Salt-Water Intrusion in a Coastal Aquifer

Abstract

A portion of a coastal aquifer was considered as a source of municipal water. It was planned to prevent brine from entering the well-field by inserting a barrier of fresh water, derived from purified sewage, on its seaward side. Sections of the boundary were porous, and because it was not known previously whether the water would be mined, fixed external potentials could not be specified. In order therefore to predict its long-term response to the planned abstraction, a mathematical model was built of the actions of the boundaries as well as those of the internal aerated and saturated zones. Measured rainfall figures acted as inputs, and initial estimates of transmissivity, vertical delay, soilwater saturation and agricultural pumpage were automatically and separately refined so as to minimize differences between individual aspects of the measured and simulated water-tables. In order to reduce ambiguity in parameter estimates, the quadrilateral computation elements were aligned to the groundwater contours and streamlines. The accuracy of the processing was checked by referring the model output back to measurements at borehole locations. When drawdowns due to postulated municipal abstraction and recharge were integrated with the simulated watertable over a long period, it was deduced that there was a danger of salt intruding from the sea into the well-field. It was confirmed that the safe abstraction rate depended critically on the rate of recharge of the fresh-water barrier, as well as on predictions of rainfall and agricultural pumpage.