Spatial distribution of triazine residues in a shallow alluvial aquifer linked to groundwater residence time
At present, some triazine herbicides occurrence in European groundwater, 13 years after their use ban in the European Union, remains of great concern and raises the question of their persistence in groundwater systems due to several factors such as storage and remobilization from soil and unsaturated zone, limited or absence of degradation, sorption in saturated zones, or to continuing illegal applications. In order to address this problem and to determine triazine distribution in the saturated zone, their occurrence is investigated in the light of the aquifer hydrodynamic on the basis of a geochemical approach using groundwater dating tracers (3H/3He). In this study, atrazine, simazine, terbuthylazine, deethylatrazine, deisopropylatrazine, and deethylterbuthylazine are measured in 66 samples collected between 2011 and 2013 from 21 sampling points, on the Vistrenque shallow alluvial aquifer (southern France), covered by a major agricultural land use. The frequencies of quantification range from 100 to 56 % for simazine and atrazine, respectively (LQ = 1 ng L−1). Total triazine concentrations vary between 15 and 350 ng L−1 and show three different patterns with depth below the water table: (1) low concentrations independent of depth but related to water origin, (2) an increase in concentrations with depth in the aquifer related to groundwater residence time and triazine use prior to their ban, and (3) relatively high concentrations at low depths in the saturated zone more likely related to a slow desorption of these compounds from the soil and unsaturated zone. The triazine attenuation rate varies between 0.3 for waters influenced by surface water infiltration and 4.8 for water showing longer residence times in the aquifer, suggesting an increase in these rates with water residence time in the saturated zone. Increasing triazine concentrations with depth is consistent with a significant decrease in the use of these pesticides for the last 10 years on this area and highlights the efficiency of their ban.
KeywordsTriazine residues Persistent organic pollutants Shallow alluvial aquifer Groundwater residence time Apparent 3H/3He age Groundwater origin Well depth Degradation rate
The PhD grant of L. Sassine was supported by the General Council of the Gard Department, the Urban Agglomeration of Nîmes Metropole, the University of Nîmes, and by the Joint Venture of the Vistrenque and the Costières Groundwaters. This work has been partially financed through the project funded by ONEMA, in the frame of the collaborative international consortium WATERJPI2013—PERSIST of the Water Challenges for a Changing World Joint Programming Initiative (Water JPI) Pilot Call.
The landowners and the stakeholders are gratefully thanked for giving us the access to the boreholes. The authors also wish to thank Marine Brogat and Amélie Sellier for their technical help in LC/MSMS analyses.
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