2.4 LEAFY VEGETABLE INGESTION


Agricultural crops may be contaminated when water (groundwater or surface water) is used as a source of irrigation water, airborne pollutants are deposited on agricultural crops or cropland (soil), or measured soil concentrations are available. The paths by which pollutants in transport media may reach the crops are shown in Figure 2.1.

For waterborne transport pathways, the exposure evaluation is performed with the following considerations.
Transport Medium:
water concentration at water treatment plant, Cswi or Cgwi, pCi/L or mg/L, expressed as a 70-year average value

Special Process:
  • loss of pollutants (environmental degradation or radioactive decay) during transport from the pumping station to the irrigation location
  • loss of pollutants during transport in the surface water body by volatilization
  • application to crops and cropland soils 
  • accumulation in soil over the exposure duration (multiple years)
  • uptake by roots from soil to edible portions of plants direct deposition onto plant surfaces and transfer to edible portions of plants
  • loss of pollutants following harvest before consumption by individuals


FIGURE 2.1 Pollutant Transfer to Edible Crops


Exposure Factors:
rate of crop ingestion and exposure duration.


    The application of irrigation water to croplands results in deposition of pollutants to soils and plants at a constant average rate over the period of irrigation. The deposition rate is given as follows:

(18)



where
Note that the irrigation water concentration is taken from the groundwater (Cgwi) or surface water (Cswi) transport analysis. When surface water is the source of irrigation water, the correction for loss of pollutant during surface water transport is made as indicated by Equation (3).

The accumulation of pollutants in soil over a multiple-year period (multiple growing periods) is accounted for by a soil accumulation factor (SAF). This factor accounts for previous years' deposition and accumulation to evaluate an average soil concentration over the exposure duration defined for the current usage location and exposed population (or individual). The factor is evaluated as a time integral of soil concentration over the exposure period. The soil accumulation factor is evaluated as the time integral of the solution to the deposition and decay differential equation, normalized to unit deposition and averaged over the deposition period. This process is represented by the following two equations:

(19)



and

(20)



where
and other terms are as previously defined.

The division by UDwi normalizes the SAF values to unit deposition rate. The above equations are used for the agricultural exposure pathways from waterborne (irrigation) deposition and for atmospheric deposition. When radionuclide decay chains are evaluated, the above equations apply to the parent radionuclide. The contributions from progeny radionuclides are evaluated similarly using the radionuclide chain decay algorithms defined in Appendix B with their dose contribution added to the parent dose. Progeny radionuclides are assumed to have no activity in the source water or air, and their contribution is limited to ingrowth during the deposition and accumulation period. When the progeny are included as explicit radionuclides in the inventory list, they are treated as a parent radionuclide, and their contribution is reported separately from their ingrowth contributions as progeny of other parent radionuclides in the inventory.

The plant contamination from irrigation deposition onto edible parts of plants will result in a contamination level at harvest that is estimated as follows:

(21)



where
The plant concentration at the time of harvest for uptake from soil via roots is estimated as follows:
Equation (22) CWRlvi

(22)



where
and other terms are as previously defined.

The total concentration in leafy vegetables is evaluated as the sum of concentrations from the two contamination routes: deposition onto plants and root uptake from soil.

(23)



where
and other terms are as previously defined. The loss rate constant for closed water systems is used to simulate loss from food products between harvest and consumption as a conservative representation for food handling and packaging. This representation is considered appropriate for frozen and canned foods and conservative for fresh foods.

The average daily dose from ingestion of chemical pollutants in leafy vegetables following irrigation water contamination is evaluated as follows:

(24)



where
and Clvi is as previously defined. The averaging time for noncarcinogenic chemicals is set to the exposure duration, and the averaging time for carcinogenic chemicals is fixed at 70 years. For radioactive pollutants, the total lifetime ingestion dose is evaluated as follows:

(25)



where
and other terms are as previously defined.

For the atmospheric transport pathway, the exposure evaluation is performed with the following considerations.
Transport Medium:
air at agricultural production location, Cari, pCi/m3, or mg/m3, expressed as a 70-year average value

Special Process:
  • deposition to crops and cropland soils
  • uptake by roots from soil to edible portions of plants
  • direct deposition onto plant surfaces and transfer to edible portions of plants
  • loss of pollutants following harvest, prior to consumption by individuals

Exposure Factors:
rate of crop ingestion and exposure duration.

The deposition rate from air to plants is given as follows:

(26)



where
The soil accumulation factor for air deposition is the same as for irrigation water deposition, as defined by Equations (19) and (20).

The concentration in edible parts of plants from airborne deposition is estimated by Equation (21), with parameters for the air pathway substituted for the irrigation pathway as follows:

(27)



where    CADlvi = pollutant i concentration in leafy vegetables at time of harvest from atmospheric deposition onto plants (mg/kg or pCi/kg)

and other terms are as previously defined.

The plant concentration at the time of harvest for uptake from soil via roots is estimated as follows:

(28)



where
and other terms are as previously defined. The annual average deposition rate to soil (DPsi) is an output parameter from the atmospheric transport analysis of MEPAS.

The total concentration in leafy vegetables is evaluated as the sum of contributions for the two contamination routes: deposition onto plants and root uptake from soil, as follows:

(29)



where    Clvi = total concentration in leafy vegetables (mg/kg or pCi/kg)

and other terms are as previously defined. The average daily dose from ingestion of chemical pollutants in leafy vegetables following atmospheric contamination is evaluated as follows:

(30)



where
and other terms are as previously defined. The averaging time for noncarcinogenic chemicals is set to the exposure duration, and the averaging time for carcinogenic chemicals is fixed at 70 years. For radioactive pollutants, the total lifetime dose is evaluated as follows:

Dlvi = Ulv DFgi Clvi Flv EDlv 365.25

(31)



where    Dlvi = total lifetime dose from ingestion of leafy vegetables for radionuclide i (rem)

and other terms are as previously defined.

For the measured soil concentration pathway, the exposure evaluation is performed for an initial soil concentration (measured) with loss and decay during the exposure period. The following considerations are included.
Transport Medium:
measured soil at the production location, Cmsi, pCi/kg or mg/kg, expressed as the concentration at the start of the exposure period
Special Process:
  • uptake by roots from soil to edible portions of plants
  • loss of pollutants from soil by volatilization or decay
  • loss of pollutants following harvest, prior to consumption by individuals
Exposure Factors:
rate of crop ingestion and exposure duration.

The change in soil concentration over the exposure period affects the total amount of pollutant ingested by the exposed individuals. The average concentration is evaluated as the time integral of the activity in the soil divided by the exposure duration, as follows:

(32)



where
and other terms are as previously defined. The average soil concentration factor is evaluated using Equation (32) for chemicals and parent radionuclides. For progeny radionuclides the average concentration from ingrowth during the exposure period is evaluated using the chain decay algorithms presented in Appendix B.

The concentration in edible parts of plants from root uptake from soil is evaluated as follows:

Clvi = SMFI Bvi

(33)



where terms are as previously defined.

The average daily intake from ingestion of chemicals in leafy vegetables is evaluated using the plant concentration from Equation (33) and intake Equation (30). For radionuclides, the ingestion dose is evaluated using the plant concentration from Equation (33) and dose Equation (31)