Persistence and toxicological effects of pesticides in topsoil: use of the equilibrium partitioning theory
1997
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 parameter. The relevance of each parameter was tested using different soil types and prolonged chemical-soil contact time by relating the results to bioavailability as measured in toxicity tests. The pesticide concentration measured in the soil pore water could be shown to correspond strongly with toxicologic effects on the soil organism Folsomia candida, irrespective of soil type and chemical-soil contact time. This relationship is predicted by the equilibrium partitioning theory and so this aspect of the theory could be validated in this study for pesticides in soil. However, the theory's assumption of equilibrium (which implies that the bioavailable fraction of the chemical is constant with time and can be calculated using one particular soil to pore-water partition coefficient) could not be justified in this study. Although organic matter normalization reduced soil-to-soil differences, it did not account for the time-related decrease in bioavailability that was found in two of the four soil-chemical combinations investigated. As a consequence, pore-water concentrations calculated from chemical content measurements in soil and a partition coefficient obtained in short-term laboratory experiments may not be a good estimate for sorption or toxicologic effects on a longer term. The suggestion is made to measure the concentration of chemical in pore water directly and to use water-only toxicity values as a basis for soil quality criteria.
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