ABSTRACT
Application of pesticides to crops can result in residues entering surface waters. After aquatic entry, the distribution and transformation processes within the aquatic system determine the ultimate extent of exposure for individual organisms. Prediction of exposure is an essential part of the risk assessment process and can be helpful for the design of mesocosm experiments. Exposure can be quantified by field measurements, or by using standard generic figures or simulation models. Models are increasingly used for exposure estimation since they have the ability to formalize and integrate knowledge of fate processes. However, the combination of a modelling approach with the use of measured data from experiments and field studies results in the most reliable exposure estimates. The various routes by which pesticides can enter surface waters, and fate processes within the aquatic system are briefly defined and an assessment is made of the state of the art with regard to predicting exposure. Spray drift, surface runoff and field drainage are short and medium term processes and groundwater pathways can result under special conditions in the long-term entry of pesticides. Because of the complexity of fate processes within aquatic systems, it is very difficult to give an accurate prediction of exposure; the main processes determining the aquatic fate are summarized and four models predicting the fate of pesticides within aquatic systems are briefly reviewed. Two approaches to exposure estimation in relation to risk assessment and the design of mesocosm studies are contrasted, namely ‘worst-case’ assessment and the ‘probabilistic approach.’ The probabilistic approach ultimately attempts to quantify the frequency and extent of exposure under a wide range of agricultural field conditions. Although difficult to apply, the approach is potentially more valuable than traditional ‘worst-case’ exposure assessment.
