Predicting herbicides concentrations in paddy water and runoff to the river basin.

This study was conducted to investigate the dissipation pattern and runoff of herbicides to the river basin from the paddy fields. Pesticide paddy field model (PADDY) was applied to predict herbicide concentration in paddy fields. A field study was conducted in a paddy farm of Higashi Hiroshima City, Hiroshima Prefecture, Japan in the year of 2003 paddy season. The herbicides were mefenacet, thiobencarb, and bensulfuron methyl. The sample water was analyzed by using gas chromatography and HPLC after solid phase extraction. Predicted dissipation rate of thiobencarb in paddy water was higher (DT50 = 4.36) than that measured, with a lower k value (-0.069). Two weeks after application no thiobencarb was detected in the drainage channel and down stream. In the down stream, thiobencarb was detected until 3 d after application, with a range of 0.02% to 0.08% of applied herbicide. The predicted dissipation rate (k) and half-life (DT50) of mefenacet was not significantly different from that of measured. In the drainage channel, upstream and downstream mefenacet was found during the whole study period. In downstream, the maximum concentration of mefenacet was present 0.61% of applied in the paddy field on 3DAH. The dissipation rate (k) of BSM varied from -0.0860 to -0.1059 to with half-life (DT50) 3.5 and 2.84 d. In upstream water, no BSM was detected except trace amounts (0.01 microg/L) at 3 d after application. However, in the drainage channel 8%, 6% and 1.58% of applied BSM was present at 0, 1 and 3 d after application respectively. In the down stream, the highest concentration was 1.06%, shortly after application.