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

众所周知，海平面上升和沿海洪水会减少淡水资源。然而，人们较少认识到海平面上升会将地下水位推高至地表以上，从而淹没低洼地带。地下水泛滥形成的湖泊和湿地使相连的含水层暴露在直接、持续的蒸发中，这可能导致水资源的大量损失。地下水泛滥和蒸发的影响将在低洼的碳酸盐岛屿上最为强烈，那里蒸发的淡水被渗入基岩的海水所取代。这些模型考虑了气候的影响，并预测在潮湿地区的岛屿下，湖泊将保持新鲜，地下水资源将持续存在。在干旱地区，湖泊将变得过咸，许多淡水地下水将被海水取代。与预期相反，在干旱的巴哈马群岛南部，由地下水泛滥形成的现代湖泊的盐度从淡水到高盐度不等。模拟的湖泊盐度和观测到的湖泊盐度之间的矛盾突出了我们对地下水泛滥如何影响岛屿水资源的缺乏了解。在这个项目中，将收集新的现场数据，并开发新的瞬变模拟工具，以检验地下水洪水将如何影响岛屿水资源的总体假设。我们将测试湖泊面积相对于地下水贡献面积的影响，湖泊与海洋的连通程度，以及与气候和海平面变化的动态相互作用对这一过程的影响。这项研究的结果应该会改善对小岛屿发展中国家(SID)淡水资源因地下水泛滥而损失的预测，地下水泛滥可能是引发它们丧失宜居性的主要环境因素。将作为该项目的一部分开发的建模工具将免费分发给水文界。将为每年访问Gerace研究中心(该项目的运作基地)的100多名学生小组编写一项实地水文地质学课程练习，其中包括沿海定量水文学的学习成果。项目成果将作为巴哈马大学客座讲师系列和国际会议的一部分介绍。该项目将通过向密歇根理工大学的两名地质学博士生和一名土木工程博士后研究员介绍沿海水文学来增强人力资源。每个PI将与他们机构的多样性办公室合作，确保有效地向代表不足的群体宣传这些职位。地下水淹没使岛屿上形成的湖泊和湿地蒸发，重新组织地下水流动路径，降低地下水位。地下水淹没和蒸发的影响将在地势较低的碳酸盐岛屿上最强烈，在那里，Ghyben-Herzberg关系导致淡水透镜(主要淡水资源)造成的损失是地下水位下降的40倍。控制地下水泛滥的主要因素是海平面上升和岛屿地形，但对淡水资源的影响可以通过气候(降水和蒸发)、含水层渗透率、地下水贡献面积与湖泊表面积的比率以及湖泊与海岸之间的距离来缓和。在这个项目中，将收集新的现场数据，并开发新的瞬变模拟工具，以检验地下水淹没将通过湖泊面积相对于其地下水贡献面积的相互作用、湖泊与海洋的连通度以及与气候和海平面变化的动态相互作用来减少淡水透镜体积的总体假设。假设检验将采取三管齐下的方法。首先，巴哈马圣萨尔瓦多岛的DEM将描述湖泊水文地质环境(集水区/湖泊面积比)。将利用对水文地质参数和气象数据的实地观测来估计地下水通量和湖泊蒸发量；将利用地下水、湖水和降水的化学和同位素组成来估计补给阈值和湖泊水源(海洋、地下水、补给降水)。其次，将开发一个用于模拟湖泊和地下水之间的动态流动和溶质运移的程序包，并与美国地质调查局合作将其整合到SEAWAT/MODFLOW模型中。该模型将用在圣萨尔瓦多岛上收集的观测数据进行参数化。第三，将使用新开发的数值模拟工具测试岛屿淡水透镜对多种海平面上升、气候变化和湖泊形成情景的瞬时响应。模拟将确定淡水透镜体积与海平面、气候变量和岛屿-湖泊水文地质环境之间关系的性质。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

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.