Distribution Characteristics and Influencing Factors of Uranium Isotopes in Saline Lake Waters in the Northeast of Qaidam Basin

Abstract

Four saline lakes in the northeast of Qaidam Basin were selected to explore the distribution characteristics and influencing factors of uranium isotopes in lake waters with high evaporation background. The 238U concentration and the activity ratios of 234U/238U ([234U/238U]AR) showed that there was no significant change in the same lake, but there was a certain degree of difference in the distribution between different lakes. We found that aqueous 238U concentration within a certain range increased with an increase in TDS (total dissolved solid) and salinity, as was also the case with pH. As in natural waters, the pH affects the speciation of 238U, but TDS and salinity affect the adsorption process of aqueous 238U. Further, the replenishment of water will also affect the uranium isotope concentration for lakes, but it is not the main influencing factor for saline lakes. Therefore, we suggest that pH is the dominant factor affecting changes in aqueous 238U concentration of the sampled saline lakes. The [234U/238U]AR in these saline lakes are closely related to the input water and the associated water–rock interactions involving sediments, atmosphere dust, and organic material, etc. during the evolution stage, metamorphous degree, and hydrochemistry of the saline lakes. Lake water samples collected in the maximum and minimum discharge water period, were used to evaluate the seasonal distribution characteristics of aqueous 238U, and we found that 238U concentration did not show an evident change with the seasons in these saline lakes. If the 238U concentration and [234U/238U]AR can remain consistent during a period of time, then the sediment ages and/or sedimentation rates could be determined by lake sediment and/or biogenic carbonate in future, thus allowing for the accurate reconstruction of the paleoclimate and paleoenvironment.

Effects of Ice Freeze-Thaw Processes on U Isotope Compositions in Saline Lakes and Their Potential Environmental Implications

“…The water chemistry results of each sampled lake are presented in Table 4, including the aqueous 238 U concentration and the [ 234 U/ 238 U] AR . Aqueous 238 U concentrations of the freshwater lakes in the northeastern part of the Qaidam Basin were generally low, which is consistent with the results of previous analyses of lake water samples collected in June, July and the beginning of December without ice coverage (Zhao et al, 2020). The 238 U concentration of ice from the five sampled lakes were less than that of water under the ice, except for samples from QHH.…”

“…They found that one end of the lake had a low uranium content and low salinity, while the other end had the opposite. The U content measured by Zhao et al (2020) in several saline lakes and our unpublished salt lakes shows the 238 U concentration varied with the range of 12.6-200 μg/L. Other studies have also shown that the U content of lakes water varies directly with TDS (salinity) and pH, the latter of which is partially controlled by evaporation and/or other secondary concentration pathways (Osmond et al, 1974;Borole et al, 1982;Yadav and Sarin, 2009;Zhang et al, 2019;Chen et al, 2020;Zhao et al, 2020).…”

“…In addition, river water and groundwater samples were collected from the rivers recharging Qinghai Lake to verify whether surface runoff would affect lake U concentrations and [ 234 U/ 238 U] AR . Previous studies showed that the U concentration and [ 234 U/ 238 U] AR of waters from the same lake in the Qaidam Basin varied little aerially or vertically within the water column or seasonally through time (Zhang et al, 2019;Zhao et al, 2020). Thus, we believe that the samples from one location of each lake are sufficient to represent the U content and the [ 234 U/ 238 U] AR distribution characteristics of these sampled lakes, and that the U data of surface water can represent the entire water column of the lake.…”

“…Thus, the sampled ice and lake waters should show the typical U distribution within the ice and the underlying water. By comparing the U composition of the sampled lake water in summer with the maximum water input published in Zhao et al (2020), we expected to gain a deeper understanding of effects of ice freeze-thaw processes on U isotope distribution in saline lake waters and the potential environmental implications on the observed U data.…”

“…The U content measured by Zhao et al (2020) in several saline lakes and our unpublished salt lakes shows the 238 U concentration varied with the range of 12.6-200 μg/L. Other studies have also shown that the U content of lakes water varies directly with TDS (salinity) and pH, the latter of which is partially controlled by evaporation and/or other secondary concentration pathways (Osmond et al, 1974;Borole et al, 1982;Yadav and Sarin, 2009;Zhang et al, 2019;Chen et al, 2020;Zhao et al, 2020). Moreover, the U concentration indicate that the length of these processes, which are controlled by the duration of water-rock interaction, is probably an important factor to consider when accounting for the variation in lake salinities within a similar geographical area.…”