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Although the economics of groundwater management is typically modelled as a single aquifer serving a single group of consumers, resource managers must often decide how to manage multiple aquifers simultaneously. We propose a temporal and spatial model to determine the efficient allocation of groundwater when two coastal aquifers are available for exploitation. Along the optimal trajectory extraction may switch from single to simultaneous use, depending on how the total marginal cost of each resource evolves over time. A numerical simulation for the South Oahu aquifer system illustrates the switching behavior