Assessing river-groundwater exchange in the regulated Rhone River (Switzerland) using stable isotopes and geochemical tracers

Modern flood protection projects are often combined with measures for river restoration, which enlarge the river bed to improve the flow capacity during peak discharge. For the planning of such projects it is essential to quantify the river-groundwater exchange. To address this question in the highly regulated upper Rhone River basin, a combination of stable isotope techniques with geochemical and transient tracers has been used. The δ18O signal in precipitation decreases towards more negative values with a slope of 0.34‰ per 100 m altitude, precipitation during winter was about 5.5‰ more negative than in summer. Since in winter about 55% of the water in the River Rhone comes from high alpine hydropower reservoirs with a known δ18O value, this isotopic signature provides direct information of the source region and the seasonality in samples from groundwater wells. On a spatial scale SO42- measurements help to constrain groundwater components, because the tributaries and groundwater sources south of the Rhone are rich in SO42- with concentrations of more than 12 mM in spring water. In winter the Rhone water reaches concentrations of up to 1.5 mM, and during snowmelt in summer, this value drops below 0.5 mM. Finally the transient tracer 3H/3He is used to estimate groundwater inflow in deep gravel pits and to calculate an average travel velocity in the alluvial aquifer of about 1.7 km a−1.