Introduction to Contaminated Site Management - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 71

An interesting and difficult issue in Risk Assessment methodologies concerns
the balance between uniformity and flexibility. Uniformity is an important regula-
tory aspect. Two different risk assessors must come to the same conclusion about
the risk appraisal of a specific site, if only from the perspective of fairness. In gen-
eral, uniformity improves when Risk Assessment procedures are more rigid, with
more fixed input parameters. However, a higher degree of freedom in the use of
Risk Assessment methodologies stimulates the incorporation of more site-specific
information, which improves the assessment, at the expense of uniformity. It is the
task of both the scientists and the regulators to strive towards an optimal balance
between uniformity and flexibility when making Risk Assessment methodologies
accessible.
Another challenge is to find a good balance between the scientific foundation and
the possibilities for pragmatic applicability. A scientist might claim that many diffi-
cult theories cannot be put into a practical format. However, practical methodologies
which are scientifically not completely mature are often better than no methodolo-
gies at all. When this kind of methodology is used, the scientific limitations need to
be made transparent. These scientific limitations are the basis of the interpretation of
Risk Assessment results, at the same time offering the possibilities for improvement
of the methodology. It takes courage, however, to follow this practical approach,
since scientists thereby make themselves vulnerable.
1.8.7.2 Conceptual Model
Every Risk Assessment is somehow related to a source-pathway-receptor approach.
With regard to risk-based soil quality assessment, it often pays off to start with
a Conceptual Model, especially in the case of contaminated aquifers. Such a
Conceptual Model gives a (usually visual) presentation of relationships between the
source, all pathways involved and the receptor. A cross-section of the contaminated
site is the most common format of a Conceptual Model. See Fig. 1.13 in which a
Conceptual Model for groundwater-to-indoor-air mass flux analysis is shown, as an
example.
In fact the Conceptual Model represents a two-dimensional contamination pat-
tern and includes all relevant pathways involved. It relates to the migration to other
compartments, for example, from soil to aquifer or to the migration within a com-
partment, for example, the migration of a contaminant plume within the aquifer. It
also represents migration of contaminants into contact media, for example, upward
transfer of volatile contaminants from the upper aquifer into a building. The source
generally relates to a location, or locations, in soil that are contaminated. The recep-
tor is a specification of the protection targets. It can relate, for example, to humans
living on the site, or to the soil ecosystem in a downstream nature reserve that might
be threatened by a lateral and upward flow pattern in the aquifer and upper soil
layers.
A Conceptual Model might serve two important purposes. First, it supports a
systematic investigation of all possible pathways, and subsequently helps to identify
all necessary Risk Assessment tools. Second, it makes the whole Risk Assessment