1. Introduction
In natural conditions, uranium (U) mainly exists as three isotopes:
234U,
235U, and
238U. Among them,
235U and
238U have long half-lives with high abundance, and the content ratio of
238U/
235U = 137.832 [
1]. Further,
234U has the shortest half-life, and the atomic abundance of
234U is about five orders of magnitude lower than that of
238U [
2]. Uranium isotopes are widely used as a typical tracer in the dating of various oceanographic processes and in the study of sources of dissolved solutes in river basins [
3,
4,
5,
6,
7,
8].
Studies of the distribution characteristic of uranium isotopes have mainly focused on glacial sedimentation at the two poles and oceanic circulation [
9,
10] and recent research has still mainly focused on the oceans. Existing research results show that the uranium concentration of seawater has a relatively consistent, linear relationship with salinity [
11,
12]. Some scholars have tried to convert oceanographic methods to inland areas, but inland research has focused on rivers and estuaries [
13,
14,
15], river bends [
16], and lakes [
17], while very few studies have explored the uranium content and uranium isotopes in saline lakes.
Limited studies on hydrological systems in saline lakes have been conducted and research methods have been systematically developed. The research dates back to Osmond, et al. [
18] who established a two-end mixing model based on the relationship between
234U/
238U, 1/U, and U/TDS (total dissolved solids) in the water of Sambhar lake. They found that one end of the lake had a low uranium content and low salinity, while the other end had the opposite. Yadav et al. [
19]. explored the U/TDS relationship in different aqueous media (including river water, lake water, and inter-crystal brine) of the Sambhar saline lake during different periods of the year, finding that there was a strong linear relationship between aqueous U concentration and TDS, which was consistent with the two-end mixing model established by Osmond et al. [
18]. Moreover, Yadav, et al. [
19] mentioned that the seasonal distribution difference of uranium isotopes was mainly controlled by the process of water dilution and evaporation. Borole et al. [
20] studied the two major water systems of Narbada and Tapti in the west coast of India and showed that the abundance ratio of the aqueous uranium content to the sum of the main cations was similar to that found in typical crustal rocks. Due to the process of evaporation and solute concentration in saline lakes, the aqueous uranium content is generally higher than that in freshwater lakes.
Reports of saline lakes in high uranium environments are limited. Since the 1950s, Zheng [
21] has carried out extensive and in-depth research on saline lakes in the Qinghai-Tibet Plateau. Zhang [
22] found that, in the mining of lithium, boron, potassium, and magnesium from the Qaidam Basin, both uranium and thorium could be enriched, meaning that uranium enrichment is one of the basic characteristics of the saline lakes of northwestern China. The concentration of uranium of soda lakes in Mongolia is 57~15,000 μg/L [
23], 320 μg/L in Gasikule salt lake in Qaidam Basin [
24], and 13.4 μg/L in Qinghai Lake [
25]. The potential mechanism of high uranium content in a saline lake has also been explored. For example, Benjamin et al. [
23] and Isupova et al. [
26] found that there exist high concentrations of uranium(U) in soda lake basins with groundwater-fed hyperalkaline. Hence, uranium-rich groundwaters are concentrated by evaporation and the U(VI) is chelated by
to form the highly soluble UO
2(CO
3 in water, which is the main cause of the enrichment of uranium content in lakes in northwestern Mongolia. Strakhovenko et al. [
27] showed that biogeochemical processes have an impact on the mobility of uranium species. Andersson et al., [
28] showed that the behavior of colloids suspended phase and colloids are beneficial for the removal of U at very low salinities. Koch-Steindl and Pröhl, [
29] indicate that the mobility of U is influenced by sorption and complexation processes on inorganic soil constituents such as clay minerals, oxides, organic matter, and biological fixation and transformation. Fredrickson et al. [
30] indicated that the Mn (III/IV) oxides in particular may significantly inhibit the microbial reduction of U(VI) and its subsequent precipitation as UO
2(s). Belli et al. [
31] thought that a certain number of OM and/or competing cations of iron (such as Ca
2+ and Mg
2+) may inhibit the adsorption effect of uranium complex on iron hydroxide.
In high-salinity environments, such high uranium features make it possible to extract uranium resources from saline lake brine. In addition, no matter when it is processed, boron lithium potassium salt products or waste liquid discharge will affect the product quality or pollute the environment.
