Kilder til BAM-forurening - Miljøstyrelsen

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 the 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.
In the Danish groundwater aquifers BAM pollution is found in concentrations
above the official limit value for drinking water in on average 10% of the analysed
wells (data from the official Groundwater Monitoring Programme,
2000). However, the degree of BAM contamination varies from county to
county from 1% in Frederiksborg County to 15% in Southern Jutland County.
The difference in the frequence of BAM contamination may be due to different
application doses of the herbicides, different land use, and different hydrogeological
conditions. Most of the BAM findings originate from groundwater
at a depth of up to 40 meter, but BAM has also been observed in
groundwater reservoirs at a depth of 100 meter.
Transport via leaky wells
Groundwater BAM contamination caused by transport of pollution through
leaky wells requires that the dichlobenil containing herbicides have been applicated
close to the well.
Investigations in this project combined with experiences from other studies
show that there is a potential risk of leaks through well constructions as a result
of drilling techniques, well construction techniques and well construction
materials. Evidence of defective well construction was recorded in 84 % of
172 wells investigated. In all wells constructed before 1980 there may be a risk
of leaks due to one or more constructional errors. Many relatively newly established
wells, however, are defective too.
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