Environmental Engineering Reference
In-Depth Information
trials, where soil-root uptake dominates, to field studies and surveys where both
soil and atmospheric uptake pathways are important. Uptake by different vegetables
varies according to metal/metalloid (and is not always consistent across sites), and in
Table 8.9 vegetables are ranked according to their potential to accumulate contami-
nants. Cobalt and molybdenum are not listed, despite being in the group of elements
likely to pose risks through food chain accumulation, due to the lack of data for
these elements. It can be seen that in general leafy vegetables tend to be 'accumula-
tors', and fruiting vegetables tend to be 'excluders' (Preer et al. 1980 ). This is likely
because leaves accumulate ions through transpiration and aerial deposition, and non-
essential elements may therefore accumulate in these tissues if the element is trans-
ported across the root membrane or deposited on the leaf surface. Fruits are prin-
cipally formed by movement of nutrients in phloem (the nutrient transport system)
into the fruit across a membrane from the xylem (the water transport system) within
the plant, and transfer of many elements from xylem to phloem may be controlled,
or affected by many counter ions, similar to root uptake processes (Section 8.4.1 ).
For aerial deposition, which is mainly important for lead, species morphology (e.g.
hairiness of leaves, surface roughness of aerial tissues) may be important in the
trapping of atmospheric particulates and aerosols (Tiller et al. 1976 , 1997 ).
8.4.3 Vegetable Cultivar
Most of the work on effects of vegetable cultivar on accumulation of metals or
metalloids has focused on cadmium, since this is the element that is most likely to
pose a food-chain hazard. There may be significant differences in cadmium uptake
by different cultivars of the same vegetable species grown on a contaminated site,
as was demonstrated for lettuce (Florijn et al. 1991 ; Thomas and Harrison 1989 ;
Wang et al. 2007 ; Xue and Harrison 1991 ; Yuran and Harrison 1986 ), potato (Harris
et al. 1981 ; McLaughlin et al. 1994b ), curcurbits (Mattina et al. 2006 ), beans (Guo
and Marschner 1996 ), carrots and peas (Alexander et al. 2006 ). Cultivar effects vary
from insignificant up to a 3- to 4-fold difference in cadmium concentrations. Cultivar
effects are therefore less pronounced than the influence of the type of vegetables, but
selection of non-accumulating culivars may provide an important Risk Management
action for minimizing exposure to humans through consumption of vegetables.
8.4.4 Soil Physical/Chemical Properties
In considering the effect of soil chemical and physical properties on metal/metalloid
availability to plants, the type of contaminant, valence and charge, and other prop-
erties that determine fate in soil (Section 8.3 ) need to be considered. Anionic
metalloids, for example, will respond differently in terms of crop accumulation
compared to cationic metals. The principal factors governing vegetable uptake are
discussed below.
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