Mercury (Hg) levels in 4 aquatic moss species (Bryophytes) from the hydrological system of the Alsatian Rhine flood plain in northeastern France (canalized Rhine, its main tributary, the Ill River with a functional flood plain, and groundwater-fed stream network) were analyzed. The location of the sampling sites was based on results obtained in former studies (Krause & Carbiener 1975, Carbiener 1977, Carbiener & Ortscheit 1987, Carbiener & Trmolires 1990) using different approaches of water quality investigation and river-aquifer interaction (hydrological tracer : Cl- ; physico-chemical parameters: (NO3- , PO43- , NH4+); phytosociological bioindication scale based on river macrophyte communities).

Characteristic differences in Hg contents in the aquatic bryophytes from the groundwater-fed streams were detected. Higher annual average Hg contents in the aquatic bryophytes up to 0.29 gram (g) Hg/g (dry weight) from the groundwater-fed streams reflected river seepage sectors which are all located close to the canalized Rhine. Low level zones with Hg contents < 0,05 g Hg/g (dw) indicated non-contaminated and drained sectors of the alluvial groundwater table. The most vulnerable sectors of the riparian alluvial aquifer close to the Rhine River, not only with regard to the historical Hg pollution, but also for chronic contamination, especially with persistent pollutants, are the sectors upstream from the hydroelectric power plants and the river bent sectors. The hydrostatic pressure of the water column leads to the infiltration of contaminated river water through the gravel bed of the Rhine, which shows high hydraulic conductivity, into the riparian aquifer.

By contrast with the canalized Rhine, the Ill River, also highly contaminated by mercury, does not show river seepage. When it is flooding, this river imports water of high quality into its aquifer. The functional flood plain of this river, due to the soil-vegetation-system, purifies the flood water : the vegetation absorbs nutrients and the soil colloids (clay, limon) adsorb cations (Sanchez-Perez et al. 1991), and thus mercury. The results of this study confirm the impact of river canalization on river-aquifer exchange activities and thus the rule of the preservation, and, perhaps, restoration, of functional flood plain systems for alluvial groundwater-quality.