SPADE

Full text: SPADE (Soil Plant Animal Dynamic Evaluation) is the name given to a suite of codes used to assess the impact of potential radioactive discharges on man through the ingestion of contaminated food. Radionuclide inputs to SPADE are results from atmospheric dispersion calculations, measured or assumed concentrations in air (Bq m-3) and/or deposition rates (Bq m-2s-1) to ground. The quantity of radionuclides reaching the above ground compartments of the plant from atmospheric sources is determined according to the interception fraction which takes account of changes in plant biomass with season. Depending on the model, plants or leaves are divided into external and internal components to allow particulate deposition to be distinguished from radioactive gases and vapours. Radionuclide distribution in plants depends on both the physiological characteristics of the plant and the physico-chemical properties of the radionuclide. Material lost from the plant by wash-off is partitioned between either soil solution and organic matter, or 'soil available' and 'soil unavailable', as appropriate. Transfers from soil to plant occur via root uptake and are assumed to vary with soil layer depth, as a function of the root distribution throughout the soil profile. Consequently the transfer of radionuclides from soil to root is represented in SPADE by a single transfer rate, normalised for each of the ten layers in the soil model according to root distribution. SPADE models radionuclide uptake by three types of fruit crops: herbaceous, shrubs and trees. Parameter values used in SPADE are those specified in the scenarios where appropriate. Where parameter values for some processes were not provided in the scenarios, SPADE default values were used. Pruning was considered for both blackcurrants and apple trees according to information supplied in the scenarios. Strawberry plants were replaced every two years and debris was removed from the field. The SPADE suite of codes is used by the United Kingdom Food Standards Agency (formerly part of the UK Ministry of Agriculture, Fisheries and Food - MAFF) for regulatory purposes. Input parameters for the models are selected to provide realistic predictions that are towards the upper end of observed concentrations in food products. On this basis the output from SPADE is a best estimate prediction. This is reinforced by the use of scenarios that are likely to produce high concentrations, e.g. deposition to crops at a time when transfer to the edible component is likely to be greatest. The fruit plant model in SPADE [Thorne and Coughtrey, 1983] consists of six compartments, representing internal leaf, external leaf, stem, fruit, storage organs and root. Movement of radionuclides within the plant model is controlled by empirically derived rate constants and parameters are derived for three broad categories of fruit plant: herbaceous, shrub and tree. Models are implemented in SPADE for 20 elements. and the following discussion considers the iodine models for fruit. Two experimental programmes have been undertaken in connection with the development of the SPADE fruit models for herbaceous and shrubby fruit crops [Kirton et al., 1987; Donnelly and Carini, 1998]. The data from these experiments provide valuable information for model validation. Foliar absorption may be an important pathway for the uptake of radionuclides deposited on external plant surfaces, and is represented by transfers between the external leaf and internal leaf compartments. Not all compartments in the model are directly linked, and in some cases transfers occur in one direction only. Ten internal transfers occur in the standard fission/activation plant model. Interception by plants takes account of changes in plant biomass with season. Depending on the model, plant or leaves are divided into external and internal components to allow particulate deposition to be distinguished from radioactive gases and vapours. Passage through the stomata and incorporation into the mesophyll is therefore represented by partitioning a fraction of the intercepted deposit to the internal compartment. The original default parameters for iodine [Coughtrey and Thorne, 1981] were based largely on data for cereals, but were modified in the case of tree and shrub fruits to allow for more rapid transfer from stem to root so that the root store could serve as a reservoir through subsequent seasons. Loss of radionuclides from external plant surfaces to the soil is modelled as transfer to the surface layer of the soil model and include losses arising from leaf fall. The parameters for the three fruit models for iodine in SPADE (herbaceous, shrub and tree) are similar with the following exceptions. Differences for herbaceous fruit crops are as follows: the root store is switched off; there are crop-specific transfers from root to stem and from stem to internal leaf; and, internal to external leaf was chosen to reflect cereals rather than fruit crops. As concerns the other two fruit crop types the return from the root store reservoir is slower for tree fruit than for shrub fruit by an order of magnitude. The process of root uptake is modelled as the transfer of radionuclides from soil solution to the plant root compartment. The transfer rate is also assumed to vary with soil layer depth, both as a function of the root distribution throughout the soil profile and as a function of the deposit distribution in soil. Consequently, the transfer of radionuclides from the soil solution to root is represented by a discrete transfer from each of the 10 layers in the soil model. The soil model is not considered further here. (author)