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

variety of chemicals show that K OC is related to K OW as discussed above. K OW is
dimensionless, thus the constant 0.41 has dimensions of L/kg. Care must be taken
to use consistent units in these calculations. For example, if the water concentration
has units of mol/m 3 , and K P is applied, the soil concentration will be in mol/Mg,
i.e., moles per 10 6 grams. The usual units used are mg/L in water and mg/kg in soil.
Units of either mass (g) or amount (mol) of solute can be used, but they must be
consistent in both water and soil.
The relationship between K OW and K OC has been the subject of considerable
investigation, and it appears to be variable. For example, DiToro (1985) has suggested
that, for suspended matter in water, K OC approximately equals K OW. Other
workers, notably Gauthier et al. (1987), have shown that the sorbing quality of the
organic carbon varies and appears to be related to its aromatic content as revealed
by NMR analysis.
Gawlik et al. (1997) have recently reviewed some 170 correlations between K OC
and K OW, solubility in water, liquid chromatographic retention time, and various
molecular descriptors. They could not recommend a single correlation as being
applicable to all substances. Seth et al. (1999) analyzed these data and suggested
that K OC is best approximated as 0.35 K OW (a coefficient slightly lower than Karickhoff’s
0.41) but that the variability is up to a factor of 2.5 in either direction. It is
thus expected that, depending on the nature of the organic carbon, K OC can be as
high as 0.9 K OW and as low as 0.14 K OW. Values outside this range may occur because
of unusual combinations of chemical and organic matter. Doucette (2000) has given
a very comprehensive review of this issue, and Baum (1997) has reviewed estimation
methods.
In summary, Z values can be calculated for soils and sediments containing
organic carbon of 0.35 Z Oy(r/1000) or 0.41 Z Oy(r/1000), where Z O is for octanol,
y is the organic carbon content, and r is the solid density, typically 2500 kg/m 3 . If
an organic matter content is given, the organic carbon content can be estimated as
56% of the organic matter content.
These relationships provide a very convenient method of calculating the extent
of sorption of chemicals between soils or sediments and water, provided that the
organic carbon content of the soil and the chemical’s octanol-water partition coefficient
are known. This is illustrated in Example 5.4 below.
Worked Example 5.4
Estimate the partition coefficient between a soil containing 0.02 g/g of organic
carbon for benzene (K OW of 135) and DDT (log K OW of 6.19), and the concentrations
in soil in equilibrium with water containing 0.001 g/m 3 , using the Karickhoff (0.41)
correlation.
benzene K OC = 0.41 K OW = 55, K P = 0.02 K OC = 1.1 L/kg
DDT K OC = 0.41 K OW = 635000, K P = 0.02 K OC = 12700 L/kg
K P and K OC have units of L/kg or m 3 /Mg, i.e., reciprocal density thus when
applying the equation below C S the soil concentration will have units of g/Mg or mg/g.
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