Moving Interface in Coastal Aquifers

The movement of the interface between fresh water and sea water in coastal aquifers caused by changes in seaward flow of fresh water is considered. Approximate solutions to the moving interface, its shape, position, and velocity of displacement are suggested. The following assumptions are made: (1) The aquifer is confined and the flow pattern is two-dimensional; (2) the aquifer is homogeneous and isotropic; and (3) the fresh water and sea water are immiscible. Assuming that the aquifer is relatively thin, the Dupuit assumption is applied to the flows in both the fresh water region above the interface and the salt water region below it. The average specific discharges obtained in this way are then combined with the continuity equation to yield partial differential equations for the time-variable thickness of the fresh water flow region. In order to solve these equations, certain assumptions regarding the variation of fresh water flow above the interface had to be made. The solutions derived in this way were compared with experimental results obtained from a Hele Shaw model. Graphs and numerical examples are presented. It was found that the approximate solutions describe the interface movement with sufficient accuracy for practical purposes.