Dissolved Fluxes of the Ebro River Basin (Spain): Impact of Main Lithologies and Role of Tributaries

Abstract

The aim of this study was to evaluate, over more than 20 years, the export fluxes for dissolved loads on the catchment scale of the Ebro River basin in Spain. Data are compiled from the databank of theConfederación Hidrográfica del Ebro (CHE). The spatial and temporal distribution of daily discharges, physico-chemical parameters and chemical data covering the last two decades (1981–2003) were investigated in the Ebro Basin on five monitoring stations along the Ebro River (Mendavia, Castejon, Zaragoza, Sastago and Tortosa), as well as six stations at the outlet of the main tributaries (Arga, Aragon, Gallego, Jalon, Cinca and Segre). The dissolved load of the rivers at the Ebro Basin scale was characterised through the EC, total dissolved solids (TDS) and the major elements chemical data. The surface water can be classified into three main categories, a clear dominance of Ca–SO4 water type, a Ca–HCO3 type mainly encountered in the upper part of the basin and some data presenting a Na–Cl water type. The TDS values are highly variable, both in time and in space, in the range 390–1,360 mg L−1. The dissolved exportations to the Mediterranean Sea and the relative contribution of the different tributaries were calculated. The Ebro basin in its upper part (upstream Mendavia) contributes around 22.4% of the total exported flux near the outlet (Tortosa) over the studied period. The tributaries that mainly contribute to the total exported load are the Cinca and Segre (19% and 17% respectively). The Aragon, Gallego and Jalon contributions are very low, often less than 5% of the total exported flux. The specific TDS flux at the outlet of the Ebro is 70 ± 23 t km−2year−1 and 108 ± 24 t km−2 year−1 upstream in Mendavia while the highest chemical erosion rate was calculated for the Arga with 251 ± 55 t km−2 year−1. The dissolved export fluxes represent the major export from the Ebro basin, and the respective contribution of carbonate and evaporite (gypsum) with respect to the TDS was then calculated using the major element concentrations and discharge data. In the upper part of the Ebro Basin, carbonate weathering is dominant compared to gypsum weathering while downstream the dissolved exportations are dominated by carbonate weathering. For the tributaries, most of them are dominated by evaporite weathering. The exportation rates at the outlet of the watershed shows dissolved exportation derived from gypsum weathering, that are about 1.5 times that derived from carbonate weathering.