M. Chapter 13 - Chapter 13 Groundwater Def'n: Groundwater...

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Chapter 13 Groundwater Def’n: Groundwater – water filling open spaces in rocks, sediment, and soil beneath the surface. Remember the hydrologic cycle (fig. 12.4, p. 337). Groundwater is a part of this cycle and represents just under ¼ (22%) of the world’s supply of fresh water. How rocks absorb water The amount, availability, and movement of groundwater is based on two properties, porosity and permeability. Def’n: Porosity – the percentage of a materials total volume that is pore space. - usually the spaces between the particles of sediment or soil - other pore spaces may be in the form of cracks, fractures, faults, or vesicles in igneous rocks. Porosity depends on the size, shape, and arrangement of the material composing the rock, so varies among rock types. - well-sorted, well-rounded sediments = high porosity (clean sandstone and siltstone) - poorly sorted sediments = lower porosity - igneous & metamorphic rocks, as well as chemical sedimentary rocks (limestone, dolostone, chert) typically have very low porosity - porosity may be increased by fracturing or chemical weathering - well-cemented rocks = low porosity Def’n: Permeability – the capacity to transmit fluids Depends on: Porosity Size of pores or fractures Interconnections of the pores or fractures Rocks may be porous without being permeable! - silt or clay is typically more porous than sand or gravel. However, the pores are extremely small, and the molecular attraction between the clay particles and the water is great, thereby preventing the movement of water. - Sand and gravel have much larger pore spaces with much less molecular attraction, making them both porous and permeable. - Rocks with interconnected fracture systems may also be highly permeable. Def’n: Aquifer – a permeable layer transporting groundwater
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Def’n: Aquiclude – materials that prevent the movement of groundwater. In the hydrologic cycle, some water precipitates on land and then: - evaporates - runs off into streams and eventually the ocean - seeps into the ground Of the water that seeps into the ground, a small amount adheres to the material it is moving through. This is suspended water . The rest seeps downward until it fills all available pore spaces. Two zones are defined by what fills their pore spaces. 1. zone of aeration – air 2. zone of saturation – water Def’n: Water Table – the surface separating these two zones. Groundwater moves by gravity. 1
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This note was uploaded on 04/16/2008 for the course GEOL 1403 taught by Professor Teague during the Spring '08 term at Tarrant County.

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M. Chapter 13 - Chapter 13 Groundwater Def'n: Groundwater...

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