MOBILITY OF SELECTED PHARMACEUTICAL COMPOUNDS IN THE HYPORHEIC ZONE OF A LOW PERMEABLE RIVERBANK
As the hyporheic zone is not yet fully understood in its function novel approaches are needed. The use of trace elements like pharmaceuticals as indicators yields relevant information on transport processes influencing the mixing of surface water and groundwater. This makes it an alternative method to classical tracers.
In the present field study in the Grand Duchy of Luxembourg the hydraulic and hydrochemical interactions of surface water and groundwater in the riverbank of an alluvial stream were investigated. During the first phase of the investigations water samples were taken from the stream and from observation wells in the riverbank under different hydraulic conditions. These samples were analysed for selected dissolved pharmaceuticals and major ions. The recorded groundwater levels in the riverbank responded with negligible delay to changes in stream stage. Frequent changes from effluent to influent aquifer conditions were observed. The detection of the pharmaceutical compounds supports the assumption that they enter the riverbank via the stream only. The chosen trace elements are therefore suitable as anthropogenic tracers for groundwater and surface water interactions at this field site and prove that water exchange also takes place in riverbanks even where the hydraulic conductivity is low.
In the next step of the study two multilevel wells were installed in order to gain more detailed information about potential zoning or preferential flow paths in the riverbank. Both wells have three filter screens at different depths. The uppermost screen is located below the lowest observed water level and the other two 1 and 2 m below the first, respectively. These wells were sampled and analysed for the selected pharmaceuticals. One multilevel well shows depth-dependent trends for the pharmaceuticals, whereas the other delivers ambiguous results, which are probably due to inhomogeneities in the riverbank.