Optimal management of a coastal aquifer under saline intrusion.

Summary Saltwater intrusion problems in coastal aquifers are now one of the main causes of deterioration in their water quality. These intrusions are most often the result of excessive withdrawals from sensitive areas of the freshwater aquifer. Seawater, which is denser, invades aquifers connected to the marine environment as soon as withdrawals are sufficiently intense to modify the hydrostatic balance. The purpose of this article is to identify the particular problems posed by the optimal management of this type of resource. To this end, the authors use a simplified model of the changes in the morphology of the freshwater table and, in particular, of the displacements of its interface with the saline water mass, in response to changes in withdrawal intensity. An increase in withdrawal intensity results in the boundary beyond which permanent withdrawals can be made inland being pushed further from the coast. This relationship between volumes withdrawn and the obligatory positioning of the withdrawal points generates a specific externality whose effects on the optimal use of the resource are analyzed. The tariff conditions that must be imposed on the concessionaire and the prices that must be charged to the users to decentralize the optimum are determined.