Simulation of pesticide persistence in the field on the basis of laboratory data - A review

Simulations of pesticide fate in soils are often based on persistence model s developed nearly 30 years ago. These models predict dissipation in the fi eld on a daily basis by correcting laboratory degradation half-lives for ac tual soil temperature and moisture content. They have been extensively appl ied, but to date no attempt has been made to evaluate existing studies in a consistent, quantitative way. This paper reviews 178 studies comparing pes ticide soil residues measured in the field with those simulated by persiste nce models. The simulated percentage of initial pesticide concentration at the time of 50% measured loss was taken as a common criterion for model per formance. The models showed an overall tendency to overestimate persistence . Simulated values ranged from 12 to 96% of initial pesticide concentration s with a median of 60%. Simulated soil residues overestimated the target va lue (50% of initial) by more than a factor of 1.25 in 44% of the cases. An underestimation by more than a factor of 1.25 was found in only 17% of the experiments. Discrepancies between simulated and observed data are attribut ed to difficulties in characterizing pesticide behavior under outdoor condi tions using laboratory studies. These arise because of differences in soil conditions between the laboratory and the field and the spatial and tempora l variability of degradation. Other possible causes include losses in the f ield by processes other than degradation, deviations of degradation from fi rst-order kinetics, discrepancies between simulated and actual soil tempera ture and moisture content, and the lack of soil-specific degradation parame ters. Implications for modeling of pesticide behavior within regulatory ris k assessments are discussed.