Radon-rich waters of the Tulinka aquifers, Novosibirsk, Russia

Abstract

The first integrated isotope and chemical data on radon-rich waters of the Tulinka aquifers (Novosibirsk, West Siberia) are presented. The Tulinka radon waters are fresh, with neutral to moderately alkaline рН (7.3–7.7), SO4–HCO3 Na–Mg–Ca major-ion chemistry. The amount of total dissolved solids is 720–910 mg/L, Si content 6.41–9.02 mg/L. Eh is within +169.1 – +250.0 mV, corresponding to oxidized conditions. The concentration of dissolved oxygen is 2.86–7.37 mg/L. Radon activity (222Rn) varies from 173 to 276 Bq/L. The concentrations of U, Th, and Ra isotopes are 0.015–0.017 mg/L238U, 9.59∙10−7 to 1.58∙10−5232Th, and up to 4.93∙10−10226Ra; the 232Th/238U ratio is in the range 5.81∙10−5 to 9.42∙10−4. Total α activity is not higher than 891 mBq/L, and β activity is 80 mBq/L. The compositions of stable isotopes are −15.4 to −15.1‰ δ18O, −114.2 to −112.8‰ δD, and −13.9 to −9.9‰ δ13C. The isotope signatures and recharge patterns of the Tulinka waters are insensitive to seasonal effects, which suggests the slow water exchange. Substantial seasonal variations of the content of dissolved inorganic carbon (DIC) are detected. The range of δ13CDIC in the analyzed samples (from −14.3 to −9.0‰) corresponds to a mixed CO2 origin: diffusion of soil CO2 approaching the atmospheric δ13CCO2 values, and biogenic CO2 released by decaying plant residues. The measured isotope characteristics provide evidence that the waters recharge by infiltration from meteoric sources. The approximate radiocarbon age of DIC in the Tulinka waters, determined for the first time, is 2663 ± 144 years. However, this age may be underestimated due to groundwater mixing with the present-day surface waters.