Page 151 - The Toxicology of Fishes

Toxicokinetics in Fishes 131

C biota = L biota K C w (3.128)
ow
This equation suggests that the contaminant concentration in the organism can be estimated from the
concentration freely dissolved in pore water. Hellou et al. (1995) suggested that K and K are not
ow
oc
equal but are instead related by a proportionality constant. A modification of Equation 3.128 was derived
by Hellou et al. (1995) by assuming that K = 0.4K and that the density of sediment is 1.6 kg/L:
ow
oc
BSAF = L biota ( OC s × 064 ) (3.129)
.
Assumptions of the EqP theory include concentration-independent uptake and an absence of biotrans-
formation or degradation of the contaminant. An implicit assumption of the EqP theory is that the
bioavailable concentration in water is the freely dissolved portion; the presence of DOC does not,
therefore, affect equilibrium partitioning. According to EqP theory, the equilibrium level accumulated
by an organism is independent of the number and types of exposure routes (e.g., sediment ingestion or
pore water exposure).
An extensive review of data supporting the EqP theory was given by Di Toro et al. (1991), who
concluded that sediment-to-sediment variation in bioavailability (assessed by toxicity) can be reduced
by a factor of two or three by application of EqP theory and that particle size effects are minimal. Using
the principles of EqP theory, the U.S. Environmental Protection Agency has developed procedures to
derive equilibrium partitioning sediment benchmarks (ESBs) for the protection of benthic organisms
from adverse effects due to nonionic organic compounds (U.S. EPA, 2003).

Kinetic Models for Chemical Accumulation from Sediment
Kinetic models of contaminant bioaccumulation from sediment have been primarily developed for benthic
invertebrates, including amphipods, insects, and mollusks (Lee, 1992). The simplest model is:
BAF = k k el (3.130)
s

where k is the sediment uptake constant (g sediment/[g tissue × time]), expressed as a clearance term,
s
and k is the elimination rate constant (1/time). This model is analogous to that given previously (Equation
el
3.124) to describe chemical bioconcentration from water. According to the model, sediment BAFs are
independent of the chemical concentration in sediment but increase with any factor that increases the
uptake rate constant or decreases the elimination rate constant.

Food Web Models of Bioaccumulation in a Sediment–Water System
Food web bioaccumulation models describe the contaminant mass balance in biota that comprise an
aquatic food web (Thomann et al., 1992b). Contaminant concentrations in biota are calculated using
mathematical equations that describe the dominant uptake and elimination processes. These processes
may include equilibrium partitioning (e.g., sediments to benthos, water to plankton), chemical uptake
from water, ingestion of contaminated food, growth, and excretion. Contaminant concentrations in source
compartments (water and sediments) are generally assumed to have a homogeneous distribution and to
be in steady-state equilibrium with biota comprising the lowest level of the food web.
Food web models are particularly useful for compounds that bioaccumulate in plankton and benthic
invertebrates and then biomagnify in fish through successive trophic transfers. These models are used
extensively to estimate BAFs, BSAFs, and BMFs and to determine the relative importance of benthic
(sediment) and pelagic (water) contaminant sources. In a regulatory setting, these models can be used
to characterize a contaminated site and evaluate various remedial options. The utility of this approach
is limited, however, by the need to incorporate a large number of parameters, many with high levels of
uncertainty. Model parameters typically include body size, temperature, feeding rate, prey selection,
lipid content, and dietary bioavailability (absorption efficiency). The proportion of time each trophic
group feeds within the contaminated food web may be an additional source of variability and uncertainty.