ABSTRACT

Monthly variations of tritium, deuterium and SO₄^2- concentrations in waters from a fumarole, seven hot springs, three rivers, and precipitation in the Nasu volcanic area were measured to investigate their origin and subsurface behavior. Most of waters in the Nasu district including fumarolic condensate are considered to be of meteoric origin on the basis of their δD values. Hot-spring waters are classified into three groups according to their tritium concentration, i.e., the Group A waters with T=30 ∼ 70TR (as of 1974), Group B waters having T=50 ∼ 100TR with seasonal variation, and Group C waters with T=3 ∼ 30TR and high δD values. A model is proposed that the water reservoirs are divided into three stratified aquifers, i.e., layers I, II and III, each of which is characterized by respective tritium concentration. The tritium concentration of the shallowest layer-I water is close to the annual average tritium concentration for precipitation at the time of sampling (1974). The age of the layer-I water is considered to be less than a few years. The layer-II water occurs deeper than the layer-I, and is 5 ∼ 12 years old. The layer-III water occurs beneath Nasu volcano, is very old and contains no tritium. Mixing of waters from the different layers produces Group A, B and C waters. Change in the mixing ratio causes seasonal variation in the isotopic and chemical compositions of some hot-spring waters.