CONTROLLED ENVIRONMENT STUDY OF THE DEGRADATION OF ENDOSULFAN IN SOILS
Mar 13, 2017

Crop Protection WCRC Croprotection-australia WCRC1
Abstract                                                                         Back to Table of contents

The degradation rate of endosulfan applied to a Black Earth soil from South East Queensland, was studied in controlled environment chambers. The degradation rates of a- and b-endosulfan were determined for the soil samples kept at the constant temperature of 30˚C under two water contents of 20% and 30% and submerged under water. The degradation rates of both isomers in all three moisture conditions followed first order kinetics with the rate constants differing significantly between submerged and non-submerged soils. For non-submerged soils the degradation rates of the two endosulfan isomers were lower for the soil with higher water content. The b-endosulfan isomer disappeared from the samples at a faster rate than the a-isomer at both water potentials. However the rates of decline for both isomers were much slower than those suggested in the literature.

Degradation of both endosulfan isomers in the submerged soil took place at a slower rate than in the non-submerged conditions, and in contrast to the results with unsaturated soil, the rate for b-endosulfan was slower than the a-isomer. Adding endosulfan to soil appears to reduce the rate of degradation of other organochlorine pesticides already present in the soil. The half-life of dieldrin was significantly increased in the presence of endosulfan. This effect was much more pronounced at the higher of the two water contents investigated. The applicability of these results to the soils of cotton farms in Queensland and NSW is under investigation.

Conclusion

Degradation of sorbed endosulfan at constant temperature appears to be well described by a first order kinetics equation under every moisture condition studied. In the soil studied, the degradation rates for both endosulfan isomers, however, are much slower than expected from the literature, half-lives being comparable with those of the most persistent members of the organochlorine family of pesticides. The main reason for such longer-than expected half lives could be the low pH value of the soil used in the experiments. It may be noted that in Australia, though not necessarily elsewhere, cotton soils have a high pH.

The degradation rates of both a- and b-endosulfan in the moist (non-submerged) soils were affected by soil moisture content, both rate constants being smaller for 30% water content than 20%. This effect was more pronounced for a-endosulfan than the b-isomer.  Similar effects of water content were observed for aldrin and dieldrin present in the soil. Combining these results with those given by Ghadiri et al. (1995) suggests that the soil water content of around 20% (-220 kPa potential) may be the optimum moisture condition for the degradation of endosulfan and other organochlorine pesticides studied. The degradation of all these pesticides decrease when the soil is wetter or drier than this optimum value.

When submerged under a deep layer of water, the degradation of b-endosulfan in soil is significantly slower than in the non-submerged condition, while the reverse was true for the a-isomer. Such an unexpected behaviour of the endosulfan isomers in the submerged soil necessitates further studies of these chemicals in the bedloads of the rivers and lakes which receive agricultural runoff.

A negative interaction was observed between the newly applied endosulfan and other organochlorine pesticides already in the soil. Applying endosulfan to the soil containing aldrin and dieldrin reduced the rate of degradation of the latter two pesticides. Interactions of this type could become an important factor in predicting the degradation rates and the half-lives of a various sorbed pesticide added to the soil. Interaction of similar nature might have contributed to the persistence of DDT and DDE in the cotton soils of Australia several years after their final application.

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