Collaborative Research: How does groundwater inundation of carbonate island interiors from sea level rise impact surface water-aquifer interactions and evaporative losses?

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Sea-level rise and coastal flooding are well-known to reduce freshwater resources. It is however less recognized that sea level rise can push water tables above the land surface to flood low-lying depressions. Lakes and wetlands that form by groundwater flooding expose connected aquifers to direct, continuous evaporation, which can result in substantial losses of water resources. Impacts from groundwater flooding and evaporation will be most intense on low-lying carbonate islands, where evaporated freshwater is replaced by seawater that percolates through the bedrock. These models incorporate impact of climate and predicted that lakes should remain fresh, and groundwater resources continuous, beneath islands in humid regions. In arid regions, lakes would be hypersaline, and much fresh groundwater would be replaced by seawater. In contrast to expectations, modern lakes that are formed by groundwater flooding in the arid southern Bahamas islands have salinities that range from fresh to hypersaline. This paradox between modeled and observed lake salinity underscores our lack of understanding of how groundwater flooding impacts island water resources. During this project, new field data will be collected, and new transient modeling tools will be developed, to test the overarching hypothesis that how groundwater flooding will impact island water resources. We will test the impact of lake area relative to its groundwater contributing area, the degree of connectivity between lakes and the ocean, and dynamic interactions with climate and sea level change on this process. The results of this study should improve predictions of freshwater resource loss of Small Island Developing States (SIDS) from groundwater flooding, perhaps the primary environmental factor triggering their loss of habitability. The modeling tools to be developed as part of this project will be freely distributed to the hydrological community. A field hydrogeology curricular exercise, with learning outcomes of quantitative coastal hydrology, will be developed to be used by the more than 100 student groups that visit the Gerace Research Centre (base of operation for this project) each year. Project results will be presented as part of a guest lecturer series at the University of the Bahamas and at international conferences. The project will enhance human resources by introducing coastal hydrology to 2 geology PhD students, and one civil engineering postdoctoral researcher at Michigan Tech. Each PI will work with their institution's diversity office to ensure that these positions are effectively advertised to under-represented groups.Evaporation of lakes and wetlands that form on islands by groundwater inundation reorganizes groundwater flow paths and lowers water tables. Impacts from groundwater inundation and evaporation will be most intense on low-lying carbonate islands, where the Ghyben-Herzberg relationship results in losses from freshwater lenses (the primary freshwater resource) being 40x the lowering of the water table. Primary controls on groundwater inundation will be sea-level rise and island topography, but impacts to freshwater resources could be moderated by climate (precipitation, and evaporation), aquifer permeability, ratio of groundwater contributing area to lake surface area, and distance between the lake and coast. During this project, new field data will be collected, and new transient modeling tools will be developed, to test the overarching hypothesis that groundwater inundation will reduce freshwater lens volumes through interactions of lake area relative to its groundwater contributing area, degree of connectivity between lakes and ocean, and dynamic interactions with climate and sea level change. Hypothesis testing will take a three-pronged approach. First, DEMs of San Salvador Island, Bahamas, will characterize lake hydrogeological setting (catchment/lake area ratios). Field observations of hydrogeological parameters and weather data will be used to estimate groundwater fluxes and lake evaporation; chemical and isotopic compositions of groundwater, lake water, and precipitation will be used to estimate recharge thresholds and lake water sources (ocean, groundwater, recharged precipitation). Second, a package for simulating dynamic flow and solute transport between lakes and groundwater will be developed and integrated into the SEAWAT/MODFLOW model in cllaboration with USGS. The model will be parameterized with observational data collected on San Salvador Island. Third, transient responses of island freshwater lenses to multiple sea level rise, climate change, and lake formation scenarios will be tested with the newly developed numerical modeling tool. Simulations will determine the nature of relationships between freshwater lens volumes and sea level, climatic variables, and island-lake hydrogeological settings.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.