Movement of water, bromide ion and the pesticides ethoprophos and bentazone measured in a sandy soil in Vredepeel (The Netherlands)

The aim of this study was to collect a data set suitable for testing pesticide leaching models in the case of a Dutch sandy soil with a shallow groundwater table. The movement of water, bromide ion and the behaviour of the pesticides ethoprophos and bentazone was studied. The substances were applied after sowing winter wheat in autumn 1990. This late application time is unusual for bentazone: it was selected on scientific grounds (without agricultural purpose). Rainfall, groundwater level and soil temperature were monitored continuously at the experimental field (80 m long and 54 m wide) until spring 1992. Soil profiles were sampled at 1, 22, 42, 103, 214, 278 and 474 d after application (16 profiles at each date). In the laboratory, pesticide transformation rates were measured with soil material from 0-25, 50-100 and 100-200 cm depth. Sorption isotherms were measured with material from 0-25 cm depth. Concentration profiles showed that mobility increased in the sequence ethoprophos - bentazone - bromide ion. Ethoprophos movement was limited to the top 25 cm layer whereas bentazone leached to below 1 m depth. At the end of the study, the concentrations of ethoprophos and bentazone were below the detection limit (0.2-2 g dm-3) in all soil layers between 25 and 120 cm depth.