Effect of runoff on pesticide concentrations in runoff water and in FOCUS streams: A model study with PRZM and TOXSWA

Runoff of pesticides from agricultural fields into adjacent surface waters may form a threat to their aquatic ecosystems. Therefore, the runoff entry route into surface waters is evaluated in the registration procedure at EU level. In this study simulations were performed for two of the four EU-FOCUS runoff scenarios. In the case considered runoff was caused by the first rainfall event after pesticide application. Running the PRZM and TOXSWA models we studied first, the effect of rainfall on the runoff volume and next, the effect of runoff volume on the concentration in runoff water and in the FOCUS stream for a range of pesticide sorption coefficients. We found that the concentration in the runoff water decreases exponentially with increasing runoff size for compounds with sorption coefficients up to 100 L/kg, while it hardly decreases for compounds with sorption coefficients of 1000 or 10000 L/kg. For compounds with sorption coefficients up to 100 L/kg the concentration in the FOCUS stream increases up to runoff size of 0.3 to 1.0 mm (originating from the treated 20 ha of the 100 ha upstream catchment feeding the FOCUS stream) and next, it decreases with increasing runoff size. For compounds with sorption coefficients of 1000 or 10000 L/kg maximum concentrations in the stream do not vary significantly as a function of runoff size, as the pesticide concentrations in the runoff water are nearly constant for these compounds.