Imaging the effect of dynamic interactions between fresh and sea water on the salinity of an aquifer-coastal wetland system: the restored la pletera salt marsh

One important issue in restoration and management projects of coastal wetlands is the challenge of how to establish the environmental conditions under which the final saline status of the system ensures a fluctuation degree compatible with the ecosystem requirements. Monitoring salinity in pre-existing and new lagoons is a crucial tool before, during and after the intervention. Nevertheless, this task should also consider knowledge of the interactions between fresh and salt water in the surface water-groundwater interphase, since periodic fluctuations in the various water input and output vectors determine the whole balance, and hence, the salinity of the entire system or parts of it. During two LIFE+ projects conducted with the aim of restoration of La Pletera coastal lagoons in Catalonia, NE Spain (http://lifepletera.com), a monitoring survey from 2016 to 2018 has been deployed in order to determine seasonal relative and accumulated variations in salinity of soil and lagoons. At a reference profile set perpendicular to the shoreline and located between a pre-existing lagoon and a newly created one, an electrical resistivity tomography profile has been measured and interpreted on a time-lapse basis. Complimentary data comprise near-surface soil sampling, measurement of water table levels and physicochemical parameters such as electrical conductivity in the lagoons, the groundwater and the sea, and compilation of daily meteorological and sea state data records. Discrete representation of the electrical resistivity section has allowed a detailed image of bulk salinity variations along space and time. This has also permitted precise delimitation of the subsurface extent of brackish groundwater and the interphase with the marine water intrusion wedge within the aquifer. Results evidence that the most pronounced fluctuations in salinity occur in the first 2 meters below the surface. This interval of depth comprises both the unsaturated zone and the upper part of the saturated zone, where flow interactions between the shallow aquifer and the lagoons occur. Here, soil undergoes strong resistivity changes reflecting the relative contribution of fresh, brackish and salty water for any given period of time, under the influence of the hydrodynamics of the system. While newly created lagoons show a seasonal pattern of variation intimately linked to that of soil, older lagoons maintain a different, higher salinity regime strongly conditioned by their low permeability muddy bottom long-time accumulated. The study has also identified the synoptic hydrometeorological scenarios that are responsible for the highand low-salinity events affecting the ecosystem.