PERSISTENCE AND TOXICOLOGICAL EFFECTS OF PESTICIDES IN TOPSOIL - USE OF THE EQUILIBRIUM PARTITIONING THEORY

Three parameters for indicating toxicologic risk of pesticides to soil organisms were compared: the traditional total content parameter, the equilibrium partitioning approach, and the pore-water concentration p arameter. The relevance of each parameter was tested using different s oil types and prolonged chemical-soil contact time by relating the res ults to bioavailability as measured in toxicity tests. The pesticide c oncentration measured in the soil pore Rater could be shown to corresp ond strongly with toxicologic effects on the soil organism Folsomia ca ndida, irrespective of soil type and chemical-soil contact time. This relationship is predicted by the equilibrium partitioning theory and s o this aspect of the theory could be validated in this study for pesti cides in soil. However, the theory's assumption of equilibrium (which implies that the bioavailable fraction of the chemical is constant wit h time and can be calculated using one particular soil to pore-water p artition coefficient) could not be justified in this study. Although o rganic matter normalization reduced soil-to-soil differences, it did n ot account for the time-related decrease in bioavailability that was f ound in two of the four soil-chemical combinations investigated. As a consequence, pore-water concentrations calculated from chemical conten t measurements in soil and a partition coefficient obtained in short-t erm laboratory experiments may not be a good estimate for sorption or toxicologic effects on a longer term. The suggestion is made to measur e the concentration of chemical in pore water directly and to use wate r-only toxicity values asa basis for soil quality criteria.