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Populations consuming saline drinking water are at greater risk of high blood pressure and potentially other adverse health outcomes. We modelled data and used available datasets to identify countries of higher vulnerability to future saltwater intrusion associated with climate change in 2050 under Representative Concentration Pathways (RCP)4.5 and RCP8.5. We developed three vulnerability criteria to capture geographies with: (1) any coastal areas with projected inland saltwater intrusion of ≥ 1 km inland, (2) > 50% of the population in coastal secondary administrative areas with reliance on groundwater for drinking water, and 3) high national average sodium urinary excretion (i.e., > 3 g/day). We identified 41 nations across all continents (except Antarctica) with ≥ 1 km of inland saltwater intrusion by 2050. Seven low- and middle-income countries of higher vulnerability were all concentrated in South/Southeast Asia. Based on these initial findings, future research should study geological nuances at the local level in higher-risk areas and co-produce with local communities contextually appropriate solutions to secure equitable access to clean drinking water.

Groundwater is the main freshwater source in many densely populated and industrialized coastal areas around the world. Growing future freshwater demand is likely to increase the water stress in these coastal areas, possibly leading to groundwater overexploitation and salinization. This situation will likely be aggravated by climate change and the associated projected sea level rise. Here, we assess the impact of sea level rise exclusively on coastal fresh groundwater resources worldwide (limited to areas with unconsolidated sedimentary systems) by estimating future decline in inland fresh groundwater volumes under three sea level rise scenarios following Representative Concentration Pathway (RCP) 2.6, 4.5, and 8.5. For that, 2D groundwater models in 1,200 coastal regions estimate the past, present and future groundwater salinity. Our results show that roughly 60 (range 16–96) million people living within 10 km from current coastline could lose more than 5% of their fresh groundwater resources by 2100 according to RCP 8.5 scenario compared to only 8 (range 0–50) million people based on RCP 2.6 scenario. We conclude that sea level rise will have severe consequences for many coastal populations heavily dependent on fresh groundwater. Plain Language Summary Sea level rise predictions for the upcoming centuries show that large strips of current coastal regions can be directly flooded. However, an often overlooked and hidden threat linked to sea level rise is the salinization of groundwater in those affected coastal regions. This can be caused by either direct lateral infiltration of saline water into the subsurface as well as by changes in groundwater pressure balance. Our modeling results show that different sea level rise magnitudes, based on future climate change scenarios, can lead to severe declines in fresh groundwater volumes in various coastal regions worldwide. Key Points Sea level rise under Representative Concentration Pathway (RCP) 8.5 scenario will severely impact fresh groundwater availability in low lying coastal regions Coastal areas with more than 5% loss of fresh groundwater by 2100 harbor around 60 million people and represent a collective gross domestic product of hundreds of billion USD Our results are globally indicative but comparison with local studies show that uncertainties are high

Coastal areas worldwide are often densely populated and host regional agricultural and industrial hubs. Strict water quality requirements for agricultural, industrial and domestic use are regularly not satisfied by surface waters in coastal areas and consequently lead to over-exploitation of local fresh groundwater resources. Additional pressure by both climate change and population growth further intensifies the upcoming water stress and raise the urgency to search for new fresh water sources. In recent years, offshore fresh groundwater (OFG) reserves have been identified as such a potential water source. In this study, we quantify, for the first time, the global volume of OFG in unconsolidated coastal aquifers using numerical groundwater models. Our results confirm previously reported widespread presence of OFG along the global coastline. Furthermore, we find that these reserves are likely non-renewable resources mostly deposited during glacial periods when sea levels were substantially lower compared to current sea level. We estimate the total OFG volume in unconsolidated coastal aquifers to be approximately 1.06 ± 0.2 million km 3 , which is roughly three times more than estimated previously and about 10% of all terrestrial fresh groundwater. With extensive active and inactive offshore oil pumping present in areas of large OFG reserves, they could be considered for temporary fresh groundwater exploration as part of a transition to sustainable water use in coastal areas on the long run.

Global data have served an integral role in characterizing large-scale groundwater systems, identifying their sustainability challenges, and informing on socioeconomic and ecological dimensions of groundwater. These insights have revealed groundwater as a dynamic component of the water cycle and social–ecological systems, leading to an expansion in groundwater science that increasingly focuses on groundwater’s interactions with ecological, socioeconomic, and Earth systems. This shift presents many opportunities that are conditional on broader, more interdisciplinary system conceptualizations, models, and methods that require the integration of a greater diversity of data in contrast to conventional hydrogeological investigations. Here, we catalogue 144 global open access datasets and dataset collections relevant to groundwater science that span elements of the hydrosphere, biosphere, atmosphere, lithosphere, food systems, governance, management, and other socioeconomic system dimensions. The assembled catalogue offers a reference of available data for use in interdisciplinary assessments, and we summarize these data across their primary system, spatial resolution, temporal range, data type, generation method, level of groundwater representation, and institutional location of lead authorship. The catalogue includes 15 groundwater datasets, 23 datasets derived in relation to groundwater, and 106 datasets associated with groundwater. We find the majority of datasets are temporally static and that temporally dynamic data peak in availability during the 2000–2010 decade. Only a small fraction of temporally dynamic data is derived with any direct representation of groundwater, highlighting the need for greater incorporation of groundwater in Earth system models and data collection initiatives across socioeconomic, governance, and environmental science research communities. A small number of countries, led by the USA, Germany, the Netherlands, and Canada, generate most global groundwater data, reflecting a global North bias in the institutional leadership of these data generation activities. We raise three priority themes for future global groundwater data initiatives, which include: data improvements through prioritizing observed and temporally dynamic data; elevating regional and local scale data and perspectives to address challenges relating to equity and bias; and advancing data sharing initiatives founded on reciprocal benefits between global initiatives and data providers.

