Coastal aquifers and processes related to Global Change: Wetlands, heat transport and floodings

Abstract

This Ph.D. Thesis investigates the hydrodynamic processes of the detrital coastal aquifers related to: (i) groundwater-surface water interactions in wetlands, (ii) the temperature distribution of the groundwater near the freshwater-saltwater interface, and (iii) coastal flooding events associated with hydrogeological processes. The aim is to advance in the knowledge of aquifers to understand their behavior under the effect of external factors (climatic, marine, and anthropic), which are expected to have more impact under the context of Global Change. The Motril-Salobreña aquifer was selected as experimental site due to its relatively good state and its location on the Mediterranean coast, which is exposed to human action and the effect of Climate Change. Regarding the groundwater-surface water interactions, the water connection of “La Charca de Suárez” wetlands with the Motril-Salobreña aquifer was determined, because of the interest related to the high degree of anthropization of the wetlands and the conservation threats. The combination of different methodologies allowed the estimation of the water balance of the lake complex, the determination of the flow directions, and the hydrochemical characteristics of the water. The obtained results showed the complex hydrological functioning of the system and the existence of variations controlled by climate conditions and human activity. In the study of the temperature distribution near the freshwater-saltwater interface in coastal aquifers, the effect of different heat sources (surface water recharge, sea infiltration, and geothermal heat) was included for the first time in the scientific literature. A numerical model was developed to propose a synthetic theoretical model of temperature distribution in coastal aquifers, which was tested in a case study with the calibration of temperature field data. Furthermore, the impact of different hydraulic and thermal parameters was determined based on a sensitivity analysis. The models indicated the connection between groundwater temperature and salinity in coastal aquifers and established the basis for future research in this field. A more detailed analysis was conducted to study groundwater temperature and salinity fluctuations in the deep zone of coastal aquifers induced by sea tides and changes in recharge. For that purpose, statistical analysis of the field data obtained at different depths was combined with numerical modeling. This methodology provided the conceptual mechanism that produces temperature oscillations and disclosed the importance of having a first picture of the temperature and salinity distribution in the aquifer. Furthermore, the most susceptible zones to temperature variations were identified as well as some of the hydrogeological characteristics of the aquifer that conditioned them. Concerning coastal floodings associated with hydrogeological processes, the role of the groundwater for the existence and the durability of these inundations was analyzed, which is frequently ignored in most flood-risk studies. The groundwater level data obtained over two years was compared to hydrometeorological data and sea wave height records to identify different types of events depending on the climatic conditions. Finally, a more detailed study of the changes in the hydrodynamics of “La Charca de Suárez wetlands due to coastal events complemented the acquired knowledge of the functioning of the lakes during extreme climatic events