Transfer patterns of pharmaceuticals used in agriculture into streams under Mediterranean climate at the catchment-scale

Livestock-breeding relies on a large array of pharmaceuticals. Many of them may pose a risk to aquatic life if they reach surface water bodies.  Depending on their physicochemical properties, some pharmaceuticals present strong sorption coefficients and are thus not expected to reach surface water bodies under most conditions. Mediterranean climate is characterized by a dry summer followed by intense storm events. We studied the effect of this climatic condition on the risk of transfer of pharmaceutical residues to streams at the catchment-scale. The study area is the 42km² Claduègne catchment in the French Ardèche department. It is characterized by extensive agricultural land-use under Mediterranean climate.

Surveys with local livestock farmers were conducted in order to identify the commercial pharmaceutical products and the active ingredients systematically used in the study area as well as their application rate, frequencies and seasonal patterns. Stream water was analyzed on high frequency (up to 3h^-1) during flood events and compared to some samples outside of flood events. A total of 32 liquid water samples were collected and analyzed for 3 veterinary pharmaceuticals systematically used in the study area as well as 14 molecules of various use. They were concentrated via solid phase extraction and analyzed using high performance liquid chromatography (HPLC) coupled to a tandem mass spectrometer. The concentration values where below the limits of detection (0.1 - 1 ng L^-1) most of the time, but peaked at high concentrations for short periods during flood events. The concentration reached up to 355 times the Predicted No Effect Concentration (PNEC) for Fenbendazole FBZ, the antiparasitic used in pork in the region. This indicates that rapid transfer processes during flood events represent an elevated risk of transfer of these molecules toward streams. Parallel transit time modelling revealed high event water fractions during flood events in the studied catchment.

We conclude that under these climatic conditions, special care should be taken after treatment application to avoid pastures that are hydrologically connected to surface water bodies. In addition, the results suggest that low-frequency monitoring is not sufficient to detect these high concentration levels that exist during very short durations of a few hours or less.