Tritium concentrations of groundwater in Naka Terrace,Ibaraki Prefecture,Japan

Levels of tritium in groundwater of the inland area of Naka Terrace were measured during 1981–1983 to establish a baseline for evaluation of any changes which may result from release of tritium from nuclear facilities in the coastal region of the terrace. Most of the samples yielded concentrations in the range 40–90 pCi litre⁻¹, which were similar to those of two rivers bordering the area of concern, whereas tritium levels in monthly precipitation samples were in the range 30–50 pCi litre⁻¹. These results imply that the rivers are fed from groundwater reservoirs, which are releasing the higher tritium levels recharged during the previous 20 years when bomb-produced tritium caused higher levels in precipitation. Only two groundwater samples yielded concentrations below detection limit, both from the deep Pleistocene gravel layer in a buried river channel underlying the terrace. Some ground-waters in the coastal area yielding more than 100 pCi litre⁻¹ are considered to be already influenced by tritium released from nuclear facilities.