The Inskie Springs: New Insights into Low-Radon Waters

Abstract

Abstract —In August 2019, four descending (gravity) springs in the Inya River valley were sampled during the field works conducted within the study of low-radon waters of the Novosibirsk urban agglomeration in the area lying away from known granite massifs. Laboratory analyses have revealed enhanced radon activity concentrations (from 5 to 149 Bq/dm3). It has been established that these waters are fresh, of HCO3 Mg–Ca chemical composition, and have a TDS value of 413 to 548 mg/dm3 and a silicon content of 4.1–8.6 mg/dm3. They are characterized by neutral to slightly alkaline pH (7.1–8.4) and oxidizing geochemical conditions with Eh from +205.3 to +231.8 mV and O2 dissolv. = 6.24–12.26 mg/dm3. The revealed predominance of SO42– over Cl– concentrations in the waters of the study area was probably due to the presence of sulfides in the water-bearing sediments, in particular, pyrite in the surface sediments. More than tenfold proportional excess of Ca concentrations over Si in the Inskie spring waters indicates the predominantly carbonate composition of the water-bearing sediments. The gross α-activity of waters is 3–4 mBq/dm3, and gross β-activity is 11–15 mBq/dm3. Natural radionuclides are found in the spring waters within the following limits (mg/dm3): 238U, from 2.83 ∙ 10–3 to 4.13 ∙ 10–3; 232Th, from 2.39 ∙ 10–6 to 1.16 ∙ 10–5, and 226Ra, from 3.83 ∙ 10–10 to 4.93 ∙ 10–10. The value of the 232Th/238U ratio for the waters ranges from 5.79 ∙ 10–4 to 3.61 ∙ 10–3, as a result of the oxidative geochemical migration-arresting capability of thorium. The uranium isotope ratio (γ) 234U/238U varies from 2.6 to 3.2 for the waters, with the uranium isotope activity determined as 117–124 mBq/dm3 for 234U and 38–48 mBq/dm3 for 238U. This indicates shallow circulation of the studied waters as compared with those of the Svyatoi spring in Verkh-Tula Village, for which γ = 1.3; the 234U activity, 147 mBq/dm3; and the 238U activity, 115 mBq/dm3. By isotopic composition, the origin of the spring waters is assigned to the infiltration type, and they are characterized by relatively narrow distribution of δ18O (from –17.5 to –16.7‰) and δD (from –128.4 to –126.2‰) values. The δ13CDIC values are from –10.3 and –10.9‰ in springs 3 and 2 and become lighter (–11.2 and –12.1‰) in springs 1 and 4, respectively. This is due to significant participation of surface waters in the recharge mechanism of springs 1 and 4, which is also consistent with the δ18O and δD data and 14C dating. The estimated age of water-dissolved carbon is 1478 ± 81 years for the waters of spring 3 (the oldest), while it is found to be only 651 ± 53 years for spring 4 and is estimated as modern for spring 1. The reported decline in the age of water-dissolved carbon down to recent age is indicative of increased contribution of surface water to the spring recharge area. The C and O isotope compositions of calcite of the host aquifer rocks are characterized by close values for most of the samples: δ13С varies within narrow limits (from –3.1 to –2.7‰), and δ18О varies from 17.2 to 18.4‰. The isotopic composition becoming lighter for carbon (up to –11.0‰) and oxygen (up to 13.9‰) was noted for weathered schist samples. Results of the carbon isotope analysis of rock samples, their organic component, and water indicate an active isotopic exchange in the water–rock–organic matter system.