FATE OF PESTICIDES IN A MODEL RICE PADDY ECOSYSTEM

摘要

The fate of C-carbosulfan was studied in a model rice paddy ecosystem. Carbosulfan was rapidly converted to carbofuran. Seventy-two hours after application on rice, the radioactivity was distributed as follows: soil > water> plant > fish > air. Rice fodder fortified with ~(14)C-carbofuran was fed to lactating goats. The ~(14)C-carbofuran equivalents were traced in the different organs and tissues of the animals, the highest concentrations being in omental fat and the liver. Carbon-14-carbofuran was metabolized and excreted in urine (77%), faeces (3%) and milk (0.05%). The residues in water were taken up and bioconcentrated by fish, the highest concentration being in the entrails. Even ducks and chickens in the vicinity of the treated rice fields contained pesticide residues. Ipomoea aquatica grown in the treated paddy water sorbed ~(14)C-carbofuran residues, with the highest uptake between days 9 and 13, declining thereafter. The pesticide residues used in the rice paddy were not detected in grains, but post-harvest treated rice grains contained high ~(14)C residues after treatment of the jute sacks, as is usual practice in warehouses. Washings, however, greatly reduced (67%) the ~(14)C-chlorpyrifos residues in grains. The bound residues of ~(14)C-isoprocarb on soil were absorbed by rice plants and by the subsequent crop, water melon. Owing to some contamination of well water collected in the vicinity of the treated rice fields, movement of monocrotophos, endosulfan and chlorpyrifos was evaluated in the field. The monocrotophos and endosulfan residues were rapidly lost in the paddy water, but significant amounts leached beyond the surface soil layer, up to a depth of 175 cm. Carbon-14 labelled monocrotophos, however, only reached a depth of 30 cm within 60 days in the soil column experiments. Likewise, ~(14)C-chlorpyrifos was found up to a depth of 20 cm on day 6 after application. Adsorption on soil was 91%, suggesting strong affinity to soil, and hence less leaching. Further experiments are in progress to examine this apparent anomaly.