Investigative approaches to determine exchange processes in the hyporheic zone of a low permeability riverbank

The hydraulic and hydrochemical interactions of surface water and groundwater in the riverbank of an alluvial stream in the Grand Duchy of Luxembourg were investigated. Hydraulic conductivity in the riverbank is low and the aquifer is confined. Gauged surface-water levels and groundwater levels were recorded over several months to observe potential effluent and influent aquifer conditions. Water samples were taken under different hydraulic conditions in the stream and from three observation wells in the riverbank. Samples were analysed for inorganic parameters and selected dissolved pharmaceuticals. The recorded groundwater levels in the riverbank responded almost without delay to changes in stream stage, which is very dynamic. Frequent changes from effluent to influent aquifer conditions were observed. The analyses for selected ions and pharmaceuticals support the assumption that the pharmaceutical compounds enter the riverbank via the stream only. The chosen trace organics are therefore suitable as anthropogenic tracers for groundwater and surface-water interactions at this field site. They prove that water exchanges also take place in riverbanks even where the hydraulic conductivity is low and for this reason indicate the existence of a hyporheic zone in the investigated section of the stream.