THE MOTION OF COASTAL GROUNDWATER IN RESPONSE TO THE TIDE

Problems of seawater intrusion into confined coastal aquifers under the influence of the tide are analysed with the use of two different models; the freshwater-saltwater interface model and the dispersion model. The freshwater-saltwater interface model, which describes the motion of the groundwater level and the freshwater-saltwater interface in response to the tide, is applied to the confined coastal aquifers in the estuary of the Naka River and Yura district of Gogo Island in Japan. The position of the outlet of the confined groundwater into the sea and also the degree of spreading of the aquifer's thickness, which is assumed to vary exponentially with horizontal distance, are estimated. From the non-dimensional analysis based on the dispersion model, it is found that salt water intrudes furthest inland at the time at which the sea level reaches the mean sea level in the ebb tide stage, consistent with the result obtained by the freshwater-saltwater interface model, and the transition zone from fresh water to salt water becomes widest at that time. © 1990.