Current and future state of groundwater in coastal unconsolidated sediment systems worldwide

The main goal of this study is to estimate the current state of fresh groundwater resources in coastal areas worldwide while also looking for new potential water sources and analyzing the effects of future sea-level rise. To achieve this goal a large set of numerical groundwater models have been built in order to estimate groundwater salinity on a global scale. Developments in computational power (and its availability) and computer codes dedicated to simulating groundwater flow and coupled salinity provided the essential tools to carry out such large-scale and complex analysis. Nonetheless, an important gap in geological information on global scale was encountered at the beginning of this study and therefore its improvement formed the essential initial step of this study. First, the thickness of the groundwater bearing geological formations (unconsolidated sediments only) is estimated. While it is an important input parameter into the groundwater models, it is shown that varying thickness has a much lower influence on simulated groundwater salinity compared to geological composition. To this end, we define geological composition and its complex nature. Several proxy parameters are defined which, combined with an approach to create synthetic geological representations, translated into sets of regional coastal geological formations formed by permeable aquifers and low-permeable aquitards. This new global coastal geological dataset is then applied to simulate groundwater salinity in coastal regions worldwide. Recent studies suggest that large offshore fresh (and brackish) groundwater reserves are located offshore near the current coastline. These could potentially serve as additional source of fresh water for coastal communities. It is assumed that these offshore fresh groundwater reserves were deposited during past low sea-level stands. To account for these sea-level changes, the groundwater model simulations cover more than 120 000 years which corresponds approximately to one full glacial-interglacial cycle accompanied by fluctuations of sea-level by about 120m. Our findings suggest that large offshore fresh groundwater reserves are present in numerous coastal regions around the world and their exploitation could either serve as input into desalination plants or directly as water source for domestic, high-end agricultural and/or industrial use. While this potential new source of fresh groundwater is yet to be explored and confirmed as suitable and profitable for human usage, rising sea levels are looming over coastal populations in the current century directly threatening current fresh groundwater reserves. Lastly, the effects of different sea-level rise scenarios on coastal fresh groundwater are studied. Our findings show a large difference between the groundwater salinization severity projected for the different sea-level rise scenarios. Coastal populations would only suffer minor consequences in the most moderate sea-level rise scenario. In contrast, the most severe sea-level rise scenario would lead to a major groundwater salinization in several regions around the globe by the end of the 21st century. If this scenario would continue in the coming centuries as well, a large fraction of all coastal regions would be rendered uninhabitable by the end of the 23rd century.