Monitoring bentazone concentrations in the uppermost groundwater after late season applications

The herbicide bentazone has been detected in groundwater in several monitoring programs with most of the findings possibly be related to applications early in the growth season. Because of a very low sorption constant bentazone can be transported in soil with the waterflow very easily. This means that the risk of leaching increases with increasing precipitation excess in autumn. Therefore in 1996 a monitoring program was performed to measure the occurrence of bentazone in the uppermost groundwater after late season (September 1st) application. On five fields were bentazone was applied the groundwater was sampled and analysed for bentazone at two points in time. On 2 out of the 5 fields a concentration of bentazone was determined in the first sampling round and on one additional field in the second sampling round.With the results of this study it was not possible to answer the research question: 'what is the threat of leaching of bentazone to groundwater after late season application?' Too few fields met the requirements of the study. In addition, extrapolation with a simulation model could not be performed, because too many parameters of the PESTRAS model had to be adjusted to match with observations in the field