Bioavalibility in Soils - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 724

bioavailability processes are driven by chemical activity, for example, sorption,
diffusion and partitioning occur from high to low activities. The use of chemical
activity as a component of bioavailability had already been incorporated into such
models as the biotic ligand model (e.g.DiToroetal. 2001 ).
The International Standards Organisation (ISO) has established a working
group on bioavailability (ISO/TC190 - Soil Quality). Harmsen ( 2007 ) summarises
their activities. The definition of bioavailability used by this organisation is
“Bioavailability is the degree to which contaminants present in the soil may be
absorbed or metabolised by human or ecological receptors or are available for
interaction with biological systems” (ISO 2005 ) which is similar to the definition
of bioaccessibility given by Semple et al. However, Harmsen ( 2007 ) argues that
bioavailability is best presented as a concept that is related to specific situations or
measurements on a case by case basis.
To conclude, a variety of definitions for bioavailability have been proposed in the
literature. There is a growing consensus that in broad terms it is a measure of uptake
of and consequent cellular interaction with a contaminant by an organism. There is
also a growing acknowledgement that bioavailability will: (1) vary between organ-
isms, (2) be a function of time of contact between the organism and contaminated
media and, (3) that further, more precise definitions will only be applicable on a
case by case basis. The rest of this chapter will review the impacts that soil proper-
ties have on bioavailability and the various methods (bioassays, chemical extractions
and models) that exist to measure or predict bioavailability.
16.3 Impact of Soil Properties on Bioavailability
To be bioavailable molecules must cross a biological membrane. In effect this means
that the molecules have to interact with the aqueous phase, either the soil's pore
water or in soil passing through the gastrointestinal tract of an organism. Therefore,
soil properties which control partitioning between the solid phase in soil and the
pore water, such as pH, organic matter content, Eh, cation exchange capacity (CEC),
and the concentration of clay minerals and oxyhydroxides, have a significant impact
on bioavailability. Generally, molecules held on exchange sites can readily become
bioavailable by desorption from these sites. Indeed many studies treat molecules
held on exchange sites as part of the bioavailable fraction despite the contradic-
tion that this generates with the above definitions of the term. As a consequence,
an increase in the number of exchange sites aids the retention of molecules in a
bioavailable form. Molecules sorbed more strongly to surfaces or in solid form are
generally not available, whilst molecules in the pore water are available, but are also
prone to being removed from the soil system as solution leaches through the soil.
From here on, inorganic and organic contaminants will be discussed separately,
because their chemical and ecotoxicological properties are rather different. This
results in different fate and consequently different interactions with living cells.
The main characteristics of each group are summarized in Table 16.1 (taken from
Campbell et al. 2006 ).