Short water transit times determine the fate of veterinary pharmaceuticals in lowland catchments

Livestock animals are commonly treated with veterinary pharmaceuticals (VPs), and their residues often enter the environment through manure applied to soil. A fraction of these residues may be further transported to surface waters through intricate transport mechanisms. Here, we examine the temporal dynamics of VPs in lowland surface waters of an agricultural catchment in the Netherlands, utilizing information on VPs concentrations in manure (2015-2020) and surface water measurements collected in 2020. We first calculate the amounts of two VPs (flubendazole and sulfadiazine) applied in the catchment of interest and analyze the local manure application practices to establish the temporal distribution of VP application. We then develop a parsimonious catchment-scale transport model for VPs that is based on time-variable water transit time distributions. The transport model considers multiple processes experienced by the VPs during their transfer to the stream network, including evapoconcentration, sorption and degradation. Despite the uncertainties associated with VP transport processes, the model accurately captures the order of magnitude of VP concentration in stream water for two VPs characterized by contrasting sorption and degradation rates. To interpret our simulations, we investigate water and solute transit times and their variability due to changing hydrological conditions. Our results suggest that, despite the mean water transit times of some years typical of lowland catchments, as well as relatively strong VP sorption or rapid degradation, detectable amounts of VPs in the order of 1-10 ng/L may reach the stream ecosystem through fast flowpaths characterized by short transit times.