Identifying groundwater discharge towards the Mar Menor lagoon (SE Spain) combining geophysical and environmental tracer methods
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Groundwater discharge is a crucial process for relevant hydrological and ecological processes in coastal environments, contributing to coastal stability, lagoon resilience, geochemical mass balances, or nutrient cycling. However, growing urban and agricultural development induce severe groundwater pollution and contributes to the degradation of coastal ecosystems. This is the case of the Campo de Cartagena aquifer – Mar Menor lagoon system (Murcia, Spain), where the lagoon has experienced severe eutrophication events linked to nutrient inputs through surface and groundwater flows. Surface water inputs are well known and consist of one permanent stream and sporadic runoff derived from heavy rainfall. The existence of permanent submarine diffuse groundwater discharge (SGD) is hypothesized since long ago. This work studies the hydrogeological conditions for the existence of SGD to the Mar Menor in different seasons, using a combination of electrical resistivity tomography (ERT) and environmental tracers (Cl-, 18O, 2H). To perform the work, four land-sea transects were studied in two field surveys carried out in April and November 2023. ERT data were generated using an amphibious electrode arrangement perpendicular to the coast, and tracer data of shallow groundwater in emerged and submerged sediments were obtained using push-point piezometers. ERT showed the existence of a huge volume of fresh water under the Mar Menor bottom, penetrating as far as 80 m from the shore. The hydraulic gradient favoured diffuse SGD across the bottom. Tracer mass balances confirmed the existence of SGD at least up to 35 m from the shoreline. The shape and size of the fresh water mass remained stable during the study and are expected to maintain along years if the piezometric levels near the coast does not decrease significantly. This work is part of projects funded by the Spanish Ministry of Science and Research, the Séneca Foundation and the European Union.