THE EXCHANGE PROCESS BETWEEN RIVER AND GROUNDWATER ON THE CENTRAL ALSACE FLOODPLAIN (EASTERN FRANCE) .2. THE CASE OF A RIVER WITH FUNCTIONAL FLOODPLAIN

The floodplain of the river Ill in the Alsace Rhine valley is used as a model to study river-groundwater exchange process. Groundwater-fed s treams located in the Ill floodplain are analysed using three methods: an analytical method based on hydrochemical variables (Cl-, NO3-, PO4 3- and NH4+), a phytosociological one based on surveys of aquatic macr ophyte communities and a biological method based on the accumulation o f mercury in the moss Fontinalis antipyretica. The results show that t he eutrophicated and polluted river Ill (660 mug l-1 N-NH4+, 500 mug l -1 P-PO43-, 0.3-0.4 mg Hg kg-1 dry weight of moss) has a negative effe ct 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 con taminate the groundwater, but this is not the case further downstream. Along a stretch of the Ill (40 km) annual floods provide eutrophicate d and polluted waters to the aquifer. However these waters are purifie d during their transfer through the soil-vegetation system. Thus in th e groundwater-fed streams the water is characterised by a low level of phosphate, ammonium nitrogen and mercury(10-20 mug l-1 N-NH4+ and P-P O43-, < 0.05 mg Hg kg-1 dry weight of moss). We demonstrate the import ance of a functional floodplain in replenishing the aquifer with poor- nutrient waters. The aquatic vegetation of groundwater-fed streams ref lects the water quality and thus can be used as a bioindicator and des criptor of river-groundwater exchange process.