Transferts rivière-nappe et effet filtre des berges. Application aux transferts de l'azote

In this article, the authors present results of a study designed to investigate hydrodynamical and geochemical environment of bank filtration. The experimental site is a well field located along the Seine river, downstream of Paris (France). At this place, hydrodynamical and geochemical features have been considerably altered from natural background by man action (intensive pumping in the aquifer, chronical pollution of the Seine river by sewage of treatment plant or by agriculture). Nitrogen species behavior during bankfiltration was more particularly examined in the experimental site. Water issued from bank filtration shows drastic changes when compared to river water, denoting a reductive trend (drastic decrease of oxygen, nitrate, sulfate concentrations and increase in ammonium, iron... concentrations). This geochemical evolution of the water in the aquifer is the reflect of intensive bacterial mineralization which happens in the first few meters of filtration through river bed sediments. As a consequence for nitrogen species, denitrification is very effective and no nitrate is found below a few centimeters depth in the sediments. On the contrary, ammonium concentration drastically increases at the same time. Away from the river bed sediments, the aquifer mainly acts as a buffer toward physico-chemical variations owing to processes like dilution, adsorption or precipitation/dissolution. The problem of ammonium transfer to the well is adressed and key parameters, as percolation velocity and organic carbon content of sediments, are discussed in conjunction with drinking water production using bank filtration.