Does groundwater move as an underground river?
People often think of groundwater as an underground river, but that is rarely true. In Florida, though, water has so thoroughly dissolved the limestone that streams travel underground and above ground. This photo shows where a large spring brings groundwater to the surface as if from nowhere.
Features of an Aquifer
To be a good aquifer, the rock in the aquifer must have good:
- porosity: small spaces between grains
- permeability: connections between pores
This animation shows porosity and permeability. The water droplets are found in the pores between the sediment grains, which is porosity. When the water can travel between ores, that’s permeability. http://www.nature.nps.gov/GEOLOGY/usgsnps/animate/POROS_3.MPG
To reach an aquifer, surface water infiltrates downward into the ground through tiny spaces or pores in the rock. The water travels down through the permeable rock until it reaches a layer that does not have pores; this rock is impermeable (Figure below). This impermeable rock layer forms the base of the aquifer. The upper surface where the groundwater reaches is the water table.
Groundwater is found beneath the solid surface. Notice that the water table roughly mirrors the slope of the land’s surface. A well penetrates the water table.
The Water Table
For a groundwater aquifer to contain the same amount of water, the amount of recharge must equal the amount of discharge. What are the likely sources of recharge? What are the likely sources of discharge?
What happens to the water table when there is a lot of rainfall? What happens when there is a drought? Although groundwater levels do not rise and fall as rapidly as at the surface, over time the water table will rise during wet periods and fall during droughts.
In wet regions, streams are fed by groundwater; the surface of the stream is the top of the water table (Figure below). In dry regions, water seeps down from the stream into the aquifer. These streams are often dry much of the year. Water leaves a groundwater reservoir in streams or springs. People take water from aquifers, too.
The top of the stream is the top of the water table. The stream feeds the aquifer.
Groundwater meets the surface in a stream (Figure above) or a spring (Figure below). A spring may be constant, or may only flow at certain times of year. Towns in many locations depend on water from springs. Springs can be an extremely important source of water in locations where surface water is scarce.
A spring in Croatia bubbles to the surface and feeds the river Cetina.
A well is created by digging or drilling to reach groundwater. It is important for anyone who intends to dig a well to know how deep beneath the surface the water table is. When the water table is close to the surface, wells are a convenient method for extracting water. When the water table is far below the surface, specialized equipment must be used to dig a well. Most wells use motorized pumps to bring water to the surface, but some still require people to use a bucket to draw water up (Figure below).
An old-fashioned well that uses a bucket drawn up by hand.
- A rock layer must be porous and permeable to be a good aquifer. An impermeable layer makes up the bottom of an aquifer.
- The water table rises and falls with additions or subtractions to the groundwater system.
- Although people get groundwater from springs, which bring water to the surface, most groundwater is accessed using wells.
Use this resource to answer the questions that follow.
- How and where does water get into the aquifer?
- What happens in the recharge zone?
- What is the artesian zone?
- What does hydrologic pressure do?
- Where are the wells located and why?
- What is a confined aquifer?
- What is the saline water zone?
- What happens when pollution leaks from a single area?
- Explain the two types of pollutants.
- What can cause each type of pollution?
- How can the pollution of aquifers be prevented?
- What happens to the water table in an extremely wet year? In an extremely dry one?
- What characteristics are needed for rock in and around an aquifer?
- What causes a spring?