Environmental Engineering Reference
In-Depth Information
In spite of the sophisticated mathematical background, a numerical model based
on a regular homogeneous porous medium is often insufficient. Examples of such
cases are heterogeneous soils or aquifers (Section 17.3.2.2 ) and the presence of
surface water bodies and of anthropogenic subsurface processes and structures in
the underground (Section 17.3.2.3 ). It is a challenge, but very important, to include
these factors in numerical models.
Homogeneity and heterogeneity of a system are concepts closely related to the
scale of the investigation. The same groundwater body can be considered as homo-
geneous on a large scale and heterogeneous on a small scale. Therefore, the scale
of investigation must be considered at the very beginning of a project. This scale
strongly depends on the relevant receptor, and is different, for example, for adjacent
clean groundwater and public water supply groundwater bodies a few kilometres
away. In case of the development of a Risk Management solution, the relevant scale
depends on the type of solution, and differs, for example, for source removal and
pathway interception.
In Section 17.4.1 , the importance of a Conceptual Model with regard to appro-
priate modelling of contaminant transport has been outlined. A correct Conceptual
Model could significantly increase the reliability of contaminant transport mod-
elling. Findings based on a wrong Conceptual Model, or on no Conceptual Model at
all, can produce results in a direction contrary to what would normally be expected.
It is generally recognized, however, that relatively large uncertainties are
involved when using models to assess risks from contaminant transport. The reason
for this is that usually drastic simplifications have to be adapted. These simplifica-
tions relate to the subsurface geometry, hydraulic and geochemical input parameters,
and the magnitude and type of the source.
Subsurface Geometry
The subsurface geometry involves huge volumes of subsoil: plumes generally move
through volumes of soil of several hundreds to thousands of cubic meters within the
time span of one year. Although these large volumes often are made up of several
irregular layers from different materials, and often include entities of another differ-
ent subsoil material, the use of contaminant transport models requires the design of
a more or less regular profile. An important question in the debate on reliability of
contaminant transport calculations is: can a layered soil at the relevant scale, with
some cracks and holes be considered as a 'regular' (homogeneous) porous medium?
Besides this, it is hard to identify locations in the subsoil that allow for very fast,
preferential water flow and contaminant transport, for example, due to cracks or soil
materials with high hydraulic permeability. Fissured soils are difficult for accurate
contaminant transport calculations. In these soils, water and contaminants follow
preferential flow paths. Bruell and Inyang (2000) described the difficulties when
modelling contaminant transport with regard to remediation of groundwater in con-
taminated rock masses. Failure to account for preferential flow paths in numerical
simulation can lead to over-estimation of the effectiveness of the remedial measure
under consideration (Zheng and Gorelick 2005 ).
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