Ditches influence on hillslope flow. Consequences on pesticides transfer
Influence d`un fossé sur les écoulements au sein d`un versant. Conséquences pour le transfert de phytosanitaires
Résumé
Man-made objects of landscape as ditches or vegetative strips, which can be found on rural watersheds, constitute hydrologic discontinuities, which may influence flows and pollutant transfer between the fields and the hydrographic network. Particularly, as pesticides may be degraded or be adsorbed during their transfer, both the way they follow and the transfer duration greatly influence their fate. In this context, according to their characteristics, ditches may behave as by-pass for the transport of pesticides, impeding some of the dissipation mechanisms as sorption by soil particles or retention of surface runoff by vegetative strips ; anyway, they may also contribute to pesticide dissipation to a certain extent, by infiltration in their bed or pollutant sorption on their substratum. This article presents a field experiment in order to assess how much pesticide flux a small ditch bordering a field can collect. This work is completed by the study of pesticide retention in such a ditch.Two ditches were equipped on the Arvalis site of "La Jaillière "(western France) : one is perpendicular to the slope, in order to maximise interception of flow coming from upslope, the other is a drainage collector, situated in a little thalweg. For both, equipment consists in piezometry, tensiometry, and discharge measurement. The hillslope where is located the first ditch is also equipped following two transects : the ditch does not influence the first one, whereas the second one is passing through this discontinuity. Moreover, water quality in the groundwater (pesticide concentrations and geochemical environment) is analysed every week or two weeks. Theses analyses aim at validating the hypothesis of ditch and hillslope transfer behaviour that the hydrological survey is leading to. The two first years of hydrological survey confirmed that two groundwater tables are present on the site : the deeper one, situated in the schist, under a layer of less permeable altered schist is present during all the year, while the other one developed itself on this less permeable layer, when water balance is positive. The obtained data and the three-dimensional saturated modelisation that was made (with Visual Modflow 2000) allowed to characterise and to quantify exchanges between the two water tables, considering the position on the hillslope and the period of the year. These results have been put in parallel with the pesticide concentrations measured in the groundwater for different positions on the slope. At the time being, as concerns water quality, only one year was monitored, and concentrations are quite low, even if they confirm the presence of pesticide deep in the regional aquifer. This result must be counterbalanced by the fact that the last pesticide applications on the studied hillslope occurred more than two years ago. Another pesticide application is scheduled at the beginning of 2003, and will be accompanied by a bromure application, in order to improve the comprehension of water flow and pollutants transfer.