9. Bioaccumulation and Trophic Transfer of Organic Contaminants
207
Within the effective molecular size range, this relative solubility between the
source compartment and the organism's lipid is a useful predictor of the potential
extent of accumulation. For contaminants that contain ionizable functional
groups, the pK of the contaminant will influence the final storage site in the
organism (Hunn and Allen, 1974). For instance, pentachlorophenol (PCP) ionizes
and has a pKa of 4.74 (Westhall, 1985); its penetration into the organism requires,
in general, that the un-ionized molecule penetrate the membrane (Stehly and
Hayton, 1990), which is reflected by the apparent lipophilicity of the contaminant
as reflected by apparent changes in the octanol/water partition coefficient with pH
(Kaiser and Valdmanis, 1982). In the Great Lakes, where the pH is about 8, PCP
would be highly ionized (approximately 0.05% in the un-ionized form) so penetration of the lipophilic membrane to enter the animal will be limited. In the case
of Diporeia, the uptake clearance for PCP (log Kow 5.01; Westhall, 1985) from
Lake Michigan water was 3.74 mi' g-l . h- 1 (Landrum and Dupuis, 1990),
whereas the uptake clearance for pyrene, a contaminant of similar log Kow (5.2),
was 131 ml . g - 1 • h - 1 (Landrum, 1988). Similarly, the distribution within the
organism may become limited because of ionization in the circulatory fluid of the
organism, which limits distribution within and, in some cases, elimination from
the organism. The impact of ionization is equally important for contaminants
having basic as well as acidic functional groups. The relative state of ionization
will dictate the degree of penetration into the lipophilic environment of the membrane due to changes in lipid solubility of the contaminant.
9.2. Prediction of Bioconcentration and Bioaccumulation
9.2.1. Bioconcentration
Once the contaminant enters the organism and is distributed to the final storage
site, the relative solubility in lipid will permit predictability. As with the distribution between the circulating fluid and the tissue, the partitioning characteristics of
the contaminant between l-octanol and the source can help predict the accumulation potential of the contaminant. This approach works as well for terrestrial
organisms as it does for aquatic organisms (Kenega, 1980). For aquatic organisms
exposed in contaminated water, nonpolar contaminants are accumulated in proportion to the octanollwater partition coefficient (Kow) of contaminants. This was
first demonstrated by a prediction of the bioconcentration of contaminants by fish.
The log of the bioconcentration factor (log BCF is defined as log of the ratio of the
contaminant concentration in the organism to the contaminant concentration in
the water) was linearly correlated with the log Kow (Neely et al., 1974). The use of
this approach was further demonstrated through the work of Veith et aI., (1980,
1979) and Mackay (1982) on fish. Subsequently, the bioconcentration in both
invertebrates and macrophytes was examined with respect to log Kow (Gobas et
aI., 1991; Connell, 1988; Hawker and Connell, 1986). In all cases, there was
evidence of nonlinearity for large, very hydrophobic contaminants. Some of this
207
Within the effective molecular size range, this relative solubility between the
source compartment and the organism's lipid is a useful predictor of the potential
extent of accumulation. For contaminants that contain ionizable functional
groups, the pK of the contaminant will influence the final storage site in the
organism (Hunn and Allen, 1974). For instance, pentachlorophenol (PCP) ionizes
and has a pKa of 4.74 (Westhall, 1985); its penetration into the organism requires,
in general, that the un-ionized molecule penetrate the membrane (Stehly and
Hayton, 1990), which is reflected by the apparent lipophilicity of the contaminant
as reflected by apparent changes in the octanol/water partition coefficient with pH
(Kaiser and Valdmanis, 1982). In the Great Lakes, where the pH is about 8, PCP
would be highly ionized (approximately 0.05% in the un-ionized form) so penetration of the lipophilic membrane to enter the animal will be limited. In the case
of Diporeia, the uptake clearance for PCP (log Kow 5.01; Westhall, 1985) from
Lake Michigan water was 3.74 mi' g-l . h- 1 (Landrum and Dupuis, 1990),
whereas the uptake clearance for pyrene, a contaminant of similar log Kow (5.2),
was 131 ml . g - 1 • h - 1 (Landrum, 1988). Similarly, the distribution within the
organism may become limited because of ionization in the circulatory fluid of the
organism, which limits distribution within and, in some cases, elimination from
the organism. The impact of ionization is equally important for contaminants
having basic as well as acidic functional groups. The relative state of ionization
will dictate the degree of penetration into the lipophilic environment of the membrane due to changes in lipid solubility of the contaminant.
9.2. Prediction of Bioconcentration and Bioaccumulation
9.2.1. Bioconcentration
Once the contaminant enters the organism and is distributed to the final storage
site, the relative solubility in lipid will permit predictability. As with the distribution between the circulating fluid and the tissue, the partitioning characteristics of
the contaminant between l-octanol and the source can help predict the accumulation potential of the contaminant. This approach works as well for terrestrial
organisms as it does for aquatic organisms (Kenega, 1980). For aquatic organisms
exposed in contaminated water, nonpolar contaminants are accumulated in proportion to the octanollwater partition coefficient (Kow) of contaminants. This was
first demonstrated by a prediction of the bioconcentration of contaminants by fish.
The log of the bioconcentration factor (log BCF is defined as log of the ratio of the
contaminant concentration in the organism to the contaminant concentration in
the water) was linearly correlated with the log Kow (Neely et al., 1974). The use of
this approach was further demonstrated through the work of Veith et aI., (1980,
1979) and Mackay (1982) on fish. Subsequently, the bioconcentration in both
invertebrates and macrophytes was examined with respect to log Kow (Gobas et
aI., 1991; Connell, 1988; Hawker and Connell, 1986). In all cases, there was
evidence of nonlinearity for large, very hydrophobic contaminants. Some of this