Geohydrology of a sector of the Bahía San Blas marsh

Abstract

The marsh of the Jabalí stream lies within the Reserva Natural Provincial de Uso Múltiple Bahía San Blas and is located in the southeast of Buenos Aires province, Argentina. The aim of the work is to analyze the main geohydrological processes that regulate the chemistry of groundwater and surface waters in this wetland, and the link of these chemical characteristics to local geomorphological and pedological features. For this purpose, a geomorphological characterization was performed and used as basis for establishing a surface and groundwater monitoring network. Fourteen phreatimeters were built along 3 transects perpendicular to the stream; the water samples collected were analyzed in situ for pH, electrical conductivity, temperature and dissolved oxygen, while major ion contents were determined in the laboratory. The data were analyzed through diagrams of ionic relationships and water classification diagrams. In each sector, soil pits were dug for the description of soil profiles and textural characteristics of the surface sediments. Results show that the chemistry of both the water from the Jabalí channel and the marsh groundwater are predominantly sodium chlorinated, with an enrichment of ionic content associated to greater distance to the mouth of the channel and more elevated topographic position. The main processes that condition groundwater chemistry are the dissolution of mineral species such as halite, gypsum and carbonates that occur as surface crusts on the soil, resulting from the total or partial evaporation of seawater and their subsequent incorporation into the aquifer through infiltration. These processes are facilitated by the semi-arid climate, the lower recurrence of flooding due to the high topographic position, and the fine texture of the soils. In turn, the oxidation of paedogenetic sulphides and the consequent acidification of the medium are proposed as additional mechanisms that condition water chemistry