Environmental Engineering Reference
In-Depth Information
concentrations. That is the reason why the second risk assessment tier was
applied.
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Tier 2
When biodegradation and partitioning are also considered, the aggregated atten-
uation results in a contaminant level in the groundwater which fulfills Hungarian
GWQC for TPH and ethylbenzene, but exceeds it for benzene, toluene and
xylene in the creek inflow point. Groundwater intervention threshold is not
exceeded when applying the moderate pessimistic scenario, but when being
highly pessimistic, only TPH and ethylbenzene fall below intervention threshold
before the inflow into the creek. From the point of view of our main goal, the
protection of aquatic life, the diluting effect of the creek should also be consid-
ered. Dilution by the creek water flow further reduces the concentrations at a
20-fold rate.
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Tier 3
A comparison of the concentration in the creek to generic surface water qual-
ity criteria raises the problem of selecting the proper criteria or standard. The
chemical-model-based criteria (yearly average or maximum permissible concen-
trations) are less frequently applied compared to standards based on biologi-
cal/ecological indicators. In Case study 2 a comparison to chemical WQC was
used: all the contaminant components meet the selected fresh water quality crite-
ria (ANZECC, 2000). Another simplified way of evaluation is the use of WHO
drinking water criteria (WHO, 2008) which can be used for orientation. This com-
parison also indicates that risk of benzene is the bottleneck in the sample case and
it needs special attention!
As a next step of the risk assessment the calculated and validated concentra-
tion in the creek can be evaluated according to ecological quality standards. The
long-term ecological assessment may prove the final conclusion on the “no risk''
situation in the creek.
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Tier 4 - uncertainties in environmental risk calculations
i)
Evaluation of the time-related and spatial context of the contamination
is necessary, including the extent and nature of the source (one-off or
continuous discharge; abandoned site, ongoing activity, etc.);
ii)
The size and geometry of the plume influence both the degradation and
the partitioning of the contaminants in the plume because concentration
reducing processes occur only on the surface of the plume;
iii)
Individual components of the pollution behave differently and their charac-
teristics are generally not additive;
iv)
The scale of conservatism can be reduced by using site-specific partition and
biodegradation values;
v)
Sensitivity of the creek's ecosystem may modify the criteria both downstream
and upstream;
vi)
Human water uses were not considered in this case, given that the water of
the creek is not used by humans for the time being;
vii)
Groundwater uses were not considered;
viii)
Recharge from the source was assumed as a constant flux, which is unlikely
the case.
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