Sorption of organic pollutants to soils is an environmentally
important process that affects properties such as bioavailability,
ecotoxicity, rate of degradation and their mobility. The affinity of
a soil for a given molecule is usually expressed as a partition constant
(Kd), which is simply the sorbed concentration divided by the
solution concentration. In general, Kd increases with the organic C
content of the soil, and hence Kd values are often normalized to the
soil C content by dividing by the organic C fraction (fOC) to give KOC.
However, the relationship between Kd and fOC is not an exact one,
and hence KOC for a given molecule can vary considerably between
soils. In a review of published KOC measurements, Wauchope et al.
(2002) reported that where KOC had been determined for a large
number of soils, the coefficient of variation in KOC was generally
around 40–60% and that the ratio of maximum to minimum KOC
was generally around 3–10. They suggested variations in the nature
of organic matter between soils may be a major contributor
to this variation.
There is ample evidence that KOC varies widely for different
types of isolated organic matter such as different plant biopoly