Dispersive mixing and sea level rise rates control depletion of freshwater in island-lake-aquifer systems undergoing groundwater and coastal inundation

Abstract:

Sea level rise can elevate water tables to inundate topographic depressions on islands and continental coasts with groundwater, forming lakes and wetlands which expose aquifers to direct, continuous evaporation. Prior steady state simulations showed that evaporation from lakes could drive upconing of saline water and even complete segmentation of freshwater lenses in arid climates, but timescales over which salinization may occur could not be determined.

Here we use computational experiments to explore interactions among sea level rise rates (SLRR), dispersion, and recharge seasonality in determining timescales of freshwater lens depletion on islands where groundwater inundation expands inland lakes. Using the MODFLOW and SEAWAT family of codes, we simulated a hypothetical island-lake-aquifer system based on the carbonate Bahamas Islands and investigated the hypothesis that seasonal recharge cycles will exacerbate losses of fresh groundwater from lake evaporation by increasing the mixing of fresh and saline water within the aquifer.

In general, evaporation of lakes formed by groundwater inundation cause rapid lens depletion (on the order of decades) relative to non-lake cases, due to greater evaporative losses which cause upconing of saline water. We found a roughly linear relationship between SLRR and salinity concentrations at selected observation points in the aquifer. As expected, increasing mechanical dispersivities from 0.1 to 10 times the gridblock size resulted in greater mixing and more rapid aquifer salinization. Comparisons between simulations with seasonal and average annual climatology show that for the lowest SLRR of 0.56 mm/yr seasonal simulations produce more rapid depletion than the equivalent average annual recharge simulations. This result is explained by substantially greater residence times for low SLRR. We also observe that when lake salinity exceeds seawater concentrations, due to evaporation, density contrasts result in miscible displacement instabilities in the form of fingering. The fingering is attenuated for the highest dispersivity case. The results of the simulations show that neglecting groundwater inundation, seasonal recharge, and lake hypersalinity can lead to underestimation of the impacts of sea level rise on island freshwater resources.