Environmental Engineering Reference
In-Depth Information
is completely different than in the water-unsaturated upper soil layer, there are often
(but not always) possibilities for risk reduction.
It must be noted, however, that the interest of the stakeholders, as well as the
public concern, diminishes when the area under investigation is approached from the
perspective of a larger scale. For obvious reasons, the stakeholders and the general
public are more committed to the site they own, live on or work on, than the wider
area around the site. The consequence for contaminated site management at the
scale of the site and the surroundings, and even more so on a regional scale, is that
a policy becomes necessary in order to force stakeholders to follow a 'large scale
thinking' approach.
1.7.9 Time Domain
In Section 1.1.4.2 , it was explained that in the phase of the Problem definition it
is very important to define the time frame for which the conclusions from the Risk
Assessment applies, since factors that impact human health risks and ecological
risks will change over time. Outcomes of the Risk Assessment should usually rep-
resent the risks over longer periods. When a site receives a positive risk appraisal,
it often is assumed that the site is suited for its purpose for many decades. As
a consequence, assumptions need to be made for factors that change over time,
mainly with regard to the layout of the site, human behaviour characteristics and the
bioavailability of the contaminants.
Moreover, the effects on humans and on bigger animals might only reveal them-
selves years or decades after exposure. Therefore, it is essential to focus on a
toxicologically relevant time frame in Risk Assessment and Risk Management.
With regard to ecological Risk Management the time frame for which ecological
restoration takes place is important. For Groundwater-related Risk Assessment, it
must be realised that transport times of contaminants might take decades or even
centuries. It is essential to be aware of these long time frames, both politically (what
time frame is relevant?) and technically.
In fact, changes in the contaminant concentrations due to migration and
degradation in the soil and groundwater also need to be considered in Risk
Assessments. Remarkably, this is not often done; the concentrations measured
are often considered constant in time. Since the concentrations in soil generally
decrease over time due to leaching, volatilisation, and degradation, this can be
considered as a worst-case approach. This is not always true for groundwater,
however, since leaching from soil could increase contaminant concentrations in
groundwater.
Many of the Risk Assessment factors also need to be adapted for the determi-
nation of exposure scenarios or risk estimates in case the Risk Assessment relates
to a different future land use or a different layout of the site under the same land
use. This situation frequently occurs, especially in densely populated areas such as
in Northwest Europe, since land use transitions are common and are often preceded
by a site investigation and, hence, a Risk Assessment.
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