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Page Title: Estimation of exposure point concentrations (Cont.)
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The octanol-water partition coefficient Kow describes the hydrophobicity of
various organic COCs, and is used to predict the partitioning of these COCs
between water and sediment. Because measuring the Kow of highly hydrophobic
compounds can be technically difficult, a range of values, representing various
techniques for measuring or modeling Kow, have been reported for most organic
COCs. For example, reported values for the log10 Kow of benz[a]anthracene range
from 5.61 to 7.50 (Mackay, Shiu, and Ma 1992a, 1992b). Comparisons of the
results of various methods that are used to determine Kow indicate that the slow-
stir flask method has less variability and experimental bias and provides the best
estimate of Kow (USEPA 1993b). Professional judgment must be used when
selecting the Kow that will be used in a risk assessment. Consequently,
uncertainties contributing to over- or underestimates of risk can occur.
The organic carbon-water partition coefficient Koc estimates the
distribution of an organic contaminant between sediment organic matter and the
freely dissolved phase in water, and is used to predict the concentrations of these
COCs in water and sediment. The Koc of a compound can be measured or
estimated from Kow based on a regression relationship (DiToro et al. 1991).
Typically, the literature provides a range of values for Koc for an individual
organic compound (Mackay, Shiu, and Ma 1992a, 1992b). For example, reported
values for the log10 Koc for benz[a]anthracene range from 4.0 to 7.30.
Professional judgment must be used when selecting the most appropriate value
from the literature. As a result, uncertainties contributing to over- or
underestimates of risk can occur.
Apparent values of Koc that are measured in field-collected sediments can be
larger than predicted. For example, PAHs that are physically sequestered into
soot particles may be less available for partitioning into soil- or sediment-
associated pore water, resulting in a Koc that is higher than predicted (Gustafsson
et al. 1997). One study reported an apparent Koc for partitioning of phenanthrene
between field-collected sediments and associated pore water that was almost
three orders of magnitude greater than a Koc reported in the literature (McGroddy
and Farrington 1995).
Alternatively, apparent values of Koc can be smaller than predicted if a greater
amount of an organic contaminant partitions into the pore water than would be
predicted based on the concentration in the sediment. Higher than expected
concentrations of organic contaminants in the pore water are taken into account
by a three-phase model, which includes partitioning of organic contaminants onto
dissolved organic carbon (DOC), such as colloids (Brownawell and Farrington
1986). For example, the lowest reported apparent log10 Koc for PCB-18 (2,2',
5-trichlorobiphenyl) in one field study was 3.05 (Brownawell and Farrington
1986), much lower than a value reported from the literature of 5.24 (Hawker and
Connell 1988).
The dissolved organic carbon-water partition coefficient Kdoc is an
important parameter in the estimation of the partitioning of organic contaminants
between the "freely dissolved" phase and the phase associated with DOC in
water. It is generally believed that only chemicals in true solution (i.e., "freely
dissolved") are bioavailable. Partitioning of a hydrophobic compound onto DOC
reduces uptake of dissolved organic contaminants from water into fish
35
Chapter 5 Uncertainty in Tier IV Risk Assessments

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