THE EXCHANGE PROCESS BETWEEN RIVER AND GROUNDWATER ON THE CENTRAL ALSACE FLOODPLAIN (EASTERN FRANCE) .1. THE CASE OF THE CANALISED RIVER RHINE

The recent canalisation (in the nineteen sixties) of the upper Rhine h as modified the exchange processes between the river and its groundwat er in the floodplain of Alsace. The Rhine seeps through its gravelly b ed 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 inf iltrations. The Rhine filtrates are characterized by a high level of c hloride and a low level of nitrate, these compounds being hydrological tracers, specific for the Alsatian floodplain (chloride resulting fro m contamination by the potash mines in the south of Alsace). They are also defined by high levels of phosphate and mercury (very localized i njection). Phosphate is responsible for eutrophication which is observ able in the appearance of specific aquatic macrophyte communities. Gro undwater 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 mac rophyte communities, is used as an ecological descriptor of the exchan ge between the Rhine and its groundwater. The maximum injection of Rhi ne filtrates occurs between two areas of stillwater (hydroelectric dam s), where the Rhine dominates its plain and where the substrate is con stituted of coarse gravels.