Uptake of Metals from Soil into Vegetables - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 355

are used to predict toxicity for plants (e.g., Thakali et al. 2006 ) will not be dealt with
here. Although models to predict toxicity are similar to metal uptake models, they
do not describe the metal concentrations or translocation to the food product.
The metal uptake models differ strongly and vary in complexity from very simple
models to detailed growth models with a high requirement for input data. Also the
scale from which they have been used differs from pot experiments to models used
on a national scale (Table 8.10 ).

8.5.1 Model Characteristics

8.5.1.1 Constant Heavy Metal Content for Each Plant Species

This assumption is used in most site-specific studies when the metal concentration
is assessed by actual site measurements (e.g. Sipter et al. 2008 ). In principle it is
assumed that the metal concentrations do not differ within a year, between years
and are not influenced by the applications of soil amendments e.g. lime or changes
in vegetable type over time.

8.5.1.2 Soil-Plant Transfer Models

A simple and commonly-used approach is the bioconcentration factor (BCF) or
bioaccumulation factor (BAF). The terms BCF and BAF originate from studies of
contaminant concentrations in water (mg/L) and biota (mg/kg dw ). Originally BCF
or BAF are expressed as the ratio between the concentration in the biota and water
and are therefore given in L/kg. However several soil Risk Assessment models
such as CLEA (Environmental Agency 2008) and CSOIL (Brand et al. 2007 )use
dimensionless BCF or BAFs, most commonly expressed as the ratio between the
metal/metalloid concentration in the vegetable (mg/kg) and that in soil (mg/kg).
Compilations of BCF values are given by Sauerbeck and Styperek ( 1985 ) and
Bechtel Jacobs ( 1998 ). The metal concentration is either expressed on a fresh or
dry weight basis. The BCF approach is rather easy to apply, but it has been shown
that BCF values are not constant, but depend on the level of soil contamination
(see Section 8.4.1.1 ). Often BCF values decrease with increasing levels of soil
contamination (Wang et al. 2004 ).
Metal concentrations in plant tissues are often linked to those in various soil
extracts by regression analysis. Linear or log transformed data for metal con-
centrations in soil, soil solution or soil extracts are linked to measured plant
concentrations. For metals like cadmium in the soil ( M soil ) and the plant ( M plant ),
Freundlich type functions are often used (Efroymson et al. 2001 ; Krauss et al.
2002 ).

10 a M soil or log [ M plant ] M plant = = a + blog[ M soil ] (8.2)

where M plant is metal concentration in the plant, M soil is metal concentration in the
soil, and a and b are constants.