Principles of Occurrence
The following discussion of the occurrence of ground water has been adapted from Meinzer's report on the subject (1923), to which the reader is referred for a more detailed discussion. A general discussion of the principles of groundwater occurrence with special reference to Kansas has been presented by Moore (1940).
The rocks that make up the crust of the earth are not solid but have many openings, called voids or interstices, which may contain air, natural gas, oil, or water. The various kinds of rock differ from one another in the number, size, shape, and arrangement of these interstices; therefore, the occurrence of water in any region is determined by the geology of the region.
The interstices of rocks in Miami County range in size from pores of microscopic dimensions to openings several inches across. These openings can be classified as either primary or secondary interstices. Primary interstices are the spaces between rock grains formed during deposition of the rock. Secondary interstices are the joints, openings along bedding planes, and solution openings that were created in the rocks after deposition.
The quantity of water that a rock will hold is dependent upon the porosity of the rock. Porosity is expressed as the percentage of the total volume of the rock that is occupied by interstices. If all the interstices of the rock are filled with water, the rock is then saturated. Specific yield is the amount of water that a saturated rock will yield under the force of gravity. The rate at which a rock will yield water to a well is determined by its permeability. Some beds of clay or shale may be very porous, but, because the interstices are small or poorly connected, they transmit little or no water and the rock is virtually impermeable.
Ground water is derived directly or indirectly from rain or snow which falls on the earth. Part of the precipitation leaves the area as surface runoff in streams, a part evaporates, and a part is transpired into the atmosphere by vegetation. Some of the precipitation, however, percolates downward through the soil and underlying strata until it reaches the zone of saturation. Water in the saturated zone below the surface of the earth is called ground water.
After reaching the zone of saturation, the water moves through the rocks in a direction determined by the lithology, the geologic structure, and the topography until it is discharged. Discharge may occur through wells and springs or into a stream or other body of water, or by evaporation and transpiration in areas where the saturated zone is relatively near the land surface.
If the upper surface of the zone of saturation is within a permeable rock (an aquifer), this surface is called the water table, and the water is said to he under water-table conditions. Ground water is said to be confined or artesian water if it occurs in permeable zones between relatively impermeable beds that confine the water under pressure. The level at which water stands in an open drill hole under artesian conditions does not represent the water table but rather the piezometric (or pressure) surface at that location.
In Miami County, many of the wells drilled into the unweathered Pennsylvanian bedrock tap confined or artesian ground water. There are no flowing artesian wells in the county, but the hydrostatic pressure in many of the aquifers is sufficient to raise the water above the point at which it is first obtained in drilling the well. The hydrostatic pressure represents the difference in altitude between points of recharge to the artesian aquifer and the well, minus pressure losses caused by resistance to flow in the aquifer.
The many structurally low areas in the surface and near-surface rocks in Miami County are favorable for the occurrence of artesian water. Wells drilled in these areas would possibly yield more water and would be more dependable than wells drilled in adjacent structurally high areas.
The water table is not a level surface nor a static surface. It is similar, though on a modified scale, to the configuration of the surface topography. Where the water-hearing materials are nearly impermeable, the surface configuration of the land and of the water table will be similar. If the water-hearing materials are very permeable, the relief of the water table will be much lower than that of the land surface. Irregularities in the permeability of the water-bearing materials from place to place will cause irregularities in the water-table surface. The water table rises and falls in response to unequal additions to or withdrawals from water in the aquifer.
Plate 1 shows the locations and depth to water of wells and test holes for which data are given in Table 4 or in the Logs of Wells and Test Holes at the end of this report. No attempt was made to draw water-table contours for the area of outcrop of Pennsylvanian rocks, because in parts of the area the water is under artesian head and in other parts the water table is discontinuous. The water level in Pleistocene deposits in the valley fill is essentially continuous and usually is not under artesian head.
Kansas Geological Survey, Miami County Geohydrology
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Web version June 2002. Original publication date June 1966.