Assessing river–groundwater exchange in the regulated Rhone River (Switzerland) using stable isotopes and geochemical tracers M. Fette a, * , R. Kipfer b,c , C.J. Schubert a , E. Hoehn b , B. Wehrli a a Limnological Research Center, Swiss Federal Institute of Environmental Science and Technology (EAWAG) and Swiss Federal Institute of Technology (ETH), Seestrasse 79, CH-6047 Kastanienbaum, Switzerland b Water Resources and Drinking Water, Swiss Federal Institute of Environmental Science and Technology (EAWAG), U ¨ berlandstrasse 133, CH-8600 Du ¨ bendorf, Switzerland c Isotope Geology, Swiss Federal Institute of Technology (ETH), CH-8902 Zu ¨ rich, Switzerland Abstract 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 d 18 O 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 d 18 O value, this isotopic signature provides direct information of the source region and the sea- sonality in samples from groundwater wells. On a spatial scale SO 2 4 measurements help to constrain groundwater com- ponents, because the tributaries and groundwater sources south of the Rhone are rich in SO 2 4 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 snow- melt in summer, this value drops below 0.5 mM. Finally the transient tracer 3 H/ 3 He 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 . Ó 2004 Elsevier Ltd. All rights reserved. 1. Introduction Alpine rivers are in the midst of a major transition. Over the last two centuries most river systems in Central Europe have been regulated to improve flood protec- tion. About 50 a ago, large hydropower schemes were developed in the Alps, Scandinavia, the Pacific North- west and in other mountain regions, which strongly modified the hydrological regimes (Johnson et al., 2001; Gleick, 2003; Bratrich et al., 2004). Today, the signs that global warming affects the water storage capacity of mountain glaciers (Haeberli et al., 1999), the seasonal precipitation patterns (Schmidli et al., 2002) and the frequency of extreme events (Frei and Scha ¨r, 2001) demand a re-evaluation of flood protection designs. In addition, the environmental impacts of hydropower use have received intense public attention. 0883-2927/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2004.11.006 * Corresponding author. Fax: +41 41 349 2168. E-mail address: markus.fette@eawag.ch (M. Fette). Applied Geochemistry 20 (2005) 701–712 Applied Geochemistry www.elsevier.com/locate/apgeochem