LECTURA DE TESIS DOCTORAL: LAURA MARTINEZ PEREZ
Programa de doctorat: Enginyeria del Terreny
Nom del doctorand/a: LAURA MARTÍNEZ PÉREZ
Data de la defensa de tesi: 09/07/2020
Títol de la tesi doctoral: Characterization of seawater intrusion and submarine groundwater discharge in alluvial coastal aquifers: field and laboratory approach
Enllaç videoconferència: https://meet.google.com/bzg-uhqo-jyi
Hora d’inici (hora local): 10:00 h
Seawater intrusion (SWI) causes not only salinization of coastal aquifers, but also a reduction submarine groundwater discharge (SGD) and nutrient fluxes to marine ecosystems. We have developed a small scale experimental field site to gain insights into both SWI and SGD. The site is located in a coastal alluvial aquifer at the mouth of an ephemeral stream in the Maresme coastline (Barcelona, Spain). Here, we describe our attempts for a detailed site characterization, which was based on the four pillars of hydrogeology: geology (Lithological description and geochemical analysis of core), geophysics (borehole logs, Electrical Resistivity Tomography and thermal monitoring), hydraulics (pumping and tidal response tests) and hydrochemistry (major and minor elements, stable isotopes and Radium isotopes for SGD assessment).
As it turned out, all four pillars yielded some surprise. The aquifer consists of alluvial sediments, rather than coastal plain deposits. Tidal loading, rather than hydraulic connection to the sea, appears to drive tidal response, in spite of the aquifer being unconfined. Hydrochemistry suggests an unusually reactive layer, with all cations reflecting some mineral dissolution process, beyond the expected cation exchange. Radium increased with salinity, but the correlation was poor, partly blurred by pH, which was low in the deep portions of the aquifer. These observations were largely explained with the help of geophysics. Both borehole logs and cross-hole ERT helped in identifying silt layers that are frequent in alluvial deposits but proved to play a critical role here. The integration of the different techniques allowed to characterize the different distribution of salt- and fresh- groundwater, that in fact yielded an inverse disposition to what is expected in coastal hydrogeology.
To improve SGD quantification, we also characterized radium behavior within the aquifer. The recovered sediment during borehole drilling was used to perform both batch and column laboratory experiments. We performed constant salted fluid injections but we also vary the salinity (SWI and SGD simulation) to infer adsorbed Ra from its mobilization. Column experiments were later simulated using a 0D model to interpret the results. We have observed that heterogeneous flux through preferential flow paths occurs in the columns. Variations in these flow paths may explain the observed fluctuations. Such variations may also occur during the freshening and salinization experiments, but the changes in sorption are so dominant that overcome the impact of changes in porosity structure.