Dissipation of pretilachlor in paddy water and sediment

A two-year field study (2001–2002) was carried out in N-W Italy to study the behaviour of pretilachlor [2-chloro-2,6-diethyl-N-(2- propoxyethyl)acetanilide] in water and sediment of a rice field. Pretilachlor is a selective pre-emergence herbicide which is effective against annual grasses, sedges and broad-leaved weeds. The herbicide was applied in pre-seeding on a flooded rice field where water circulation was stopped for about 3 weeks after treatment. Pretilachlor concentration in the paddy water decreased by more than 90% during the first three weeks after the treatment. The amount of the herbicide in the paddy water gradually fell to levels below the sensitivity of the analytical method when water circulation was re-established. The pretilachlor concentration in the sediment gradually increased after the treatment, reaching the highest value 5 to 6 days later. The average DT50 in water and sediment were 6.77 and 28.76 days in 2001, 4.68 and 15.01 days in 2002, respectively. The low percolation rate (0.95 and 0.79 mm day–1 in 2001 and 2002, respectively) and the high ratio of the herbicide adsorption on the sediment suggest that pretilachlor disappearance from the water was mainly the result of degradation.