Predicted Concentrations for Pesticides in Drainage Dominated Catchments Patrik Fauser & Marianne Thomsen & Peter B. Sørensen & Søren Petersen Received: 29 December 2006 / Accepted: 25 August 2007 / Published online: 14 September 2007 # Springer Science + Business Media B.V. 2007 Abstract Regulators need a reliable, precise and easy to use tool for predicting the occurrence of pesticides in drain water and catchments in agricultural areas. Occurrence depends on a wide range of substance and site specific factors and this study presents a simple model built on the results from simulations of a detailed model system that does not neglect or omit any of these factors. A drainage dominated sub- catchment (0.03 km 2 ) of the Lillebaek catchment (4.4 km 2 ) on Funen, Denmark, represented by the catchment model MIKE SHE is considered. Detailed analyses have been made with respect to geological and hydrodynamic conditions as well as measure- ments of pesticide concentrations in ground and surface waters. Maximum concentrations in drain water, the time for reaching this concentration and the time interval for exceeding the limit value have been derived empirically from MIKE SHE simula- tions using degradation rates and sorption coefficients values for 37 pesticides included in the Danish PATE database. The relatively hydrophilic bentazon and hydrophobic pendimethalin are used as model pesti- cides for illustration. A simple tool applicable for a wide range of pesticides has thus been designed based on detailed analyses of a limited number of pesticides. The user requirements are degradation rates, sorption coefficients, application rates and regulatory limit values for the pesticides of interest. Keywords Bentazon . Pendimethalin . Pesticides . Exposure model . Drainage catchments . MIKE SHE . PEC 1 Introduction Risk assessment of pesticides in soil, groundwater and surface waters is often performed in a tiered approach, where the initial step is to consider a simple conservative worst-case scenario with respect to pesticide input and distribution in the environment. More detailed transport, accumulation and degrada- tion processes are therefore not taken into account. If risk is shown, more refined steps, including more advanced modelling or experimental studies, are Water Air Soil Pollut (2008) 187:149–156 DOI 10.1007/s11270-007-9503-0 DO09503; No of Pages P. Fauser (*) : M. Thomsen Department of Policy Analysis, National Environmental Research Institute, Frederiksborgvej 399, Post Box 358, 4000 Roskilde, Denmark e-mail: paf@dmu.dk P. B. Sørensen Department of Terrestrial Ecology, National Environmental Research Institute, Vejlsøvej 25, Post box 314, 8600 Silkeborg, Denmark S. Petersen Water and Environment, Danish Hydraulic Institute, Agern Allé 5, 2970 Hørsholm, Denmark