The U. Geological Survey website has a map of important aquifers in the contiguous United States. Groundwater can be found in a range of different types of rock, but the most productive aquifers are found in porous, permeable rock such as sandstone, or the open cavities and caves of limestone aquifers. Groundwater moves more readily through these materials, which allows for faster pumping and other methods of extracting the water.
Aquifers can also be found in regions where the rock is made of denser material — such as granite or basalt — if that rock has cracks and fractures. Dense, impermeable material like clay or shale can act as an "aquitard," i.
Through groundwater might move through such material, it will do so very slowly if at all. Faults or mountains can also block the movement of fresh groundwater, as can the ocean, Phillips said. An aquitard can trap groundwater in an aquifer and create an artesian well. When groundwater flows beneath an aquitard from a higher elevation area to a lower elevation, such as from a mountain slope to a valley floor, the pressure on the groundwater can be enough to force the water out of any well that's drilled into that aquifer.
These aquifers are also known as water table aquifers. They receive recharge directly from the infiltration of rainfall and surface water. Confined aquifers are those that are covered confined by an impermeable or semi-permeable layer of rock.
Confined aquifers are not directly recharged by vertical infiltration. Confined aquifers need to be connected to an unconfined area through which recharge can occur. The confining impermeable layers rarely form a complete barrier to groundwater. There is generally some transfer or flow of groundwater between the confined aquifer and the confining layers. Water from these sediments can reach a well if they are in contact with permeable layers that are intersected by the well.
So, although clays are not usually thought of as aquifers, they may be a key part of the storage in an aquifer system.
Perched aquifers occur where groundwater is perched above unsaturated rock formations as a result of a discontinuous impermeable layer. Perched aquifers are fairly common in glacial sediments. They also occur in other sedimentary formations where weathered layers, ancient soils or caliched a calcareous layer common in semi-arid areas have created impermeable zones. The water that reaches these chambers is usually much cleaner than the water of reservoirs at the earth's surface.
Almost no bacteria live in aquifers. Many pollutants are filtered out as the water passes through the soil on its way to the aquifer. There is no silty mud to cloud the water, no pollution from boaters, and no evaporation of the water supply by the sun. However, aquifers can become polluted due to human actions and when an aquifer becomes polluted this is very difficult to remedy. To tap the groundwater in an aquifer, wells are dug until they reach the top layer of the aquifer, the water table.
The water table is not flat as its name makes it seem. It has peaks and valleys that echo the shape of the land above it. Further precipitation adds water into the porous rock of the aquifer. The rate of recharge is not the same for all aquifers, though, and that must be considered when pumping water from a well.
Pumping too much water too fast draws down the water in the aquifer and eventually causes a well to yield less and less water and even run dry.
In fact, pumping your well too fast can even cause your neighbour's well to run dry if you both are pumping from the same aquifer.
Water levels in most aquifers vary with the season and during droughts. For purposes of wise groundwater protection policy, we should consider as aquifers, the full vertical and horizontal extent of seasonally dewatered or over-pumped rock formations. Water flowing into recharge areas--land covered with soil and trees-- refills the aquifer.
Bogs and swamps may absorb and store water that later slowly drains into aquifers. When recharge areas and wetlands are replaced by parking lots and highways, less water reaches the aquifer. Oil and road salt from paved roads may trickle down with rain and snowmelt and pollute an aquifer.
High levels of chemical-use and waste generation in recent decades are slowly poisoning supplies of groundwater — the major source of our freshwater needs. It is a silent disaster spreading through many parts of the world. The relentless contamination of groundwater will make the supplies of usable water tighter still. Our increasing demand on water has made it a resource critical to a degree that even gold and oil have never been.
Since , there has been a dramatic expansion in irrigated agriculture. In India, the leading country in total irrigated area and the world's third largest grain producer, the number of shallow tubewells used to draw groundwater surged from 3, in to 6 million in While India doubled the amount of its land irrigated by surface water between and , it increased the area watered by aquifers fold.
Today, aquifers supply water to more than half of India's irrigated land. Other industries have been expanding their water use even faster than agriculture - and generating much higher profits in the process. Thus as the world has industrialized, substantial amounts of water have been shifted from farms to more lucrative factories.
The amount of water available for drinking is thus constrained not only by a limited resource base, but by competition with other, more powerful users.
On almost every continent, many major aquifers are being drained faster than their natural rate of recharge. Most groundwater, including a significant amount of our drinking water, comes from aquifers. In order to access this water, a well must be created by drilling a hole that reaches the aquifer.
While wells are manmade points of discharge for aquifers, they also discharge naturally at springs and in wetlands.
Groundwater can become depleted if we use it at a faster rate than it can replenish itself. The replenishment of aquifers by precipitation is called recharging. Depletion of aquifers has increased primarily due to expanding agricultural irrigation. Groundwater can become contaminated when an excessive amount of pesticides and herbicides are sprayed on agricultural fields, septic tanks leak, or landfills are improperly lined or managed and toxic materials seep through the soil into the aquifer.
Aquifers naturally filter groundwater by forcing it to pass through small pores and between sediments, which helps to remove substances from the water. This natural filtration process, however, may not be enough to remove all of the contaminants. Pesticides can be fungicides which kill harmful fungi , insecticides which kill harmful insects , herbicides which kill harmful plants , or rodenticides which kill harmful rodents.
The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit. The Rights Holder for media is the person or group credited.
How Does an Aquifer Work? An aquifer is filled with moving water and the amount of water in storage in the aquifer can vary from season to season and year to year. Ground water may flow through an aquifer at a rate of 50 feet per year or 50 inches per century, depending on the permeability. But no matter how fast or slow, water will eventually discharge or leave an aquifer and must be replaced by new water to replenish or recharge the aquifer. Thus, every aquifer has a recharge zone or zones and a discharge zone or zones.
Figure 2 is a simple cartoon showing three different types of aquifers: confined, unconfined, and perched. Recharge zones are typically at higher altitudes but can occur wherever water enters an aquifer, such as from rain, snowmelt, river and reservoir leakage, or from irrigation.
Discharge zones can occur anywhere; in the diagram, discharge occurs not only in springs near the stream and in wetlands at low altitude, and also from wells and high-altitude springs. The amount of water in storage in an aquifer is reflected in the elevation of its water table. If the rate of recharge is less than the natural discharge rate plus well production, the water table will decline and the aquifer's storage will decrease.
A perched aquifer's water table is usually highly sensitive to the amount of seasonal recharge so a perched aquifer typically can go dry in summers or during drought years. Why is Groundwater So Clean? Aquifers are natural filters that trap sediment and other particles like bacteria and provide natural purification of the ground water flowing through them.
0コメント