Hydrogeochemistry and stable isotopes in radon-rich thermal waters of Belokurikha (Altai, Russia)

The first integrated isotope and chemistry results have been obtained for radon-rich thermal waters from the Belokurikha field which are used at a large spa resort in Altai, Russia. The waters reside in an unconfined aquifer composed of Quaternary soft sediments and in a confined (artesian) aquifer of monolithic to weathered Upper Paleozoic granites. The waters belong to three geochemical groups: low-radon nitrogen-silicic interstitial waters in weathered Paleozoic granites; groundwaters of REE-enriched and background compositions; surface waters of the Belokurikha River. The interstitial waters in granites have HCO₃-SO₄ Na and SO₄-HCO₃ Na major-ion chemistry, total salinity from 198 to 257 mg/L, pH = 8.6-9.6, silica contents of 19.8 to 24.6 mg/L, and ²²²Rn activity from 160 to 360 Bq/L (290 Bq/L on average). Judging by their oxygen and hydrogen (deuterium) isotope compositions (-17.5 to -14.2 ‰ and -126.9 to -102.7 ‰, respectively), the Belokurikha aquifers recharge with infiltrating meteoric water, especially the winter precipitation. The carbon isotope composition of dissolved inorganic carbon (-9.7 to -25.6 ‰ δ¹³С_DIC) corresponds to biogenic origin. Comparison of radon-rich mineral waters from different areas of southern Siberia shows that the change from oxidized to reduced environments leads to ²³²Th/²³⁸U increase from 4.20∙10^-5-7.39∙10^-2 to 0.0022-26, respectively, with an intermediate range of 2.63∙10^-5-0.20 in transitional conditions.