Groundwater - Wikipedia
Falling Water Tables · IGRAC International Groundwater Resources Assessment Centre · SahysMod aquifer model. The water table and an aquifer are terms used when discussing groundwater. The major difference between the two terms is that the water table references a specific portion of Definition of a Seasonal High Water Table. USGS Office of Groundwater · UK Groundwater Forum · IGRAC, International Groundwater Resources Assessment Centre · IAH.
In the Punjab region of Indiafor example, groundwater levels have dropped 10 meters sinceand the rate of depletion is accelerating. Groundwater is also ecologically important.
The importance of groundwater to ecosystems is often overlooked, even by freshwater biologists and ecologists. Groundwaters sustain rivers, wetlandsand lakesas well as subterranean ecosystems within karst or alluvial aquifers. Not all ecosystems need groundwater, of course. Some terrestrial ecosystems — for example, those of the open deserts and similar arid environments — exist on irregular rainfall and the moisture it delivers to the soil, supplemented by moisture in the air.
Types of aquifers
Water flows between groundwaters and surface waters. Most rivers, lakes, and wetlands are fed by, and at other places or times feed groundwater, to varying degrees. Groundwater feeds soil moisture through percolation, and many terrestrial vegetation communities depend directly on either groundwater or the percolated soil moisture above the aquifer for at least part of each year.
Hyporheic zones the mixing zone of streamwater and groundwater and riparian zones are examples of ecotones largely or totally dependent on groundwater. Groundwater-related subsidence Subsidence occurs when too much water is pumped out from underground, deflating the space below the above-surface, and thus causing the ground to collapse.
The result can look like craters on plots of land. This occurs because, in its natural equilibrium state, the hydraulic pressure of groundwater in the pore spaces of the aquifer and the aquitard supports some of the weight of the overlying sediments. When groundwater is removed from aquifers by excessive pumping, pore pressures in the aquifer drop and compression of the aquifer may occur. This compression may be partially recoverable if pressures rebound, but much of it is not. When the aquifer gets compressed, it may cause land subsidence, a drop in the ground surface.
The recharge zone of an aquifer near the seacoast is likely to be inland, often at considerable distance. In these coastal areas, a lowered water table may induce sea water to reverse the flow toward the land. Sea water moving inland is called a saltwater intrusion. In alternative fashion, salt from mineral beds may leach into the groundwater of its own accord. Pollution[ edit ] Iron III oxide staining caused by oxidation of dissolved iron II oxide and precipitation, from an unconfined aquifer in karst topography.
PerthWestern Australia. Groundwater pollution Polluted groundwater is less visible, but more difficult to clean up, than pollution in rivers and lakes. How quickly will a contamination incident reach a well or spring? Computer models can be used to test how accurately the understanding of the aquifer properties matches the actual aquifer performance. Porous aquifer properties depend on the depositional sedimentary environment and later natural cementation of the sand grains.
Aquifer - Wikipedia
The environment where a sand body was deposited controls the orientation of the sand grains, the horizontal and vertical variations, and the distribution of shale layers.
Even thin shale layers are important barriers to groundwater flow. All these factors affect the porosity and permeability of sandy aquifers. Groundwater flow directions can be determined from potentiometric surface maps of water levels in wells and springs.
Aquifer tests and well tests can be used with Darcy's law flow equations to determine the ability of a porous aquifer to convey water. A groundwater flow rate of 1 foot per day 0. Karst[ edit ] Water in karst aquifers flows through open conduits where water flows as underground streams Karst aquifers typically develop in limestone. Surface water containing natural carbonic acid moves down into small fissures in limestone.
This carbonic acid gradually dissolves limestone thereby enlarging the fissures. The enlarged fissures allow a larger quantity of water to enter which leads to a progressive enlargement of openings.
Abundant small openings store a large quantity of water. The larger openings create a conduit system that drains the aquifer to springs. These conventional investigation methods need to be supplemented with dye tracesmeasurement of spring discharges, and analysis of water chemistry.
Geological Survey dye tracing has determined that conventional groundwater models that assume a uniform distribution of porosity are not applicable for karst aquifers. Locating a well in a fracture trace or intersection of fracture traces increases the likelihood to encounter good water production. For example in the Barton Springs Edwards aquifer, dye traces measured the karst groundwater flow rates from 0.
This may occur in eroded limestone areas known as karst topographywhich make up only a small percentage of Earth's area. More usual is that the pore spaces of rocks in the subsurface are simply saturated with water—like a kitchen sponge—which can be pumped out for agricultural, industrial, or municipal uses.
If a rock unit of low porosity is highly fractured, it can also make a good aquifer via fissure flowprovided the rock has a hydraulic conductivity sufficient to facilitate movement of water. Porosity is important, but, alone, it does not determine a rock's ability to act as an aquifer. Areas of the Deccan Traps a basaltic lava in west central India are good examples of rock formations with high porosity but low permeability, which makes them poor aquifers. Similarly, the micro-porous Upper Cretaceous Chalk Group of south east England, although having a reasonably high porosity, has a low grain-to-grain permeability, with its good water-yielding characteristics mostly due to micro-fracturing and fissuring.
Human dependence on groundwater[ edit ] Center-pivot irrigated fields in Kansas covering hundreds of square miles watered by the Ogallala Aquifer Most land areas on Earth have some form of aquifer underlying them, sometimes at significant depths. In some cases, these aquifers are rapidly being depleted by the human population. Fresh-water aquifers, especially those with limited recharge by snow or rain, also known as meteoric watercan be over-exploited and depending on the local hydrogeologymay draw in non-potable water or saltwater intrusion from hydraulically connected aquifers or surface water bodies.
This can be a serious problem, especially in coastal areas and other areas where aquifer pumping is excessive. In some areas, the ground water can become contaminated by arsenic and other mineral poisons.
Aquifers are critically important in human habitation and agriculture.
The Difference Between an Aquifer and the Water Table | Sciencing
Deep aquifers in arid areas have long been water sources for irrigation see Ogallala below. Many villages and even large cities draw their water supply from wells in aquifers. Municipal, irrigation, and industrial water supplies are provided through large wells. Multiple wells for one water supply source are termed "wellfields", which may withdraw water from confined or unconfined aquifers. Using ground water from deep, confined aquifers provides more protection from surface water contamination.
Some wells, termed "collector wells", are specifically designed to induce infiltration of surface usually river water. Aquifers that provide sustainable fresh groundwater to urban areas and for agricultural irrigation are typically close to the ground surface within a couple of hundred metres and have some recharge by fresh water.
This recharge is typically from rivers or meteoric water precipitation that percolates into the aquifer through overlying unsaturated materials.
- Groundwater & Aquifers
Occasionally, sedimentary or "fossil" aquifers are used to provide irrigation and drinking water to urban areas. As the saturation zone changes, so does the water table level. For example, if the weather is dry, the water table may become deeper as less water is available. An aquifer is the water beneath the water table. Aquifer An aquifer is a body of saturated rock through which water can easily move, according to the Idaho Museum of Natural History.
Water moves through the pores of the rock. The pores act as a natural filtration system, removing even viruses and bacteria from the water. Aquifers can be considered unconfined or confined.