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The formation of evaporites associated with the final stages of the precipitation sequence, such as the case of halite, is frequent in marshes in arid areas, but it is not to be expected in those humid climates. This work, by means of the study of the hydrological, climatic and land use conditions, identifies the factors that allow the formation of saline precipitations in a marsh located in a humid climate area. The results obtained show that the exclusion of the marsh as a result of the embankment is the main reason for the presence of halite. It is to be expected that in the future the growth of the embanked marsh areas, together with the climatic and tidal condition tendencies recorded, will favour a higher rate of formation of evaporite salts. The identification of these factors makes it possible to set basic sustainable management guidelines to avoid soil salinisation., (Copyright © 2016 Elsevier Ltd. All rights reserved.)

Coastal wetlands are complex hydrogeological systems, in which saline groundwater usually occurs. Salinity can be attributed to many origins, such as dissolution of minerals in the sediments, marine contribution and evapotranspiration, among others. The aim of this paper is to evaluate the processes that condition the hydrochemistry of an arid marsh, Playa Fracasso, located in Patagonia, Argentina. A study of the dynamics and geochemistry of the groundwater was carried out in each hydrogeomorphological unit, using major ion and isotope ((18)O and (2)H) data, soil profiles descriptions and measurements, and recording of water tables in relation to the tidal flow. Water balances and analytical models based on isotope data were used to quantify the evaporation processes and to define the role of evaporation in the chemical composition of water. The results obtained show that the groundwater salinity of the marsh comes mainly from the tidal inflow, to which the halite and gypsum dissolution is added. These mineral facies are the result of the total evaporation of the marine water flooding that occurs mostly at the spring high tides. The isotope relationships in the fan and bajada samples show the occurrence of evaporation processes. Such processes, however, are not mainly responsible for the saline content of groundwater, which is actually generated by the dissolution of the typical evaporite facies of the arid environment sediments. It is concluded that the evapotranspiration processes condition groundwater quality. This is not only due to the saline enrichment caused by the evapotranspiration of shallow water, but also because such processes are the main drivers of the formation of soluble salts, which are then incorporated into the water by groundwater or tidal flow., (Copyright © 2014 Elsevier B.V. All rights reserved.)

Three wetland‐shallow lakes were studied with the aim of analysing the processes and factors that control hydrochemistry and evaporite deposits in them in basins of the Pampean Plain, Argentina. In this sense, water balances and analysis of the water content of the lakes with normalized water difference index were made, and geologic‐geomorphological characteristics and groundwater flows were defined. Groundwater, surface water and evaporite samples were taken, along with sediment samples from wetland‐shallow lakes and surroundings. In situ pH, electrical conductivity and temperature, and laboratory chemical determinations of major ions of the water were measured. x‐ray diffraction (XRD) and scanning electron microscope (SEM)‐energy dispersive spectroscopy (EDS) analyses of evaporite and sediment samples were carried out. The results obtained allow the identification of two different systems. One of them is represented by Leubucó wetland‐lake basin, which is topographically higher and smaller than the other system, with Na‐HCO3 and Na‐Cl/SO4 groundwater inflows, with high concentrations of Ca2+ and Mg2+. In periods of high evaporation, the regional water table drops, and due to its topographic position, the lake dries. These determine the formation of thick layers of halite associated with magnesium salts on surface, and gypsum layers interbedded with clastic sediments. The other system involves de la Sal and Chasilauquen wetland‐lake basins, which have larger extensions. They receive Na‐HCO3 and Na‐Cl/SO4 groundwater inflow, but with low Mg2+ and Ca2+. Their lower topographic position determines that even during deficit periods, the water table intercepts the lakes surface and therefore, lakes have a shallow surface water level almost all the year, which is hypersaline, Na‐Cl type. These systems only have thin evaporite crusts, composed of halite and thenardite at the edges of the lakes, and the formation of Ca‐Mg sulfates is not observed. It is concluded that topographic position, basins size, geology, groundwater inflow and evaporation are the main processes and factors controlling hydrochemistry and evaporitic deposits. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Salt marshes are some of the most important wetlands in many regions of the world. Soil and groundwater salinity plays an important role in coastal wetland ecosystems because of the differences in tolerances of plant species to salinity and tidal inundation. Given that the salinity of these environments is mostly dominated by the chloride anion, it is the aim of this study to identify the geochemical processes that regulate its content in groundwater. A comparison of two intertidal wetlands under different environmental conditions was carried out in Peninsula Valdés and in Samborombón Bay, both on the Atlantic coast of Argentina. The tidal influence over the groundwater marsh dynamics was analyzed from continuous records of groundwater levels and electrical conductivity. Besides, major ion and environmental isotope data were used to identify the geochemical processes that determine the chloride content, based on the study of ion ratios and analytical models. The results show that, despite the hydrological differences between the two studied marshes, the processes regulating the Cl − contents are similar: evaporation, transpiration and halite dissolution. Among them, evaporation/transpiration are the processes that could continuously increase the chloride concentration. However, it is expected that those are not processes that greatly increase the groundwater saline content if compared to the dissolution of halite. [ABSTRACT FROM AUTHOR]

