Page 235 - Partition & Adsorption of Organic Contaminants in Environmental Systems

226 CONTAMINANT UPTAKE BY PLANTS FROM SOIL AND WATER

The results in Table 8.2 reveal that although the a pt values for compounds
with high lipid-to-water solubility ratios (i.e., those with high K ow values) are
significantly less than 1, their concentration factors from soil interstitial water
(C w or C som /K som ) to the plant (C pt ) (i.e., the C pt /C w ratios) are markedly higher
than for relatively water-soluble compounds (e.g., atrazine). As manifested in
Table 8.2, the C pt /C w ratio increases largely with increasing K ow because a net
increase in K ow outweighs the resulting decrease in a pt , as exemplified, for
example, by the data of dieldrin and DDT. As such, extremely water-
insoluble DDT with a small a pt = 0.11 exhibits, nevertheless, a large concentra-
tion factor, about 2600, from soil interstitial water into the barley, based on
the assumed lipid content (1%) and lipid–water partition coefficient (K ow ).
Here the concentration of DDT inside the plant is only about 10% of the equi-
librium value with respect to the soil-interstitial-water concentration; the the-
oretical concentration factor at equilibrium would be about 10 times greater
than observed. The small a pt values for DDT and other compounds with large
lipid-to-water solubility ratios may be attributed to insufficient amounts of
external-water transport into the plant circulatory system for these com-
pounds to achieve the equilibrium partition capacities.

8.6 UPTAKE BY ROOT CROPS FROM DIFFERENT SOILS

A very instructive work on the effect of soil type on contaminant uptake by
crops is that of Harris and Sans (1967), who measured the uptake of dieldrin
and DDT by several root crops grown in three contaminated field plots of
widely different soil types. Each of the three field plots maintained stable
levels of dieldrin and DDT during the growing season. The soils studied were
as follows: a Fox sandy loam (1.4% SOM), a clay soil (3.6% SOM), and a muck
(66.5% SOM). The plots were seeded during mid-May; the growing season
varied with the crop type and lasted, for example, about one month for
radishes (Sparkler White Tip) and three months for carrots (Nantes). Soil
insecticide-residue levels before seeding and after harvesting were measured
and no significant changes were observed. The levels of dieldrin and DDT in
crops and soils observed by Harris and Sans (1967) are given in Table 8.3. In
addition to the original insecticide concentrations in soils (C s ), the SOM-
normalized soil concentrations (C som ) are also presented.
Calculations of the a pt values for dieldrin and DDT with each soil–crop
system have been made using the K ow and K som values from Table 8.2 and the
assumed compositions of the root crops. Among the root crops studied
(carrots, radishes, turnips, and onions), carrots showed the highest uptake from
the soils and radishes exhibited trace levels, while the uptake by turnips and
onions was near or below the detection limit (<0.01ppm). For DDT, the
observed levels in these crops were mostly below the detection limit, and thus
the data were more limited and less precise. The present analysis is confined
largely to the dieldrin levels in carrots and radishes and to a lesser extent