Nitrate removal by denitrification in alluvial ground water: Role of a former channel

Abstract Chemical variations of ground water in an ancient meander of the Garonne river (southern France) were compared with that of ground water located under the intensively cultivated fields of the surrounding alluvial plain. Nitrate, Mn[II] and Fe [II] concentrations were found to be very different in the two situations. The denitrification process in the former channel was identified through the natural 15 N isotope enrichment on residual nitrate induced by isotope fractionation and by the in situ measurement of gaseous nitrogen losses using the acetylene method. A recent embankment leads to the progressive drying of the waterlogged soils thereby decreasing the capacity of this natural riparian wetland to regulate nitrate transfers from the agricultural plain to the river.