Лекция: Groundwater

Some precipitation seeps into the ground. Some of this infiltrating water accumulates as soil moisture and partially fills pores between soil par­ticles and rocks within the upper soil and rock layers of the earth's crust. Most of this water is eventually lost to the atmosphere by evaporation from the upper layers of soil and by evapotranspiration from leaves.

Under the influence of gravity, some infiltrating water slowly percolates through porous materials deeper into the earth and completely saturates pores and fractures in spongelike or permeable layers of sand, gravel, and porous rock such as sandstone. These water-bearing layers of the earth's crust are called aquifers, and the water in them is known as ground­water. Aquifers are recharged or replenished naturally by precipitation, which percolates downward through soil and rock in what is called a recharge area. The recharge process is usually quite slow (decades to hundreds of years) compared to the rapid replenish­ment of surface water supplies. If the withdrawal rate of an aquifer exceeds its recharge rate, the aquifer is converted from a slowly renewable resource to a non­renewable resource on a human time scale.

There are two types of aquifers: confined and unconfined. An unconfined, or water-table, aquifer forms when groundwater collects above a layer of rel­atively impermeable rock or compacted clay. The top of the water-saturated portion of an unconfined aqui­fer is called the water table. Thus groundwater is that part of underground water below the water table, and soil moisture is that part of underground water above the water table. Shallow, unconfined aquifers are recharged by water percolat­ing downward from soils and materials directly above the aquifer.

To obtain water from an unconfined aquifer, a water table well must be drilled below the water table and into the unconfined aquifer. Because this water is under atmospheric pressure, a pump must be used to bring it to the surface. The elevation of the water table in a particular area rises during prolonged wet periods and falls during prolonged drought. The water table can also fall when water is pumped out by wells faster than the natural rate of recharge, creating a vacated volume known as a cone of depression.

A confined, or artesian, aquifer forms when groundwater is sandwiched between two layers of rel­atively impermeable rock, such as clay or shale. This type of aquifer is completely saturated with water under a pressure greater than that of the atmo­sphere. In some cases the pressure is so great that when a well is drilled into the confined aquifer, water is pushed to the surface without the use of a pump. Such a well is called a flowing artesian well. With other confined-aquifer wells, known as nonflowing artesian wells, pumps must be used, because pressure is insuf­ficient to force the water to the surface. Confined aqui­fers cannot be recharged from directly above them; they receive water from areas without overlying impermeable rock layers. Thus recharge areas for con­fined aquifers can be hundreds of kilometers away from wells where water is withdrawn, and the rate of natural recharge is not governed by local precipitation at the point of withdrawal as it is for unconfined aquifers.

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Water is our most abundant resource, covering about 71% of the earth's surface. This precious film of water – about 97% salt water and the remainder fresh – helps maintain the earth's climate and dilutes environmental pollutants. Essential to all life, water constitutes from 50% to 97% of the weight of all plants and animals and about 70% of your body. Water is also essential to agriculture, manufactur­ing, transportation, and countless other human activities.

Because of differences in average annual pre­cipitation, some areas of the world have too little fresh water and others too much. With varying degrees of success, human beings have corrected these imbalances by capturing fresh water in res­ervoirs behind dams, transferring fresh water in rivers and streams from one area to another, tap­ping underground supplies, and attempting to reduce water use, waste, and contamination.

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