Isotope study on surface water seepage during a flood event 

in the upper Rhine valley

P. Königer, Ch. Leibundgut & R.-D. Lohner

University of Freiburg, Institute of Hydrology, Fahnenbergplatz, 79098 Freiburg, Germany

to the poster

Especially during large flood events there may be significant interactions between surface- and groundwater. In such cases the aquifer is penetrated by river water of low quality. The range of surface/groundwater interactions is usually monitored by measurements of groundwater levels. These continuous measurements allow groundwater storage estimations, that include different water components like infiltrating precipitation, normal groundwater discharge, river seepage. However, it is not easy to spatially separate normal groundwater discharge that is blocked by infiltrating river water from the induced recharge itself. Hence in the present study the separation and identification of river water in the aquifer was done applying environmental isotopes.

The study site is located in the flood plain of the upper Rhine valley, 20 km south of Freiburg (Southwest Germany). At this site the Rhine is divided into a bypass channel (Grand Canal d´Alsace) and the former river bed. Due to an agreement between the French and the German governments, most of the discharge flows through the bypass channel (hydropower). A minimum of about 20 m³/s is regulated to remain as discharge in the former river bed, that, in contrast to the bypass channel, is connected to the groundwater. During large floods huge amounts of water (up to 2000 m³/s) are conducted through the former river bed. Then the bank filtration of river water into the aquifer is considerable.

Since groundwater mainly originates from the black forest and the river water is dominated by its alpine origin, isotopic compositions differ significantly and isotope investigations are promising. During a four week lasting flood event in November 1998, hydrometric data of river discharge (Hartheim river gauge station) and groundwater levels from a transect of five perpendicular situated piezometers were evaluated for a calculation of groundwater storage. In addition sampling was intensified during this flood event: water samples were collected at least every second day compared to twice a week during the rest of the year. Isotope contents were analysed in the laboratory for Oxygen-18 and Deuterium as  d values in relation to V-SMOW.

The observed maximum level of the flood event in the Rhine rose by 3,90 m and lasted 30 days, reaching a maximum discharge of 1110 m³/s. The results show groundwater level fluctuations more than 1500 m apart from the river bed. Isotope concentrations in the river water are considerably lower ( d¹⁸ O -10,6 ‰ and  dD -78 ‰ in a yearly mean) than the mean values of groundwater ( d¹⁸ O -9,3 ‰ and  dD –65 ‰). They changed significantly during the event up to a distance of 300 m apart from the river.

In this case study a combination of hydrometric measurements and tracer data showed the extent of surface water seepage into the groundwater of the upper Rhine valley aquifer system. The approach demonstrates a method to assess the vulnerability of water supply systems, which are using the upper Rhine valley aquifer system. It can also contribute to the calibration or validation of groundwater models.