An Application of a Transfer Model to the Drainage System of Tritium in a River Basin in the Vicinity of a Nuclear Site

A prototype of a tritium transfer model in the environment was developed for the purpose of dose estimation in our institute in 1995. The model consists of many compartments representing tritium reservoirs in a hydrological cycle in the local hydrosphere, and reservoirs in food chains in the aquatic and terrestrial environments. Underground tritium reservoirs are defined as three aquifers in the model, although the number of aquifers might depend on the site. Our present work was focused on an application of the model to an area with nuclear facilities, Tokai-mura, Japan, where there is only one aquifer. The Shinkawa (Shin River) flows through the Muramatsu area between two nuclear facilities, and then into the Pacific Ocean. About 2000 people live in the area and some use groundwater as a convenient daily water source as well as village tap water. Samples at 16 rainwater and 20 groundwater sites from the Muramatsu area (5 km²) as well as 3 groundwater sites from an inland area were taken during 1983–90, and tritium concentrations were measured. In order to investigate the scale of the drainage system of the Shinkawa basin, time series data of excess tritium in rainwater and groundwater were applied to our transfer model. As a result, the volume of the aquifer was calculated as half of the annual rainfall in the area. This is so small that the tritium concentration of the groundwater has a quick time trend response to input of tritium from rainwater. The results can be used to estimate population dose of the residents in the Muramatsu area.