Paleo-Hydrogeological Modeling to Understand Present-Day Groundwater Salinities in a Low-Lying Coastal Groundwater System (Northwestern Germany)

Abstract

Groundwater abstraction and drainage are considered to be the main drivers for the salinization of low-lying coastal groundwater systems, while the role of past boundary conditions is less clear. In this study, 3-D paleo-hydrogeological variable-density groundwater flow and salt transport modeling (“paleo-modeling”) is applied to reconstruct the evolution of groundwater salinities during the Holocene, that is, the last 9,000 years, in Northwestern Germany. Novel aspects of this study include the consideration of highly resolved time-variant boundary conditions in a 3-D paleo-modeling framework, for example, sea-level rise, surface elevation and coastline changes, development of drainage networks and groundwater abstraction, as well as the quantification of isolated processes impacting salinization. Results show that salinization was a function of sea-level rise from 9000 BP until 1300 CE. The creation of the dike line ∼1300 CE set the starting point for increasing anthropogenic control of the hydro(geo)logical system: changes in surface elevation and drainage of low-lying marshes have become main drivers for salinization after 1600 CE when peat was artificially degenerated. Moreover, changes in the dike line caused by storm floods impacted the salinities. Model results for 2020 CE match well with present-day salinity observations. Yet, salinization will continue in the future, as the hydro(geo)logical system has not reached an equilibrium. The presented paleo-modeling framework can be viewed as a blueprint for similar low-lying coastal groundwater systems, influenced by marine transgression and human development. Thereby, it enables the reconstruction of meaningful present-day salinity distributions, serving as a vital basis for modeling future groundwater systems in a changing climate.