Introduction to Contaminated Site Management - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 34

with Critical Exposure values, and many more Risk Assessment tools. A Decision
Support System (DSS) does include selected Risk Assessment tools, but also policy
points of view. A DDS could for example be used to calculate national Soil Quality
Standards on the basis of several selected Risk Assessment tools and national policy
points of view.
It is often said that Risk Assessment is an objective process and that scientists
need to operate independently of the interests of any stakeholder. To a certain extent
this is truth, since scientific independence is the key to an objective risk qualifi-
cation. This independent position, however, certainly does not justify a strict 'no
communication policy'. The reason for this is that Risk Assessment includes several
policy decisions, for example, as to the degree of conservatism and the required level
of protection of human health, the soil ecosystem, the groundwater or agricultural
products.
The independent status of scientist will not be affected by the adaptation of spe-
cific political boundary conditions, as long as it is made transparent what these
boundary conditions are. Risk assessors can do an excellent and objective job when
they, for example, commit themselves to the political boundary condition that a Risk
Assessment for an industrial site should focus on 'average adult workers' and not
relate to children or other sensitive groups. Again, it is important to make these
boundary conditions and, hence, the validity range of the conclusions from the Risk
Assessment, transparent. Therefore, this political boundary condition needs to be
clearly described in the Risk Assessment report. This enables regulators to guar-
antee the safety of these sensitive groups, for example, by fencing of the site with
anti-trespassing controls in order to protect children as in the case just mentioned.
In the USA, there is a standardised procedure for performing Human Health
and Ecological Risk Assessment called Risk-Based Corrective Action (RBCA) .A
summary of this three-tiered Risk Assessment procedure is given in ASTM ( 2009 ).
It is generally acknowledged that the total concentration is not the optimal mea-
surement with regard to risk to the soil ecosystem, the groundwater and, to a lesser
extent, for human health. Especially exposure and leaching are strongly related to an
'effective' fraction of the contaminant in soil, this is, the bioavailable fraction with
regard to ecological risks and a specific part of human health risks (like the risk
through vegetable consumption) and the available fraction with regard to leach-
ing from the upper soil into the groundwater. In Mallants et al. ( Chapter 18 of this
topic) a detailed description of the leaching process is given. The (bio)available rel-
evant fraction depends on the type of organism and, last but not least, the relevant
timeframe.
An enormous number of papers have been written on calculating and measuring
bioavailability, in particular with regard to metals. An example is given in Alvarenga
et al. ( 2008 ), who determined two bioavailable metal fractions, that is, a 'mobile
fraction' and a 'mobilisable fraction' using a sequential extraction, with the purpose
to assess the risks in an acid metal-contaminated soil from the Aljustrel mining area
in Southwest Portugal, in the Iberian Pyrite Belt. An example with regard to plant
uptake is given in Kalis et al. ( 2007 ), who described a procedure for assessing metal
uptake by Lolium perenne. To this purpose they used a four-step approach, starting
with the total metal content in soil, including the calculation of the concentration