Comparison of methods measuring electrical conductivity in coastal aquifers

authored by

Nico Skibbe, Thomas Günther, Kai Schwalfenberg, Rena Meyer, Anja Reckhardt, Janek Greskowiak, Gudrun Massmann, Mike Müller-Petke

Abstract

Coastal aquifers, the transition zone between freshwater and saltwater, show large salinity contrasts in the subsurface. Salinity is a key parameter to understand coastal groundwater flow dynamics and consequently also geochemical and microbial processes. For mapping porewater salinity, a variety of methods exists, mainly using electrical conductivity as a proxy. We investigate methods including hydrological/geochemical (well sampling, fluid logger) as well as geophysical method (direct push, geoelectrics) utilizing measurements near the high-water line of a high-energy beach at the North Sea island of Spiekeroog. We compare the methods, discuss their benefits and limitations and assess their spatial and temporal resolution. One key to enable a comparison is the estimation of formation factors transforming bulk conductivity measured by geophysical tools in to fluid conductivities obtained from direct measurements. We derive depth-dependent formation factors derived from time-series measurements of fluid loggers and a vertical electrode installation. Using these formation factors, the vertical electrode chain proves to provide reliable salinities at high spatial and temporal dimension. Direct-push profiling data provide the highest vertical resolution. However, a careful calibration is needed to allow for salinity quantification. On the other hand, electrical resistivity tomography (ERT) exhibits the lowest spatial resolution, but can image two-dimensional salinity distributions. We found ERT to fit very well to all other methods, but the data analysis should be aimed at salinities instead of bulk conductivities, i.e. including formation factors and temperature models into the inversion process.

External Organisation(s)

Leibniz Institute for Applied Geophysics (LIAG)
Carl von Ossietzky University of Oldenburg

Type

Article

Journal

Journal of hydrology

Volume

643

ISSN

0022-1694

Publication date

11.2024

Publication status

Published

Peer reviewed

Yes

ASJC Scopus subject areas

Water Science and Technology

Sustainable Development Goals

SDG 6 - Clean Water and Sanitation

Electronic version(s)

https://doi.org/10.1016/j.jhydrol.2024.131905 (Access: Open)