Evaluation du niveau de contamination des eaux de surface par les produits phytosanitaires

Assessment of the contamination degree of surface waters by pesticides at a regional scale

¹ Service Régional de Protection des Végétaux de Bretagne B.P. 29, 35650 Le Rheu
² Laboratoire d'Etude et de Recherche en Environnement et Santé. Ecole Nationale de la Santé Publique, av. du P r-Léon-Bernard, 35043 Rennes Cedex

Résumé

Avec l'intensification rapide de l'agriculture bretonne, l'usage des produits phytosanitaires s'est considérablement développé durant ces dix dernières années. La première partie de ce travail donne une évaluation qualitative et quantitative des matières actives utilisées, et l'évolution de la contamination par les pesticides de cinq rivières bretonnes. Après une discussion des conséquences sur les usages de l'eau, il examine à l'échelle d'un sous bassin versant, les facteurs géoclimatiques et les modalités d'occupation des sols qui influencent le niveau de contamination des eaux aux divers exutoires. La distance des lieux d'épandage au chevelu du réseau hydrographique apparaît comme un facteur prépondérant de la contamination des eaux. Un suivi des flux d'atrazine au cours d'une crue montre l'impact des fortes précipitations sur le transfert sol-eau des phytosanitaires.

Abstract

The use of pesticides in Brittany over the last ten years has considerably increased due to the rapid intensification of agriculture. To gather information on the quantity and types of pesticides used an enquiry was carried out. Since the results of the enquiry showed that the quantity of pesticides used was high, a study on five Breton rivers was carried out in order to investigate the possible effects of this level of pesticide use on water quality. The results show that the contamination is above all due to the use of herbicides for treating maize and cereal crops and more rarely due to the use of insecticides for soil treatment. Atrazine and simazine were present as were other major river pollutants. Atrazine was detected in 96% of samples, the maximum concentration observed being 14 µg. l^-1. Substituted ureas have also been frequently observed, but generally at lower concentrations than those associated with triazines. This level of contamination, which may be attributed only to a minor degree (0.1 - 0.2%) to the presence of applied agricultural pesticide residues in superficial water, would seem to have negative consequences for water use, and in particular for domestic water quality. Such contamination has effects on the aquatic environment in general and we have also observed harmful effects on tomato plants when contaminated water was used to irrigate tomato crops. Further studies were carried out within the Cantache subbasin to investigate to what extent and how certain geoclimatic factors and country planning regulations could influence the level of river water contamination. Monitoring atrazine flow while the Cantache was in spate has shown how heavy rainfall may affect soil-water transfer of pesticides. In such conditions concentrations increase extremely rapidly, values of 30 µg. l^-1 being reached after approxiamately two hours. The fall in atrazine levels is much slower and a day was needed to regain initial levels. In general a comparative examination of the results within the different subbasins would seem to show that there is a relationship between the degree of proximity of the area of pesticide application to the surface water network and the quantity of pesticides transfered into the river water.