ch15 copy - Chapter 15: Groundwater Water In The Ground (1)...

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Chapter 15: Groundwater
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Water In The Ground (1) Groundwater is defined as all the water in the ground occupying the pore spaces within bedrock and regolith. The volume of groundwater is 40 times larger than the volume of all water in fresh-water lakes or flowing in streams. Less than 1 percent of the water on Earth is ground water. Most ground water originates as rainfall.
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Depth of Groundwater Water is present everywhere beneath the land surface, but more than half of all groundwater, including most of what is usable, occurs above a depth of 750m. Below a depth of about 750 m, the amount of groundwater gradually diminishes. Russian scientists encountered water at more than 11 km below the surface.
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The Water Table (1) The zone of aeration (also called the unsaturated zone ) is a layer of moist soil followed by a zone in which open spaces in regolith or bedrock are filled mainly with air. Beneath the unsaturated zone is the saturated zone , a zone in which all openings are filled with water. The upper surface of the saturated zone is the water table.
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Figure 15.1
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The Water Table (2) Normally, the water table slopes toward the nearest stream or lake. In fine-grained sediment, a narrow fringe as much as 60 cm thick immediately above the water table is kept wet by capillary attraction . Capillary attraction is the adhesive force between a liquid and a solid that causes water to be drawn into small openings.
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The Water Table (3) In humid regions, the water table is a subdued imitation of the land surface above it. It is high beneath hills and low beneath valleys because water tends to move toward low points in the topography under the influence of gravity.
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How Groundwater Moves (1) Groundwater operates continuously as part of the hydrologic cycle. As rain seeps into the ground it enters the groundwater reservoir.
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How Groundwater Moves (2) Most of the groundwater within a few hundred meters of the surface is in motion. Groundwater moves so slowly that velocities are expressed in centimeters per day or meters per year. Groundwater must move through small, constricted passages, often along a tortuous route.
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Porosity and Permeability (1) Porosity is the percentage of the total volume of a body of regolith or bedrock that consists of open spaces, called pores. It is porosity that determines the amount of water that a given volume of regolith or bedrock can contain.
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The porosity of a sedimentary rock is affected by several factors: The sizes and shapes of the rock particles. The compactness of their arrangement. The weight of any overlying rock or sediment. The extent to which the pores become filled with the cement that holds the particles together. The porosity of igneous and metamorphic rocks
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ch15 copy - Chapter 15: Groundwater Water In The Ground (1)...

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