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A model of tritium transport in the environment near an atmospheric source of tritium is presented in the general context of modelling material cycling in ecosystems. The temporal and spatial scales of the model were chosen to allow a comparison with environmental monitoring data for tritium collected in the vicinity of the Savannah River Plant. Initial simulations with the model showed good agreement with the monitoring data, which included measurements of atmospheric and vegetation tritium concentrations. The model can also simulate values of tritium in organic matter in vegetation if the key parameter distributing the source of organic hydrogen is varied to fit the data. However, because of the lack of independent confirmation of the distribution parameter, there is still uncertainty about the role of organic movement of tritium in the food chain, and its effect on the dose to man. Simulations with the model have been used to estimate the effect of different values of the distribution parameter on the dose to man. The simulation suggests that the dose from tritiated hydrogen is about 4 % of the dose from tritiated water vapour when direct incorporation of tritium into organic matter is not included. The relative dose can be considerably higher for cases where the distribution parameter is set to include very small amounts of organic tritium from atmospheric tritiated hydrogen.