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

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.

Kombination geophysikalischer und hydrogeologischer Methoden zur gezielten Erkundung feinkörniger Talfüllungen

“…Schulmeister et al 2003) und in der Salinität des Porenwassers (z. B. Herd und Schafrik 2017;Rahman et al 2021) geben. In feinkörnigen Lockersedimenten ist jedoch die Eindringtiefe geophysikalischer Oberflächenmessungen stark limitiert und die räumliche Auflösung der identifizierten geophysikalischen Eigenschaftsverteilungen nimmt mit der Tiefe stark ab (Van Nostrand 1953;Spies 1989;Ismail et al 2012).…”

Current and future state of groundwater salinization of the northern Elbe-Weser region

“…The chloride concentration in the Oste River varies between 100 mg/l (upstream) and 3000 mg/l (downstream, near the Elbe) whereas the concentration in the Weser River was set as 9700 mg/l because no data were available for that part of the Weser, which is present in the model area. The initial chloride distribution in each layer was obtained from Rahman et al (2021). Effective porosity and dispersivity were assigned according to parameters in Table 2.…”