Interannual climate variability and spatially heterogeneous improvement of agricultural management impede detection of a decreasing trend in nitrate pollution in an agricultural catchment

International audienceDespite widespread implementation of the nitrate directive in the European Union since the 1990s, the impact onnitrate concentration in rivers is limited (Bouraoui and Grizzetti, 2011). To assess whether this lack of response isdue to the long time lags of nitrate transfer or to inadequate programs of measure, long term river and groundwatermonitoring data are necessary.This study analyses 15 years of daily nitrate concentration data at the outlet of an intensively farmed catchmentin Western France (Kervidy-Naizin, 5 km2) and quarterly nitrate concentration data in the groundwater of twohillslopes equipped with piezometers (Kerroland and Gueriniec) within the same catchment. In this catchmentgroundwater contribution to annual stream flow is dominant. The objectives of this study were to i) disentanglethe influence of interannual climate variability and improvement of agricultural practices (i.e. reduction in Nsurplus) in the stream chemistry and ii) discuss the reasons for slow catchment recovery from nitrate pollution bycomparing trends in groundwater and stream concentrations.Analysis of stream data showed that flow-weighted mean annual concentration at the outlet of the Kervidy-Naizincatchment has decreased by 1.2 mg NO3- l-1 yr-1 from 1999 to 2015. This decrease was slow but significant (pvalue 100 kg N ha-1yr-1).We conclude that, despite the lags due to pluri annual nitrate transfer through the unsaturated and satured zones incatchments of Western France, significant decrease in nitrate concentration in groundwater and streams should bevisible within less than 10 years after implementation of an efficient program of measures. Spatial heterogeneityin the implementation of programs of measures (i.e. reduction of N surplus) is a likely cause of slow, sometimesundetectable, reduction in nitrate concentration.Bouraoui, F., and Grizzetti, B.: Long term change of nutrient concentrations of rivers discharging in Europeanseas, The Science of the total environment, 409, 4899-4916, 10.1016/j.scitotenv.2011.08.015, 2011