The transport of radionuclides through terrestrial and aquatic biota can result in the contamination of the human diet. Ingestion of radionuclides in foods can be an important contributor to the total dose received by an individual or critical (population) group.

The models presented here are intended to be simple screening models for the purpose of estimating the potential impacts, of specific water use scenarios, to hypothetical critical groups in the absence of any site-specific data.

The radionuclides of concern are the long-lived radionuclides of the ²³⁸U and ²³²Th decay chains.

Since simple transfer factors based upon concentration factors are used, steady state conditions are assumed with regard to the long term build-up of contaminants in the various environmental compartments i.e. it is assumed for example that the use of contaminated water has been a long term process.

The models are not intended for use in modeling infrequent events e.g. a discrete batch discharge. They are intended for situations in which the long term average radionuclide concentrations in water are reasonably well known; i.e. they are not intended to predict doses from single grab sample results.

The result merely indicates the range of potential hazard that may arise if the water is put to a particular use. This value is compared to a defined criteria (e.g. indicated dose >250 µSv.y^-1) to assist in a decision making process to determine whether further investigations are required e.g. initiate further sampling programme; identify source: identify actual critical groups: implement control over source and/or initiate remedial action.

In reality an exposed group of individuals will receive widely varying doses from an exposure pathway, these models are intended to indicate the potential range of dose to a critical group or person for each exposure pathway.

Since the consumption factors and transfer factors are required to cover all possible situations and types of critical groups and taking into account the lack of data on transfer factors in semi-arid environments the maximum values used are conservative (in order to ensure that potential doses are not underestimated); and merely indicate the maximum likely dose within a poorly quantified scenario.

Where significant doses are indicated by the model, the implication is that the actual situation on the ground should be investigated further to define mitigating factors and the actual water uses.

If site specific data is available the results obtained using the default values can be scaled to give a more realistic estimate of dose. For example if the water consumption of an identified group of adults is known to be only 250 litres from the contaminated source the resulting dose can be scaled i.e.

µSv/(a.Bq.l^-1) x (250/730)

Scaling can be applied to any consumption rate, occupation factor, irrigation rates, transfer parameter etc where site specific information is available.

It is proposed that where a value exceeds 250 µSv/a that consideration be given to site specific investigations to better quantify the dose e.g. determine actual critical groups, their water use and food consumption rates.

These have been taken from ICRP-72 and indicate the committed effective dose per unit intake in µSv.Bq^-1.

The Dose Coefficients used in the model are given below for a child (age range 1-2 years) and for adult members of the public.

In the absence of data specific to arid South African conditions default values have been based mostly upon a literature survey of data applicable to temperate European and North American situations. The values selected are conservative and represent in most cases the upper bound of reported values (95% CL).

The more important references reviewed to obtain parameter and consumption values are given in the reference section at the end of this document.

Ranges of Parameter values are given in a number of cases to indicate the range of recorded variability and the degree of conservatism: these values are taken from various IAEA summaries. It should be noted that these ranges reflect variations associated with different parts of the world as well as between different crop species or types of farm animals. The values used in the simple model are not species specific but refer to very broad generic categories of edible pasture, vegetables, fruits, etc (e.g. leafy vegetables applies to all types of leafy vegetables and pasture includes grass, wild forage, browse, hay etc).

The soil to plant concentration factor, F_v, can be applied to animal feeds (e.g. pasture, forage, browse and plant based feed) and is referred to as F_v1. In addition F_v2 values are defined for fresh food crops consumed by humans.

The concentration factor reflects only the uptake of radionuclides from the soil via roots and excludes the effects of deposition of nuclides onto plant surfaces by resuspension, deposition and fallout.

F_v1 = (Bq.g^-1 Dry weight plant) / (Bq.g^-1 dry weight soil)

F_v2 = (Bq.g^-1 Fresh weight plant) / (Bq.g^-1 dry weight soil)

Dietary composition varies widely around the world and can vary widely within the same country e.g. South Africa. The great majority of default values reported in the literature and used in modeling are based upon European or North American diets with very limited data available on African diets and consumption rates.

