appendix o exposure scenarios and unit dose factors ... - Hanford Site

HNF-5636 Rev. 0
HNF-SD-WM-TI-707 Revision 1
Living plants supplying food for people fall into two broad categories, aquatic plants and
terrestrial plants. It will be assumed that aquatic plants contribute very little to the typical human
diet -- either directly or indirectly. If exceptions are identified then a suitable set of parameters
and models for contaminant uptake by aquatic plants and subsequent consumptions by humans
will be utilized. All plants eaten are assumed to be terrestrial rather than aquatic.
The calculation of radionuclide concentrations in living terrestrial plants uses three main
routes, (1) root uptake, (2) resuspension to leaves (also called "rain splash"), and (3) direct
deposition of irrigation water on foliage. Each of these will be considered separately below.
The three uptake routes are then combined to obtain the total concentration in edible portions of
plants.
2.4.1 Root Uptake
The model for root uptake of a contaminant into terrestrial plants assumes that the
concentration in the edible portion is proportional to the concentration in the soil at the time of
harvest. The constants of proportionality are known as the soil-to-plant concentration ratios.
These concentration ratios are measured as the concentration of the dry produce item divided by
the soil concentration. They have no units, since the soil and food items have the same massbased
concentration units, eg. pCi/kg.
Because the human consumption rates shown in Tables 13 and 14 are the wet weights, it is
necessary to select suitable constants to convert from dry weight to wet weight. These constants
are known as "dry-to-wet ratios". They are simply the dry weight of the food item divided by the
wet weight of the item. The "dry-to-wet ratios" from three sources are listed in Table 24. The
values chosen for the ILAW PA are from PNWD-2023 for leafy vegetables and
NUREG/CR-5512 for the others. The chosen values for the ILAW PA appear in the last column
of Table 24. The values under the "GENII" column have been used in prior Hanford Site
performance assessments.
The GENII dry-to-wet ratios for grains differ greatly from the other collections. However,
it has been assumed in prior performance assessments that grains would be unlikely to become
contaminated in the intruder or irrigation scenarios. The intruder would probably not raise grains
in his home garden, and the principal grain crop in this area (dry-land wheat) would not be
irrigated. For the ILAW PA grains are included as contaminated vegetable intakes. The basis for
this approach is that some grains (e.g. corn) are irrigated. In addition, the 25% contamination
fraction incorporates the presence of non-irrigated grains (e.g. wheat) in the individual's diet.
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