Estimating the thickness of unconsolidated coastal aquifers along the global coastline

Abstract

Abstract. Knowledge of the thickness of aquifers is crucial for setting up numerical groundwater flow models in support of the management and control of groundwater resources. 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 must have better information about subsurface 10 conditions in coastal zones. Here, we propose a method that combines available global datasets to estimate, along the global coastline, the thickness of aquifers formed by unconsolidated sediments. To validate our final estimation results, we collected both borehole and literature data. Additionally, we performed a numerical modelling study of 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 15

The coastal transition zone is an underexplored frontier in hydrology and geoscience

“…Henceforth, for definiteness we focus on the application of geophysical, drilling and modeling techniques based on coastal hydrogeology as the prime example of the need for white ribbon characterization, although each of the foregoing application areas, and others, are of comparable significance. In hydrogeology, the lack of deep subsurface information beneath the coastal white ribbon impedes an accurate accounting of the partitioning and volume transfers between terrestrial groundwater and sub-ocean reservoirs 19 . Herein we must differentiate between groundwater that discharges directly into the ocean as submarine groundwater discharge (SGD) and groundwater that remains stored underground beneath the coastal zone.…”

“…Geological heterogeneity as an OFG driver breaks the paradigm that sea-level lowstands are a prerequisite for the formation of fresh groundwater reserves below the continental shelf. Recent global compilations of offshore sedimentary systems 19,90 have shown that both heterogeneity and sealevel changes influence OFG systems and the balance between these spatial and temporal drivers may be reserve-dependent. Modeling can help evaluate the persistence of OFG bodies, particularly in response to both onshore and offshore pumping 91,92 .…”

Technical note: Displacement variance of a solute particle in heterogeneous confined aquifers with random aquifer thickness fields

“…stochastic analysis of solute transport in heterogeneous porous formations has focused on relating the spatial variability of the hydraulic conductivity field to that of the flow velocity field, and thus to the spatial variability of the displacement of a solute particle. However, natural aquifers at regional scales often exhibit nonuniform aquifer thickness (e.g., Masterson et al, 2013;Zamrsky et al, 2018;DeSimone et al, 2020), and spatial variability in the aquifer thickness field has also been shown to have an important influence on flow field variability (e.g., Hantush, 1962;Cuello and Guarracino, 2020;Chang et al, 2021). Thus, the underlying motivation for this work is to provide an analytical stochastic method for improved quantification of the variability of solution displacement at the regional scale in heterogeneous aquifers under more realistic field conditions, i.e., taking into account the effects not only of the spatial variation of the hydraulic conductivity field but also of the thickness field of the confined aquifer.…”

Tide‐induced head fluctuations in coastal aquifers of variable thickness

“…The deposition pattern of the sediments, the pre‐depositional topography and the post‐depositional erosion can lead to a widening or a thinning of the aquifer in the coastal zone (Rotzoll et al, 2013; Trapp, 1992). Zamrsky, Oude Essink, and Bierkens (2018) estimated the thickness of unconsolidated aquifers along the global coastline using available datasets. They assumed that this type of aquifers has an increasing thickness towards the coast and constitutes around 25% of the coastal ribbon of the world.…”