Abstract-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.
2‐Methyl‐2‐(methylsulphinyl)propionaldehyde O‐methylcarbamoyloxime (aldicarb sulphoxide), aldoxycarb, oxamyl and methomyl were incubated at 10°C in soil samples taken from layers above and below shallow ground‐water tables at four locations in the Netherlands. Soil samples from above the water table were incubated under moist and aerobic conditions. The anaerobic conditions below the water table were simulated by incubating the soil samples under 0.5‐1 cm of ground water, and a nitrogen atmosphere. During incubation, the pH and redox potentials were measured. Less than 5% of the oxamyl and methomyl remained after one day in four water‐saturated, anaerobic subsoils. The half‐lives of aldicarb sulphoxide and aldoxycarb ranged from 5.1 to 131 days in the four anaerobic subsoils. Conversion rates in the aerobic soil layers above the water table were from 8 to more than 100 times lower than in the water‐saturated layers in the same soil profile. Half‐lives in the aerobic soils ranged from 26 days for oxamyl in loamy fine sand (pH 8.0), to 1100 days for aldoxycarb in fine sand (pH 5.0). When soil from below the water table was incubated aerobically, the conversion rates of oxamyl and aldoxycarb were drastically reduced. The opposite was found when an originally aerobic soil was incubated anaerobically. Autoclaving the incubation systems retarded the conversions.
Aldicarb was incubated in seven soils at 15°C and its loss was well described by firstorder kinetics. Rate constants varied between 0.078 day-1 in a peaty sand to 0.35 day-l in a clay loam. The concentration-time relationships for aldicarb, its sulphoxide and its sulphone were approximated by a computation model which was used to analyse the importance of the various consecutive and simultaneous reactions. It was computed that 91 to 100% of the aldicarb would be oxidised to its sulphoxide.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.