from precipitation replenishes the water content of an aquifer in the process of recharge. Groundwater recharge from precipitation and runoff sometimes replaces the water withdrawn from wells. However, if withdrawal of groundwater exceeds the aquifer's recharge rate, the drawdown increases until all wells in the area become dry.

Figure 10.12 Overpumping from one well or multiple wells can result in a cone of depression and a general lowering of the water table.

Ordinary Wells And Cone Depression

Before heavy pumping

Before heavy pumping session

Lowered water table After heavy pumping

Figure 10.13 An artesian aquifer contains water under pressure. Identify the features that cause the primary difference between an ordinary well and an artesian well.

Artesian wells An aquifer's area of recharge is often at a higher elevation than the rest of the aquifer. An aquifer located between aqui-cludes, called a confined aquifer, can contain water that is under pressure. This is because the water at the top of the slope exerts gravitational force on the water downslope, as you will learn in the Problem-Solving Lab on this page. An aquifer that contains water : under pressure is called an artesian aquifer. When the rate of recharge ; is high enough, the pressurized water in a well drilled into an artesian aquifer can spurt above the land surface in the form of a fountain : known as an artesian well. The level to which water in an open well can rise is called its pressure surface, as shown in Figure 10.13. Similarly, a spring that discharges pressurized water is called an artesian spring. The name artesian is derived from the French province of Artois, where such wells were first drilled almost 900 years ago.

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Responses

  • matilda
    How high can artesian wells spurt?
    8 years ago

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