A multidisciplinary approach to characterizing coastal alluvial aquifers to improve understanding of seawater intrusion and submarine groundwater discharge

Coastal aquifers are affected by seawater intrusion (SWI), which causes their salinization, and yield submarine groundwater discharge (SGD), which feeds marine ecosystems. Characterizing groundwater dynamics in coastal aquifers is fundamental for understanding both processes and their interaction. In order to gain insights into SWI and SGD, we developed a 100 m-scale experimental field site located in a coastal alluvial aquifer at the mouth of an ephemeral stream on the Maresme coastline (Barcelona, Spain). Given the complexity of coastal aquifers and the dynamism of the processes occurring therein, understanding of the coupled processes can be achieved by combining methods and approaches across different hydrogeological disciplines. In this study, we conduct a detailed aquifer characterization based on the four pillars of hydrogeology: geology (lithological description and core samples analyses), geophysics (downhole and cross-hole measurements), hydraulics (pumping and tidal response tests) and hydrochemistry (major and minor elements, together with stable and Ra isotopes). Each discipline contributed to the characterization of the aquifer: (1) geological characterization revealed that the aquifer consists of fluvial sediments, organized in fining upwards sequences with alternating layers of gravel, sand and silt; (2) geophysics helped in identifying silt layers and their continuity, which play a segmenting role in the aquifer hydrodynamics; (3) hydraulics tests, specifically tidal response tests, evidenced that tidal loading, rather than hydraulic connection to the sea, drives the tidal response; and (4) hydrochemistry revealed a surprising high reactivity, as most ions reflect some reaction, beyond the expected cation exchange. The summary is that the aquifer, which initially looked like a homogeneous unconfined aquifer 22 m thick, effectively behaves as a multi-aquifer and reactive system with freshwater discharging beneath saltwater at several depths. The fact that thin silt layers caused such a significant impact opens new paths beyond this study both for coastal aquifer management (the possibility of transient pumping for freshwater resources) and marine ecology (expect diffuse groundwater discharge).