Surface water and groundwater response to the tide in coastal wetlands: Assessment of a marsh in the outer Río de la Plata estuary, Argentina

Abstract

On the coast of the outer Río de la Plata estuary (Argentina) there is a vast coastal wetland occurring in a marsh environment. The hydrodynamics of this wetland is subject to a semidiurnal microtidal regime which causes a wedge of estuary salt water to penetrate below the freshwater coming from continental discharge. This paper analyzes the hydrodynamics of surface and groundwater, and it assesses how anthropogenic changes affect the natural hydrological behavior of the coastal wetland in the southern sector of the outer Río de la Plata estuary. Water level and temperature measurements were carried out on water from the canals flowing into the estuary and the phreatic aquifer located in the marsh. The salinity of the water column was measured at one high tide which was above and one which was below the regional groundwater discharge level. The results show that in natural conditions the wetland has a complex hydrological behavior conditioned by the tidal flow. Surface and groundwater saline stratification is a distinctive characteristic, and the variations in level, temperature and salinity of groundwater depend on its interaction with estuary and continental water. At present, 47 % of the marsh is excluded from the tidal cycle due to anthropogenic action (levees, roads and canals with floodgates), causing a major alteration to the hydrological behavior and the environmental characteristics of the wetland.