EXCHANGES AT THE SEDIMENT-WATER INTERFACE IN THE RIVER SEINE, DOWNSTREAM FROM PARIS

The exchanges at the sediment-water interface were studied in the rive r Seine, downstream from Paris. Various species of nitrogen were consi dered. A budget of exchanges could be established between the sewage t reatment plant at Acheres and Poses, 140 km downstream. For each of th e three sites studied the sediments were characterized by their granul ometry and their particulate organic carbon and particulate nitrogen c ontents. To quantify the exchanges between the sediments and the water two techniques were used: study of cores interstitial water and of di alysis cells water allow to establish vertical profiles for nitrogen a nd phosphorus, essential to calculate molecular diffusion; global exch anges at the interface were determined with a benthic bell-jar, whose water was regularly sampled until anoxy is reached. That study allowed the clarification of the exchange kinetics at the sediment-water inte rface with regards to the sludge deposited in the river from the sewag e treatment plant. Denitrification played a minor role while ammonific ation of organic nitrogen seemed to be partially responsible for the a mmoniacal nitrogen concentrations in the interstitial water. A notable release of phosphorus was also observed. The concentration gradients measured in the dialysis cells experiments were used to calculate the exchanges exclusively due to molecular diffusion, while the measuremen ts obtained with benthic bell-jars took into account all the chemical, biological and physical phenomena. Thus the discrepancies found betwe en the values of molecular diffusion and the exchange values obtained with benthic bell-jars were very marked. These last values allowed the establishment of the mean release budgets in the studied area: ammoni acal nitrogen, 13.4 t d(-1); phosphorus, 3.7 t d(-1).