Abstract

With reference tot the Memorandum Environmental Criteria for the Protection of Soil and Water against Chemical Pollutants, the objective of this study was to indicate whether dilution and degradation in the ground water had to be accounted for in case of Approval of Pesticides. In this report the possible occurrence of pesticides in the upper (1 m) layer of the ground water and the deep, extracted ground water has been evaluated based on the actual use of soil and pesticides in a number of vulnerable agricultural areas in the Netherlands. Dilution, defined as the physical process of mixing of water streams in the extraction well, is determined by two factors: the actual use of a pesticide and the acreage fractions of the crops to which a pesticide may be applied. In case of public drinking water supplies the dilution factor, defined as the ratio of the pesticide concentration in the upper (1 m) layer of the ground water and that in the deep, extracted ground water, has a minimum value of 3.7 for pesticides which leaks into the ground water with resulting concentrations higher than 0.1 mg.m -3. This dilution factor can be much higher for the various combinations of pesticide and crops. This dilution cannot prevent that the EC standard (for all pesticides together) of 0.5 mg.m -3 in the extracted ground water is exceeded (by factors of 10 to 1000) and that for 13 of the 130 pesticides evaluated the individual EC standard of 0.1 mg.m -3 is exceeded. For private water extraction wells the situation may be somewhere between two extremes: no dilution (in case of very shallow extractions) or dilutions comparable to those of a public drinking water supply. It was concluded that degradation of the pesticides in the saturated zone may have a substantial effect on the concentrations in the extracted groundwater. The extent is determined by the relation between the residence time in this zone and the degradation rate of the pesticide in this zone. Only very limited information is available on the transformation of pesticides in the water satured zone and a reasonable certainty about the formation of harmless metabolites does not exist. Therefore the influence of transformation in the subsoil on the levels of pesticides in deep ground water has not been evaluated. In the discussion attention has been given to the uncertainties of the model (parameters) and the overground processes which determine the fraction of the dosage reaching the soil.