METHODS FOR REPRESENTING LAKE FORMATION IN AN ISLAND SETTING WITH SEA LEVEL RISE: A COMPARISON OF ALTERNATIVE APPROACHES

It is well understood that rising sea levels can lead to salinization of groundwater and flooding of coastal areas, however, the impact of sea level rise (SLR) on surface water features away from the shoreline are less commonly considered. Rising sea level can elevate groundwater tables above the land surface, flooding low-lying depressions to form lakes and wetlands; a process known as groundwater inundation. Groundwater inundation can also expand existing surface water bodies. These newly formed or expanded surface water features expose aquifers to direct, continuous evaporation which can reduce freshwater lens volumes relative to higher elevation islands that do not experience groundwater inundation. Groundwater inundation is especially significant for low-lying carbonate islands where freshwater resources can be limited. To explore the effects of sea level rise and groundwater inundation on island freshwater lens depletion, we developed 2D hypothetical island models using two different approaches for lake simulation. The first approach uses SEAWAT, a coupled variable density groundwater flow and transport program. Because SEAWAT lacks the ability to represent lakes directly, the lake is modeled as an area of high hydraulic conductivity and high porosity, compared to the surrounding aquifer. The second approach uses a development version of MODFLOW 6 and a modified Lake Package. In the MODFLOW 6 development version, an instantaneous mixing equation is used to simulate lake concentration and lake density, and the effects of density variations on lake-aquifer interactions. Variations between the high hydraulic conductivity approach and the modified Lake Package are explored. The resulting changes in freshwater lens volume in response to alternative sea level rise scenarios are examined within the context of these differing methods.