The suitability and usage of O-18 and chloride as natural tracers for bankfiltrate at the middle River Elbe

Flow times and infiltration behaviour are very important for water extracti on from riverbank filtration. Both the chloride content and the isotopic co mposition of oxygen (delta(18)O) were found to be suitable indicators for t he conditions encountered in the middle course of the River Elbe near Torga u. The ranges for Elbe water were measured to be 20-43 mg Cl/l (1995-97) an d -11 to -8.5 parts per thousand delta(18)O (1993-97). Both methods permitt ed flow-time spectra at two adjacent sampling profiles incorporating river and production wells to be obtained, indicating the preferred flow paths an d the vertical extension of the riverbank filtrate plume in the aquifer. Ho wever, since the differences between the mean values for river water and re gional groundwater were too small the percentage of river water abstracted from production wells could not be calculated.