Variability of pesticides and nitrates concentrations along a river transect: chemical and isotopic evidence of groundwater – surface water interconnections
Résumé
Groundwater quality is increasingly monitored in Europe where various levels of nitrate and pesticide and/or
metabolite contamination have been demonstrated (Loos et al., 2010, Stuart et al., 2012). The Groundwater
Daughter Directive (2006/118/EC) to Water Framework Directive (WFD) particularly requires measures to
prevent or limit inputs of pollutants into groundwater and compliance with good chemical status criteria (based
on EU standards of nitrate and pesticides). The WFD mentioned the need to protect groundwater but also to
have a particular regard to its impact and interrelationship with associated surface waters and directly dependent
terrestrial Ecosystems.
The Ariège river basin (SW France – 538 km
2
) is an alluvial plain under high agricultural pressure leading
to a contamination of the aquifer by several pesticides and metabolites (Amalric et al., 2013). The Crieu is an
allochtone river, crossing the plain (
10 km length) before joining the Ariège River. The Crieu is often dry in its
middle section suggesting water leakage from surface water towards groundwater. At the opposite, the permanent
flow observed downstream suggests an input of groundwater into surface water. In May 2014, while the Crieu
flow was continuous through the plain, 7 river samples were collected and analyzed for pesticides, major ions,
strontium concentration and isotopes. In situ measurements of electric conductivity were also performed as well
as flow gauging. Two groundwaters close to the river were also sampled.
The flow gauging measurements show a decreasing river discharge in the central area of the Crieu River,
suggesting surface water leakage towards groundwater. Nevertheless, the electric conductivity increases along the
river flow as well as some pesticides and nitrates concentrations. This chemical evolution of the river water is
thus inconsistent with a simple water infiltration and another source of dissolved solutes is required to explain the
increased of concentration. Finally, downstream the quantified pesticides were different from those observed in
the upper part of the Crieu but similar to those observed in groundwater.
Sr isotopes together with major elements and Sr concentrations allow to identify 3 distinct end-members to explain
the river quality evolution : 1) surface water, 2) groundwater and 3) sub-surface water. On this basis, we first
demonstrate that the contribution of the different end-members to the river flow is highly variable from upstream
to downstream. Secondly, we evidence water exchanges between the river and the groundwater compartment and
vice-versa.
The combination of the isotopic and geochemical approaches was essential to understand the complex relations
and exchanges between surface and ground-waters occurring in few kilometers along the Crieu River. This under-
standing allows the comprehension of spatial variability of surface water quality. This is of primary importance
when to help water managers to select relevant sampling points to be monitored in the framework of the WFD.
Amalric L., et al. (2013). International Journal of Environmental Analytical Chemistry, 93: 1660-1675
Loos R. et al. (2010). Water Research, 44: 4115-4126
Stuart M. et al. (2012). Science of the Total Environment, 416: 1-21