groundwater

groundwater - This page last updated on 12-Apr-2011 EENS...

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Unformatted text preview: This page last updated on 12-Apr-2011 EENS 1110 Physical Geology Tulane University Prof. Stephen A. Nelson Groundwater Groundwater is water that exists in the pore spaces and fractures in rock and sediment beneath the Earth's surface. It originates as rainfall or snow, and then moves through the soil into the groundwater system, where it eventually makes its way back to surface streams, lakes, or oceans. z Groundwater makes up about 1% of the water on Earth (most water is in oceans). z But, groundwater makes up about 35 times the amount of water in lakes and streams. z Groundwater occurs everywhere beneath the Earth's surface, but is usually restricted to depths less that about 750 meters. z The volume of groundwater is a equivalent to a 55 meter thick layer spread out over the entire surface of the Earth. z It is an important resource for potable water, irrigation, and industry. z Because it is largely hidden from view, it is often forgotten and subject to contamination by careless humans. z Groundwater is a primary agent of chemical weathering and is responsible for the formation of caves and sinkholes. The Groundwater System Groundwater resides in the void spaces of rock, sediment, or soil, completely filling the voids. The total volume of open space in which the groundwater can reside is porosity . Porosity determines the amount of water that a rock or sediment can contain. Porosity In sediments or sedimentary rocks the porosity depends on grain size, the shapes of the grains, and the degree of sorting, and the degree of cementation. Groundwater 4/12/2011 Page 1 of 11 z Well-rounded coarse-grained sediments usually have higher porosity than fine-grained sediments, because the grains do not fit together well. z Poorly sorted sediments usually have lower porosity because the fine-grained fragments tend to fill in the open space. z Since cements tend to fill in the pore space, highly cemented sedimentary rocks have lower porosity. z In igneous and metamorphic rocks porosity is usually low because the minerals tend to be intergrown, leaving little free space. Highly fractured igneous and metamorphic rocks, however, could have high porosity Secondary porosity is porosity that developed after rock formation. Processes such as fracturing, faulting, and dissolution can create secondary porosity. Permeability is a measure of the degree to which the pore spaces are interconnected, and the size of the interconnections. Low porosity usually results in low permeability, but high porosity does not necessarily imply high permeability. It is possible to have a highly porous rock with little or no interconnections between pores. A good example of a rock with high porosity and low permeability is a vesicular volcanic rock, where the bubbles that once contained gas give Groundwater 4/12/2011 Page 2 of 11 the rock a high porosity, but since these holes are not connected to one another the rock has low permeability....
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This note was uploaded on 02/01/2012 for the course EENS 1110 taught by Professor Staff during the Fall '10 term at Tulane.

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groundwater - This page last updated on 12-Apr-2011 EENS...

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