干旱区生态环境知识资源中心

The origin of the high salinity in the groundwater of a coastal wetland in an arid climate was studied in the Playa Fracasso marsh, located on the northwest coast of the extra-Andean Patagonia. Research was carried out by means of the design of a network of soil pits and short piezometers in the marsh and the surrounding landforms. Continuous fluctuations of the water table, in situ physical and chemical properties, major ions (Ca2+, Mg2+, Na+, K+, Cl-, SO4 (2-), HCO3 (-)) and stable isotopes (O-18 and H-2) in groundwater, as well as soil salinity, were measured. The combined analysis of the hydrodynamics, the ion ratios rCa(2+)/rCl(-) and rMg(2+)/rCa(2+) vs. rCl(-) and the isotopic composition made it possible to recognize an area within the high marsh in which the origin of groundwater is mainly marine and another in which the contributions are of mixed origin. By means of the analysis of rCl(-) vs. delta O-18, a salinization process with no change in isotopic composition was identified. Its interpretation, together with those of the soil salinity profiles and the records of the fluctuations in electrical conductivity associated with extraordinary tides, was used to define a conceptual model of salinization which could be useful to understand other coastal wetlands under similar arid climatic conditions. It consists in a cyclical mechanism of evapotranspiration, precipitation, dissolution and transport of salts during tides.