Wash-out processes of evaporitic sulfate salts in the Tinto river: Hydrogeochemical evolution and environmental impact

This work deals with the wash-out processes of evaporitic salts in the Tinto river in October 2005, with the arrival of the first rainfall after the summer. In order to monitor water levels and electrical conductivity, a datalogger was set up in the river, while sampling was performed by a portable autosampler. Thirty-two samples were selected for analysis for a wide range of elements by ICP-AES. Three different flood events, with a maximum discharge of 8.1 m3/s, were monitored. River waters suffered from a dilution effect at the beginning of the first event, recording a concentration decrease of most elements, just before the wash-out of soluble salts precipitated during the summer took place. Wash-out processes provoked a sharp increase in most element concentrations coinciding with an intense decrease in Na and Sr. After the first event, there was strong enrichment of As, and to a lesser extent in Fe, Cr and Pb, due probably to the redissolution/transformation of Fe oxyhydroxysulfates. During the third event, evaporitic sulfate salts were depleted from riverbanks ending wash-out processes, and a decrease in most element concentrations was observed. Barium exhibited different behaviour to the rest of elements owing to the solubility control exerted by barite. Lead also showed a different pattern throughout the study period, its concentration decreasing due probably to its great affinity for coprecipitate on jarosite-group minerals, and increasing when schwertmannite precipitation is thermodynamically favoured. In October 2005, the Tinto river carried around 8100 t SO4, 1300 t Fe, 400 t Al, 100 t Zn and Cu, etc., highlighting the importance of wash-out processes of soluble salts in the Rيa de Huelva ecosystem and metal fluxes into the Atlantic Ocean.