English Abstract In 1990 several permanent wells, placed below
agricultural land used for potato, maize and flower-bulb culture, as well as
wells placed below uncropped land, have been sampled and analysed for
pesticide residues. The sampling took place on vulnerable soil types as
well as soil types that were expected to be less vulnerable for leaching of
pesticides. Below both soil types several pesticide residues haven been
found. Since the start of this sampling program in 1988, 26 different
organic compounds have been detected in the wells. The compounds are
residues of nine pesticides allowed for use in groundwater protection areas
and six that are not allowed in these areas. The highest concen- trations
were found for transformation products and admixtures. In particular
transformation products are a threat for the quality of groundwater and the
drinking water supply.
