Bioavailability Of Pesticide Residues In Soil To Earthworms

Soils serving as a reservoir for many organic pollutants such as pesticides of potential ecological concern, the large fraction of a pesticide applied in the fields would enter soils directly and indirectly. The pesticides become less bioavailable and thus less toxic as they persist in soil under field conditions. However, the declining bioavailability is not reflected by the currently used analytical methods based on vigorous extractions with organic solvent. As a result, such analytical methods overestimate exposure, toxicity, and hence risk from toxic compounds in contaminated sites. Butachlor, myclobutanil, and chlorpyrifos were extensively used in China. There is increasing concern on their ecotoxicological effects on terrestrial, groundwater and aquatic ecosystems due to their persistence in soil, and potential to cause adverse chronic effects at low levels. However, little information on bioavailability of them to earthworms can be found. Therefore, in this study, the earthworm was used as a model soil organism to assess the bioavailability of butachlor, myclobutanil, and chlorpyrifos in soil.For predicting bioavailability of butachlor, myclobutanil, and chlorpyrifos in soil to earthworms (Eisenia foetida and Allolobophora caliginosa), several solvent systems, including methanol, methanol-water (9:1), methanol-water (1:1), acetone-water (5:3), petroleum ether and water were assessed. The results show that extractability and bioavailability of the three pesticides decreased with increase in aging time. Butachlor and myclobutanil extracted by water was well correlated with and quite similar to the fraction available to earthworms, suggesting that water might be useful for predicting bioavailability of these two pesticides to earthworms. Similarly, methanol-water (1:1) and petroleum ether also might be used to be indicator of bioavailability of chlorpyrifos to earthworms because of their extracted fraction approximating uptake of chlorpyrifos by earthworms. Throughout the experiments, E.foetida always uptook more butachlor, myclobutanil, and chloprpyrifos than A. caliginosa did, suggesting that availability of the soil-sequestered pesticide is species dependent.For examining effect of soil properties on bioavailability and extractability ofbutachlor, myclobutanil, and chlorpyrifos sequestered in soil, five soils and the indigenous earthworm A. caliginosa, water-extraction for butachlor and myclobutanil and petroleum ether-extraction procedure for chlorpyrifos were used. The results show that sequestration (decrease in extractability and bioavailability) of butachlor, myclobutanil, and chlorpyrifos varied with soil properties. Simple regression analyses show that sequestration of pesticides tested was highly correlated with organic matter content. Multiple linear regression analyses show that an interaction of organic matter content with CEC, sand content, or pH determined sequestration of pesticides involved in this study. This suggests that organic matter content, CEC, and other properties of soil may be useful predictors of sequestration of some compounds. Good correlation and quite equivalency between extractability and bioavailability validated again that water, water, and petroleum ether can be used as a reagent for extracting the bioavailable fraction of butachlor, myclobutanil, and chlorpyrifos in soil to earthworms, respectively.For investigating relationship between adsorption/desorption and bioavailability of butachlor, myclobutanil and chlorpyrifos, the adsorption and desorption of butachlor, myclobutanil and chlorpyrifos on five soils was examined using a batch equilibration technique. The results show that the adsorption and desorption isotherms of the three pesticides on five soils could be described well with the Freundich equation. The adsorption and desorption behaviors of butachlor, myclobutanil, and chlorpyrifos were highly controlled by both octanol/water-partitioning coefficient (Kow) of pesticides and soil organic matter content (OM). Uptake routes, expect for properties of both pesticides and...