Pumping Optimization under Uncertainty in an Island Freshwater Lens using a Sharp‐Interface Seawater Intrusion Model

Pumping optimization under uncertainty is a powerful approach for the management of groundwater resources, and its implementation would be valuable in island aquifers where freshwater lenses are affected by seawater intrusion. Sharp-interface numerical models are especially well suited for the task as they offer fast simulation times, but to date they have not been used because of a lack of guidelines and due to the specific challenges associated with this approach. This study presents a methodology for pumping optimization under uncertainty for island freshwater lenses using a sharp-interface model (MODFLOW-SWI2) and demonstrates it for a real case (Magdalen Islands, Quebec, Canada). The total pumping in a well field was maximized while avoiding well salinization due to upconing. To do so, the sharp interface simulated below the pumping wells was corrected successively for cell-to-well upconing and for dispersion. Pumping optimization under uncertainty was then conducted using PESTPP-OPT, considering parameter and observation uncertainty, and was repeated for 23 reliability levels to illustrate a large range of risk-averse, tolerant and neutral stances. The maximum pumping was obtained as a function of risk of well salinization. This approach enabled quantification of the tradeoffs between pumping and risk, allocation of pumping amongst wells, and an examination of the ability of the well field to meet the water demand while maintaining an acceptable level of risk. Ultimately, this framework allows groundwater managers to select the final pumping scenario themselves, depending on their attitude toward risk.