Page 222 - Partition & Adsorption of Organic Contaminants in Environmental Systems

INFLUENCE OF SORPTION ON CONTAMINANT ACTIVITY 213

a sharp rise in the vapor fluxes of volatile pesticides (trifluralin and triallate)
from the soil surfaces. Although the increased vapor fluxes may not be signif-
icant for nonvolatile chemicals with a similar soil wetting, their chemical activ-
ities should rise as sharply as those of the volatile chemicals as a result of their
desorption from soils.
A change in the chemical activity of a soil-incorporated pesticide by soil
moisture leads invariably to a change in the apparent pesticidal toxicity.
Upchurch (1957) found that diuron was more toxic to cotton (Gossypium hir-
sutum L.) under moist than under dry soil conditions. Barlow and Hadaway
(1955) observed that chlorinated insecticides (lindane, DDT, and dieldrin)
were inactivated for mosquito control by dry clay but were reactivated under
high humidities. Gerolt (1961) reported that the toxicity of dieldrin in a soil
to insects increased sharply with an increase in ambient humidity. Harris
(1964) observed correlations of the insecticide toxicities of heptachlor, DDT,
diazinon,V-C 13 (dichlofenthion), and parathion on a sandy soil (f om = 0.0052)
and a muck soil (f om = 0.65) with soil-moisture and organic-matter contents
(Table 7.22). As shown, heptachlor was 7.8, DDT 9.9, parathion 24.4, diazinon
132, and V-C 13 189 times more toxic to crickets when the sandy soil was moist
(5.5% water on a dry weight basis) than when it was dry. By contrast, mois-
ture in the muck soil (162% water) had only a marginal effect on the insecti-
cide toxicity. In moist soils all the insecticides were found to be strongly
inactivated by the SOM content, with the extent of inactivation depending on
the specific insecticide; in dry soils there was no obvious correlation between
SOM content and pesticide toxicity. Harris thus concluded that inactivation of
the insecticides in moist soils was proportional to the SOM content, while the
inactivation with dry soils was related to the adsorptive capacity of the mineral
fraction.
The sharp contrast of the effects of soil moisture on the apparent toxicity
of pesticides with mineral and organic soils is closely related to the recognized
effects of soil moisture on the contaminant adsorption on soil minerals and
partition into soil organic matter. The remarkable effect of moisture to sup-
press the adsorption of organic compounds on soil minerals agrees vividly with
our common experience that the air in the field has a pleasant fragrance fol-
lowing a rain shower that succeeds a long period of drought. This phenome-
non does not occur if the field has been sufficiently wet prior to the rain
shower. By such recognition, we now have a good perspective of the role of
humidity in influencing the activity of soil-incorporated contaminants in terms
of the distinct mechanisms through which contaminants are sorbed by soil
organic matter and minerals.