Strontium isotopes as tracers of groundwater-induced floods: the Somme case study (France)

Major ions, trace elements, and O, H and Sr isotopes have been determined for surface water and groundwater in the Somme catchment (northern France). The present study, using a coupled hydrological and geochemical (stable- and Sr isotopes) approach, focuses on the interactions between ground- and surface water. Stable water isotopes (delta 18O, delta 2H) show that all waters have a purely local origin from precipitation, without significant evaporation or water-rock interaction, as all points plot close to the general meteoric-water line. Water chemistry in the different catchments shows large variations in major-element contents. Plotting Na+ vs. Cl- contents and Mg2+, NO3-, K+, SO(4)2-, Sr2+ concentrations reveals: (a) slight variations in groundwater composition, (b) agricultural input into surface waters, and (c) some similarities between ground- and surface waters suggesting the former to be responsible for the chemical signature of the dissolved river load. Strontium-isotope ratios measured in groundwater range from 0.70769 to 0.70808, clearly higher than that of Chalk matrix water (0.707337). Variations of Sr-isotope signatures in groundwater from the Chalk aquifer are significant and help identifying different groundwater bodies. The different signatures in groundwater are better explained by matrix heterogeneity, which can present lateral facies variations, than by a strong anthropogenic influence. The Sr-isotope compositional range in surface waters is very wide, varying from 0.70775 to 0.70816 and defining a trend towards a more radiogenic Sr-isotope signature with high Sr contents and high Mg/Sr ratios. The relationship between 87Sr/86Sr and the Mg/Sr ratios allows different end-members to be defined: the rivers are influenced by anthropogenic disturbance, even during flood periods, and river water has geochemical characteristics very similar to those of groundwater, indicating a strong influence from groundwater feeding the rivers.