Sustainable Management of Groundwater Resources in Island Aquifers

Island aquifers are coastal groundwater systems sustained most often solely by groundwater recharge from local precipitation. These aquifers are typically lens-shaped with a freshwater thickness of only a few meters and result particularly vulnerable to seawater intrusion (SWI), sea level rise, storm surges, and unsustainable groundwater pumping. These effects may ultimately lead to the depletion of freshwater resources, aquifer salinization, and increased groundwater supply costs, The management of fragile freshwater resources in island aquifers, requires identifying pumping strategies that trade off the groundwater supply cost against SWI. In this work, these trade-offs are explored quantitatively using an optimization framework that aims at minimizing the groundwater supply operation cost, which accounts for pump operation and water desalination treatment, subject to SWI constraints. We focus on the San Salvador Island aquifer, Bahamas, for which a variable density flow model is constructed. Pumping strategies are characterized by the pumping distance from the shoreline (WL), the pumping depth (D) and the pumping rate (Q), which represent the decision variables of the optimization problem. SWI is quantified using indicators such as (a) the aquifer drawdown over the pumping location; (b) the reduction in aquifer freshwater volume; and (c) the salt mass increase in the aquifer. Major findings indicate that, increasing D and decreasing WL reduce aquifer drawdown, which preserves the hydrogeologic stability of the system and reduces the pumping operation portion of the groundwater supply cost. On the other hand, this also increases the salt concentration in the abstracted groundwater, increasing the water treatment requirements, which have a major impact on the overall groundwater supply cost. From a mere financial perspective, groundwater abstraction by in proximity of the island center and at shallow depth seems the most convenient strategy. However, the analysis of the optimization constraints reveals that strategies where the pumping system is situated by the island center tend also to cause a more severe SWI, and highlights the precarious balance between the cost of water supply and the sustainability and vulnerability of these groundwater systems. 

Oral Presentation at:  American Geophysical Union's 2022 Fall Meeting Conference, Chicago, 12-16 December 2022