Bioavalibility in Soils - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 740

Kp Kp

Soil solids Soil solution

feeding

Effects on: growth,
metabolism,
reproduction, etc.

BCF

Biota

=

Fig. 16.2 Schematic representation of the equilibrium partitioning theory (EPT) (Kp partition-

ing coefficient, BCF = bioconcentration factor)

It should be noted that the fugacity concept and the concept of equilibrium parti-
tioning share many communalities and in fact, Reichenberg and Mayer ( 2006 ) added
another concept: the thermodynamic concept of the chemical potential. In practical
terms, the concepts have in common that it is the concentration of the contaminant
in the pore water that is to be used as the basis for modelling uptake and effects of
contaminants.
A model developed by Belfroid et al. ( 1995 ) in which estimates of accumulation
of organic contaminants by earthworms (inputs: bioconcentration in water, sorption
and elimination constants, rate of soil ingestion, and uptake efficiency) were gen-
erated, showed that in most cases uptake from pore water is the dominant uptake
route. However, for contaminants with Log K ow exceeding 5, oral uptake may con-
tribute significantly: about 10% for soils with an average organic matter content
of 3%, and about 50% for soils with an organic matter content of 20% (Belfroid
et al. 1995 ). This implies that at maximum, the modelled concentration of organic
contaminants exceeds the EPT prediction by a factor of 2. As reported by Jager
( 2003 ), this is similar to the differences found in general when comparing values
for the bioaccumulation factor (BAF) and the bioconcentration factor (BCF). The
BAF and BCF represent one of the most simplified single-compartment models for
bioaccumulation, predicting partitioning between exposure medium and biota. Both
BCF and BAF are generally calculated as the ratio, at equilibrium, of internal biota
concentration to exposure concentration, with BCF being related to accumulation in
organisms arising from water only, and BAF to accumulation from water and dietary
sources (McGeer et al. 2003 ). In general, BAF is derived from measurements in the
field and BCF is more readily measured under laboratory conditions.
Further support, although not for a soil inhabiting species, is obtained from exper-
iments with Lumbriculus variegatus . Sormunen et al. ( 2008 ) showed that these
sediment-ingesting worms have access to an additional bioavailable fraction of
PCBs on top of the pore water pool that was especially evident when pore-water
concentrations of PCB 77 approached the solubility limit. Thus, feeding may mod-
ify the bioavailable fraction in a way that cannot be explained by simple equilibrium
partitioning models.
In laboratory tests, the validity of the EPT has been confirmed for various organ-
isms like earthworms, enchytraeids, and nematodes. Almost the same evidence
has been found for collembolans and isopods, while no prediction was possi-
ble for snails due to specific uptake mechanisms. Thus, evidence exists that pore
water is the main uptake route for the above-mentioned organism taxa, at least for