The following should be noted:

1) Due to the difficulty in establishing an "average" South African diet given the first-world/third-world mixture of potential critical groups; two default diets have been derived (refer to IAEA 57 and IAEA 57 redraft) based upon per caput consumption values considered to be representative of different geographical regions. Diet 1 corresponds to African per caput values and Diet 2 to a European diet: the major difference in these diets being in the milk, meat and vegetable consumption factors. (Note: default water and fish consumption is assumed to be the same).

2) These consumption values are for use in screening assessments and should ensure that all types of potential critical groups are covered e.g. rural dwellers, subsistence farmers, modern farmers and other groups with a variable dietary intake.

3) Values for infants have been derived by scaling from default adult consumption rates based upon values from the literature (e.g. ICRP-29).

4) The values were derived after review of a wide range of literature (e.g. IAEA 1982, 1986, 1994, 1996, ICRP 1978). It should be noted that they are not the highest per capita consumption values reported in the literature. Extensive use was made of interpolation and rounding in deriving these values.

Table 2: Annual Food Consumption Parameters for Diet 1

Table 3: Annual Food Consumption Parameters for Diet 2

** (Applicable to both recreational and subsistence fishermen: there is uncertainty as to whether this value adequately represents the intake of certain subsistence groups utilising this resource in the Gauteng area)

a. Feed, pasture, browse or forage

The above default values cover all breeds and intensities of dairy farming or beef production. Since dairy cows have higher intake demands the defaults are based on dairy cows. Ranges reported in the literature for all breeds and climates and milk production rates are as follows:

Water = 20 - 110 Litres per day.

Dry feed = 5 - 25 kg per day.

Models involving irrigation were calculated in a three-stage approach:

Calculating the concentration (C_iv) in pasture and feed and crops

Calculating the concentration in milk, meat

Calculating the effective dose

An example of the approach used is given in Appendix 7A.

The various parameter values used to calculate C_iv in pasture, forage, feed and crops are given in the text.

A brief overview is given of the structure of the models used in the database. Relevant diet values from Diet 2 are provided as example values since these are the highest of the two diet groups.

Activity Concentration: 1 Bq.L^-1 per radionuclide

No dilution: No removal: No treatment prior to consumption

Water consumption:

Adult = 730 L.y^-1

One year old = 260 L.y^-1

µSv y^-1 = 1 (Bq.L^-1) X 730 (L.y^-1) X DC_Adult (µSv.Bq^-1)

µSv.y^-1 = 1 (Bq.L^-1) X 260 (L.y^-1) X DC_Child (µSv.Bq^-1)

CED = water concentration x consumption x dose coefficient

(CED = Committed Effective Dose)

Activity Concentration: 1 Bq.L^-1 of relevant radionuclide

No dilution: no removal: no treatment.

Assumes a long-term stable contamination situation.

Fish Consumption (edible wet weight)

Adult = 25 kg.y^-1

One year old = 1 kg.y^-1

 

µSv.y^-1 = 1 (Bq.L^-1) X Bp (Bq.L^-1 / Bq.Kg^-1) X 25 (Kg.y^-1) X DC_Adult (µSv.Bq^-1)

µSv.y^-1 = 1 (Bq.L^-1) X Bp (Bq.L^-1 / Bq.kg^-1) X 1 (kg.y^-1) X DC_Child (µSV.Bq^-1)

CED = concentration x bioaccumulation factor x annual consumption x

dose coefficient.

Under equilibrium conditions, the incorporation of radioactivity into fish can be expressed as the bioaccumulation factor B_p defined as:

"The ratio of the activity concentration in fish tissue to that in water which is normally expressed as Bq.kg^-1 wet weight fish per Bq.kg^-1 (or L^-1) water (units of L.kg^-1)". The activity concentration in fish tissue usually refers to the edible portion of the fish wet mass.

The values can be used to predict activity levels in edible fish tissue from activity levels in water under steady state conditions.

*Higher values have been reported recently up to 60000 in a French study.

