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，人们很少认识到，海平面上升可以将地下水位推到地表以上，淹没低洼的洼地。由地下水泛滥形成的湖泊和湿地使相连的含水层直接、持续蒸发，这可能导致水资源的大量损失。地下水的泛滥和蒸发对地势低洼的碳酸盐岛屿的影响最为严重，在那里，蒸发的淡水被渗透到基岩中的海水所取代。这些模型考虑了气候的影响，并预测在潮湿地区的岛屿下面，湖泊应该保持新鲜，地下水资源应该是连续的。在干旱地区，湖泊的含盐量会很高，许多新鲜地下水会被海水取代。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新鲜到高盐不等。模拟和观测到的湖泊盐度之间的矛盾凸显了我们对地下水洪水如何影响岛屿水资源的理解不足。在该项目中，将收集新的现场数据，并开发新的瞬态建模工具，以测试地下水洪水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其地下水贡献面积的影响，湖泊与海洋的连通性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。这项研究的结果应能改进对小岛屿发展中国家因地下水泛滥而丧失淡水资源的预测，地下水泛滥也许是导致这些国家丧失可居住性的主要环境因素。作为该项目的一部分，将开发的建模工具将免费分发给水文界。将编制一个实地水文地质学课程练习，其中包括定量海岸水文学的学习成果，供每年访问Gerace研究中心（该项目的作业基地）的100多个学生小组使用。项目结果将作为巴哈马大学和国际会议的客座讲师系列的一部分提出。该项目将通过向密歇根理工大学的两名地质学博士研究生和一名土木工程博士后研究员介绍海岸水文学来加强人力资源。每位PI将与所在机构的多样性办公室合作，确保这些职位有效地向代表性不足的群体进行宣传。由于地下水淹没而在岛屿上形成的湖泊和湿地蒸发，重新组织了地下水流动路径，降低了地下水位。地下水淹没和蒸发对低洼的碳酸盐岛屿的影响最为强烈，在那里，Ghyben-Herzberg关系导致淡水透镜（主要淡水资源）的损失是地下水位下降的40倍。地下水淹没的主要控制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例以及湖泊与海岸之间的距离来缓和。在该项目中，将收集新的实地数据，并开发新的瞬态建模工具，以验证地下水淹没将通过湖泊面积相对于地下水贡献面积的相互作用、湖泊与海洋之间的连通性程度以及与气候和海平面变化的动态相互作用减少淡水透镜体体积的总体假设。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/湖泊面积比率）。水文地质参数和气象资料的实地观测将用于估计地下水通量和湖泊蒸发；地下水、湖水和降水的化学和同位素组成将用于估计补给阈值和湖泊水源（海洋、地下水、补给降水）。其次，将与美国地质调查局合作开发一个模拟湖泊和地下水之间动态流动和溶质运输的软件包，并将其集成到SEAWAT/MODFLOW模型中。该模式将用在圣萨尔瓦多岛收集的观测数据进行参数化。第三，利用新开发的数值模拟工具测试岛屿淡水透镜体对多种海平面上升、气候变化和湖泊形成情景的瞬态响应。模拟将确定淡水透镜体体积与海平面、气候变量和岛屿-湖泊水文地质环境之间关系的本质。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值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.