The biggest reservoir of fresh, liquid water on earth is groundwater. There is more than 20 times as much water stored within the ground as in all the lakes and rivers combined. In many parts of the world, agricultural, industrial, and domestic water demands can only be met by pumping water out of the ground.
As you'll see in the diagram below, wells work only if they penetrate the water table - the name given to the subsurface boundary that separates saturated and unsaturated ground. Above the water table, cracks and spaces between soil and rock particles contain at least some air. Infiltrating water moves more or less vertically as it passes through this unsaturated zone. Below the water table, all openings in the ground are filled with water. Once groundwater reaches this saturated zone, it begins to travel both horizontally and downward, and can flow into wells, rivers, and eventually the sea.
The water table is an irregular surface that generally resembles a gentler version of the overlying ground surface. It is not fixed, but moveable, rising when rainfall adds more water to the ground, and falling when drought reduces the water supply. The depth and shape of the water table can also change dramatically when groundwater is pumped out of the ground. Pumping can quickly draw down the local water table right around a well. Over time, excessive pumping can also lower the water table over a wide region.
You are the city water master responsible for supplying water to local homes and farms. In the past, groundwater supplies have been adequate. But now a growing population has forced you to drill more wells, and you are concerned that increased pumping will affect water levels. In the simulation below, you can see what will happen when your pump station comes on line. Each pump can be started and stopped by clicking on it. Try running 1, 2, and even all 3 pumps at once, to determine the impact of various rates of pumping on the groundwater and the wells.
Note: The behavior of the water table in the simulation has been simplified to make it easier to understand. For more detailed information on groundwater and wells, go to:
The U.S. Environmental Protection Agency
Overuse of groundwater can cause wells to dry up. This often leads to expensive and ultimately futile attempts to keep up with the dropping water table by drilling deeper and deeper wells. Other serious consequences can also follow groundwater overuse.
When too much water is withdrawn from the ground, the land can collapse, a process called subsidence. When groundwater fills spaces in the soil, it supplies part of the internal strength of the ground. When the water is removed, leaving openings filled only with air, the weight of the overlying earth compacts and crushes the spaces.
In this photograph of California's San Joaquin Valley, the dates on the pole mark the former heights of the ground. In the span of 50 years, water pumping for irrigation led to nearly 30 feet of subsidence.