Uptake of Organic Contaminants from Soil into Vegetables and Fruits - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 388

concentrations in the peel of carrots, when grown at a PCDD/F-contaminated site.
No increase of the concentration in lettuce and peas was found. In a similar study,
it could be shown that the transfer of PCDD/F into apples and pears is exclusively
from air (Müller et al. 1993 ). The results from these studies confirm the model
predictions in which uptake of lipophilic contaminants into above-ground plant parts
is primarily from air. However, there are exceptions. Hülster et al. ( 1994 ) could proof
a transfer of PCCD/F into zucchini and pumpkins in field experiments, which was
much higher than for other fruits.
Another example is benzo(a)pyrene (BaP). The range of measured concentra-
tions of BaP in food crops is quite large, from 0.01 to 48 gkg − 1 . Generally, leafy

vegetables and lettuce had the highest concentrations, followed by grains, potato
and root vegetables (Kazerouni et al. 2001 ; Samsøe-Petersen et al. 2002 ;SCF 2002 ).
This indicates air as the primary source for contamination of vegetables with PAHs.
Uptake of organochlorine pesticides and polychlorinated biphenyls (PCBs) from
soil and air into radishes were measured at a contaminated field site in the Czech
Republic (Mikes et al. 2009 ). Root concentration factors ( RCF ) and bioconcentra-
tion factors for leaves ( BCF L ) were determined by linear regression. The transfer
from soil into leaves (average BCF L 0.08 kg kg dw − 1 ) was rather constant for all
contaminants and could best be explained by soil particle attachment. Generally,
uptake from air was higher than uptake from soil. Uptake from air into radish
roots and bulbs was observed. The example in Section 9.3.1 was taken from this
study.

μ

9.5.4 Dissipation from Soil

The simulations above were done for the steady-state situation, with constant con-
centration in soil. However, contaminants can rapidly dissipate from soil by several
removal processes, such as degradation, plant uptake, leaching, volatilization and
sequestration.
An example is shown in Fig. 9.7 . Ortho-xylene is a moderately lipophilic con-
taminant which is rapidly degraded in soil. Søvik et al. ( 2002 ) give a first-order
degradation rate constant of 0.11 d − 1 , i.e. a half-life of about 1 week. After a few
weeks, the concentration in soil has approached zero. The simulated concentration
in roots closely follows this pattern: About four days after the pulse input, the calcu-
lated concentration in root is already close to equilibrium to soil, and parallel with
the concentration in soil, concentration in roots falls to very low values within a few
weeks, in any case before the harvest. A steady state simulation with constant soil
concentration would overpredict concentrations in the harvest product by several
orders of magnitude.
The degradation or total dissipation rate is a key variable. To some extent,
elimination in soil is related to physico-chemical properties. Bacterial degradation
depends on the bioavailable fraction of the contaminants (Alexander 2000 ;
Reichenberg and Mayer 2006 ), which is typically higher at low sorption and low
sequestration. Plant uptake and leaching are important for soluble contaminants