 |
Business | Industries | Finance | Tax |
| Index: > A B C D E F G H I J K L M N O P Q R S T U V W X Y Z |
|
|||||
| First Prev [ 1 2 ] Next Last |
Groundwater is a long-term 'reservoir' of the natural water cycle, as opposed to short-term water reservoirs like the atmosphere and fresh surface water. It is naturally replenished from above, as surface water from precipitation sinks into the ground.
Groundwater is often contained in aquifers, which are subterranean areas (or layers) of permeable material that channel the groundwater's flow. Aquifers can be confined or unconfined. If a confined aquifer follows a downward grade from a recharge zone, groundwater can become pressurized as it flows. This can create artesian wells that flow freely without the need of a pump. The top of an unconfined aquifer is called the water table or phreatic groundwater, where water pressure is equal to atmospheric pressure. The region between the land surface and the water table is called the vadose zone (Latin for shallow). Within the vadose zone water is held to soil particles by adhesion and in pore spaces by capillary action.
Groundwater is a highly useful and abundant resource, but it does not renew itself rapidly. If groundwater is extracted intensively, as for irrigation in arid regions, it may become depleted. The most evident problem that may result from this is a lowering of the water table beyond the reach of existing wells. Wells must consequently be deepened to reach the groundwater; in places like India, the water table has dropped hundreds of feet due to over-extraction. A lowered water table may, in turn, cause other problems such as subsidenceSubsidence is a term used in geology, engineering and surveying to denote the motion of a surface (usually, the earth's surface) downwards relative to a datum such as sea-level. In meteorology, subsidence refers to the movement of air downwards. Surfaces.
The film of ground water around particles of an aquifer of unconsolidated sediment actually holds the particles apart, and the removal of this water will compact the sediment. Thus the aquifer is permanently reduced in capacity, and the surface of the ground may also subside. The city of New Orleans, LouisianaNew Orleans is the largest city in the state of Louisiana, United States of America. By law and government, the city of New Orleans and the parish of Orleans Parish are one and the same 6. It is an industrial and distribution center, a major seaport, and is actually below sea level today, and its subsidence is partly caused by removal of ground water under it.
Generally (but not always) ground water flows in the same direction as the slope of the 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 seawaterSea water is water from a sea or ocean. On average, sea water in the world's oceans has a salinity of ~3. This means that for every 1 liter (1000mL) of sea water there are 35 grams of salts (mostly, but not entirely, sodium chloride) dissolved in it. to reverse the flow toward the sea. Sea water moving inland is called a saltwater intrusion . Alternatively, saltFor other meanings of the word salt see salt (disambiguation In chemistry, a salt is a composed of positively charged cations and negatively charged anions, so that the product is neutral and without a net charge. They are typically the product of a chemi from mineralMinerals are natural compounds formed through geological processes. The term "mineral" encompasses not only the material's chemical composition but also the mineral structures. Minerals range in composition from pure elements and simple salts to very comp beds may leach into the groundwater of its own accord.
Sometimes the water movement from the recharge zone to the place where it is withdrawn may take centuries. When the usage of water is greater than the recharge, it is referred to as mining water. Under those circumstances it is not a renewable resource.
In India and BangladeshThe People's Republic of Bangladesh is a country in South Asia that forms the eastern part of the ancient region of Bengal. Bangladesh literally means "The Country of Bengal". Lying north of the Bay of Bengal, on land it borders India almost exclusively,, a drop in the water table has been associated with arsenic contamination. It is thought that irrigation for rice production since late 1970s resulted in the withdrawal of large quantities of groundwater, which caused the local water table to drop, allowing oxygen to enter the ground and touching off a reaction that leaches out arsenic from pyrite in the soil. The actual mechanism, however, is yet to be identified with certainty.
Not all groundwater problems are caused by over-extraction. Pollutants dumped on the ground or in landfills may leach into the soil, and work their way down into aquifers. Movement of water within the aquifer is then likely to spread the pollutant over a wide area, making the groundwater unusable. Areas of karst topography on limestone bedrock are especially vulnerable to surface pollution. Sinkholes and underground caverns allow direct groundwater flow without the filtering effect of a permeable aquifer. See environmental engineering and remediation.
Water table conditions are of great importance to agricultural irrigation, waste disposal (including nuclear waste), and other ecological issues.
| Politics | Business | Finance |
 |
 |
 |
|