5Supp%205%250porosity%20permeability%20hydraulic%20cond -...

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Hydrology Lecture 4 Porosity, Permeability, and Darcy’s Law Finally, we begin our study of groundwater. First, we need to consider the variables that affect the storage and movement of water in aquifers . Porosity Groundwater occurs in the void spaces of earth materials: soils, unconsolidated sediments, and rock. Unconsolidated and semi-consolidated sediments typically have primary porosity in the void spaces between the grains. Lithified sedimentary rock is typically low in primary porosity due to the presence of crystalline cement (quarty or calcite) filling the primary void spaces. Crystalline igneous and metamorphic rock typically has little primary porosity. Bedrock with little primary porosity will form secondary porosity as the rock is fractured due to tectonic stress or pressure. Igneous rock may also have fractures due to cooling joints and carbonate rocks may have extensive solution cavities and fractures. These fractures provide space for the storage and movement of groundwater. Porosity is defined as the percentage of a volume of rock (the all purpose term hydrologists use for ‘earth materials’) that is empty space. n = 100 V void V total Effective porosity : void spaces that are too small to admit water molecules are of little interest to hydrologists. The amount of voidspace available for fluid flow is the effective porosity. Even large voidspaces that are interconnected by small pore throats are unavailable for fluid flow. Fortunately, studies have shown that even fine clays have pore throats that are larger than water molecules, so that, at least in sediments, effective porosity is equal to total porosity. Total porosity can be computed from measurements of density: n = 100 1 − ρ bulk ρ pd ( ) [ ] p bulk is the density of the bulk aquifer material p pd is the density of the particles that make up the aquifer material
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For most rock and soil, the particle density is about 2.65 g/cm 3 , roughly the density of quartz and clay minerals. Factors affecting porosity Grain size : In and of itself, grain size has no effect on porosity. Well rounded sediments that are packed into the same arrangement generally have porosities from 26% to 48% depending on the packing. A room full of bowling balls and a room full of BBs would have the same porosities if the spheres were packed the same way. Sorting : Well sorted sediments generally have higher porosities than poorly sorted sediments for the simple reason that if a sediment is a range of particle sizes then the smaller particles may fill in the voids between the larger particles. Sorting is measured as a ratio of the larger to smaller particle sizes in the sediment. This measure is called a uniformity coefficient . C
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This note was uploaded on 12/25/2010 for the course GLY 4288 taught by Professor Root during the Fall '10 term at FAU.

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5Supp%205%250porosity%20permeability%20hydraulic%20cond -...

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