Site-Specific Ecological Risk Assessment - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 700

operational and reliable methodologies, i.e. the Triad approach which combines
three lines of evidence (Fig. 15.2 ).

15.3 Weight of Evidence and the Triad Approach

In order to deal with uncertainties in the process of ERA in a pragmatic and responsi-
ble way, it has been proposed to use a weight of evidence (WoE) approach (Chapman
et al. 2002 ; Hull and Swanson 2006 ; Long and Chapman 1985 ; Rutgers and Den
Besten 2005 ; Suter et al. 2000 ). The rationale is that multiple and independent ways
to arrive at the same type of conclusions will provide a stronger 'evidence' for
ecological effects, substantially improving the reliability of ERA. Unfortunately,
precise definitions and application schemes of WoE in ERA are unclear (Weed
2005 ), but a series of papers edited by Chapman et al. ( 2002 ) addressed several
issues. In this chapter we will not focus on clarifying these issues, but instead we
present relevant scientific developments and practical considerations for the appli-
cation of a WoE at contaminated sites (Critto et al. 2007 ; Dagnino et al. 2008 ;
Faber 2006 ; Jensen and Mesman 2006 ; Weeks and Comber 2005 ). In addition, we
illustrate these considerations by a newly adopted framework in the Netherlands
(Mesman et al. 2007 ; Rutgers et al. 2008b ).
For terrestrial ecosystems, WoE approaches are still in an exploration and devel-
oping stage (Critto et al. 2007 ; Jensen and Mesman 2006 ; Rutgers and Den Besten
2005 ; Semenzin et al. 2007 , 2008 ; Suter et al. 2000 ). The Triad approach relates
to a specific form of a WoE (Fig. 15.1 ; step 5d). It is based on the simultaneous
deployment of three independent types of assessment tools:

• site-specific chemical characterization often combined with the estimation of

ecotoxicological effects using literature data, e.g. by calculating a risk index;

• application of bioassays or biomarkers in order to determine de novo and ex situ

toxicity in soil samples from the site;

• on-site ecological observations or other monitoring data that provide insight in

the plausible effects of the contamination.

The major assumption is that WoE using a combination of tools from these three
independent disciplines will lead to a more detailed and correct assessment than an
approach, which is solely based on one of these, for example the total concentra-
tions of contaminants at the site. A multidisciplinary approach will thereby help to
minimize the chance on false positive (incorrectly assuming that there are effects,
whilst in reality there are no effects) and false negative (incorrectly assuming that
there are no effects) conclusions.
The advantage of the Triad approach can also be stated as follows: the combina-
tion of three simple instruments enables the reduction of model uncertainties, which
is compatible or better than reducing model uncertainties using one sophisticated
tool. Information about model uncertainties can be deduced from results of tools