Evaluation of predictive models for pesticide behaviour in South African soils

Abstract

The re-evaluation of pesticide use patterns is a high priority internationally. The process has led to a reduction in the numbers of pesticides allowed for use in many countries. This withdrawal of pesticides is aimed at consumer and environmental protection. Pesticide mobility and persistence is of major importance especially when considering the protection of water sources. In order to evaluate the suitability of a pesticide for use it is essential that its environmental behaviour is understood and predictable. In this thesis several case studies in which damage to crops may have been caused as a result of herbicides migration are described. Pesticide adsorption and persistence determinations showed that the adsorption coefficients do not differ from those published. Pesticide half lives are however likely to be prolonged in South African soils. Field migration studies show that pesticides are vertically and horizontally mobile in South African soils, seemingly independent of soil type. Both migration as well as upward movement was found, due to the mobility of the pesticides in the test soils. Mobility evaluation was identified as an aspect of importance for registration of pesticides under local conditions. To this end a system is proposed whereby migration can be incorporated into the existing pesticide evaluation framework. The system proposed used the migration model PESTAN as a predictive tool for pesticide migration in the evaluation process. The model is used for the evaluation of pesticides, according to Proposed Pesticide Migration Categories (PPLC). Migration evaluation should be conducted in conjunction with GPS systems to aid in determining potential risk areas where certain pesticide should not be used.