Aquifer PropertiesAn aquifer is a geologic formation, a part of a formation, or a group of formations that will yield water. The quantity of ground water that an aquifer will yield to wells depends partly on its thickness, extent, continuity, and homogeneity, and partly on its physical properties of permeability and porosity.
The field coefficient of permeability (P) is defined as the number of gallons of water that will move in 1 day, at the prevailing water temperature, through a vertical section of the aquifer I foot square under a hydraulic gradient of 100 percent or 1 foot per foot. Coefficients of permeability of less than 100 gallons a day per square foot are considered low, coefficients of 100 to 1,000 are medium, and those of more than 1,000 are considered high. The coefficient of transmissibility (T) is equal to the field coefficient of permeability multiplied by the thickness (m) of the aquifer.
The coefficient of storage (S) is defined as the volume of water, measured in cubic feet, released from storage in each column of the aquifer having a base I foot square and a height equal to the thickness of the aquifer, when the water table or other piezometric surface is lowered 1 foot. In water-table aquifers the coefficient of storage for long periods of pumping is approximately the specific yield and has a range from about 0.1 to 0.3. The specific yield is defined as the ratio of the volume of water a saturated material will yield by gravity to its own volume. For artesian aquifers the coefficient of storage generally is very small, ranging from about 10-5 to 10-3.
The coefficients of transmissibility and of storage are used in making quantitative estimates of water available in an aquifer, and of the water-level decline that will result from pumping. Controlled aquifer (or pumping) tests can be made to obtain the data required to determine these coefficients.
Drawdown in a well is the lowering of the water table or piezometric surface caused by pumping or artesian flow. The specific capacity of a well is the discharge expressed as rate of yield per unit of drawdown, generally gallons per minute per foot of draw-down.
Hydrologic data on the principal aquifers, obtained by pumping and laboratory tests, and information supplied by well owners are summarized in Table 6.
Kansas Geological Survey, Geohydrology of Douglas County
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Web version Aug. 1999. Original publication date Dec. 1960.