** A lower default value (1000) was used in the model based on South African data.

Bio-accumulation parameter values vary widely according to many factors e.g. fish type, type of water body, feeding patterns and feeding habits (e.g. carnivorous, omnivorous, herbivorous), trophic level, water chemistry, stable element concentrations, pH, nutrient levels, eutrophic level of water body, water temperature, size of fish, age, migratory behaviour, physico-chemical form of the radionuclide, dissolved mineral content, sedimentation and resuspension processes as well as with different radionuclides, in addition reported values in the literature for specific radionuclides in different freshwater environments can vary by several orders of magnitude. In addition to the above factors inadequate sampling and radiochemical analysis programs can contribute to uncertainties.

It should also be noted that the transfer factors express the summed fractional uptake into the fish via many pathways e.g. direct uptake from water through the gills, skin and gut: uptake from sediment; uptake through various trophic feeding levels and routes.

The range of this potential variation for fish is indicated in the above Table.

The availability of site specific data is of great importance to provide realistic estimates of effective dose through the consumption of fish.

Concentration: 1 Bq.L^-1 of relevant radionuclide.

No dilution: no removal: no treatment of water prior to irrigation.

The sole source of all milk is obtained from the cow i.e. no dilution with uncontaminated milk.

Cows always graze on contaminated land e.g. pasture, lucerne etc.

Assumes an irrigation rate of 750 L.y^-1 per m^-2 (source: mean value for irrigation farmers in Lower Vet river). The model assumes only 1 year of irrigation with contaminated water prior to the cows commencing grazing.

Steady state conditions: i.e. minimal leaching of activity from soil.

Excludes uptake through spray irrigation through deposition on leaves.

Covers all types of irrigation.

Excludes uptake arising from eating soil.

Accounts for decay.

Milk Consumption (litres)

Adult = 250 L.y^-1

One year old = 300 L.y^-1

Feed consumption = 25 000 g.d^-1 (dry)

 

µSv.y^-1 = C_iv X 25000 (Dry g.d^-1) X F_M X 250 (L) X DC_Adult (µSv.Bq^-1)

µSv.y^-1 = C_iv X 25000 (Dry g.d^-1) X F_m X 300 (L) X DC_Child (µSv.Bq^-1)

CED = concentration in feed (Bq.g^-1) x dry feed consumption: grams per

day x f_m (feed to milk transfer factor: per Bq.L^-1 milk per Bq.day

intake) x annual milk consumption (L) x dose coefficient.

 

Note: Refers to one year of irrigation: assumes a 15 cm plough depth containing 240 kg of dry soil per m².

The transfer of radionuclides from an animal's feed to milk is commonly described by using the transfer coefficient F_m defined as:

"The fraction of the animals total daily intake of a radionuclide that is transferred to each litre of milk per day".

Note: The values given below may be applied to cow, goat or sheep milk.

Concentration = 1 Bq.L^-1 of relevant radionuclide.

The sole source of all milk is obtained from the cow i.e. no dilution with uncontaminated milk.

Cows only drink contaminated water.

Steady state conditions apply.

Concentration = 1 Bq.L^-1 of relevant radionuclide.

No dilution: no removal: no treatment of water prior to drinking.

Milk is obtained direct from the animal i.e. no dilution with uncontaminated milk.

Cow Water Consumption = 75 L.d^-1

Milk Consumption (litres)

Adult = 250 L.y^-1

One year old = 300 L.y^-1

µSv.y^-1 = 1 (Bq.L^-1) X 75 (L.d^-1) X F_m X 250 (L) X DC_Adult (µSv.Bq^-1)

µSv.y^-1 = 1 (Bq.L^-1) X 75 (L.d^-1) X F_m X 300 (L) X DC_Child (µSv.Bq^-1)

CED = concentration in water x w_c (daily water consumption) x F_m

(water to milk transfer factor: Bq.day intake per Bq.L^-1) x

annual milk consumption (L) x dose coefficient.

For F_m values refer to Table 7.

Concentration = 1 Bq.L^-1 of relevant radionuclide.

