WHAT IS GROUNDWATER?
Water is the life blood of every living creature on earth. Approximately 70 percent of the earth's surface is covered with water. Through the wonders of nature, water can take on many different forms. It is easy to understand the significance water plays in our lives but it may be difficult to understand the water that exists below the earth's surface. This water is called groundwater.
Groundwater is often thought of as an underground river or lake. Only in caves or within lava flows does groundwater occur this way. Instead, groundwater is usually held in porous soil or rock materials, much the same way water is held in a sponge.
When rain falls to the ground, the water does not stop moving. Some of it flows along the surface in streams or lakes, some of it is used by plants, some evaporates and returns to the atmosphere, and some sinks into the ground. Groundwater is water that is found underground in cracks and spaces in soil, sand and rocks. The area where water fills these spaces is called the saturated zone. The top of this zone is called the water table. The water table may be only a foot below the ground's surface or it may be hundreds of feet down.
Groundwater can be found almost everywhere. The water table may be deep or shallow and may rise or fall depending on many factors. Heavy rains or melting snow may cause the water table to rise or an extended period of dry weather may cause the water table to fall. Groundwater is stored in, and moves slowly through, layers of soil, sand and rocks called aquifers. The speed at which groundwater flows depends on the size of the spaces in the soil or rock and how well the spaces are connected.
Aquifers typically consist of gravel, sand, sandstone, or fractured rock, like limestone. These materials are permeable because they have large connected spaces that allow water to flow through. Water in aquifers is brought to the surface naturally through a spring or can be discharged into lakes and streams. This water can also be extracted through a well drilled into the aquifer. A well is a pipe in the ground that fills with groundwater. This water then can be brought to the surface by a pump. Shallow wells may go dry if the water table falls below the bottom of the well. Some wells, called artesian wells, do not need a pump because of natural pressures that force the water up and out of the well.
Groundwater supplies are replenished, or recharged, by rain and snow melt. In some areas of the world, people face serious water shortages because groundwater is used faster than it is naturally replenished. In other areas groundwater is polluted by human activities. In areas where material above the aquifer is permeable, pollutants can sink into the groundwater. Groundwater can be polluted by landfills, septic tanks, leaky underground gas tanks, and from overuse of fertilizers and pesticides. If groundwater becomes polluted, it will no longer be safe to drink.
When groundwater becomes contaminated, it does not cleanse itself as surface water tends to do. The turbulent action of a flowing river, which mixes oxygen into river water and tends to dilute contaminated water with clean water, is absent in groundwater flows. Likewise, there are fewer bacteria in contact with groundwater, compared to surface water, so bacterial action does not cleanse groundwater as it might surface water. Furthermore, groundwater does not have the benefit of sunlight, and it is also relatively cool. For all these reasons, contaminated groundwater tends to remain contaminated for long periods of time measured in decades or centuries or aeons. On a normal human time scale, groundwater contamination can be considered permanent.
After an aquifer has been contaminated it is difficult to entirely define or isolate a contaminant plume. It is also difficult and extremely costly to remove it. Even after the source of contamination has been removed, an aquifer may remain contaminated for anywhere from a few years to a few centuries. Thus, it is often unrealistic to talk about a "cure" for groundwater contamination. Prevention is the key and finding sources of contamination and learning to control them.
Half of all the people in the U.S. derive their drinking water
supplies from groundwater. Because of connections between
groundwater and surface water, contaminated groundwater can
affect the quality of surface waters and can thus impact wildlife (including fish, and mammals that rely on surface water for drinking). Therefore, groundwater is an essential ingredient for life, a national resource that should be protected.
Groundwater is used for drinking water by more than 50% of the people in the United States, including almost everyone who lives in rural areas. The largest use for groundwater is to irrigate crops.
How Aquifers are Replenished
Recharge is the process by which aquifers are replenished with water from the surface. This process occurs naturally as part of the hydrologic cycle as infiltration when rainfall infiltrates the land surface and as percolation of water into underlying aquifers. A number of factors influence the rate of recharge including physical characteristics of the soil, plant cover, slope, water content of surface materials, rainfall intensity, and the presence and depth of confining layers and aquifers.
Surface water bodies may also recharge groundwater. This occurs most often in arid areas. Lakes and dry creek beds may fill up with water during heavy rains. If the water table is low in underlying aquifers, water may seep from the sides of these water bodies and percolate into the groundwater.
In some places, artificial recharge is used to replenish aquifers. This is accomplished through the pumping, or injection, of water into wells where it replenishes the aquifer directly or through the spreading of water over the land surface where it can seep into the ground. Artificial recharge is done to replenish the groundwater supply when rains are heavy in order to preserve water for later use or, in the case of injection wells, to dilute or control the flow of contaminated groundwater.
Where does Water from Aquifers Go?
Gravity is the dominant driving force in groundwater movement in unconfined aquifers. As such, under natural conditions, groundwater moves "downhill" until it reaches the land surface at a spring or through a seep in the side or bottom of a river bed, lake, wetland, or other surface water body. Groundwater can also leave the aquifer via the pumping of a well. The process of groundwater outflowing into a surface water body or leaving the aquifer through pumping is called discharge. Many rivers, lakes, and wetlands rely heavily on groundwater discharge as a source of water. During times of low precipitation, these bodies of water would not contain any water at all if it were not for groundwater discharge. It is important to note that because of discharge, contaminants in groundwater can flow into surface water bodies. This process can make the removal of contamination very complex. Discharge from confined aquifers occurs in much the same way except that pressure, rather than gravity, is the driving force in moving groundwater to the surface. When the intersection between the aquifer and the land's surface is natural, the pathway is called a spring. If discharge occurs through a well, that well is a flowing artesian well.
Too Much Water Going Out
When the withdrawal of groundwater in an aquifer exceeds the recharge rate over a period of time, the aquifer is considered to have overwithdrawal. There are two possible effects from the overwithdrawal of water from an aquifer. First, when the amount of fresh water being pumped out of an aquifer in a coastal area can not be replaced as fast as it is being withdrawn, saltwater migrates towards the point of withdrawal. This movement of saltwater into zones previously occupied by fresh water is called saltwater intrusion. Saltwater intrusion can also occur in inland areas where briney water underlies fresh water. Secondly, in some areas, overwithdrawal can make the ground sink because groundwater pressure helps to support the weight of the land. This is called subsidence. Sinkholes are an example of this effect.
It is important for all of us to learn to protect our groundwater!