Coastal groundwater systems: mapping chloride distribution from borehole and geophysical data

Jörg Elbracht,Reinhard Kirsch,Qian Zhao,Eva González,Nico Skibbe,Mohammad Azizur Rahman,Nico Deus,Helga Wiederhold,Bernhard Siemon

doi:10.1007/s00767-021-00475-1

Abstract

Information on chloride (Cl) distribution in aquifers is essential for planning and management of coastal zone groundwater resources as well as for simulation and validation of density-driven groundwater models. We developed a method to derive chloride concentrations from borehole information and helicopter-borne electromagnetic (HEM) data for the coastal aquifer in the Elbe-Weser region where observed chloride and electrical conductivity data reveal that the horizontal distribution of salinity is not uniform and does not correlate with the coastline. The integrated approach uses HEM resistivity data, borehole petrography information, grain size analysis of borehole samples as well as observed chloride and electrical conductivity to estimate Cl distribution. The approch is not straightforward due to the complex nature of the geology where clay and silt are present. Possible errors and uncertainties involved at different steps of the method are discussed.

Highlights

Worldwide coastal aquifers are increasingly endangered by saline water intrusion from oceans and rivers (van Weert et al 2009)
Because of the invalidity of Eq 1 for cohesive materials, the apparent formation factor of clay and peat were taken from the literature
Measured resistivities (Rb) from helicopter-borne electromagnetic (HEM) data were grouped in a box plot, which shows that the median resistivity value of most petrography classes increases with the increase of grain size, except in the case of peat and gravel (Fig. 2a)

Summary

Introduction

Worldwide coastal aquifers are increasingly endangered by saline water intrusion from oceans and rivers (van Weert et al 2009). Changes in rainfall patterns and sea level rise driven by climate change interact with increased groundwater exploitation, land drainage, and urbanization (White and Kaplan 2017). The effects of these driving forces make sustainable coastal zone groundwater management very complicated. Accurate groundwater resource assessment and information on the spatial distribution of salinity is required (Barlow and Reichard 2010). Groundwater models can support this planning action (Dogrul et al 2016) but for density-dependent groundwater modelling, information on existing freshwater head and chloride distribution is required at sufficient resolution

Methods

Results

Discussion

Conclusion