Tracing surface water mixing and groundwater inputs using chemical and isotope fingerprints (δ18O-δ2H, 87Sr/86Sr) at basin scale: The Loire River (France)

Abstract

The spatial and temporal distributions of major elements, stable isotopes of water molecule and strontium isotope ratios were investigated in the surface waters of the Loire River basin. The present study, using a coupled hydrological and geochemical approach, focuses on surface water mixing and the interactions between ground- and surface water. Stable isotopes (δ18O, δ2H) provide good evidence for the complex hydrological behavior of the Loire River system at the basin scale, and if binary mixing of surface water masses explain the longitudinal evolution of the Loire signature in the upstream part of the basin, groundwater contributions are required to explain locally the Loire signature. The water chemistry in the different zones of the basin shows large variations in major-element contents. The NO3 content in the Loire River roughly increases from up-to downstream and is mainly controlled by the input from tributaries. The highest concentrations are observed in the middle part of the Loire basin, in close relation with the diffuse agricultural sources. In terms of water-rock interaction, using Na as the reference element, the Loire Basin data are scattered between the three main end-members representing the major lithologies i.e. basalts, granite/gneiss and carbonate. Strontium-isotope ratios measured in water range from 0.70691 to 0.71395 and plotted vs. the Ca/Na ratios, the 87Sr/86Sr ratios clearly discriminate the three main lithological endmembers. In the Middle Loire section, the Loire flow rate increases without significant inputs of surface water, previous studies attributed this increase to groundwater inputs. The relationship between 87Sr/86Sr and the Cl/Sr ratios clearly shows the groundwater inputs from the Beauce carbonate aquifer along a 90 km river profile, between Orléans and Amboise. These conservative tracers can be used to calculate up to 20% groundwater mixing during low flow period.