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Kilder til BAM-forurening og forureningsudbredelse Summary and conclusionsThe pesticide (or pesticide degradation product) which has hitherto polluted most water extraction wells in Denmark is 2.6-dichlorbenzamide (BAM). BAM derives from the herbicides Prefix and Casoron and is a degradation product of the active substances dichlobenil and chlorthiamide in these herbicides. The herbicide products were used in the period from 1965 to 1997 but are now prohibited. The average annual consumption in Denmark of chlorthiamid and dichlobenil was 29,000 kg/year in the period 1965-1997. The application dose of dichlobenil was around 4-30 kg active substance per ha. Prefix and Casoron were used widely by many different users in different areas (not in order of priorities):
Weed control using Casoron and Prefix occurred both in urban and agricultural groundwater catchment areas. The number of BAM contaminant sources, however, is typically much higher in urban areas and the contaminant sources closer compared to agricultural areas. Investigations of the BAM contaminant sources show concentrations of typically up to 100 µg/kg of the parent pesticide dichlobenil in soils. The average concentration in the top 1 meter is at approx. 55 µg/kg. Dichlobenil is found in soil samples in more than half of the investigated sites, regardless of knowledge of dichlobenil-containing herbicides having been used at the sites or not. It is assumed that the herbicides have not been used since 1997, and that the observed concentrations of dichlobenil represent residual concentrations of dichlobenil that has not been degraded. Small amounts of BAM is also detected in the soil, which shows that dichlobenil is still degraded and that BAM continues to infiltrate from the surface. BAM is also detected in pore-water from vadose zone (in concentrations of up to 37 µg/l) as well in groundwater aquifers near the ground surface, which shows that BAM is still leaching through vadose zone to the groundwater. Often, 2.6-dichlorobenzoic acid also is detected in pore water samples from the vadose zone, indicating that BAM is degraded since 2.6-dichlorobenzoic acid in the literature is reported as being a metabolite from BAM. Calculations show that the residuals of dichlobenil in soils from the upper few meters of the investigated sites can still cause groundwater contamination for many years to come. However, the main part of BAM is considered to have already infiltrated into deeper parts of vadose zone or into the groundwater aquifer. Apart from the direct infiltration of BAM from the contaminant sources there might be a risk of spreading BAM via surface water (streams) and rainwater runoff from roads and paved surfaces. For instance, rainwater basins without firm bottom imply a risk of infiltration into the groundwater of BAM polluted rainwater. Investigations of selected groundwater catchments have shown that the many BAM contaminant sources in urban and agricultural areas have caused widely spreaded BAM-pollution in the groundwater aquifers. The spreading of BAM-pollution is caused by leaching of BAM from all these contaminant sources resulting in many small pollution plumes which often seem to combine to larger coherent plumes due to the high mobility of BAM. The complex distribution of contaminant sources in the catchment areas combined with the spreading of BAM via surface water and runoff rainwater pattern makes it very difficult to identify preferential pathways from the contaminant source to the groundwater.
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