AN APPROACH TO QUANTIFY THE FLUCTUATION CHARACTERISTICS OF GROUNDWATER TABLE

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Groundwater table fluctuations reflect the physical properties of the aquifer of a region. Especially, in a coastal region, sinusoidal fluctuations of groundwater levels occur in response to tide. If the level of bodies of surface water varies with a simple harmonic motion, a chain of sinusoidal waves may be propagated inland from the shore. Consequently, the water yield of a flowing well under the influence of tidal activity fluctuates correspondingly. So the author attempted to quantify the fluctuation char-acteristics of groundwater table levels in the flowing wells located at various distances from the river shore of the Tone. Data concerning the variation of groundwater levels were collected from three flow-ing wells in one line towards inland from the shore. The author's procedures are given below. (1) The fluctuation characteristics of daily and monthly yields of each well are expressed by the following equations respectively. _??_ where, Di denotes an absolute value of deviation from the mean monthly yield. Q is the discharge of a flowing well (l/min) M is the mean value of Q Qr, is the mean deviation of water yield (2) The statistics of frequency distribution of daily yield throughout a year is represented by Pd which is defined as the modal value of the Q. A cumulative curve like that of a flow duration curve is drawn, and from the graph the standard deviation of Q is calculated as the mean of the first and the third quartile deviation of water yield. The other measures proposed are as follows, QH : the yield which equalled or exceeded 95 days of the year. QM: the yield which equalled or exceeded 185 days of the year. QL : the yield which equalled or exceeded 355 days of the year. (3) The equation governing the groundwater behaviour in porous, isotropic media may be written in the form: _??_ where, Δh is the amplitude of groundwater fluctuations during a half tide cycle, and this value can be obtained from the flow duration curve and it generally tends to decrease with the distance from the shore. ΔH is the tidal range. x is the distance inland from the shore. σ is the angular velocity 28.98/hr during a semidiurnal fluctuation. k is a parameter which is equivalent to the coefficient of permeability and governed by the porosity of the aquifer. t is time. The above analysis suggests that the elastic property of confined aquifers results in changes in hydrostatic pressure when changes in loading occur. That is, an additional load compresses the aquifer and increases the hydrostatic pressure which is transformed into the potential head of a flowing well. After then, the pressure decreases and ap-proaches its original value as groundwater moves away. Field measurements of groundwater table variations in coastal region show that the parameter k proposed by the author in this work is a satisfactory representation of the groundwater dynamics for the studied area.