FRESHWATER RESOURCE DEPLETION ON OCEAN ISLANDS AS LAKES FORM AND EXPAND WITH SEA LEVEL RISE

In addition to the impact of coastal inundation on the depletion of freshwater lenses on small islands due to sea level rise, lakes formed as water tables rise above topographic lowlands can also impact the freshwater lenses. For example, lake evaporation can cause upconing of seawater, segmenting of the lens, and increased lake salinity. In this study, we assess the importance of mixing processes on the freshwater lens, focusing on mixing from dispersion and from transient climate inputs to the lake and aquifer. We simultaneously assess the impacts of sea level rise rates on freshwater lens depletion. We use three different sea level rise rates (0.56, 6.22, and 11.78 mm/yr) and dispersivity values (0.1, 1, and 10 m²) and compare transient and constant annual climates. The analysis was conducted through running the variable-density groundwater flow and transport model, SEAWAT. We adopted 30-year climate data collected on San Salvador Island, the Bahamas, as either the constant annual or transient climate inputs. The lake expands from the island center to the coast at the same rate of the coastal inundation that is proportional to the sea level rise rate. We found that the transient climate patterns significantly widen the mixing zones for all scenarios with the widest mixing zone formed at the highest sea level rise rate and highest dispersivity. Episodic wet and dry seasons caused pulsed vertical movement to the groundwater flow field and enhanced mixing. Transient climate patterns also enhance lake water salinity as more solute is discharged into the lake from a more salinized inland aquifer. As expected, the highest dispersivity resulted in greater mixing and faster depletion of freshwater lens. Freshwater lens depletion also accelerates as the lake expands faster with a faster sea level rise rate. We conclude that freshwater lens will deplete fastest when seasonal recharge patterns are combined with the highest dispersivity and sea level rise rate, a condition that future modeling study and policy makers have to consider when conceiving sustainable groundwater management strategies for tropical islands.