Uptake of Organic Contaminants from Soil into Vegetables and Fruits - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 394

log K OW (Trapp et al. 2007a ; Zohair et al. 2006 ). The results from the experiments
are in good accordance with the model predictions.
Note that the ingestion of soil attached to the daily vegetable diet may be higher
than the current estimate for direct soil ingestion used in Exposure Assessments
for humans. The direct ingestion of soil is 50 mg/day for adults and 100 mg/d
for children (ECETOC 2001 ). The average lettuce consumption in Denmark is
6 g/d for children and 9 g/d for an adult woman. A transfer of 1% (10 mg soil
g plant − 1 (fresh weight)) due to attached soil corresponds to 60 mg/d and 90 mg/d
soil ingestion with lettuce only. Other vegetables and fruits are consumed at 389 g/d
(children) and 475 g/d (mother). If on average 0.1% soil were attached, this cor-
responds to 389 mg/d and 475 mg/d of soil, which is much more than the value
of direct ingestion of soil and deserves consideration in Human Health Exposure
Assessment.

9.6.4 Variation of Partition Coefficients

The RCF regressions for barley (Briggs et al. 1982 ) and bean plants (Trapp and
Pussemier 1991 ) differ (Fig. 9.3 ). Also, the partition coefficients between leaves and
air, K LA , vary with plant species. The K LA depends both on plant-specific parameters
(such as plant lipid content and empirical exponent b (Eq. 9.10 )) and contaminant-
specific parameters ( K OW and K AW or K OA ). Kömp and McLachlan ( 1997 ) found
differences in the uptake of PCB between five different plant species of up to a
factor of 20. There was a linear relationship between log K PA and log K OA values
within each plant species, but the slopes of the regressions ranged from 0.57 to 1.15.
The standard value for leaves (slope

= b = 0.95, Eq. 9.9 ) was determined by Briggs

et al. ( 1983 ) and lies in this range.

9.6.5 Permeability

The velocity of diffusive uptake from air and loss to air is controlled by the per-
meability of leaves (synonyms exchange velocity, conductance or transfer velocity).
Three major resistances control the exchange: air boundary layer resistance, stom-
ata resistance and cuticle resistance (Riederer 1995 ). Their importance depends on
the chemical properties but varies for each leaf. Volatile and gaseous contaminants
will prefer stomata for the entry to or escape from plants, while lipophilic contami-
nants will preferably diffuse through cuticles. The stomata resistance for water and
contaminants can be calculated from the transpiration rate, leaf area, temperature
and humidity (Trapp 1995 , 2007 ). The cuticle resistance depends on thickness and
diffusivity inside the cuticle (Schönherr and Riederer 1989 ).Thelatterishighly
variable with species and temperature (Buchholz et al. 1998 ): at high temperatures,
the surface waxes liquidize and get more permeable.
A method for estimation of conductance g (m s − 1 , related to gas phase) is given
by Trapp ( 1995 , 2007 ). Values for g estimated with this method range from 9

10 − 3

×