Estimation of exposure point concentrations (Cont.)

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

Chapter 5 Uncertainty in Tier IV Risk Assessments