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Who hasn’t heard about the ecological disaster of the Mar Menor in recent years?

The Mar Menor is an example of the environmental impact of human activities. This iconic ecosystem, once known as a tourist destination, is now sadly famous for its ecological degradation. Since 2016, the largest lagoon in the western Mediterranean has suffered several episodes of eutrophication, a process of increased primary productivity that depletes the oxygen in the water and can cause high mortality among species, such as the well-known fish die-off that occurred in 2019. Traditionally, most of the pollution has been attributed to phosphate- and nitrate-rich agricultural fertilizers, which reach the Mar Menor through the Albujón stream, the only surface watercourse that flows into the lagoon.

To better understand the main pollutant pathways entering the lagoon, at the Physics Department and at the Institute of Environmental Sciences and Technology of UAB (ICTA-UAB), in collaboration with the Polytechnic University of Cartagena (UPCT) and the Institute of Marine Sciences (ICM-CSIC), we set out to investigate what is invisible: groundwater. To do this, they used radium isotopes to trace the pathways of submarine groundwater discharge to the lagoon. This allowed us to quantify the three main groundwater discharge processes. Firstly, freshwater, which mainly comes from irrigation water infiltration and precipitation into the aquifer, followed by its discharge to the lagoon. Also, the continuous recirculation of saline lagoon water, produced by the inflow of Mar Menor water into the aquifer and its subsequent discharge due to processes such as variations in the lagoon’s water level; and, finally,porewater exchange, which occurs at the bottom of the lagoon and consists of rapid recirculation of lagoon water through the uppermost centimeters of the sediments. 

What we have found in this study is that the recirculation of saline water and interstitial water exchange play a much more important role than previously considered, as the majority of groundwater fluxes entering the Mar Menor occur through these two processes. Moreover, we have identified that interstitial water exchange is a process with strong seasonal variability, with fluxes increasing considerably during the summer. These processes can act as pollutant transport mechanisms, including nutrients, metals, and emerging contaminants, which can threaten the fragile health of the Mar Menor. Therefore, this study highlights the need to identify the main contaminants in groundwater and their preferential transport pathways to the lagoon. Future environmental policies must take these new transport pathways into account to effectively mitigate the invisible pollution that continues to affect this ecosystem.

Finally, the study pays tribute to Professor Jordi Garcia-Orellana, from the Universitat Autònoma de Barcelona, who conceived and led the project. This research has been carried out within the framework of the OPAL project (Origin and Pathways of Anthropogenic Solutes into Coastal Lagoons: Groundwater, Sediments, and Episodic Events), funded by the Ministry of Science, Innovation, and Universities (PID2019-110311RB-C21).