EVIDENCIAS HIDROQUIMICAS, ISOTÓPICAS E HIDRODINÁMICAS DE LA INFLUENCIA MARINA Y CONTINENTAL SOBRE UN HUMEDAL COSTERO. PLAYA FRACASSO, PENÍNSULA VALDÉS

ALVAREZ, MARIA DEL PILAR; DAPEÑA, CRISTINA; BOUZA, PABLO; RIOS, STELLA MARIS; HERNANDEZ, MARIO

GEOQUÍMICA DE LA SUPERFICIE ARGENTINA

Universidad Nacional del Sur Ediuns

Lugar: Bahia Blanca; Año: 2014; p. 127 - 144

Coastal wetlands may be affected by the tide and be under the greater or lesser influence of continental water contributions. Even though their occurrence is usually associated with estuaries or deltas, in the case of the Playa Fracasso (Fracasso Beach) marsh in Península Valdés (Valdés Peninsula), which occurs in an arid climate, there are no permanent surface water courses and the continental surface water fluxes are only ephemeral. In the area, geoecological research has been carried out, but the hydrogeological system is not known. The main objective of this work is to determine the influence of marine and continental waters on the Playa Fracasso marsh by means of hydrochemical, isotopic and hydrodynamic evidence or properties. A groundwater monitoring network was set up, consisting of 13 wells located in different physiographic positions (topography and vegetation) in which the seasonal and daily water tables were measured, and samples were extracted to determine major ions, Br and stable isotopes (18O and 2H). The results obtained indicated an unsaturated zone of variable thickness, between 0.45 and 2 m from the marsh to the bajadas surrounding it from the continent, and a saturated level composed of beach sediments of sandy grain-size. Besides, a preferential recharge area was identified, represented by the coastal bajada and sandy layer units, an area of circulation directly associated with the marsh, and a discharge area on the coastline. The level fluctuations measured reveal the tidal influence up to the central area of the high marsh. Groundwater hydrochemistry is highly variable depending on the location and timing of the monitoring, with high salinity characterizing the whole area (in the marsh it is twice as high as in marine water) and the water type is markedly sodium chloride. When used individually, the rMg/rCa, rSO4/rCl and rCl/rBr ion relations and the stable isotopes 2H and 18O showed limitations to define marine influence. However, when interpreted together with the hydrodynamics, they indicated that there is an area within the high marsh in which the source of groundwater is mainly marine and another one in which the contributions are of mixed origin. The area which the indicators used coincide in characterizing as dominated by marine influence is associated with a vegetation of Limonium brasiliense, and the area of mixed origin (central area and the area adjoining the bajadas) is indicated by a vegetation of Sarcocornia perennis and Spartina densiflora.