Review of measurement methods
*The time shown is the "technical time" needed for analysis (processing of the dry sample and measurement). It does not integrate organisational constraints, such as the fact that heavy instrumentation (e.g. AMS) can generally only be used on a batch of samples, by campaign and on fixed dates.
Usual radioecological models (at equilibrium): terrestrial environment
The conventional evaluations of the radioecological impact of 14C in terrestrial ecosystems remain very empirical, implicitly or explicitly assuming a 14C/12C isotopic equilibrium in the soil-plant-atmosphere system. Recently, models related to biomass have been developed that take into consideration the dynamic nature of acute exposure (Tamponnet, 2005b, Galeriu et al., 2007).
Given that 14C is easily incorporated in food products and that humans mainly assimilate carbon by ingestion, the soil is not modelled as an exposure environment.
Models based on this assumption overlook the various soil compartments that may influence carbon isotope equilibrium. In addition, these models do not consider the physical/chemical form of the source term (whether the addition to the soil is liquid or solid), which will undergo a process of incorporation in the organic material cycle and/or be mineralised. A simplified model can be employed that uses default transfer factor values based on the assumption of 14C/12C isotopic equilibrium between organic matter in soil and in plant, but this substitute approach appears to be very conservative for evaluating plant transfer.
However, the importance of the soil is reflected in certain models, given that it is a potential source of contamination for plants, as indicated in the following section.
Carbon-14 transfer to plants is modelled with the assumption that the soil-atmosphere-plant equilibrium is perfectly reached. Most models only integrate atmospheric contamination subsequent to gaseous releases. During photosynthesis, CO2 is incorporated in organic matter, for which it contributes to the carbon skeleton. Equilibrium is then rapidly established between the specific activities of atmospheric CO2 and CO2 of organic plant material under construction.
The atmosphere -> plant transfer is modelled with the assumption that the specific activity (Bq of 14C per kg of 12C) of plant carbon is the same as the atmospheric specific activity. The underlying assumption is that the transfer of the trace radionuclide 14C is identical to 12C transfer and that the two compartments are at equilibrium. This results in the following equation (IAEA, 2010):
Proportion of stable carbon in terrestrial plants ( *1000, g C/kg wet)
The irrigation water -> plant transfer is modelled with the assumption that the 14C from irrigation is incorporated via photosynthesis, as a result of emanation of CO2 from the soil. The plant's activity concentration is evaluated by considering the 12C and 14C fluxes emanating from the soil, and the fraction of the carbon flux that comes from the soil and that participates in photosynthesis.
Transfer to plants from soil that is contaminated, in particular via liquids, is still poorly understood. The only recent work in this area was conducted as part of radioactive waste disposal studies (BIOPROTA, 2010).
Transfer to animal products is modelled based on the transfer pathway (atmospheric and/or liquid contamination pathway for plants included in the animals' food ration) according to the respective expressions:
* Factor fcont is used to integrate the fact that animals can be fed with concentrates or from far-away sources that are not contaminated. The value of this factor should be based on local agricultural practices. If a site-specific value is not available, must be conservatively set to 1.
Proportion of stable carbon in animal products ( *1000, g C/kg wet)
The effect of food processing is quantified by a transfer factor, also called a retention factor. This factor gives the fraction of radionuclide remaining in the product after processing.
Food processing transfer factor (Bq/kgwet of product processed by Bq/kgwet of unprocessed product)
Usual radioecological models (at equilibrium): freshwater environments
Water and sediment
For drinking water, 14C concentration is assumed equal to the concentration in the original stream, river or well, with correction by a factor related to the treatment used, if necessary. The efficiency of drinking water treatment processes at removing 14C from supply water has not been studied.
Sediment is rarely taken into account in models for calculating doses resulting from exposure to 14C. The fraction of pollutant linked to the dissolved phase can, however, be calculated based on a Kd value of 2 x 103 m3/t dry weight.
The equation for phytoplankton and underwater plants is as follows:
For fish (or other animals), the released chemical forms, whether inorganic (proportion pm in liquid effluents) or organic (po = 1-pm), can be taken into account in the evaluation of transfers. Due to the slowness with which organic forms are transformed into inorganic forms, inorganic carbon-14 at equilibrium in the stream or river water comes only from liquid releases. Part of this carbon-14 has been used for photosynthesis; the inorganic forms available for animals are present at a proportion lower than pi, and the expression Cmin(water) = p1.Cwater can be considered conservative.
For organic forms, the balance at equilibrium is more complex, as a result of two competing effects. The first is that integration by the microbial loop of organic carbon-14 from the water in the stream or river tends to reduce the 14C proportion below po, and the second is that decay of contaminated preys and excrements of contaminated organisms tends to increase this proportion to above po. The following expression is only an approximation of organic carbon-14 activity in the stream or river water: . Given these assumptions, the following expression is used for initial approximation of the activity concentration of 14C in animals:
Proportion of stable carbon in aquatic plants and animals (kg of C/kg wet)
(Garnier-Laplace et al., 1998)
Dose conversion coefficients (DCCs), expressed in wet weight
nd*: DCC not determined due to the distance or to the shield effect of sediment
nd*: DCC cannot be determined because of distance