Relation concentration-débit et évolution temporelle du nitrate dans 25 rivières de la région Bretagne (France)

The study of the twenty-five largest rivers of Brittany showed that nitrate concentrations are positively correlated with river flow, in 90 % of the cases. The hydrogeological characteristics of the watersheds had an influence on these relationships. In this respect, river basins characterized by prevalent surficial water runoff exhibited higher correlation coefficients and lower lag times between river flow sequences and nitrate concentrations. The multi-year trend in riverine nitrate concentrations indicated a fourfold increase over 30 years. Nevertheless, 80% of rivers exhibited a recent decrease in nitrate concentrations, independent of inter-annual river flow fluctuations. The highest annual rates of nitrate decrease corresponded to river basins for which rapid surficial runoff, a sign of lower hydrogeological "inertia", was prevalent. This result may be due to a decrease in agricultural pressure (i.e., a decrease of organic manure and mineral fertilizer use since the 1990's) in watersheds with a fairly short reaction time (between 2 and 10 years). Nevertheless this decrease in nitrate concentrations must continue in order to reach concentrations low enough to limit macroalgal eutrophication in coastal zones and to attain levels that are safe for human consumption as drinking water.