The processes regulating the relationship between tidal flows and shallow groundwater dynamics, temperature and salinity in a coastal wetland in an arid climate are analysed in a detailed field study carried out in the marsh located at Playa Fracasso (Argentina). The continuous records of groundwater level, temperature and electrical conductivity from a transect perpendicular to the coastline were studied during a period ranging from summer to winter, together with the information obtained in hydrogeomorphological field surveys and soil profiles. An assessment of the processes conditioning marsh hydrology was carried out contemplating seasonal (summer–winter) and periodical variations caused by tidal flows. The study showed that the dynamics of groundwater in relation to tidal flows depends almost exclusively on the infiltration of tidal water when the marsh is flooded during spring tides (syzygy), with an increase in the groundwater discharge level at the onset of syzygy. The differences in temperature between sea and continental water were very useful in defining the origin of the different contributions. Groundwater salinity is mainly associated with the leaching of the soil salts that enter with the sea water infiltrating during flood events. The presence of saline soils in the marsh is regulated by the evapotranspiration predominating in arid zones. The conceptual hydrological model suggested may help in the understanding of the hydrological processes in other similar marshes of Patagonia, as well as in coastal wetlands of arid zones worldwide. [ABSTRACT FROM AUTHOR]

Coastal lagoons and marshes are among the most fluctuating and productive ecosystems in the world, which provide a wide range of ecosystem. The objective of this work was to evaluate the factors and processes that condition the hydrogeochemical dynamics in salt marshes associated with the development of small coastal lagoons located at the southern end of the outer estuary of the Rio de la Plata, where it confluences with the Atlantic Ocean. Four samplings were carried out where major ions composition was determined in seawater, surface water of the lagoon and tidal channel, and in groundwater of the marsh, measuring in situ water pH and the electrical conductivity (EC). The chemical data were analyzed considering the conditions and processes associated with both water balance and tidal regime. The coastal lagoon develops within a sandy spit with dunes which is connected to the sea by a narrow tidal channel where the entry of the tidal flow is limited. The tide that enters has strong chemical variability, reflected in EC ranging from 19.1 to 37.5 mS/cm, depending on the dominant source (ocean or estuary). In periods with evapotranspiration domain, tidal water in the lagoon evaporates resulting in soluble salts precipitation such as gypsum and halite. The subsequent tidal flood dissolves these salts causing that in these periods both surface water of the lagoon and groundwater in the marsh present EC values between 8.4 and 57.1 mS/cm. Conversely, when evapotranspiration is low, surface water of the lagoon and tidal channel and groundwater of the marsh have EC between 3.5 and 18.1 mS/cm, and freshwater discharge from the dune sector where the rain preferentially infiltrate becomes relevant. The results obtained show the strong chemical variability that this type of coastal wetlands presents, providing the basis for its conservation and management of natural reserve areas. • Coastal lagoons and salt marshes are environments with high hydrochemical variability. • Water balance and tidal flood composition condition the hydrochemistry of wetland. • The tidal flow composition in estuary – ocean transition areas is very variable. • Dissolution of salts formed by evaporation of tidal water increase the salinity. • Rains and groundwater discharge control hydrochemistry when evapotranspiration is low. [ABSTRACT FROM AUTHOR]