REPORT OF INVESTIGATIONS NO. 75
HYDRAULIC CHARACTERISTICS OF THE BOULDER ZONE
RELATION BETWEEN AQUIFER CHARACTERISTICS AND SHORT - TERM CLIC WATER -LEVEL FLUCTUATIONS
The hydraulic characteristics of an aquifer can be determined from short-term natural water-level fluctuations in a well. Ferris (1951) described a method that related cyclic (sinusoidal) water-level fluctuations in wells and nearby surface-water bodies to hydraulic diffusivity (the ratio of transmissivity to storage coefficient T/S, or, equivalently, the ratio of hydraulic conductivity to specific storage KISS of an aquifer). The method assumes that the aquifer is homogeneous, of uniform thickness, and of great areal extent. Furthermore, the aquifer is assumed to be bounded along a straight line on one side by a surface-water body. Within the aquifer, water storage is assumed to change instantaneously with and at a rate proportional to the change in head. Tidal fluctuations are, therefore, transmitted horizontally through the aquifer from the surface water-aquifer contact. The fluctuation decreases exponentially with time and distance from the source.
Carr and Van Der Kamp (1969) modified the method to permit the determination of hydraulic conductivity and specific storage for a confined aquifer underlying the ocean. Tidal fluctuations in inland wells are induced by the inland migration through the aquifer of the pressure waves produced by the cyclic loading of the ocean. Fluctuations of water levels in the inland wells are less than those of the pressure head in the part of the aquifer beneath the sea, owing to the energy losses'that accompany the to-and-fro movement of water in the aquifer. Later Van Der Kamp (1972) showed that the tidal efficiency at the coastline would be about one-half the loading efficiency; and that the loading efficiency is essentially the tidal efficiency expressed in equation 2.
The dampening of a oceanic tidal fluctuation in an artesian aquifer during transit from the shoreline to an inland well (Ferris, 1962, p. 133, equation 63) can be written
_x NI-ff _S'
IT T
hw = hoe (4)
when hw Fluctuation in.the well at distance x from the shoreline
ho =
fluctuation in the aquifer at the shoreline (x = 0)