No dilution: no removal: no treatment of water prior to irrigation.

The sole source of all meat consumed is obtained from the cattle.

Cattle always graze on contaminated land e.g. pasture, lucerne etc.

Assumes an irrigation rate of 750 L.y^-1 year per m^-2 (source: mean value for irrigation farmers in Lower Vet River). The model assumes only 1 year of irrigation with contaminated water prior to the cows commencing grazing.

Steady state conditions: i.e. no leaching of activity from soil.

Excludes uptake through spray irrigation through deposition on leaves.

Covers all types of irrigation.

Excludes uptake arising from eating soil.

Meat Consumption (kg)

Adult = 100 kg.y^-1

One year old = 20 kg.y^-1

Feed consumption

25 kg.d^-1

µSv.y^-1 = C _iv X 25000 (Dry g.d^-1) X F_F X 100 (kg) X DC_Adult (µSv.Bq^-1)

µSv.y^-1 = C _iv X 25000 (Dry g.d^-1) X F_F X 20 (kg) X DC_Child (µSv.Bq^-1)

CED = concentration in feed (Bq.g^-1) x dry feed consumption g per day

x F_F (feed to meat transfer factor: per Bq.kg^-1 meat per Bq.day

intake) x annual meat consumption (kg) x dose coefficient.

 

The transfer of radionuclides from an animal's feed (pasture, grass, forage) to edible animal products is commonly described by using the transfer coefficient F_f defined as:

"The fraction of the animals total daily intake of a radionuclide that is transferred to each kg of flesh at equilibrium or at time of slaughter".

The values below are derived for beef meat but may be may be applied to all types of edible beef and cow products as well as pigs, goats, horses and game animals.

Assumptions

Concentration = 1 Bq.L^-1 of relevant radionuclide.

The sole source of all meat is obtained from the cattle.

Cattle only drink contaminated water.

Steady state conditions apply.

No dilution: no removal: no treatment of water prior to drinking.

Water Consumption = 75 L.d^-1

Meat Consumption (kg)

Adult = 100 kg.y^-1

One year old = 20 kg.y^-1

µSv.y^-1 = 1 (Bq.L^-1) X 75 (L.d^-1) X F_F X 100 (kg) X DC_Adult (µSv.Bq^-1)

µSv.y^-1 = 1 (Bq.L^-1) X 75 (L.d^-1) X F_F X 20 (kg) X DC_Child (µSv.Bq^-1)

CED = water concentration x w_c (daily water consumption) x

F_F (water to meat transfer factor: Bq.day intake per Bq.kg^-1) x

annual meat consumption (kg) x dose coefficient.

Concentration = 1 Bq.L^-1 of relevant radionuclide.

No dilution: no removal: no treatment of water prior to irrigation.

The sole source of all food consumed is irrigated with contaminated water.

Assumes an irrigation rate of 750 L.y^-1 year per m^-2 (source: mean value for irrigation farmers in Lower Vet River). The model assumes only 1 year of irrigation with contaminated water)

Steady state conditions: i.e. no leaching of activity from soil.

Excludes uptake through spray irrigation through deposition on leaves.

Covers all types of irrigation.

Annual Plant Consumption by Humans (f_c)

Cereals and Grains:

Adult = 150 kg

1 year old = 60 kg

Root Crops

Adults = 170 kg

1 year old = 70 kg

Leafy Vegetables

Adults = 55 kg

1 year old = 22,5 kg

Fruits and other vegetables

Adults = 75 kg

1 year old = 30 kg

Root crops, leafy vegetables and fruits, nuts etc refers to fresh weight consumption.

The generic equation applies to the consumption of grains or leafy vegetables or root vegetables or fruits. Insert the appropriate f_v2 value for the crop to determine the concentration (C_iv)(Bq.g^-1) in the crop (refer to attachment A) and the age specific annual consumption values f_ca (adult) and f_cc (child).

µSv.y^-1 = C_iv X f_ca (g.y^-1) X DC_Adult (µSv.Bq^-1)

µSv.y^-1 = C_iv X f_cc (Dr