Modellering af optagelse af organiske stoffer i grøntsager og frugt
Bilag E
Direct soil contact
Given that the amount of attached soil to the plant is wS (kg wet weigth), and that the weight of the plant is wP (kg wet weight).
The net transfer rate (TFnet) is defined as the relative contribution from the attached soil to the total concentration in plant. It can be calculated from
| TFnet = fefficiency × wS/wP | Equation 25 |
where
| fefficiency | is the fraction of the chemical in the attached soil particles actually entering the plant |
| CB | is the chemical concentration in bulk wet soil. |
This means that the concentration in washed plant (CP) is calculated from
| CP = TFnet · CB + CP,0 | Equation 26 |
where
| CP | is the concentration of a chemical in plant |
| CP,0 | is the background concentration, i.e. the concentration in the plant if no soil attachment occurs |
Let R be the ratio of attached soil to the total plant weight (inclusive soil):
| R = wS / (wS+wP) | Equation 27 |
giving
| TFnet = R/(1-R) · fefficiency | Equation 28 |
Once deposited on a leaf, the chemical may undergo several fate processes:
 | It may volatilize |
| It may be washed off together with the particle |
| It may be degraded |
| It may migrate into the leaf |
Particles deposited to leaves seem to migrate partly into the cuticle. A significant fraction may remain in foliage even after intense washing (Kaupp 1996). The net transfer factor for a washed plant that has taken up all chemical from the attached soil (fefficiency = 1) is
 |
Equation 29 |
For R << 1 it follows that TFnet » R, meaning that the
contribution from the attachment of soil particles to the soil BCF for the plant
(concentration in plant divided by the conentration in bulk wet soil), can be calculated
from relative mass loading (mass of soil per mass of plant) on the plant.
Table E.1
Empirical values for mass loadings of soil particles on plants (g soil per g
plant dry weight)
Plant |
Mass loading |
Reference |
Pasture grass |
0.0045 |
Pinder III et al. (1991) |
Pasture grass |
0.00024 |
Pinder III et al. (1991) |
Broccoli |
0.01 |
Pinder III & McLeod (1989) |
Cabbage |
0.0011 |
" |
Lettuce |
0.260 |
" |
Sunflowers |
0.0026 |
" |
Tobacco |
0.0021 |
" |
Turnips |
0.032 |
" |
Grass |
0.018 |
Li et al. (1994) |
Tomatoes |
0.017 |
" |
Soybean |
0.0021 |
" |
Wheat |
0.0048 |
" |
Corn |
0.0014 |
" |
Broadbean |
0.0095 |
" |
Ryegrass |
0.0058 |
" |
MEAN |
0.025 |
|
Table E.1 gives an overview of empirical values of soil mass loadings on plants. The
highest loading has lettuce, with 0.26 g soil attached to 1 g lettuce (dry weight). The
mean of all values is 0.025 g soil per g dry plant. For a dry weight of 20%, this
corresponds to 0.005 g soil per g fresh plant. The default value in the ECOSYS model for
nuclear risk assessment is 1% for grass (Paretzke & Garland). The default value of
CSOIL is 1% on a dry weight basis, corresponding to 0.1% on a fresh weight basis (Johannes
Lijzen, personal communication 2002). RIVM uses 9.8 % average plant dry weight.