Human Exposure Pathways - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 460

transport, accumulation and possibly degradation of contaminants. It is important to
realise that the representative concentration in vegetables relates to contaminants in
vegetables that originate from soil at the time of harvesting and after food prepara-
tion. The uptake routes can be via the roots, via the leaves after rain splash or the dry
deposition of soil particles on plant surfaces, and via air. Generally, root uptake is
the most important route, especially for contaminants that are mobile within the soil.
For contaminants that are immobile in the soil, uptake via the leaves can contribute
significantly to the contaminant concentration in leafy vegetables. It is possible to
include the relevant routes of uptake and the impact of food preparation in the cal-
culation of exposure. When contaminant concentrations in vegetables are measured,
it is very difficult to exclude contaminants taken up via external routes that are not
related to the contaminated site. Uptake via the leaves, after atmospheric deposition
is usually the most important source of contaminants originating from outside the
contaminated site. However, measuring makes it easier to deal with the impact of
food preparation (mainly washing).
Calculation of the representative concentration in vegetables is described in
detail in other parts of this topic (see Chapter 8 by McLaughlin et al., for metals;
Chapter 9 by Trapp and Legind, for organic contaminants, this topic). Therefore,
this section will focus on the calculation of human exposure through consumption
of contaminated vegetables only.
11.2.2.2 Differences Between Vegetable Types
Obviously, exposure through consumption of contaminated vegetables depends
strongly on the amount of vegetables consumed. The uptake and accumulation of
contaminants differs greatly between vegetables. There are plants that can accu-
mulate large amounts of metals in above-ground plants and roots and those that
are not effective in the uptake of metals. Generally, fast growing leafy vegetables,
like spinach, endive and broccoli, show high metal uptake and accumulation rates.
Cadmium, however, shows a relatively high accumulation for all vegetables. In addi-
tion, different genotypes of the same crop can exhibit substantial differences in
uptake and accumulation rates. Wu et al. ( 2004 ), for example, demonstrated differ-
ences in cadmium uptake among different cotton genotypes. Specific plant types that
readily take up contaminants, like Brassicaceae (cabbage), Papilonaceae (pods),
Poaceae (grasses), are even used for phytoremediation (Gawronski 2000 ). The lat-
ter group also includes cereals, but they generally do not hyperaccumulate. Some
plants are extremely tolerant for soil metals. These plants grow in areas where natu-
ral ores of heavy metals occur in the upper layers of the soil. A well-known example
of a heavy metal adapted plant, although not an edible plant, is the zinc violet.
The uptake of organic contaminants is related to the root lipid content, which
varies between crops. Concentrations of organic contaminants in vegetables should
therefore be normalised to the lipid content of the vegetables (Simonich and Hites
1995 ). As with metals, the uptake of organic contaminants can also vary between
different varieties within certain plant types. As a consequence, there is a large vari-
ation in the uptake and accumulation of organic contaminants between vegetables.