Approche des facteurs de contamination des eaux souterraines par l'atrazine et la simazine. Cas des aquifères de la Manche

Approach of contaminating factors of groundwaters by atrazine and simazine. Study of aquifers of the "Département de la Manche"

¹ Département Environnement et Santé, E.N.S.P., 35043 Rennes Cedex
² D.D.A.S.S. de la Manche, B.P. 399, 50008 Saint-Lô Cedex
³ D.D.A.F. de la Manche, Cité administrative, 50009 Saint-Lô Cedex

Résumé

La recherche de triazines a été effectuée dans l'eau de 149 captages du département de la Manche. Seules l'atrazine et la simazine ont été retrouvées et sur un faible nombre de points d'eau uniquement. Une étude statistique montre que les ouvrages dont les teneurs en atrazine ou en simazine sont supérieures au seuil de détection sont situés dans les terrains sédimentaires carbonatés. Les captages implantés dans les formations du socle semblent bénéficier d'une protection pouvant s'expliquer par la fixation des triazines par les argiles et par leur dégradation compte tenu de la faible perméabilité des terrains mais aussi pour partie en raison d'une moindre intensification agricole dans ces secteurs.

La recherche des métabolites de ces molécules s'avère cependant indispensable pour appréhender correctement les modalités de transfert de celles-ci.

Abstract

Waters from 149 wells of the "departement de la Manche" were analyzed for eleven triazines. Atrazine and simazine were only found. Atrazine was traced above minimum detection limit in 34 wells and simazine only in 11 wells. Wells contaminated by simazine are also contaminated by atrazine.

A statistic study shows that high levels of atrazine or simazine are observed in alkaline waters entrained in permeable carbonated aquifers. Acidic waters of bedrock aquifers are on the whole uncontaminated by atrazine or simazine. Ionisation of triazines in acidic conditions and their according removal by ion exchange on clays in the weathered top of the rocks and degradation of these pesticides account for this result. Less intensified crops on the watershed can also explain why these waters are pollution free.

The contaminated wells were sampled during the years 1991-1993. The concentrations of atrazine and simazine in three shallow wells, contaminated by side non agricultural pesticide use, widely throughout the year. The highest concentrations are observed at the end of spring and of summer after any rainy periods. In deeper wells in agricultural and urban surroundings concentrations vary slightly. The control of water quality from a well located in a granitic watershed did not show any atrazine or simazine contamination in spite of an intensive corn production. The sudden fall in atrazine concentrations in the shallow wells coincides with an increase in deethylatrazine concentrations. Therefore the analysis of metabolites in water is necessary to provide a proper set of data on pesticide transfert through soil and aquifers. The occurence of hydroxyatrazine in acidic waters can be expected but this metabolite is not easily, therefore seldom, analyzed.