Groundwater Resources, continued
Aquifer PropertiesAn aquifer is a geologic formation, a part of a formation, or a group of formations that will yield significant quantities of water to wells and springs. The properties of an aquifer that determine its capacity to transmit water and to release water from storage are its permeability, transmissibility, and storage. These factors depend in part on the thickness, extent, continuity, and homogeneity of an aquifer and on the size and nature of the pore spaces in the aquifer.
The field coefficient of permeability (P) is defined as the number of gallons of water per day, at the prevailing water temperature, that will move in 1 day through a vertical section of the aquifer 1 foot square under a hydraulic gradient of 100 percent or 1 foot per foot (Stearns, 1928). Coefficients of permeability of less than 100 gpd (gallons per day) per sq ft (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 saturated thickness (in), in feet, of the aquifer (Theis, 1935, p. 520).
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 1 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 equal to the specific yield and has a range of 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 g.
Drawdown in a well is the lowering of the water table by pumping or of the piezometric surface by artesian flow. The specific capacity of a well is the discharge expressed as rate of yield per unit of drawdown, generally gallons per minute (gpm) per foot of drawdown. Specific capacity is a better measure of an aquifer's capacity to yield water than is the actual yield of a well, because it relates yield to drawdown in the well.
Hydrologic data for aquifers in Johnson County and the source of the data are summarized in table 8. The availability of groundwater in the unconsolidated and consolidated rock aquifers and the important aquifer properties known or inferred are described in the following pages.
Kansas Geological Survey, Johnson County Geohydrology|
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Web version April 2002. Original publication date Dec. 1971.