Известия Томского политехнического университета: Инжиниринг георесурсов (Dec 2022)

DISCOVERY OF LOW-RADON WATERS – SEDOVA ZAIMKA

  • Dmitry A. Novikov,
  • Aleksandr N. Pyrayev,
  • Anastasia A. Maksimova,
  • Anna F. Sukhorukova,
  • Fedor F. Dultsev,
  • Anton S. Derkachev,
  • Anatoliy V. Chernykh,
  • Albina A. Khvachshevskaya

DOI
https://doi.org/10.18799/24131830/2022/12/3735
Journal volume & issue
Vol. 333, no. 12
pp. 61 – 73

Abstract

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Relevance of the investigation is in obtaining actual isotope-geochemical data on natural waters and water-bearing rocks in the Sedova Zaimka, an occurrence of radon waters. Objective: to study the features of chemical composition of natural waters and water-bearing rocks, and to obtain the first data on the activity of 222Rn and on the isotope composition: δD, δ18O, δ13С, 234U, 238U, 226Ra and 228Ra. Methods. Laboratory studies of the chemical composition by means of titration, ion chromatography, mass spectrometry with inductively coupled plasma were carried out at the Research Laboratory of Hydrogeochemistry at the School of Earth Sciences and Engineering of the Tomsk Polytechnic University. Determination of the set of parameters δD, δ18O, δ13СDIC in waters and dissolved inorganic carbon was carried out at the Shared Equipment Center if the IGM SB RAS with the help of Isotope Ratio Mass Spectrometer FinniganTM MAT 253, equipped with sample preparation attachments H/Device (to determine δD) and GasBench II (to determine δ18O and δ13СDIC). 222Rn concentration in waters was determined using the Alfarad Plyus instrument in the Laboratory of Hydrogeology of Sedimentary Basins of Siberia at the IPGG SB RAS. The data on the total β-activity of waters, as well as on the activities of 234U, 238U, 226Ra and 228Ra, were obtained after preliminary radiochemical sample preparation using an alpha spectrometer ALPHA-ENSEMBLE-8 (Ametek, ORTEC, USA); gamma-spectrometric system arranged on the basis of well coaxial HPGe semiconductor detector with low-background cryostat EGPC 192-P21/SHF 00-30A-CLF-FA (EURISYS MEASURES, France) and alpha-beta radiometer UMF-2000 with a silicon detector (Doza, Russia) to measure low activities. Data separation into homogeneous geochemical sets was performed with the help of Са/Na, Са/Mg, Ca/Si, Mg/Si, Na/Si coefficients. To determine the extent of element concentrating in natural waters, the coefficients of concentration and water migration (according to A. I. Perelman) were calculated. Results. Radon water occurrence at Sedova Zaimka was discovered, and isotope-geochemical investigation was carried out. Radon waters (with 222Rn activity up to 428 Bq/dm3) are mainly characterized by HCO3 Mg-Na-Ca composition with TDS 158 to 581 mg/dm3 and silicon content 4,34 to 30,84 mg/dm3. The geochemical parameters of the environment vary from reductive to oxidative settings with Eh from –40,2 to +28,4 mV; pH from 7,5 to 7,6 and O2dissolved from 2,99 to 5,24 mg/dm3. Geochemical coefficients are equal to: Ca/Na 77,17; Ca/Mg 6,63; Ca/Si 11,42; Mg/Si 1,48; Na/Si 0,92; Si/Na 15,34; rNa/rCl 2,12; SO4/Cl 4,02, which is a regular indication of the formation of radon water composition in the embedding rocks of mainly aluminosilicate composition. Among trace components, the highest average content is characteristic of (mg/dm3): Si=17,77; Fe=1,18; Mn=0,16; Zn=0,020 and W=0,0036. No significant coefficients of component concentrations were revealed. Strong migration ability in solution is exhibited by Sr, medium – by Si, Mn, Ba, Cs and U. The total β-activity of the waters is 32 mBq/dm3. The concentrations of natural radionuclides vary (mg/dm3): 238U from 3,91∙10–4 to 6,39∙10–4; 232Th from 6,02∙10–6 to 2,37∙10–5 and 226Ra from 6,66∙10–11 to 1,09∙10–10. The 232Th/238U ratio in the waters varies from 1,02∙10–2 to 3,71∙10–2, which is a consequence of the oxidative geochemical setting, in which thorium does not migrate. The uranium isotope ratio (γ) 234U/238U is 5,75, with the activity of uranium isotopes (mBq/dm3): 234U (115±7), 238U (20±2), which points to not very deep circulation of the studied waters. The activity of radium isotopes in the waters is 70±7 mBq/dm3 for 226Ra and 51,8±3,9 mBq/dm3 for 228Ra. The 226Ra/228Ra ratio in HCO3 Mg-Na-Ca radon waters is equal to 1,35. The isotope composition of radon waters (from –126,3 to –121,1 ‰ for δD and from –16,8 to –16,3 ‰ for δ18O) points to their meteoric infiltration origin. The isotope composition of carbon δ13CDIC points to the biogenic origin of carbon dioxide and its participation in carbonate-silicate weathering of the rocks.

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