Page 237 - Partition & Adsorption of Organic Contaminants in Environmental Systems

228 CONTAMINANT UPTAKE BY PLANTS FROM SOIL AND WATER

ties. As may be seen, if the C s values are not normalized to the SOM contents,
the C s/C pt ratios vary radically, showing no apparent correlation. Similar results
are noted for dieldrin with radishes and for DDT with carrots and radishes;
however, the data here show more uncertainties where the contamination
levels in the crops were either near or below detection limits (<0.01ppm)
(Harris and Sans, 1967).

8.7 EFFECT OF PLANT COMPOSITION

The generally lower uptake of dieldrin by radishes than by carrots appears
to result largely from the difference in their lipid contents. This effect is
illustrated by the finding that the C pt/C w ratios for dieldrin with a given soil
for radishes are about one-half the values for carrots, which are in accord
with the presumed difference in their lipid contents. Moreover, the present
dieldrin concentration factor (C pt/C w) with carrots correlates well with the
dieldrin concentration factor with barley seedlings described earlier, in
which a fivefold difference in crop lipids leads to about a fivefold difference
in C pt/C w values. Although the close agreement of the data in both cases
seems a bit fortuitous for reasons stated earlier, the results suggest that plant
lipids are the major factor for the observed difference in plant uptakes of
the lipophilic contaminants. This view is further supported by the results of
Lichtenstein (1960) on the uptakes of lipophilic aldrin, dieldrin, heptachlor,
and heptachlor epoxide from a soil by carrots, radishes, beets, potatoes, onions,
and lettuce. In this experiment, the field plots of a Carrington silt loam
were treated with high insecticide levels on which various crops were seeded,
and soil and crop contaminant levels were analyzed at harvest; however,
information on the SOM content of the soil was not available. An essential
portion of Lichtenstein’s data on soil and crop contaminant levels is given in
Table 8.4.
Data in Table 8.4 reveal that the whole-crop levels of these relatively water-
insoluble pesticides in radishes (Early Scarlet Globe), beets (Detroit Dark
Red), potatoes (Russet Sebago), onions (Yellow Globe Danvers), and lettuce
(Great Lakes) were all significantly lower than in carrots (Red Cored Chante-
nay), despite significant variations in relative contaminant levels with differ-
ent crops. Although the lipid contents for all these crops are very low, carrots
appear to have a comparatively greater lipid content than the others accord-
ing to the USDA Nutrient Database. Thus, the levels for each of the pesticides
in these crops seem to be influenced most by the plant lipids. Significant vari-
ations in relative contaminant levels among crops may stem from various
unspecified sources, such as the analytical sensitivity, the sample preparation
loss with soil and crops, and other factors mentioned previously. Detailed eval-
uations of individual contaminant levels with these crops require the SOM
content of the soil, accurate lipid data, and accurate K ow and K som values which
are not readily available.