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# GROUNDWATER2

الكلية كلية الهندسة     القسم  الهندسة البيئية     المرحلة 4
أستاذ المادة عدي عدنان جهاد الخيكاني       06/04/2013 07:59:15
GROUNDWATER
When many people think of a water source, they think of lakes, rivers and streams; in other words, surface water. However, of all of the usable freshwater in the world, approximately 97 percent of it is groundwater. According to the United Nations, 10 million cubic kilometres of water are stored underground. The United States Geological Survey states that there is about 4.2 million cubic kilometres of water within 0.8 kilometre of the earth’s surface. Environment Canada cites a study that estimates that all of the groundwater in the world would cover the surface of the earth to a depth of 120 metres, while all of the surface freshwater would only cover the earth to a depth of 0.25 metre! While groundwater estimates can vary, scientists agree that there is a lot of water under the earth’s surface!
What is groundwater and where is it found?
Groundwater is water that accumulates underground. It can exist in spaces between loose particles of dirt and rock, or in cracks and crevices in rocks. Different types of rocks and dirt can contain different amounts of water. The saturation zone is the portion of the soil and rock that is saturated with water, while the unsaturated zone is the portion of the soil and rock that is not saturated. The top of the saturated zone is called the water table. The diagram below illustrates these terms.
When it rains, the water infiltrates the soil and percolates downwards until it reaches the water table. Some types of soils allow more water to infiltrate than others. Permeable surfaces, such as sand and gravel, allow up to 50 percent of precipitation to enter the soil. Rainwater can take years or even decades to reach the water table. Due to the immense volume of groundwater, once rainwater reaches the water table, it often remains there for an extremely long period of time. Some water that is currently stored in the ground may be rain that fell hundreds or thousands of years ago. www

المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .