(1) The global level of tritium is currently much lower than during the period of nuclear testing and concentrations have now fallen to near natural background levels. Nevertheless, some areas in the vicinity of chronic atmospheric or sub-surface sources of tritium still show elevated concentrations. Modelling the environmental transport of tritium from chronic sources, and the subsequent concentrations in different environmental media, cannot be carried out by direct application of multi-purpose generic assessment models, which are not suitable due to the unique properties and behaviour of tritium compounds in the environment.
Whilst specific models for short term or accidental releases of tritium have previously been developed and tested in various international fora (e.g. see BIOMOVS II, 1996 a, b; Barry et al, 1999), this has not been the case for models for chronic releases. When the TWG was inaugurated within the IAEA BIOMASS programme in December 1996, the participating modellers and experimentalists agreed that their activities should concentrate in four main areas related to chronic sources of tritium.
(i) Testing whether the existing models for calculating long term air concentrations are suitable for assessing concentrations of HTO and HT in air following chronic atmospheric releases.
(ii) Testing and modifying appropriate models for calculating HTO concentrations in near-surface soil moisture and plants following chronic atmospheric releases.
(iii) Modifying or developing models for processes involved in tritium transport down through deeper soil layers to groundwater and up through unsaturated media to the soil surface as a result of long term atmospheric or sub-surface sources respectively.
(iv) Identifying a key modelling problem for which observational data were lacking so that experimentalists and modellers could collaborate in the development of a suitable sampling programme to provide the required data.
(2) Six different model test scenarios (four concerned with atmospheric pathways and two with sub-surface pathways) and a 20-month environmental sampling programme were undertaken to address these four main areas of interest. Conclusions arising from these activities are provided below together with recommendations for model improvements, data acquisition methods and future studies.
ATMOSPHERIC PATHWAYS
(3) Four different model test scenarios were developed for chronic atmospheric releases of tritium either as HTO or HT. The first scenario was a relatively simple model-model test exercise that was undertaken whilst sets of relevant observational data from sites in Canada, Russia and France were collated to provide the basis for three model-data exercises. In each case, modellers were asked to calculate concentrations of HTO in air moisture, soil moisture and plant tissue free water (TFW) and concentrations of organically bound tritium (OBT) in dry plant matter. Each of these exercises presented the modellers with specific challenges. For example, the scenario based on Canadian data required the modellers to consider multiple emission sources and to explain temporal and spatial variability in the observations. The scenario based on Russian data was concerned with a non-continuous, but chronic, source where emissions decreased significantly over the twenty years of observations. The scenario based on French data required consideration of multiple sources and the effects of rolling terrain on air concentrations. A comparison of the four sets of results enabled a number of conclusions to be drawn concerning modelling HTO concentrations in each environmental
medium. Most participants used models based on simple equilibrium concepts but some employed time-dependent codes that simulated the transport processes in detail.
HTO releases: Air moisture concentrations
(4) All the participating models were based on the traditional Gaussian dispersion formulae. The first scenario (a model-model inter-comparison exercise) considered a single stack that continuously emits a constant concentration of HTO under well-defined average weather conditions. The weather statistics were provided in the scenario description and all the modellers used a sector-averaged approach to the dispersion calculations. Predicted concentrations of HTO in air moisture were all within a factor of two, except in the immediate vicinity of the stack where the concentrations are very low if re-emission from the contaminated soil was neglected and much higher if the re-emission was taken into account.
The divergence in the results that were obtained at more than a few hundred metres from the source was considered to be acceptable. However, the question remained as to how the models would perform in ‘real’ assessment type situations where modellers must interpret data and adapt their model to reflect conditions at the site of interest.
(5) The three subsequent scenarios were blind test exercises in which predictions from the different models were compared with observations. In most cases predictions agreed with the observed air concentrations within a factor of three for the Canadian and French sites, and within a factor of five for the Russian site. The results are considered acceptable for the scenarios based on Canadian and French data. It is interesting to note that in the case of the French site with rolling terrain, some participants actually obtained better results when ignoring topography than those modellers who included it. On the basis of these results, it appears that topographic effects need not be taken into account, at least for sites of moderate relief and for receptors up to a few kilometres from the source(s). The more divergent results for the Russian site were attributable to two main causes. The first was a problem with the meteorological data which were not supplied in the normal format used in the models. As a consequence the modellers interpreted the data differently and used different techniques to obtain suitable meteorological statistics as input for their models. Secondly, results for air moisture concentrations were most divergent for just one of the sampling locations (Sampling Point 4). In fact, calculations for this sampling point actually helped to demonstrate that there was a problem with the observational data. Upon further investigation it was found that there was a second source of emission near Sampling Point 4 that was not known to the scenario developer until the last phase of the scenario test exercise. So, although the additional emission was much weaker than the main source it nevertheless affected the air concentrations. The modelling results would have been closer to observations had modellers been able to take the second source into account. Overall, it appears that the existing models are satisfactory for predicting HTO concentrations in air moisture.
(6) In theory, the models should account for the contribution of re-emitted tritium to the air concentration. In fact, the calculation of secondary air concentrations is not a simple matter.
Summing the contribution from all ground level sources is really the only way to estimate contributions from this process. Although the dispersion part of such calculations is relatively straightforward there are a number of uncertainties involved, the most notable of which is the height at which to measure wind speed. It is also a non-trivial matter to estimate re-emission fluxes. But secondary re-emission to air from soil probably contributes less than 10% to the air moisture concentrations of HTO under most circumstances. So, it was considered that re-emission could be ignored unless calculations are required for areas very close (within 500 m) to an elevated point source or when the source is below ground.
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HTO releases: Soil moisture concentrations
(7) In contrast to results for air concentrations, predictions for HTO concentrations in soil water were more divergent. For the first scenario, results varied by more than a factor of seven. For the scenario based on Canadian data, the predicted soil water concentrations differed from the observations by a factor of less than ten, although this was reduced when soil concentrations were normalised to air concentrations in order to eliminate the influence of different modelling approaches on atmospheric dispersion. In the scenario based on Russian data, snow lying on the soil surface for any length of time was shown to prevent the ingress of HTO into the soil until snow melt occured in spring. Snowmelt also appeared to cause some retention of tritium in the soil but more information is required before firm conclusions can be made on this.
(8) The differences in predicted soil concentrations were due to differences in calculated air concentrations (see above) combined with differences in modelling approach. Some modellers used an activity balance approach in their calculations. This generally gives soil/air ratios close to unity, a result much higher than most observations. It appears that a steady state assumption is unrealistic, and/or that the method for formulating the balance of activity in soil is oversimplified. Some modellers ignored the contribution of dry deposition to soil concentrations. Others included semi-empirical representations of both wet and dry deposition processes. The predictions underestimated the observed soil/air ratios if only wet deposition was included in the models, and overestimated the ratios when both dry and wet deposition were included. It appears that dry deposition must be considered in calculating long term average soil concentrations but it is not yet clear how to quantify it properly.
(9) It was decided to commission a field survey with the specific objective of collecting data on the relative contributions of wet and dry deposition of HTO to soil water concentrations (see Objective 4 above). The work was undertaken collaboratively by CEA/DAM/DASE, France and ZSR of the University of Hannover, Germany and carried out at a site near Bruyeres-le-Chatel, France. Concentrations of HTO were measured monthly in air moisture, rain and soil moisture at two stations that received either mainly dry or mainly wet deposition. The results of the 20-month sampling programme showed that dry deposition does contribute to soil moisture concentrations. The experimental work also showed that a better estimate of soil moisture concentrations is obtained if they are related to HTO concentrations in air averaged over the period of study rather than to average concentrations in rainwater as often previously assumed. Moreover, the ratio of soil/air concentrations showed a spatial variability that depended on the joint frequency of precipitation occurrence and wind direction. These data will provide the first step toward the development of better models for predicting soil concentrations. Until then, from the ensemble of data obtained from the Bruyeres-le-Chatel field experiment (Part G) and the Chalk-River field survey (Part B), the following recommendation can be made. The concentration of tritium in soil moisture (Bq l–1) can be assumed to be 0.3 times the concentration of tritium in atmospheric moisture (Bq l-1); a higher ratio of 0.5 possibly could be used for conservative assessments. It is recommended that further work is undertaken on the activity balance and semi-empirical modelling approaches in the hope of finding a more general, theoretically-based model that can account for variations among sites. Although uncertainties in calculations of soil moisture concentrations are larger than uncertainties in predictions for other environmental compartments, HTO concentrations in soil do not contribute much to dose estimates and so a moderate level of uncertainty in modelling soil concentrations can be tolerated.
(10) It is often assumed that, where field observations and model predictions disagree, it is the predictions that are at fault. However, this is not always the case since field data are always subject to errors. This is examined further in the discussion below on how to improve field survey data.
HTO releases: Plant aqueous and organic phase concentrations
(11) The spread of results for predictions of plant tissue free water tritium (TFWT) and organically bound tritium (OBT) for all the atmospheric pathway scenarios was smaller than the divergence of results for soil moisture concentrations. Predictions and observations were generally in agreement within a factor of two to three for the model-model inter-comparison and the Canadian and French scenarios, and less than a factor of five for the Russian scenario.
This is not surprising as model calculations depend primarily on air, not soil, concentrations.
All the participating models for plant leaf uptake are similar. Overall, the models over-estimated the proportion of HTO taken up from the atmosphere probably because they are based on the assumption that all the water in plant leaves is accessible for exchange with water in the atmosphere and this is not true for all plants. Generally, the TFWT and OBT over-estimates were relatively small because plant water concentrations are forced by specific activity concepts to be lower than concentrations in air humidity. The relatively small errors are probably acceptable if assessments require conservative estimates of doses to humans from consumption of foodstuffs. However, care needs to be taken that summing of conservative estimates over all the environmental compartments does not result in unrealistic predictions. The observational data for OBT concentrations in tree leaves and tree rings (in the French scenario) emphasised that information is lacking on the behaviour of OBT in plants and trees.
(12) Overall, it is considered that models for plant leaf uptake of HTO into TFW are acceptable. However, transport of TFWT to other plant parts such as to tubers, root vegetables, shrub and tree fruits, and particularly development of models for OBT in plants (and animals) used for food by humans all require further study.
HT releases: Air moisture concentrations
(13) HT releases were only considered in the first, model-model intercomparison, scenario.
Concentrations of HTO in air moisture following an HT release are entirely controlled by oxidation of HT at the soil surface and subsequent re-emission of HTO to the atmosphere.
Model predictions were spread over a factor of ten. This is not surprising given the difficulty of simulating the oxidation and re-emission processes. This is one area where more information and data are required. Until this is forthcoming, it is suggested that the empirical ratios derived from the 1994 HT chronic release experiment conducted at Chalk River Laboratories (CRL), Canada are used, namely that HTO in air (Bq m-3) due to an HT release is 0.04 times the predicted HT concentration in air (Bq m-3). Uncertainties that would arise by using this ratio are likely to be lower than those that would occur in trying to model deposition and retention in soil, re-emission and dispersion in the atmosphere. However, it should also be recognised that there are uncertainties associated with the data obtained from the experiment and that the ratios may not be appropriate for sites where the climate, plants or soil differ markedly from those at CRL.
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HT releases: Soil moisture concentrations
(14) As for air moisture concentrations, it is suggested that empirical ratios derived from the 1994 HT chronic release experiment at CRL provide the best way to calculate soil moisture concentrations. The recommended tritium concentration in soil moisture (Bq l-1) is 6 times the concentration of HT in air (Bq m-3). Once the HTO concentrations in air and soil are known, the rest of the model for HT releases is identical to the model for HTO releases.
SUB-SURFACE PATHWAYS
(15) An effort was made to model the transport of tritiated water both down to, and up from aquifers in the vicinity of tritium facilities. There was no ambition to be totally comprehensive in the processes to be studied as this was neither a hydrological nor a soil study. The aim was to improve models for tritium assessment purposes. The first problem given to modellers involved tritium transport down through the soil and the consequent contamination of a previously uncontaminated shallow aquifer. The second problem involved a sub-surface source of tritium that had resulted in the contamination of an aquifer and the resulting upward movement of tritium through a previously uncontaminated soil. These are issues that must be considered either for the protection of groundwaters or for the isolation of buried wastes from the overlying soil medium.
(16) Although detailed soil and hydrological models are generally available, there has been no previous attempt to couple these with the specific problem of modelling tritium transport in unsaturated media. At the start of the TWG four-year programme, it was hoped that at least one data set would become available for testing the models. Despite a concerted effort to obtain such data from various sources this goal was not realised. Consequently, two model-model inter-comparison exercises were developed so that model-modellers could develop or adapt appropriate models and discuss the modelling problems that arose. Most importantly, these two exercises helped modellers to develop modelling tools that can now be applied to the problem of tritiated water movement in saturated and unsaturated media.
(17) Different models were applied to the two problems posed in the scenarios. Some modellers adapted flexible models of a commercial type that are normally used to solve a wide range of environmental problems, including flow, heat and mass transport in geological media. Other participants developed their own codes based on process models and numerical techniques formerly developed by others. A few modellers attempted to elaborate analytical models. The application of the different models to the two problems led to results that were relatively close together, and in many cases were grouped within a factor of two. Nevertheless, some important issues were highlighted during the study. The numerical models were found to be more flexible than the analytical models and could be more readily adapted to the complex processes involved in tritium transport. But, great care is required with temporal and spatial discretisation. Use of small time steps and a large number of thin soil layers appeared to be most suitable for the problems in hand. However, one modeller was able to ‘tune’ certain parameters and transfer coefficients in order to retain a model with a small number of thick soil layers that gave comparable results to the more complex models. Regardless of the number of layers used, how to treat the variability within the layers still needs to be addressed.
The two modelling exercises were approached in a research context and the question remains as to whether complex models would perform adequately in an assessment situation where all the relevant data required for their operation might not be available. Certainly in the two inter-comparison scenarios the simple models also appeared to perform well. Indeed, the steep
tritium concentration gradients imposed in the second sub-surface scenario were rather artificial. It may be that with the shallower gradients seen in nature, models with a small number of thick layers would perform adequately. So, without actual observational data it is not possible to unequivocally recommend the highly sophisticated models.
(18) A number of other conclusions were drawn during the work on the two scenarios. First, there is a need to examine closely if the models conserve the activity balance during the evolution of the systems involved in either downward or upward transport of tritium.
Secondly, it was evident that water infiltrating down through the soil, or water being drawn up and evaporating from the soil surface, together with mechanical dispersion are the key factors in the transport of tritium in saturated or unsaturated media. Thirdly, the transport of tritium is very sensitive not only to water flow, but also to the value assigned to the dispersivity
Secondly, it was evident that water infiltrating down through the soil, or water being drawn up and evaporating from the soil surface, together with mechanical dispersion are the key factors in the transport of tritium in saturated or unsaturated media. Thirdly, the transport of tritium is very sensitive not only to water flow, but also to the value assigned to the dispersivity