Various water tracers were used for the determination of flow characteristics such as residence time, velocity of flow, and dispersion phenomena especially under the aspect of stagnant water regions. The tracers applied were bromide, uranin, eosin, lithium salt, and in one case tritiated water as a reference. Bromide as a chemically very stable anion with no detectable retardation proved under these conditions as very reliable. Uranin and eosin showed low retardation but were subject to losses by sorption, degradation, and photolysis in uncovered regions. Lithium can not be recommended for such applications.

The results showed that the mean residence time in the filter was between 6 and 40 days with respect to hydraulic load, evapotranspiration, soil material and the hydraulic gradient. The determined conductivity of the filters was approximately 10−4 to 10−5 m/s and did not vary during the investigation period.