Pergamon 0045-6535(94)00412-9 Chemosphere, Vol. 30, No. 3, pp. 485-500, 1995 Elsevier Science Ltd Printed in Great Britain 0045-6535/95 $9.50+0.00 PREDICTION OF SURFACE WATER INPUT OF CHLORIDAZON AND CHLORPYRIFOS FROM AN AGRICULTURAL WATERSHED IN CHILE Ricardo Barra 1, Marco Vighi 2* and Antonio Di Guardo 2 (1) EULA-Chile Environmental Studies Center. University of Concepci6n P.O Box 156-C Concepci6n, Chile. (2) Group of Ecotoxicology-lnstitute of Agricultural Entomology, University of Milan Via Celoria 2. 20133 Milano, Italy. (Received in Germany 3 June 1994; accepted 1 December 1994) ABSTRACT Two compartmental fugacity-based models (Agrifug and SoilFug) for the prediction of pesticide runoff from agricultural fields were applied and experimentally validated in a river watershed, of about 106 km 2, in central Chile. Chloridazon and chlorpyrifos, two of the most widely used pesticides in the study area, ware selected for their different physico-chemical properties, in order to test a range of environmental fate patterns by means of these models. Theoretical values were compared with analytical determinations in river water and soil and a good predictive capability was found for both models, at least at the order of magnitude level. Value and limitations of the application of this approach at the basin scale and its usefulness for the management of pesticides are discussed. INTRODUCTION The development of new strategies for controlling water pollution from diffuse sources is a key point in natural resources management. This is particularly true in developing countries where water pollution from pesticides is of growing concern (Cepal,1988), even in the frame of a sustainable development stategy. In the last decade several simple multicompartmental evaluative models were proposed as a tool to predict the environmental distribution and fate of organic chemicals. Among them the fugacity approach (Mackay, 1979; Mackay and Paterson, 1981, 1982) has proved to be very effective due to its versatility (Mackay, 1991). In particular, on the basis of the original fugacity approach, some models specifically designed for the prediction of pesticide runoff and transport to surface waters (Agrifug, SoilFug) have been recently proposed (Di Guardo et al., 1993, 1994a). These models have been experimentally validated and have proved capable of producing reliable results in experimental fields up to some hundreds of hectars (Di Guardo et al., 1994a, b). Notwithstanding these promising results, there is still a lack of information about the validity on a larger scale (basin or regional scale) of models derived from the fugacity approach. 485