Facteurs de transfert vers les eaux de surface de quelques phytosanitaires dans le contexte agricole breton

In order to put into context the relative importance of the various factors that determine the level of transfer of agro-chemical products from treated areas to surface water, two complementary approaches were implemented for the purposes of this study. The transfers were studied in two small sub-basins within the Coët-Dan Basin near Naizin, in order to examine other parameters introduced by the situation of plots of land compared to the stream. The observations were carried out over a dry period, during the use of atrazine in 1997, and during a very damp period during application in 1998. Similar measurements were implemented at the outlet from the basin as a whole over six full years in order to study the impact of rainfall and the hydrology of the basin on the concentration and volume of chemicals transferred. In both cases, field surveys carried out among farmers indicated the nature and quantity of active matter used, with the application dates. An analysis of the results obtained shows that, near the places in which the products were applied, high water levels in rivers and streams generate short-lived but intense concentrations in the streams. The loss of active matter during these occurrences depends greatly on the proximity of the stream to the plots of land on which the chemicals were used and on the level of rainfall, which determines hydrological levels. In conditions of high rainfall, atrazine concentrations amounting to several hundred microgrammes per litre can be observed. During periods when water levels are high, the concentration of agro-chemicals correlates very strongly with variations in the rates of flow of the streams and the time elapsed since chemical use. This determines the level of active matter still resident in the soil. Losses appear to be mainly associated with rises in water level but a significant quantity is nevertheless transferred other than during these periods. As far as atrazine in a small sub-basin is concerned, the fraction transferred represents 30% of the total exported, of which 40% consists of degradation products. For the Naizin Basin as a whole, annual atrazine runoffs observed between 1991 and 1997 represent from 0.1% to 0.6% of the quantities applied. They continue to remain below 1% and appear to be closely linked to rainfall, which determines the hydrological conditions in the basin. Pour situer l'importance relative des divers facteurs qui déterminent l'intensité des transferts des produits phytosanitaires des zones traitées vers les eaux des surfaces, deux approches complémentaires sont mises en oeuvre dans ce travail. Les transferts sont étudiés sur deux petits sous-bassins différenciés choisis dans le bassin du Coët-Dan près de Naizin pour examiner d'autres paramètres d'influence de la situation des parcelles par rapport au ruisseau. Ces observations sont effectuées sur une période sèche lors des applications d'atrazine en 1997 et très humide pour celles de l'année 1998. Des détermination analogues sont réalisées à  l'exutoire de l'ensemble du bassin sur six années complètes pour étudier l'impact de la pluviométrie et de l'hydrologie du bassin sur les concentrations et les masses exportées. Dans les deux cas, des enquêtes de terrains effectuées auprès des agriculteurs permettent de connaà®tre la nature et la quantité des matières actives utilisée ainsi que les dates d'application. L'analyse des résultats acquis montre qu'à  proximité des lieues d'application, les crues génèrent dans les ruisseaux des pointes de concentration brèves mais intenses. Les pertes en matières actives lors de ces événements dépendent fortement de la proximité des parcelles traitées au ruisseau et de l'intensité des précipitations qui détermine leur régime hydrologique. Dans des conditions pluviométriques défavorables, des pointes de concentrations en atrazine de quelques centaines de µg/L peuvent être observées.