Isotopic Age and Composition of Streamflow in the Weser Basin, Germany and Possible Implications for Groundwater Sustainability

Long term isotope time series in streamflow collected by the German Federal Institute of Hydrology (BfG) were used to estimate residence times of water in the Weser Basin, Germany. Tritium recession in streamflow of large basins indicate contributions of slowly moving water. For an appropriate interpretation of the recession of tritium, it is necessary to consider different runoff components (direct runoff, fast groundwater and slow groundwater) and varying residence times of tritium in these components. Spatially distributed catchment models (TAC^D — tracer aided catchment model, distributed; WaSiM-ETH) and the tritium balance model (TRIBIL) were combined to model process based tritium concentrations in streamflow of the Weser Basin (46 240 km²) and seven embedded sub-basins. Runoff separations using the TAC^D model led to baseflow quantities of 54% to 71% (including fast and slow groundwater), 76% to 95% using WaSiM-ETH and 59% to 71% with a simple digital filter method (BFLOW). Mean residence times of 14 to 50 years and 15 to 26 years were derived with TRIBIL-TAC^D and TRIBIL-WaSiM, respectively. Stable isotopes (deuterium and oxygen-18) in streamflow that are available for the period of 2003 to 2008 were used in combination with simple black box modelling approaches (dispersion model with FLOW-PC, sine-wave method) to calculate mean residence times. The stable isotopes input function was calculated using four GNIP precipitation stations (Wasserkuppe, Bad-Salzuflen, Braunschweig and Cuxhaven) that are located within the Weser basin. In contrast to tritium stable isotopes on the large scale mainly characterize fast runoff components (seasonal variations) and estimated stable isotope mean residence times were in the order of several months for the dispersion model and close to one year for the sine-wave method. An interpretation of groundwater sustainability from isotope observations and mean residence times alone is difficult because in the case of tritium, they vastly dependent on the used model and the hydrological interpretation of the system. (author)