Page 60 - The Toxicology of Fishes

40 The Toxicology of Fishes

bioavailability of TCDD to organisms in the food chain. Theoretically, more organic carbon in the water
and sediments of an aquatic ecosystem should result in reduced bioavailability of TCDD to fish. A
consequence of low amounts of organic carbon in an aquatic ecosystem would be increased bioavailability
and thus bioaccumulation of TCDD.
In water, the bioavailability of a nonpolar, hydrophobic organic chemical is proportional to the fraction
of chemical that is freely dissolved (f ), which is related to the concentrations of POC and DOC as
fd
follows:
f fd = 1 ≈ 1 (2.8)
(
+
( )
1 + ( POC K poc) + ( DOC K doc) 1 + ( POC DOC /110) K ow)
( )
where organic carbon partition coefficients for POC (K ) and DOC (K ) can be approximated as
doc
poc
K and K /10, respectively (Eadie et al., 1992). Slightly different values of K are commonly used
ow
poc
ow
based on laboratory studies, such as those of Karickhoff (1981), which determine organic carbon
partition coefficients for sediment organic carbon (K ) from sorption/desorption rates for sediment
soc
rather than suspended solids. As a first approximation, one may assume K ≈ K . In the past decade,
poc
soc
considerable debate occurred among environmental chemists concerning the nature of organic chemical
binding to particles in water and the degree to which equilibrium models for the distribution between
water and organic carbon on particles are appropriate for modeling hydrophobic organic chemical
distribution in natural systems (Schrap and Opperhuizen, 1992). Measurement of K or K for TCDD
soc
poc
is complicated by the inability to conduct experiments with no organic carbon in the water. Extrapo-
lation of TCDD sediment desorption measurements to zero aqueous organic carbon resulted in an
estimate of log K of 7.25 to 7.59 (Lodge and Cook, 1989), a value significantly greater than the
soc
estimated log K . Aside from the possibility that log K has been underestimated for TCDD, this
ow
ow
discrepancy might be due to higher affinities of planar aromatic compounds such as TCDD for black
carbon* in sediments (Barring et al., 2002) than for other sediment carbon, for which TCDD parti-
tioning more closely follows K .
ow
The degree to which values of K are less than K or K is also uncertain and may vary with the
doc
soc
poc

chemical composition of dissolved organic carbon in different ecosystems (Burkhard, 2000; Chin and
Gschwend, 1992; Eadie et al., 1990). Clearly, accuracy in the prediction of concentrations of freely
dissolved TCDD in different waters depends heavily on the accuracy of the determination of K for
ow
TCDD. Figure 2.14 demonstrates how, under different POC conditions, f varies for TCDD with an
fd
estimated log K of 7 vs. an estimated log K of 8. A major impediment to the direct determination
ow
ow
of concentrations of freely dissolved TCDD in water is the difficulty of separating freely dissolved
TCDD from bound TCDD in water samples, combined with the challenge of detecting and measuring
concentrations on the order of femtograms/liter (parts per quintillion).
Because a major fraction of TCDD in aquatic systems is associated with organic carbon, exposure of
aquatic organisms to TCDD and the resulting bioaccumulation depend greatly on each organism’s
connectivity to sediments or suspended particles. Benthic organisms have direct contact with sediment
and accumulate hydrophobic aromatic chemicals to a greater extent through ingestion of sediment than
from exposure to pore water (Leppänen and Kukkonen, 1998). If the concentration of TCDD varies with
depth in the sediment, the depth to which the organism burrows in the sediment will influence exposure.
Ingestion of TCDD with food by oligochaetes (Lumbriculus variegatus) and midges (Chironomus
tentans) results in accumulation of TCDD in the organisms at levels close to equilibrium with the food
(West et al., 1997). Slow rates of elimination from these benthic invertebrates suggest an inability to
metabolize TCDD. Bioaccumulation of organic chemicals with logK values greater than 5.5 by pelagic

ow
fish can be strongly influenced by the presence of a benthic food chain component in the diet (Burkhard,
* Black carbon refers to carbon from a variety of sources such as combustion particles (soot), fly ash, coal dust, and tire dust.
The different forms of black carbon can have different partitioning properties than carbon originating from low-temperature
biogenic processes. For sediments, the term organic carbon includes both biogenic and black carbon, in contrast to inorganic
forms of carbon such as carbonates. Black carbon usually comprises a small fraction of the total organic carbon measured yet
has the potential to exert a greater relative influence on partitioning and bioavailability of planar organic chemicals such as
TCDD and BaP.