Abstract
Numerous coastal areas worldwide already experience fresh water shortages due to overexploitation and salt water intrusion. Future climate change and population growth will further intensify this threat in more areas in coming decades. Therefore, it is necessary to explore any potential fresh water source, such as offshore fresh groundwater, that could alleviate this fresh water shortage and provide valuable time for adaptation measures implementation and changes in water management strategies. Recent evidence suggests that a disproportionally large portion of human population living in coastal areas relies on groundwater resources stored in underlying unconsolidated groundwater systems. These systems are often very heterogeneous, combining numerous high permeability aquifers interlaid with low permeability aquitards with varying total thickness. This heterogeneity is a major control on the fresh groundwater volume and groundwater salinity distribution within such systems. Thus, the quantification of geological heterogeneity is often the limiting factor when estimating fresh groundwater volumes, both inland and offshore, along the global coastline. To overcome this obstacle, we combine conceptual geological models with available state-of-the-art global datasets to derive a set of geological heterogeneity parameter distributions quantifying geological heterogeneity of coastal unconsolidated groundwater systems as formed over last 1 Ma. These are then used in an algorithm designed to build synthetic heterogenic parameterizations of coastal unconsolidated groundwater systems along the global coastline. These, in turn, provide key input for modelling variable-density groundwater flow and coupled salt transport to analyze changes in groundwater salinities and offshore fresh groundwater volume. Such an analysis is performed over one full glacial-interglacial cycle (the last 0.13 Ma) to account for oscillating sea-level conditions and shifts in coast-line positions and salinity incursions. Our simulation results show a close match between the modelling scenarios and values presented by literature sources demonstrating the potential of the hereby presented methodology to be applied in similar future studies.
Highlights
More than two billion people live in the coastal areas worldwide (Ferguson and Gleeson, 2012) and are directly dependent on local fresh water resources
The need for a better understanding and definition of heterogeneous coastal hydrogeology at the global scale is addressed in this study by combining various conceptual geological models with state-of-the-art freely available global datasets
The combination of these is used as input into the global geological heterogeneity algorithm
Summary
More than two billion people live in the coastal areas worldwide (Ferguson and Gleeson, 2012) and are directly dependent on local fresh water resources. An increased pressure is observed on both shallow and deep (modern and fossil) inland fresh groundwater resources which results in depletion and quality deterioration (e.g., due to rising salinity), especially in arid regions (Custodio, 2002; Gleeson et al, 2017) These fragile fresh water sources are threatened by natural hazards such as seawater-overwash events (Yang et al, 2013, 2018; Cardenas et al, 2015; Chui and Terry, 2015; Yu et al, 2016; Gingerich et al, 2017) and sealevel rise (Carretero et al, 2013; Rasmussen et al, 2013; Sefelnasr and Sherif, 2014; Mabrouk et al, 2018), further stressing the need to adjust water management strategies and find potential additional sources of fresh water. Rapidly growing population numbers (e.g., United Nations, 2017) will lead to increased urbanization and to coastal aquifer over-exploitation and lower groundwater recharge rates into the aquifers due to surface sealing (e.g., Custodio, 2002). Michael et al (2017) stress the need to rapidly improve current water management strategies in coastal areas worldwide to adapt to the threats mentioned above
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