3. Groundwater - Ground water Part II AQUIFER PROPERTIES...

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Ground water Part II AQUIFER PROPERTIES
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Summary –part 1 Sources: precipitation Types :Meteoric , connate and juvenile Distribution of groundwater: zone of aeration (vadose zone), capillary zone, saturated zone Water table:- upper surface of the zone of saturation. water surface that is constantly adjusting itself towards equilibrium. where the hydrostatic pressure equals the atmospheric pressure Aquifer – geologic unit that stores and transmits water Types of aquifers: confined, unconfined, semi- confined(leaky), perched
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Names of confining layers: Aquitards- formations wwhers only seepage is possible, thus yielsd is insignificant compared to an aquifer (partly permiablee.g sandy clay Aquicludes- formation which is essentially impermeable to the flow of water e.g clay and Aquifuges- formation which neither porous nor permeable. There are no interconnected opening and hence cannot transmit
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Artesian well: A well whose source of water is a confined aquifer The water level in artesian wells is at some height above the water table due to the pressure of the aquifer (piezometric head)
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E. Potentiometric Surface and Water Table
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AQUIFER PROPERTIES Porosity The spaces where groundwater occupies are known as voids, interstices, pores or pore spaces. They are fundamentally important to the study of groundwater because they serve as water conduits. Their nature is dependent on their geology. Porosity is a measure by the ration of the contained voids in a solid mass to its total volume. Porosity is expressed as either a fraction, or a percentage: η = V voids /V total , η = (V voids /V total ) x 100
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Porosity Porosity : Percent of volume that is void space. Sediment : Determined by how tightly packed and how clean (silt and clay), (usually between 20 and 40%) Rock : Determined by size and number of fractures (most often very low, <5%) 1% 5% 30%
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Primary porosity- created at the time of origin of the rock in which they occur e.g intergranular Secondary porosity - results from the actions of subsequent geological, climatic or biotic factors upon the original rock e.g fractures, faults, cavities, etc. The term effective porosity refers to the amount of interconnected pore space available for fluid flow and is also expressed as the ratio of the interstices to total volume. (excludes fraction of pores too small to hold water, or those that are not interconnected)
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Permeability : Ease with which water will flow through a porous material Sediment : Proportional to sediment size Gravel Excellent Sand Good Silt Moderate Clay Poor Rock : Proportional to fracture size and number. Can be good to excellent Measured by constant head and falling head permeameters in lab.
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