Uptake of Organic Contaminants from Soil into Vegetables and Fruits - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 387

BCF s for hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) (McKone and Maddalena
2007 ) confirm model predictions for polar non-volatile contaminants.
Contrarily, trichloroethene (TCE) is a volatile chlorinated solvent ( K AW :0.5
LL − 1 ) that does not accumulate in leaves. A study on trichloroethylene uptake by
apple and peach trees and transfer to fruit was performed by Chard et al. ( 2006 ).
No TCE could be detected in fruits, but 14 C from unidentified metabolites was
found. In leaves, the metabolites dichloroacetic acid (DCAA) and trichloroacetic
acid (TCAA) could be detected. The article cites a field study where TCE could be
detected in several fruits, but only in traces. Overall, the findings confirm the model
prediction (Fig. 9.6a ) that volatile contaminants do not show high accumulation in
above-ground plant parts like leaves.

9.5.3 Uptake from Air Versus Uptake from Soil

A frequent experimental result is that contaminants are found in moderate or even
high concentrations in plants even though concentrations in soil are low (Delschen
et al. 1996 , 1999 ; Mikes et al. 2009 ). This is typically the case when uptake is mainly
from air (compare Section 9.3.1 ). The simulations displayed in Fig. 9.6b were done
for identical conditions as for Fig. 9.6a , except that the concentration in air was set to
phase equilibrium to soil (i.e., C Air = C Soil /K SW ) , with concentration in soil

equal to 1 mg kg − 1 ). The development of the concentration in plants is completely
different from Fig. 9.6a (note that the figure was rotated and the z-axis crosses now
at C Leaf equal to 1 mg kg − 1 ). The concentration in leaves is higher than in Fig. 9.6a
where there was no contaminant present in the air, in particular for volatile contam-
inants ( K AW :0.2LL − 1 ). Also, the concentration is less variable, with most values
between 1 and 10 mg kg − 1 . This is because for most contaminants the system is
close to equilibrium in regard to soil with air and air with leaves. An exception are
the non-volatile contaminants, their predicted concentration does not change sub-
stantially. For the polar and non-volatile contaminants, the calculated concentration
in leaves is particularly high.
From Fig. 9.6a and b it can be seen that the partition coefficient between air
and water (also known as the dimensionless Henry's Law constant) is a very
important parameter for calculation of the accumulation in leaves, because K OA
(the ratio of K OW and K AW ) determines partitioning into leaves. Leaves have a
very high exchange with air (that is their role in plant physiology), and any
volatile contaminant (with high K AW ) will escape from leaves into air and will not
accumulate.
The pattern of uptake of contaminants from soil into fruits is very similar (not
shown), although the level of concentrations is typically about a factor of 10 lower
(Trapp 2007 ). This means that also, in fruits, polar and non-volatile contaminants
have the highest potential for accumulation from soil.
Uptake into fruits of lipophilic contaminants is preferably from air. An exam-
ple is the transfer of PCDD/F from contaminated sites into field crops, which
has been intensively studied. Müller et al. ( 1994 ) found an increase of PCDD/F

K AW ×