Exchanges at the sediment-water interface in the river Seine, downstream from Paris

The exchanges at the sediment-water interface were studied in the river Seine, downstream from Paris. Various species of nitrogen were considered. A budget of exchanges could be established between the sewage treatment plant at Achères and Poses, 140 km downstream. For each of the three sites studied the sediments were characterized by their granulometry and their particulate organic carbon and particulate nitrogen contents. To quantify the exchanges between the sediments and the water two techniques were used: study of cores interstitial water and of dialysis cells water allow to establish vertical profiles for nitrogen and phosphorus, essential to calculate molecular diffusion; global exchanges 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 interface with regards to the sludge deposited in the river from the sewage treatment plant. Denitrification played a minor role while ammonification of organic nitrogen seemed to be partially responsible for the ammoniacal 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 measurements obtained with benthic bell-jars took into account all the chemical, biological and physical phenomena. Thus the discrepancies found between 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: ammoniacal nitrogen, 13.4 t d−1; phosphorus, 3.7 t d−1.