Introduction to Ecological Risk Assessment - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 595

exposure itself is generally not determined in Ecological Risk Assessment, when
considering soil organisms. For insight into Ecological Risk Assessment, however,
it is useful to understand the exposure of the soil ecosystem. Typically, soil organ-
isms reside in the (partly water-filled) soil pores in close contact with contaminants,
practically all their lives. As a consequence, unlike human beings, soil organisms are
more or less constantly exposed to contaminants. However, the intensity of expo-
sure changes over time and is very different between species. Generally, two type of
exposure occur, that is, oral and dermal exposure. Oral exposure happens through
the consumption of other organisms, their excrements and soil particles with or with-
out organic matter, which contain contaminants. This oral exposure is dependent
on the gut conditions of the organisms (see, for example, Vijver ( 2005 )). Dermal
exposure mainly occurs via dissolved organic contaminants in the pore water. Some
species either are exposed through dermal or oral exposure only, while other species
are exposed through both oral and dermal exposure.
Ideally, the combined effect of all contaminants present on the ecosystem is
assessed (e.g., De Zwart and Posthuma 2005 ). In the Netherlands, for example, the
local toxic pressure of mixtures is included in the latest revision of the Dutch Soil
Protection Act, by applying the ms (multi substance)-PAF method (Boekhold 2008 ),
based on the Species Sensitivity Distribution concept (see Section 13.6.2 ).
13.5.3.2 Bioavailability
An important phenomenon with regard to exposure, most certainly in terms of der-
mal exposure, is bioavailability (Peijnenburg et al. 1997 ). Bioavailability relates to
a specific fraction of contaminants in soil that is 'operational', that is, the bioavail-
able or effective fraction . This fraction is potentially available to be taken up by
organisms, including plant roots, while other fractions are (temporarily) connected
to solid soil phases such as organic matter particles, clay minerals, or (hydr)oxyde
surfaces, and hence (temporarily) not available. The bioavailable fraction is mainly
relevant with regard to dermal exposure. For oral exposure, the bioaccessibil-
ity of contaminants adsorbed to organic matter in the organisms controls internal
exposure.
The bioavailability of a contaminant is a dynamic variable, dependent on the
chemical characteristics of the contaminant and the soil, and on the characteristics
of the organism involved. Bioavailability is related to pore water concentration and
to the desorption or dissolution rate of a contaminant.
It is important to realize that the relevant bioavailable fraction depends very much
on the time frame that is of interest. In fact, part of the contaminants are read-
ily available, another part will become available within a period of days, weeks,
months or years, while others virtually never will become available. The theory of
bioavailability is strongly dependent on the type of contaminant. A very useful clas-
sification with regard to bioavailability characteristics relates to the two main classes
of contaminants, that is, metals versus organic contaminants.
There is, however, some political controversy with regard to bioavailability. This
controversy relates to the time factor. On the one hand, a higher bioavailability of