McKay, Donald. "Front matter" Multimedia Environmental Models ...

5.5 ENVIRONMENTAL PARTITION COEFFICIENTS AND Z VALUES
5.5.1 Introduction
Our aim is now to use the physical chemical data to predict how a chemical will
partition in the environment. Information on air-water-octanol partitioning is invaluable
and can be used directly in the case of air and pure water, but the challenge
remains of treating other media such as soils, sediments, vegetation, animals, and
fish. The general strategy is to relate partition coefficients involving these media to
partitioning involving octanol. We thus, for example, seek relationships between
K OW and soil-water or fish-water partitioning.
5.5.2 Organic Carbon-Water Partition Coefficients
Studies by agricultural chemists have revealed that hydrophobic organic chemicals
tended to sorb primarily to the organic matter present in soils. Similar observations
have been made for bottom sediments. In a definitive study, Karickhoff
(1981) showed that organic carbon was almost entirely responsible for the sorbing
capacity of sediments and that the partition coefficient between sediment and water
expressed in terms of an organic carbon partition coefficient (K OC) was closely related
to the octanol-water partition coefficient. Indeed, the simple relationship was established
to be
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K OC = 0.41 K OW
This relationship is based on experiments in which a soil-water partition coefficient
was measured for a variety of soils of varying organic carbon content (y) and
chemicals of varying K OW. The soil concentration was measured in units of mg/g or
mg/kg (usually of dry soil) and the water in units of mg/cm 3 or mg/L. The ratio of
soil and water concentration (designated K P) thus has units of L/kg or reciprocal
density.
K P = C S/C W (mg/kg)/(mg/L) = L/kg
If a truly dimensionless partition coefficient is desired, it is necessary to multiply
K P by the soil density in kg/L (typically 2.5), or equivalently multiply C S by density
to give a concentration in units of mg/L. A plot of K P versus organic carbon content,
y (g/g), proves to be nearly linear and passes close to the origin, suggesting the
relationship
K P = y K OC
where K OC is an organic carbon-water partition coefficient.
In practice, there is usually a slight intercept, thus the relationship must be used
with caution when y is less than 0.01, and especially when less than 0.001. Since
y is dimensionless, K OC, like K P also has units of L/kg. Measurements of K OC for a