Groundwater, Earth’s largest nonfrozen freshwater reservoir, is vital for water supply security. Groundwater models help to manage complex domestic, agricultural, and industrial water demands while preserving ecosystem health under climate change. The community-driven groundwater model portal (GroMoPo) hosts groundwater model metadata to analyse biases and distribution of groundwater models. Over 450 models are currently featured on GroMoPo, with most models from high-GDP countries at local-to-regional scales. The GroMoPo initiative addresses current knowledge gaps and facilitates future collaboration and data sharing.

Knowledge of aquifer thickness is crucial for setting up numerical groundwater flow models to support groundwater resource management and control. Fresh groundwater reserves in coastal aquifers are particularly under threat of salinization and depletion as a result of climate change, sea-level rise, and excessive groundwater withdrawal under urbanization. To correctly assess the possible impacts of these pressures we need better information about subsurface conditions in coastal zones. Here, we propose a method that combines available global datasets to estimate, along the global coastline, the aquifer thickness in areas formed by unconsolidated sediments. To validate our final estimation results, we collected both borehole and literature data. Additionally, we performed a numerical modelling study to evaluate the effects of varying aquifer thickness and geological complexity on simulated saltwater intrusion. The results show that our aquifer thickness estimates can indeed be used for regional-scale groundwater flow modelling but that for local assessments additional geological information should be included. The final dataset has been made publicly available (https://doi.pangaea.de/10.1594/PANGAEA.880771).

Groundwater, Earth’s largest nonfrozen freshwater reservoir, is vital for water supply security. Groundwater models help to manage complex domestic, agricultural, and industrial water demands while preserving ecosystem health under climate change. The community-driven groundwater model portal (GroMoPo) hosts groundwater model metadata to analyse biases and distribution of groundwater models. Over 450 models are currently featured on GroMoPo, with most models from high-GDP countries at local-to-regional scales. The GroMoPo initiative addresses current knowledge gaps and facilitates future collaboration and data sharing. Les eaux souterraines, le plus grand réservoir d’eau douce non gelée de la planète, sont essentielles à la sécurité de l’approvisionnement en eau. Les modèles d’eaux souterraines aident à gérer les demandes complexes en eau domestique, agricole et industrielle tout en préservant la santé des écosystèmes face au changement climatique. Le portail communautaire de modèles d’eaux souterraines (GroMoPo) héberge des métadonnées de modèles d’eaux souterraines pour analyser les biais et la distribution des modèles d’eaux souterraines. Plus de 450 modèles sont actuellement présentés sur GroMoPo, la plupart provenant de pays à PIB élevé, à l’échelle locale ou régionale. L’initiative GroMoPo comble les lacunes actuelles en matière de connaissances et facilite la collaboration et le partage de données futurs. La mayor reserva de agua dulce no congelada de la Tierra, el agua subterránea, es vital para la seguridad del abastecimiento de agua. Los modelos de aguas subterráneas ayudan a gestionar la compleja demanda de agua para uso doméstico, agrícola e industrial, preservando al mismo tiempo la salud de los ecosistemas en un contexto de cambio climático. El portal de modelos de aguas subterráneas orientado a la comunidad (GroMoPo) contiene metadatos de modelos de aguas subterráneas para analizar los sesgos y la distribución de los modelos. En la actualidad, GroMoPo incluye más de 450 modelos, la mayoría de los cuales proceden de países con un PIB elevado y se elaboran a escala local y regional. La iniciativa GroMoPo aborda las actuales lagunas de conocimiento y facilita la colaboración y el intercambio de datos en el futuro. 地下水是地球上最大的非冰冻淡水储备，对于水供应安全至关重要。地下水模型有助于管理复杂的家庭、农业和工业用水需求，同时在气候变化的背景下保护生态系统健康。社区驱动的地下水模型门户（GroMoPo）汇集了地下水模型的元数据，以分析地下水模型的偏差和分布。目前，GroMoPo上展示了超过450个模型，绝大多数模型来自高GDP国家，涵盖从地方到区域的规模。GroMoPo倡议旨在填补当前的知识空白，促进未来的合作与数据共享。 As águas subterrâneas são os maiores reservatórios de água doce não congelada da Terra, e são essenciais para a garantia de segurança hídrica. Os modelos de águas subterrâneas auxiliam não só no gerenciamento de complexas demandas hídricas domésticas, agrícolas e industriais, como também na preservação dos ecossistemas frente às mudanças climáticas. O portal colaborativo de modelagem de águas subterrâneas (GroMoPo) armazena metadados diversos modelos hidrogeológicos para analisar os vieses e as distribuições desses modelos. Atualmente, mais de 450 modelos estão reunidos no GroMoPo, sendo a maioria proveniente de países de alto PIB e em escala local a regional. A iniciativa GroMoPo busca preencher lacunas de conhecimento e promover a colaboração e o compartilhamento de dados no futuro.