Predicting the hydrochemical impacts of sea level rise and intrusion in the lagoon ecosystem of the Maspalomas coastal dune field Natural Reserve (Gran Canaria, Spain) through reactive transport modelling

Water resources and ecosystems in volcanic islands frequently exhibit significant vulnerability to water contamination from anthropogenic or natural sources, seawater intrusion, or freshwater scarcity. This vulnerability stems from the typically limited water storage capacity and the reduced recharge and volume of freshwater bodies. In the south of Gran Canaria Island (Canary Islands, Spain) is found the Maspalomas Coastal Dune Field Natural Reserve, a protected and unique aeolian landscape and ecosystem in Europe. Within this coastal dune system, a lagoon is found, whose ecosystem survival is highly dependent on salinity, pH, oxygenation, and organic matter content, as well as processes of evaporation, anoxia (lack of oxygen), and eutrophication. Furthermore, the projected sea level rise according to the IPCC within the next 100 years could enhance seawater intrusion into the freshwater aquifer that feeds the lagoon.

A reactive transport model has been performed in the context of the European project NATALIE, using the code PHAST to assess the potential effect of enhanced seawater intrusion into the Maspalomas lagoon. The model was implemented through a 2D mesh representing the lagoon formed in a unit of aeolian sands. Two water samples of the lagoon and seawater were used as input solutions and the sea level rise up to 1 m was simulated, displacing the mixing zone up to 10 m inland. The evolution of saturation indices (SI) of calcite, gypsum and halite and the chemical reactions of pH buffering and mineral precipitation in the lagoon were implemented in PHREEQC and introduced in the PHAST model. The simulation showed that a seawater fraction up to 72 % could be reached by mixing with the freshwater feeding the Maspalomas lagoon. This enhanced intrusion could lead to electrical conductivities (EC) up to 38,000 µS/cm, and neutral to alkaline pH up to 7.8, conditions to which several present acuatic plants as Juncus acutus and Tetraena fontanesii are not adapted. Besides, with seawater fractions over 65 % the lagoon reached oversaturation in calcite and was close to oversaturation in gypsum and halite, whose precipitation could affect the hydraulic properties of the connection between the lagoon and the shallow aquifer. A managed periodical freshwater recharge of the lagoon with urban runoff through SUDS (Sustainable Urban Drainage Systems) is proposed as part of the tasks of the European project NATALIE.