Tracing water cycle in regulated basin using stable δ18O–δ2H isotopes: The Ebro river basin (Spain)

Abstract

Variations in the stable-isotope compositions of O and H in a catchment's water balance are mainly caused by natural variations in the isotopic composition of rainfall, through mixing with pre-existing waters and evaporation. Stable isotopes were analysed in surface waters along the main course of the Ebro River (Spain), in its main tributaries and in some groundwaters as part of a year-long monthly survey at the outlet. Meteoric input data over the basin from five IAEA/GNIP stations surrounding the Ebro catchment showed that the mean weighted δ18O–δ2H rain input showing enriched values for four stations and a depleted value for the fifth. Most surface- and ground-waters collected in the Ebro catchment plot close to the global and local meteoric-water lines, reflecting a meteoric origin and a lack of significant evaporation or oxygen-isotope exchanges between water and the rock matrix. Although the main tributaries show large variations of δ18O and δ2H signatures (respectively −5.0 to −9.6‰ and −38.5 to −63.6‰) only the Guadalope river shows an evaporated signal. All tributaries have more depleted δ18O and δ2H values than the mean rainwater δ18O and δ2H signal for the Ebro catchment given by local rain monitoring stations. The depleted values in the Ebro tributaries originate from runoff from the Pyrenees, whereas normal Mediterranean rains yield mostly enriched values. The Ebro River itself shows δ18O and δ2H values that fall between those of its tributaries and no variation is related to the location along the river course. The river water at the outlet (Amposta) display a range of δ18O–δ2H values from respectively −7.2 to −8.4‰ and −47.5 to −53.9‰, though with a few enriched values. There is no link between river discharge and Cl content. Temporal δ18O–δ2H variations at the Ebro outlet reflect input of enriched water from Mediterranean rains and of depleted waters from Pyrenean runoff. The geochemical signal at the Ebro basin outlet was deconvolved through a conceptual scheme of the Ebro hydro system, highlighting the major role played by dam impoundments, not only on water quantity, but also on its quality.