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In the first stage of a programme of sampling and analysis for pesticide residues in groundwater in The Netherlands, the upper groundwater below four vulnerable soils was analysed for nearly 2,5 years in eight sampling rounds. Of 18 compounds analysed, including some metabolites, 1,3-dichloropropene, aldicarb, ethoprophos dinoseb, metamitron, atrazine, desethyl- and desisopropylatrazine, metolachlor and ethylenethioureum were repeatedly detected in the groundwater in concentrations above 0.1 micrograms.dm-3, the limit for pesticides in drinking water set by the EC. These observations were made below fields with potatoe-, maize- and bulb flower culture, all on low-humic to moderately humic sandy soils. No residues were found below a cracked light clay on a sandy subsoil. On the locations sampled evidence was found for complete in situ removal in the upper groundwater of 1,3-dichloropropene, high persistence of aldicarb residues and partial in situ degradation of dinoseb.