Environmental Engineering Reference
In-Depth Information
Source
One of the most uncertain factors in contaminant transport is the quantifica-
tion of the source, that is, the emission to soil and groundwater. Examples of
uncertainties with regard to the source relate to the necessary simplifications of
the irregular contaminant inputs. In models, these inputs must be translated into
a homogenous contaminant load in a specified layer of the (upper) soil or into a
steady-state contaminant input at the surface of the soil or into the water-saturated
soil compartment.
17.4.3.2 Dealing with Uncertainties
Most of the issues described in Section 1.5.4.2 on dealing with uncertainties from
a general Risk Assessment perspective, also apply to Groundwater-related Risk
Assessment. An important conclusion is that outputs from contaminant transport
models must be regarded as indications. Nevertheless, these models are extremely
useful tools, as long as they are intelligently used. A few recommendations for the
use of contaminant transport models are:
relatively simple worst-case calculations can be used in a first
tier Risk
Assessment;
generally, trends can be assessed in terms of a protection target that might be
threatened by contaminant transport within a specific time range (e.g., 'within a
few years');
identification of the most sensitive processes and input parameters supports good
modelling practice;
in specific cases the modeller could consider performing a probabilistic model
calculation;
in sensitive cases peer review or second opinions may be established;
the uncertainties must be made transparent and must be communicated to all
stakeholders;
methods can be indicated to reduce the uncertainties during subsequent project
phases.
17.4.4 Good Modelling Practice
As with any model, for the reasons previously mentioned, no existing contaminant
transport model can ensure the correct prediction of the development of transport
modelling, that is, the evolution of groundwater plumes or the breakthrough curves
of a contaminant. Any practitioner should realise this when applying models for
the secondary purpose of contaminant transport modelling as described in Section
17.4.2.1 , that is, understanding contaminant transport systems and supporting
decision making on the basis of calculated concentrations.
Modelling of contaminant transport is especially complex when several ground-
water plumes interact, a situation that is often found in intensively urbanised areas.
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