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What is Ground Water?

Ground water is all the water stored in the spaces, voids, pores and fractures in soil and rock at some depth below the earth's surface. Geologic formations capable of yielding usable quantities of ground water to a well are called "aquifers". Ground water is recharged as rain, snow and other surface water such as rivers and streams, seeps downward through surface soils and fills open spaces below ground.

Ground water moves significantly slower that surface water. The rate of ground water flow is determined by a variety of factors, including porosity, aquifer gradient and outlets in the aquifer system, such as rivers, lakes, streams, springs and production wells. Ground water movement is commonly measured in terms of feet per day but in some systems, movement can be measured only in terms of inches per year!

Ground Water

A number of factors account for the amount of water available in an aquifer, including aquifer thickness, amount of precipitation, topography, surface soil types, porosity and permeability.

Porosity refers to the capacity of earth materials to hold water in spaces found within and between sediments and rock. Well sorted sediments, i.e. evenly distributed sediments of uniform size, typically contain 26% to 48% open space. These open spaces, or pores, allow for the storage and movement of water. If an aquifer contains a mixture of sediment sizes and types such as intermixed clay, sand and gravel, the smaller sand and clay particles will fill pore spaces, reducing the amount of open spaces available for storage and movement of water. Thus, well sorted sediments have higher porosity while poorly sorted sediments have low porosity.

Permeability refers to how easily water passes through sediments and rock. Permeability is a function of the size of pore openings. If sediments are well sorted, pore spaces are uniformly sized, thereby allowing for groundwater flow readily between individual sediment particles. If sediments are poorly sorted, pore spaces are filled with smaller sediments, thereby reducing the movement of water through the deposits. These deposits will supply lower volumes of ground water.



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