Precipitation/dissolution of marine evaporites as determinants in groundwater chemistry in a salt marsh (Península Valdés, Argentina)

Abstract

Evaporites are mineral facies frequently occurring in salt marsh environments in arid climates. Considering that groundwater and soil salinity play an important role in the development of salt marsh ecosystems, the aims of this work are to study the occurrence and precipitation/dissolution processes of evaporites of marine origin and to evaluate how such processes influence groundwater chemistry in the Fracasso Beach salt marsh (Península Valdés, Argentina). Groundwater and seawater samples were collected for isotopic and chemical analysis. Ionic speciation and saturation indices were estimated, and the modelling of theoretical seawater evaporation was undertaken. The estimation of the isotopic variations due to evaporation was carried out according to an analytical model. To study the evaporites, soil salt crust samples were collected in the different sectors of the marsh and then observed in a scanning electron microscope with an X-ray energy-scattering micro-analyzer. The evaporite minerals occurring in different areas of the salt marsh show that there are changes in the dominance and type of evaporites depending on the topographic height. In the most elevated areas, abundant halite and gypsum/anhydrite precipitates are registered, accompanied by epsomite and bischofite, whereas in the lower salt marsh, mainly halite can be observed and, to a lesser extent, gypsum and scarce epsomite. The dissolution of all the evaporites is verified in the groundwater chemistry; the results show that the process of precipitation/dissolution of marine evaporites is the main determinant of groundwater salinity in the Fracasso Beach salt marsh. The variation in ion content registered within the salt marsh is related to the recurrence of the tidal flooding in each sector, as it renews the groundwater. This indicates that in the highest sectors the evaporation percentages are higher, and it confirms that the occurrence of evaporite minerals is determined by their closeness to the sea, the topographic height and, therefore, by the flooding frequency.