Uptake of Metals from Soil into Vegetables - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 354

uptake of anionic metals/metalloids, due to displacement of these ions from sorb-
ing surfaces in soil (Cao and Ma 2004 ; Creger and Peryea 1994 ). High additions of
potassium (K) fertiliser as KCl can enhance cadmium uptake by crops due to the
chloro-complexation issue described in Section 8.4.4.4 (Sparrow et al. 1994 ).
In many urban garden soils, fertilisers are often added in excess of plant
requirements, so this factor may play an important role in metal accumulation by
vegetables. In particular, it is important that N (from either manufactured fertilisers
or organic sources of N) is not used in excess, as this rapidly acidifies the soil and
will enhance cationic metal uptake by vegetables.

8.5 Models to Predict Contaminant Uptake by, or Toxicity
to, Vegetables

Different models with increasing complexity can be used to predict metal concentra-
tions in vegetables (Table 8.10 ). Among the easiest to apply are those that assume a
constant plant metal concentration or the ones that relate the plant metal concentra-
tion to certain soil parameters: an equilibrium approach. In more process-oriented
approaches, the concentration is not necessarily constant throughout the growing
period. Here also the uptake of metals is a function of certain soil parameters, but
also specific plant parameters that for instance regulate water uptake, such as root
growth and length.
An important aspect related to Risk Assessment is that only the metal con-
centrations in the edible products of the plant (root, stem or fruit) are considered
relevant, although metals are present in all plant parts. Identification of those pro-
cesses responsible for uptake of metals have been described in papers such as by
Clemens ( 2001 ) and Hall ( 2002 ) and will not be dealt with here. Also models that

Table 8.10 Overview of model approaches to predict heavy metal contents in vegetables

Type of approach Examples Scale of use Advantages/problems

1. Constant heavy
metal content

Site-specific Risk
Assessment

2. Soil-plant transfer
models

BAF, BCF,
regression
models

Site-specific/national
Risk Assessment

Advantage: available
soil parameters

3. Free ion activity
model

FIAM

4. Biotic ligand
model

BLM Experimental Problem: parameters

are not available at
regional/national
scale,

5. Physiological
models

Site-specific Advantage: predicts

differences
between years

6. Barber-Cushman
mechanistic model

Experimental