Many of Earth’s groundwater supplies are threatened, mainly by human population growth and contamination. Groundwater is freshwater that resides underground; it collects following the movement of water from the surface down through soil and rock.
As the water moves downward, a zone or line is formed underground, above which the spaces in the soil and rock are filled with air and water (called the zone of infiltration) and below which water occupies every available opening (called the zone of saturation). The water table is at the top of the zone of saturation, and groundwater lies beneath the water table.
In a desert, groundwater is a vital source of life to plants, which can produce roots that are dozens of feet (meters) long, to reach down from the dry surface to the water below. Groundwater is also important to humans for drinking water, growing crops (irrigation), and other uses.
For example, in the year 2000, about 21% of the 408 billion gallons of water used in the United States each day came from underground sources. Over the course of a year, that adds up to about 32 trillion gallons of groundwater!
Threats to groundwater
The main reason that groundwater is threatened has to do with the chemistry of water. Many other compounds (substances formed by the joining of two or more elements) can easily dissolve into the liquid form of water. Liquids into which other compounds dissolve in are called solvents, and water is known as the “universal solvent.”
“Out of sight, out of mind”
Because many areas have a plentiful supply of groundwater, this resource can easily be overlooked. In Canada for example, before 1990, water scientists found that although 30% of Canadians relied on groundwater for their drinking water, the government had not formed laws to help protect groundwater quality. Since that time, the number of Canadians depending upon groundwater for drinking water has increased, but Canada still has no groundwater protection policy.
Individual versus community groundwater use
A well that supplies one house does not usually harm a groundwater source. This is because the amount of water that is taken from the ground is more than balanced out by the amount of water that goes back into the ground as rainfall, melting snow, floodwater or other sources.
However, when a community relies on one or a series of wells for its water supply, the total amount of water that is withdrawn from the groundwater source can be large. If an area experiences a drought (prolonged shortage of rainfall), water consumption rises, or wells are placed too close together, then more groundwater can be taken than is replenished.
Rather than drawing water from different parts of the geographical area where water from the region collects (the aquifer), wells very near one another can draw water from the same part of the aquifer, depleting it faster than if the wells were further apart.
These imbalances cannot continue indefinitely or the groundwater supply will run out. The city of Las Vegas, Nevada is an example of a community whose demand for water is causing problems to the underground supply of water. So much groundwater has been taken from the ground around Las Vegas due to a growing population’s demand for water, that areas of the land have dropped by over five feet (1.5 meters) in the past century.
Other sources of groundwater depletion
Some industries use large amounts of groundwater. In the oil industry, for example, groundwater (along with surface water) is forced down into oil wells to help bring the hard-to-pump oil up to the surface.
While much of this water is eventually returned to the ground, some is lost, mainly through evaporation, or is so polluted that it cannot be put back into the environment. Mines that are made by digging huge holes in the ground (quarries) also help deplete groundwater. Groundwater will often leak out of the walls of the quarry and pool in the bottom of the hole.
|digging huge holes in the ground|
The quarry acts as a sponge, drawing water out of the surrounding ground. Even though the water can be pumped out and returned to the surrounding ground, the rate of water loss from the ground is sometimes higher than the rate at which the water returns to the ground.
Nature is another source of groundwater depletion, in the form of a drought. Without rainfall to replenish the groundwater, an underground water source can quickly run dry with continued use.
Most of the chemicals and microorganisms in water will be removed as it seeps down through layers of soil and rock to reach the water table. Soil and rocks filter the chemicals and microorganisms in water by sticking to the contaminants or because they are too large to pass through the tiny cracks in the rock.
This cleaning effect is not, however, always foolproof. There can be breaks in the soil-rock barrier that allow chemicals and microorganisms to rapidly move down to the groundwater, where they collect and contaminate the groundwater.
Pesticides in groundwater.
Only a few generations ago, the contamination of groundwater by chemicals designed to kill weeds and other pests (pesticides) was not a concern. But as the population of the United States has grown, and with the demand to produce more crops from each acre of land, scientists began to develop chemical pesticides.
The use of pesticides has increased the amount of food that can be grown. Indeed, the United States has become the largest producer of food in the world, partially due to the fact that crops are protected from destruction by insects and other pests with the use of pesticides.
This increase in production has come with a big disadvantage, the contamination of groundwater (along with surface bodies of water and soil) by the pesticides. Because pesticides can take decades to get from the surface to groundwater, the problem with pesticides in groundwater could become worse in the future.
Organizations such as the U.S. Environmental Protection Agency are working to understand pesticide contamination of groundwater and to try to set standards for how much pesticide can be safely applied to the land.
Other sources of groundwater contamination.
Another example of how groundwater can be contaminated is via a sinkhole; a crater formed on the surface when the rook of a cavern located directly below collapses. The sinkhole creates a kind of water freeway; water can move easily down through the many open spaces in the rock to the groundwater zone.
Sinkholes often occur in limestone rock. In states enriched in limestone, such as Iowa, sinkhole-related contamination of groundwater is a concern. In just the northeast part of Iowa, almost 13,000 sinkholes are known to exist.
Groundwater can also be contaminated when underground storage tanks, such as the tanks located underneath gas stations, develop a leak. As the toxin leaks out of the tank, it can make its way to the groundwater.
When the chemicals disperse (spread) in the groundwater, a wide area (many city blocks) around the tank can become contaminated. As this type of groundwater contamination is harder to detect, most communities have systems in place to check underground storage tanks for leaks on a regular basis.
Since water moves downward to the groundwater, any contaminant that is on the surface may also be carried down. Along with pesticides, surface contaminants can also include wells in fields that are designed to help drain excess water, water containing salt that is applied to roads in northern climates to prevent ice from forming, old uncovered wells that contain animal or other waste, and the burial of waste material in landfills.