Page 220 - Partition & Adsorption of Organic Contaminants in Environmental Systems

INFLUENCE OF SORPTION ON CONTAMINANT ACTIVITY 211

compounds, the amount and type of dissolved and suspended organic matters.
The individual sorption coefficients of the polar contaminants may also be
subject to change to a certain extent with their own concentrations and with
the in situ concentrations of other compounds. In nonaqueous systems, the
important factors are the mineral type and content, the ambient humidity (or
soil-moisture content), and the contaminant and solvent (medium) polarities.
An important consequence of the suppression of mineral adsorption by
water and the partition into SOM of contaminants in soil-water systems is that
the contaminant mass sorbed to a soil (or sediment) should usually be only a
small fraction of the SOM content, regardless of the contaminant level in
water. This is because most contaminants of interest have a small solubility in
SOM. For example, Choi and Chen (1976) found that concentrations of chlo-
rinated hydrocarbons (DDT, DDE, and PCBs) in sediments of a marine site
were well correlated with the organic carbon contents of the sediments; total
chlorinated hydrocarbon concentrations ranged from 0.3 to 3.5mg per kilo-
gram of sediment, whereas total organic carbon contents ranged from 4.5 to
17g per kilogram of sediment on a dry weight basis. Similarly, Goerlitz et al.
(1985) found that sandy aquifer sediments, which contained trace amounts of
organic matter, in a creosote-contaminated groundwater site exhibited little
retention of substituted phenols and PAHs, despite the fact that the ground-
water was contaminated with significant levels of these compounds. By this
account, a more sensible criterion for evaluating the extent of sediment con-
tamination or the sediment quality in an aquatic system should be based on
the contaminant level normalized to the SOM content rather than on the level
with respect to the whole sediment.
The contaminant activity in a terrestrial environment is expected to be
influenced most sensitively by a change in soil-water content from above to
below the saturation capacity in the field, because the drying–wetting cycle
sharply affects the contaminant uptake by soils. The very top layer of surficial
soils undergoes a drying–wetting transition, the extent of which depends on
ambient humidity and field operation. Under relatively dry conditions, strong
adsorption by soil minerals (particularly, high-surface-area clays) along with
the partition into SOM lowers the activity (as measured by P/P°) of a con-
taminant in the soil. Upon wetting, the chemical activity would rise rapidly
and sharply because of the displacement by water of those species previously
adsorbed on soil minerals (Goring, 1967; Chiou and Shoup, 1985). Volatile
compounds sorbed initially to dry soils and clays are thus readily released by
wetting the soil and clay (Chisholm and Koblitsky, 1943; Stark, 1948; Hanson
and Nex, 1953; Wade, 1954; Goring, 1967; Chiou and Shoup, 1985; Pennell et
al., 1992;Thibaud et al., 1993). In fields to which pesticides have been applied,
the rainfall event and the dew deposition on soil surfaces trigger large and
sudden increases of volatile pesticides into the air (Glotfelty et al., 1984;
Grover et al., 1988; Majewski et al., 1993). If the soil surface is relatively dry,
the evaporative fluxes of pesticides are usually small. An example illustrating
this phenomenon is given in Figure 7.48, where rainfall in a field brought about