Kilder til BAM-forurening - Miljøstyrelsen

- The ability of the soil and aquifer to attenuate the pollution (degradation
and sorption of dichlobenil and BAM).
The evaluation was carried out by means of mathematical modelling using a
linked aquitard- and groundwater model FRAC3dvs and MOD-
FLOW/MT3D, which include the above mentioned factors of importance for
the durability of the pollution. Model simulations were carried out for geologically
different types of aquifer in order to describe the extent and durability of
BAM under various typical Danish hydrogeological conditions. The model
evaluation therefore provides an overall picture of the average extent and durability
of BAM in groundwater aquifers with rough intervals for concentrations
and duration of contamination.
The modelling shows how BAM-pollutions appear and develop in the
groundwater, and the future evolution is highly varying and dependent on e.g.
the thickness of the overlaying clay aquitard and of the water balance.
The modelling indicates that the maximum BAM concentrations will occur in
groundwater aquifers with only a thin or no protecting clay aquitard above.
The highest concentrations are expected in approximately year 2000 and the
concentrations will decrease to below 0.1 µg/l after a period of 20-50 years.
The modelling indicates that groundwater with thick protecting clay aquitards
above will have a breakthrough of BAM pollution after many years. For
groundwater aquifers with 30 m of protecting clay aquitard above BAM contamination
is only expected after approximately 50 years and hereafter the
BAM concentration will slowly increase until the middle next century. However,
the modelling shows that the lowest maximum concentrations in
groundwater occur where the thick inter-layered clay layers are without fractures.
The modelling moreover indicates that the groundwater will be more exposed
to BAM pollution underneath aquitards of glacially disturbed clay and heterogenous
clay types containing abundant thin sand layers. In such areas the
model indicates that, other things being equal, there is or there will soon be a
breakthrough of BAM to aquifers and following long duration of BAM contamination.
The highest BAM concentrations appear in groundwater under the urban areas
of the model catchment, while the BAM concentrations in the agricultural
areas in general are lower. Lower BAM-concentrations and longer distances
between the BAM contaminant sources in the agricultural catchment areas
mean a lower frequency of BAM findings and shorter durability compared to
the urban catchment areas.
It shall be emphasized that the results of the model are very sensitive and dependent
on the degradation potential of BAM. In the laboratory studies no
degradation of BAM was observed in experiments representing groundwater
aquifers. However, if BAM is degraded in groundwater, the degradation rate
must be very low since the model calibrations with ”no degradation” seem to
fit the actual findings of BAM in the groundwater. Nonetheless, the predicted
durabilties of BAM pollution are very long, and even a very slow degradation
of BAM (e.g. half life of 10 years) in the groundwater would reduce the predicted
extend and duration of BAM pollution dramatically.
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