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The sea water penetrates more or less into a coastal aquifer. When the ground water table in the aquifer is sufficiently high compared with sea level, the fresh water dischages into the sea and hence the sea-water intrusion can extend little. The increasing demand for ground water in many coastal areas, however, may lower the ground water table. As a result, the seaward flow of fresh water is decreased or even reversed and the sea-water intrusion is allowed to extend far in the. aquifer. Several methods have been suggested for controlling or preventing the seawater intrusionl. In order to find the effective applications of those methods and to realize their limitations, many analytical models have been proposed for studying the behavior of sea-water intrusion with respect to the difference between the ground water table and sea level2). Some of them treated the effects of the infiltration of rainfall on preventing the intrusion. They have given better understandings about the length of intrusion, the shape of fresh-salt water interface and the amount of fresh-water discharge into the sea. However, it should be remarked that most of these analytical models were studied under steady condition. The problems related to the behavior of sea-water intrusion are mostly unsteady ones. Namely, if the sea-water intrusion responds quickly to the change in fresh water flow, steady solutions may provide good approximations to the shape change of fresh-salt water interface at any moment. In reality, any change in the fresh water flow causes the sea-water intrusion to move slowly toward its new equilibrium position. Thus, the study of the intrusion under unsteady condition becomes important especially when the changes in the fresh water flow are recurred. This paper reports numerical solutions for the changes in the ground water table and fresh-salt water interface in a coastal aquifer which are induced by a local accretion through the phreatic surface. The mathematical statement of this phenomenon consists of two nonlinear differential equations which are related to each other by the boundary condition on the interface. To describe the unsteady behaviors of the water table and interface, two differential equations should be solved "simultaneously". Exact analytical solutions for these differential equations are not yet possible. One difficulty is due to their nonlinearity and the other is owing to the unsteady condition involved in the problem. Beside trying to find exact analytical solutions, the experiments may be performed by employing the sand box model or viscous flow analog to observe the phenomenon in a coastal aquifer. In this case, the difficulties obviously come out of the scalings of length and time, especially when the research subjects involve the accretion through the phreatic surface. Therefore, the numerical study may be a worthy approach to realize the overall behavior of the unsteady phenomenon in a coastal aquifer. In this work, the explicit finite difference method3 is employed to solve two "simultaneous" differential equations, expecting that its applicability may be examined through the investigation of numerical solutions for the problem studied herein. The results clarify the unsteady behaviors of ground water table and interface due to a local replenishment, and prove the applicability of the numerical method. It is also noticed that the unsteady condition could not be replaced by the steady condition except when it is done with great care. The numerical results show good qualitative agreement with the experimental results.

NUMERICAL ANALYSIS OF THE GROUND WATER MOUND AND FRESH-SALT WATER INTERFACE IN A COASTAL AQUIFER