APPLICATION OF A SIMPLE-MODEL TO ASSESS THE GROUND-WATER CONTAMINATION POTENTIAL OF PESTICIDES

A simple model is described for assessing the ground water contaminati on potential of pesticides. This model, based on simple processes of l inear, equilibrium and singular partition between solution and soil or ganic matter, steady convective flow, and first-order degradation, cal culates fractions of pesticides remaining in the soil as they leach to greater depth. Possible effects by preferential flow that may be pres ent in structured soils are ignored. The soil profile is divided into a number of layers which may differ in bulk density, moisture content at field capacity, organic C content, and degradation half-life. The l eaching depths predicted by this model generally agreed with the mean leaching depths measured in a field study, and with those predicted by the CALF model. The leaching depths by the simple model were also sta tistically related to those simulated by the LEACHM model, although th e values by LEACHM were found to be much higher than those by the simp le model and by CALF. Under the soil and environmental conditions give n, the simple model predicted that chlorpyriphos and chlorthal dimethy l would not leach to any noticeable depth before being completely degr aded, that metribuzin, prometryne, propyzamide and simazine would most ly degrade to insignificant concentrations before reaching 500 cm, and that traces of fenamiphos, metalaxyl and linuron might reach beyond 1 000 cm.