Facing geological heterogeneity impact on reciprocal coastal systems

Abstract

In western Mediterranean areas, seawater intrusion (SWI) used to be a major issue during the 1970’s decade due to the agricultural exploitation of the fertile lands located near to the coast. Proper pumping management nearly solved the problem, but the later economic development carried out by industries and tourism added an extra pressure to these sensitive systems, reactivating SWI in some cases. A good example of this situation can be found in the northeastern coast of Spain. In this area, the topographic configuration is controlled by medium reliefs and sloped piedmonts that discharge precipitation through ephemeral streams towards the sea. These torrential flows have a groundwater component that generates a coeval submarine groundwater discharge (SGD) with the intrusion of seawater. SGD strongly determines the health of marine environments, reason why is classically neglected in SWI research: in this area, SGD has been characterized measuring natural radioactive tracers (223Ra, 224Ra, 226Ra, 228Ra) in open sea campaigns, but the impact of mixing of this seaward flow and SWI within the aquifer remains unknown. The presence of different lithologies also determines the kind of geochemical reactions that modify the conservative behavior of the tracers used to measure SGD, as well as the localization of preferential flowpaths, making hard to quantify and predict both types of reciprocal flows (SGD and SWI). To be able to understand these interactions, a detailed characterization of the solid and liquid phase of the aquifer at different scales has been performed. To do so, we built a dedicated experimental site in a coastal alluvial aquifer in the Maresme coast line (Barcelona, Spain): the Argentona experimental site. Different types of geophysical logging (induction, spectral gamma ray and magnetic susceptibility) were deployed in the drilled wells to characterize the salinity gradient and distinguish sedimentary bodies. Also, the lithology and the geochemistry of discrete samples recovered from the drilling stage, were determined at smaller scale using X-ray diffraction, rock total analysis for chemical composition, cation exchange capacity, surface area, 226Ra and 228Ra content and grain size distribution. Moreover, groundwater composition was characterized performing mayor and minor elements analysis and radium and radon isotopes quantifications. This characterization of the geological heterogeneity will serve as a basis for future interpretation of field tests and modelling studies focused on the Argentona experimental site.