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
DPwi = rate of deposition of pollutant i in irrigation water to cropland soil and plants (mg/m2/d or pCi/m2/d)
Ciri = pollutant i concentration in irrigation water(mg/L or pCi/L)
IR = irrigation water application rate during irrigation periods (L/m2/mo)
30 = units conversion factor (d/mo).
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
Cawi = soil concentration from irrigation water deposition as a function of time (mg/m2 or pCi/m2)
UDwi = unit deposition rate to soil (mg/m2/d or pCi/m2/d)
l di = environmental degradation and decay constant for pollutant i in surface soil (d-1)
SAFi = soil accumulation factor for an exposure duration of EDkk years for pollutant i (d)
EDkk = exposure duration for pathway kk (yr)
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
CWDlvi = pollutant i concentration in leafy vegetables at time of harvest from water deposition onto plants (mg/kg or pCi/kg)
TVlv = translocation factor from plant surfaces to edible parts of the plant for leafy vegetables (dimensionless)
rlv = fraction of deposition retained on plant surfaces (dimensionless)
l ei = effective weathering and decay constant for pollutant i (d-1)
TClv = duration of the growing period for leafy vegetables (d) l ei = ldi + l w l w = weathering decay constant for losses from plant surfaces (d-1)
Ylv = yield of leafy vegetables (kg/m2) 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:
Equation (22) CWRlvi
(22)
where
CWRlvi = pollutant i concentration in edible parts of leafy vegetables at harvest from root uptake (mg/kg or pCi/kg)
FIlv = fraction of year that irrigation occurs for leafy vegetable crops (dimensionless)
Bvi = soil-to-plant transfer factor for pollutant i (kg dry soil/kg wet weight plant)
P = area soil density of farmland (kg dry soil/m2 farmland)
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
Clvi = leafy vegetable plant concentration at time of consumption (mg/kg)
l gi = loss and decay rate constant for pollutant i in closed water systems (d-1)
THlv = holdup time between harvest of leafy vegetables and consumption by humans (d)
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
Dlvi = average daily dose from ingestion of leafy vegetables (mg/kg/d)
Ulv = ingestion rate of leafy vegetables by the exposed individual (kg/d)
Flv = fraction of days per year that leafy vegetables are consumed (dimensionless)
EDlv = exposure duration for the leafy vegetable ingestion pathway (yr)
ATlvi = averaging time for leafy vegetable ingestion pathway for exposure to pollutant i (yr)
BWlv = body weight of individuals exposed by leafy vegetable ingestion pathway (kg).
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
Dlvi = total lifetime dose from ingestion of leafy vegetables (rem)
DFgi = ingestion dose conversion factor for radionuclide i (rem/pCi ingested)
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
DPai = rate of deposition of pollutant i from air to plants (mg/m2/d or pCi/m2/d)
86,400 = unit conversion factor (s/d).
Cari = average concentration of pollutant i in air at the location of crop production (mg/m3 or pCi/m3)
Vdi = deposition velocity for pollutant i (m/s).
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
CARlvi = pollutant i concentration in edible parts of leafy vegetables at harvest from root uptake (mg/kg or pCi/kg)
DPsi = annual average deposition rate of pollutant i to soil from atmospheric transport and deposition (mg/m2/d or pCi/m2/d)
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
Dlvi = average daily dose from ingestion of leafy vegetables for chemical pollutant i (mg/kg/d)
EDlv = exposure duration for the leafy vegetable ingestion pathway (yr)
ATlvi = averaging time for the leafy vegetable ingestion pathway for exposure to pollutant i (yr)
BWlv = body weight of individuals exposed by the leafy vegetable ingestion pathway (kg).
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
SMFi = average soil concentration for pollutant i over the exposure duration (mg/kg dry soil or pCi/kg dry soil)
Cmsi = measured concentration of pollutant i in soil (mg/kg dry soil or pCi/kg dry soil)
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)