Combining terrestrial and marine electrical resistivity methods to improve data acquisition in the land-sea transition zone.

In recent decades, there has been a growing interest in accurately characterizing the process of fresh groundwater discharge into the coastal ocean since it represents a significant pathway for solute from land to sea. Although SGD research has focused on quantifying water and solute fluxes, little is known about the physical forces, mechanisms, and distribution of SGD.

Investigating interaction processes between the fresh and saltwater under the sea bottom represents a logistical and technical challenge. Traditionally, electrical geophysical methods have been solely used in the terrestrial part, and during the last decades, the method techniques have been introduced to marine environments. However, these methods are limited by their characteristics in obtaining subsurface resistivity data in the first few meters of the coast. Especially in the land-sea transition zone of microtidal environments, where many of the most critical processes take place, this data is traditionally not available. In this study, we developed a combination of methods to bridge the data gap between the terrestrial and marine realms. This study aims to characterize the FSGD in two aquifers from different geological contexts (alluvial and karstic) on the Mediterranean coast near Barcelona.

To study the transition zone of the aquifer with a good spatial resolution, the amphibious electrical tomography method has been chosen, combining a terrestrial and aquatic line fixed in contact with the marine sediment. Profiles perpendicular to the coastline has been made in a shallow water area to obtain electrical resistivity data of the seabed at a local scale. The configuration used is Dipole-Dipole and Wenner-Schlumberger, with an investigation depth of 10 m. Based on the results, designing a 3D model of electrical resistivity in marine sediments was possible. In parallel to getting electrical resistivity data, manual piezometers were used to bring porewater samples at different points.

We have been able to measure the presence of freshwater or brine, and we have identified differences in the geometry depending on the geological context, where the karst environment is the one that presents a more significant proportion of freshwater saturating the marine sediments. Therefore, amphibious electrical tomography is a non-invasive method that detects resistive zones of marine sediments associated with discharge processes and can be instrumental in characterizing the presence and distribution of SGD.

 

Acknowledgments

This work was partly funded by the Spanish Government (grant no. PID2019-110212RB-C22) and the project TerraMar (grant no. ACA210/18/00007) of the Catalan Water Agency.