Introduction to Contaminated Site Management - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 72

BV (= L x w x h)

Site
Building

F ia2

ER

h

F ia1

Mass Balance
F sg = F gw1 − F gw2
F sg = F ia1 − F ia2

Unsaturated
Soil

L

F sg

F gw2

F gw1

Ground-water
Bearing Unit

Source
Area

v

Fig. 1.13 A Conceptual Model for groundwater-to-indoor-air mass flux analysis, as an example
of a Conceptual Model (source: McHugh et al. 2003 ; reproduced with permission). Fgw1 = mass

flux through groundwater at upgradient edge of building, Fgw2 = mass flux through groundwater

at downgradient edge of building, Fsg = mass flux from groundwater to soil gas under building,

Fia1 = mass flux from soil gas to building, Fia2 = mass flux from building to outdoor air

process accessible, even to non-experts. For this reason, a Conceptual Model is an
excellent basis for discussions in an early stage of any Risk Assessment project,
along with the involvement of all stakeholders.

1.8.7.3 Tiered Approach

The most elegant way of dealing with Risk Assessment is in the form of a tiered
approach. In such an approach several assessment steps ( tiers ) are described. In
each tier, an assessment is performed with generally two possible outcomes: either
a judgment of the absence of unacceptable risks can be given, and the total assess-
ment is finished, or unacceptable risks cannot be excluded and the assessment has
to be followed into the next tier. When the presence of unacceptable risks cannot
be reputed in the final tier, unacceptable risks cannot be excluded, the total assess-
ment is also finished and Risk Management needs to follow the tier-based Risk
Assessment. Given the nature of a tiered approach, in each step the assessment
becomes less conservative, is based on more site-specific information and, hence, is
more complex, time-consuming and often more expensive. The philosophy behind
this is: simple when possible (only the first tier) and more complex when neces-
sary (higher tiers) . A tiered approach is an efficient way of risk assessing, without
compromising scientific integrity.
A tiered approach often starts with a generic Risk Assessment, that is, comparing
measured value with Soil Quality Standards (screening levels). The purpose could