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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The floodplain of the river Ill in the Alsace Rhine valley is used as a model to study river-groundwater exchange process. Groundwater-fed streams located in the Ill floodplain are analysed using three methods: an analytical method based on hydrochemical variables (Cl-' NO,, PO: ~ and NH2 ), a phytosociological one based on surveys of aquatic macrophyte communities and a biological method based on the accumulation of mercury in the moss Fontinalis antipyretica. The results show that the eutrophicated and polluted river Ill (660 pg I-' N-NH2 , 500 pg l-' P-PO; -,0.3-0.4 mg Hg kg-' dry weight of moss) has a negative effect on the groundwater via the bed, depending on the level of the river bed in relation to the groundwater table level. Upstream of Colmar in the south of the Alsace floodplain, the Ill waters infiltrate and contaminate the groundwater, but this is not the case further downstream. Along a stretch of the Ill (40 km) annual floods provide eutrophicated and polluted waters to the aquifer. However these waters are purified during their transfer through the soil-vegetation system. Thus in the groundwater-fed streams the water is characterised by a low level of phosphate, ammonium nitrogen and mercury (lo-20 pg 1~ ' N-NH,+ and P-PO: -, < 0.05 mg Hg kg-r dry weight of moss). We demonstrate the importance of a functional floodplain in replenishing the aquifer with poor-nutrient waters. The aquatic vegetation of groundwater-fed streams reflects the water quality and thus can be used as a bioindicator and descriptor of river-groundwater exchange process.

Les rivières phréatiques situées à proximité du Rhin canalisé révèlent des teneurs irnportantes en Hg, ce qui démontre l'existence d'infiltrations directes des eaux du Rhin (<>, comme celle par le mercure (forte pollution dans la décennie 1970). L'aval des barrages au niveau des canaux de fuite constitue par contre et bien évidemment un site privilégié de drainage de la nappe.Dans le cas de la rivière III, tout le réseau phréatique du lit majeur ordinaire fonctionnel (champ d'inondation) est très peu contaminé par le mercure, bien que l'III le soit fortement. L'Ill, à l'inverse du Rhin draine ordinairement la nappe phréatique, du moins dans Mousses aquatiques, bioindicateurs du niveau de pollution chimique 243 le secteur d'étude à la hauteur de l'axe Rhinau -Benfeld. En période de crueinondante elle assure la recharge de la nappe en eau d'excellente qualité grâce au système d'é-puration efficace << sol-végétation >> que constitue le champ d'inondation : la rétention du mercure provenant des eaux d'inondation est très forte dans les sols riches en colloïdes.Abstract. -Mercury (Hg) levels in 4 species of aquatic rnosses (Bryophytes) from the hydrographical system of the Alsace Rhine plaine were analysed ( figure l a , b, c)