The recent canalisation (in the nineteen sixties) of the upper Rhine has modified the exchange processes between the river and its groundwater in the floodplain of Alsace. The Rhine seeps through its gravelly bed and in this way feeds the nearby groundwater table by means of the so-called 'Rhine filtrates'. Using a few groundwater stream examples, this paper presents the characterization and localisation of these infiltrations. The Rhine filtrates are characterized by a high level of chloride and a low level of nitrate, these compounds being hydrological tracers, specific for the Alsatian floodplain (chloride resulting from contamination by the potash mines in the south of Alsace). They are also defined by high levels of phosphate and mercury (very localized injection). Phosphate is responsible for eutrophication which is observable in the appearance of specific aquatic macrophyte communities. Groundwater contamination by mercury is reflected by its accumulation in the bryophyte Fontinalis antipyretica sampled in groundwater streams. Thus aquatic vegetation, and more particularly the distribution of macrophyte communities, is used as an ecological descriptor of the exchange between the Rhine and its groundwater. The maximum injection of Rhine filtrates occurs between two areas of stillwater (hydroelectric dams), where the Rhine dominates its plain and where the substrate is constituted of coarse gravels.

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