Estimating nitrate and pesticide transfer mode within the unsaturated zone of a fluvioglacial aquifer and its implication on spatial and temporal concentration variability

Laurence Gourcy (1) , Nicolas Surdyk (1) , Laurent Cadilhac (2) , Nicole Baran (1) , Marie Rousseau (1) , Jean-Jacques Seguin (1)

1 BRGM - Bureau de Recherches Géologiques et Minières (BRGM)
2 Agence de l'eau Rhône Méditérranée Corse

The Meyzieu fluvioglacial aquifer of 113 km 2 located at the eastern part of Lyon is intensively exploited and subject to multiple pressures (urban, industrial, agriculture). Nitrate concentrations are quite high while pesticides contamination is low considering the past and today's pressure. A project was initiated in 2011 with the objective of determining the role of the unsaturated zone in the transfer of NO 3 and pesticides from soil to groundwater. Monthly monitoring of major elements, some pesticides, and stable water isotopes is carried out at three sampling points located along the flow lines. Additional information such as hydrogeological functioning, pressure inventory was gathered and additional data from three water sampling campaigns was collected. The temporal variation of NO 3 , metolachlor and metabolites and atrazine and metabolites is quite different at the three monitoring points. At the sampling point located upstream, where the unsaturated zone has a thickness around 40m, the nitrate concentrations are comprised between 20 and 40 mg.l-1 from 2004 to mid-2013. From mid-2013 nitrate concentrations are increasing regularly and are now reaching 70 mg.l-1. This point presents the highest level in atrazine metabolites (DEA) of the area but with concentrations lower than 0.05 µg.l-1 and very low detection of metolachlor or its metabolites. In the centre of the basin, where agriculture pressure is the highest and unsaturated zone of up to 30m in depth, nitrate concentrations are above 50 mg.l-1 with low seasonal variations. Metolachlor is lower than 0.03 µg.l-1 but values of 0,39 µg.l-1 max. of ESA metolachlor (MESA) were reached. Downstream, just before the discharge area, the sampling point shows the lowest and more stable nitrate concentrations (around 40 mg.l-1). The water level in this sector is at less than 20 m deep. Metolachlor have been detected only once while MESA is nearly always detected and can reach 0.24 µg.l-1 .

Mots clés: eau souterraine; transfert nitrate; zone non saturée

Domaines: Recherche d'information [cs.IR]; Milieux et Changements globaux

Laurence Gourcy, Nicolas Surdyk, Laurent Cadilhac, Nicole Baran, Marie Rousseau, et al.. Estimating nitrate and pesticide transfer mode within the unsaturated zone of a fluvioglacial aquifer and its implication on spatial and temporal concentration variability. LuWQ 2017, May 2017, La Haye, Netherlands. ⟨hal-01443995⟩