Disequilibrium between uranium and its progeny in the Lake Issyk-Kul system (Kyrgyzstan) under a combined effect of natural and manmade processes

Abstract

The Kadji-Sai abandoned field of U-bearing brown coal on the southern coast of Lake Issyk-Kul (Kyrgyzstan) poses a threat of radioactive pollution to the world's fifth deepest and second largest pristine highland lake. The valleys of ephemeral streams in the lake catchment are filled with coarse-grained sand and clay, with a background U–Ra activity of 35–55 Bq kg −1. High activity areas vs. this background come from three sources: (1) scarce outcrops of uraniferous brown coal and mining wastes containing fragments of this coal with 238U/ 226Ra ratios of 0.8 due to uranium losses through weathering; (2) manmade anomalies caused by a radioactive waste dump, where U was extracted from the ash of coal burnt at a coal-fired power plant. As a result, the 238U/ 226Ra ratios become 0.15–0.25; (3) six catch pools terraced below the mine, where U activity decreases downslope, and 238U/ 226Ra ratios reach 150–200. Uranium lost in the extraction process may have been retained on the terraces. The distribution pattern of radionuclides in the bottom sediments of the lake is controlled by water depth and offshore distance. The upper section of homogeneous limy–argillic deposits in the lake center remains undisturbed by currents, as indicated by regular sub-exponential distribution of atmospheric 137Cs and 210Pb atm. Sedimentation rate in the lake center for the past century, found from 210Pb, was 0.32 mm yr −1. 238U/ 226Ra in deep-water sediments was about 3. The activity of uranium adsorbed by sediments from the lake water was estimated by subtraction of the Ra-equilibrium component from the total U activity. Thus, the flux of dissolved U to the bottom sediments was as 2.07 × 10 −7 g cm −2 yr −1. The upper section of near-shore deposits was disturbed by currents, with 137Cs and 210Pb atm more or less uniformly distributed in this layer. Peaks of 226Ra and 210Pb occur at different depths from 5 to 20 cm below the sediment surface, with 238U/ 226Ra ratios 0.28–0.44. The presence of mullite in these sediments indicated that radioactive ash penetrated into the lake in the past. At present, 226Ra in the ash is buried under a non-radioactive cap.