| Popis: |
The fate of metobromuron and aclonifen, two sunflower herbicides, in soil was studied in laboratory and in field lysimeters under different water regimes. Laboratory degradation followed first-order kinetics, with half-lives ranging from 19 to 44 days for metobromuron and from 40 to 49 days for aclonifen under different incubation conditions. The influence of temperature on degradation was evident for metobromuron and negligible for aclonifen, while the influence of soil moisture was negligible for both herbicides. Adsorption was well described by an S-type isotherm with a 1/n value near unity and koc values, ranking metobromuron as a ‘transition’ pesticide and aclonifen as a ‘non-leacher’, according to the classification proposed by Gustafson (Environ. Toxicol. Chem. 8 (1989) 339). The movement in lysimeters depended on the water regimes adopted. In lysimeters that received water by the drip irrigation system (6 mm h−1) to counter evapotranspiration, metobromuron reached 60 cm depth at days 17 and 31 from treatment, while aclonifen remained in the top 20 cm layer. In lysimeters with a groundwater table at 60 cm constant depth, metobromuron reached a maximum depth of 30 cm, while aclonifen did not exceed 10 cm depth. In lysimeters that underwent three leaching events, metobromuron reached 120 cm depth at day 17 from treatment, while aclonifen remained within 30 cm depth. After three leaching events (for a total of 80 mm percolation), a total of 0.48% metobromuron (540 μg) was found in the water samples, while aclonifen was never detected. It can be concluded that, under the experimental condition tested, the two sunflower herbicides do not represent a real hazard for groundwater pollution. |
