Origin of thermal waters on the basis of their radioisotopic content

Abstract

The content of the isotopes of U, Ra, Th and also Rn 222, Ac 227 and Pb 210 was investigated in the waters of active volcanism (Kamchatka, Kuril Islands) and in the thermal waters of neovolcanic regions (Caucasus). The content of radioisotopes in the condensates of the volcanic vapours is very like that of the depth gases. Here the average content of radioisotopes is U 2.5 × 10 −7 g/l, Ra 1.0 × 10 −13 g/l, Pb 210 8 × 10 −15 g/l. The Ra/Rn (or U/Rn) varies from 10 −3 to 10 −5 (activity units). In the fumarole waters the content of U and Pb 210 diminished by several times (dilution in the meteoric waters) and the content of Ra by tens of times (precipitation of the elements from the sulphatic waters). The average ratios: U 234/U 238 = 1.3; Io/Th = 1.05; RdTh/Th = 2.0; MsTh/Ra = 10; Pb 210/Ra 226 ≥ 30 are increased. The same results are obtained in the fumarole minerals too. The Pu 239 indicates the existence of depth matter. The thermal waters (Causcasus) are distinguished by increased value of U 234/U 238 (the average 2.0, but in some cases reaches to 3.5). The content of shortlived isotopes of Th and Ra can be very high (RdTh/Th = 16 and ThX/MsTh = 105). This indicates the proximity of the formation region, the recent age of the water and the absence of contamination from waters of different genesis. The content of Ra 226 is small and U 238/Rn decreases as compared to volcanic waters. It means that the Rn emission from the rocks is here less essential than in active volcanism rocks. The time of circulation of the gas vapour flows of the volcanoes can be estimated on the Rn/Ra ratio as 10 ÷ 100 days, and for thermal waters as much as hundreds of years. The difference of the radiochemical parametres allows us to determine the genesis of thermal waters.