Thus, it is necessary to determine the distribution characteristics of aqueous uranium isotopes in lakes with different salinity gradients in order to understand the influencing factors of uranium isotope distribution in lake waters. Previous research has shown that ion concentrations, TDS, salinity, pH, and biogeochemical process are factors that influence the aqueous uranium content distribution and the [234U/238U]AR. For this study, we selected several lakes of different salinities with low human disturbance in the northeast of Qaidam Basin of the Qinghai-Tibet Plateau for water chemistry (anions, cation, TDS, salinity, and pH) and uranium isotope analysis to gain insight on the overall distribution pattern and possible seasonal variations in the distribution characteristics of uranium in the saline lakes of China. In particular, we wanted to gain insights into the controlling factors and underlying mechanism of aqueous uranium concentrations in the lakes and to explore the potential environmental applications of uranium isotopes with the goal of promoting the widespread use of radioactive isotopes as environmental indicators.
3. Results
The water chemistry results of each sampled lake are presented in
Table 2, including the aqueous uranium isotope concentration and the [
234U/
238U]
AR. Given that
238U is the dominant uranium isotope, accounting for 99.275% of the total U content, the uranium concentration in this paper refers to the
238U concentration, expressed as
238U hereafter. In general, freshwater lakes (KLK, XLH 2) are lower than saline lakes (GH, TS, XLG 1) for all measured parameters.
(a) Given that the salinity in the GH waters was above the detection limit of the instrument, we estimated these GH samples salinity by converting the measured conductivity as described below, resulting in an estimated salinity of 72.36‰ for the GH samples.
Conductivity can be converted to salinity between 0–40 °C using the following conversion formula:
where
y NaCl is the salinity calculated as NaCl,
x is the conductivity in μs/cm, and
t is the water temperature in °C. Salinity calculated using the above formula is given in units of ppm, and must be subjected to a basic unit conversion for expression in ‰.
(b) These contents of
and
have been calculated by these two formulas [
40].
3.1. Water Chemistry Characteristics in the Northeast of Qaidam Basin
As shown in a Piper diagram (
Figure 2), the cations in these lake waters of the northeast of Qaidam Basin were mainly Na
+, jointly accounting for 78.5%, 54.5%, 73.6%, 72.4%, and 71.0% of the total cations (TZ
+) in GH, KLK, TS, XLG1, and XLG2 lakes, respectively. Na
+ and K
+ are followed by Mg
2+ with the second highest concentration, while Ca
2+ had the lowest cation concentrations measured. Of note, the XLG 2 waters had a slightly higher concentration of Ca
2+ than Mg
2+. The anions were mainly Cl
−, accounting for 76.5%, 52.2%, 63.4%, and 67.9% of all anions (TZ
−) in GH, KLK, TS, XLG1, and XLG2 lakes, respectively. Cl
− was followed by
with the second highest concentration, while
,
,
, and
jointly had the lowest anion concentrations measured. The concentration of an ion (or ion pair) remained essentially the same throughout the same water body.
3.2. Distribution Characteristics of Uranium Isotopes in These Lake Waters in the Northeast of Qaidam Basin
Multiple sampling sites were set at different depths in KLK and TS so that the horizontal and vertical distribution characteristics of
238U could be analyzed for these lakes. The horizontal distribution characteristics of the
238U concentration and the [
234U/
238U]
AR in the KLK and TS waters are shown in
Table 3.
Aqueous 238U concentration of the freshwater lakes in the northeast of Qaidam Basin were generally low. In contrast, the concentration of 238U in these saline lake waters were generally higher. There is no significant difference in the [234U/238U]AR between freshwater lakes and saline lakes, with the values generally ranging from 1.460 to 1.798. A comparison of all five sampled lakes data revealed that 238U (TS) was the highest of all, while [234U/238U]AR (TS) was the lowest. There was no significant horizontal distribution difference among the three sampling sites for the 238U concentration and the [234U/238U]AR.
The vertical distribution characteristics of the
238U concentration and [
234U/
238U]
AR in the KLK and TS waters are shown in
Table 4.
Three sampling sites were set in KLK. At sampling sites 1 and 2, water samples were collected at three depths referred to as T (surface), M (middle), and B (bottom) from the surface water to the bottom water, while only two sampling depths (T and M) were collected at sampling site 3.
In summary, in natural lakes, in the absence of interference from other aqueous factors, the 238U concentration increased with an increased water depth, while the [234U/238U]AR showed an initial increase, then decreased gradually with depth. However, the ranges of these changes were very small, indicating that aqueous 238U concentration and the [234U/238U]AR were essentially evenly distributed vertically and horizontally within the same lake.
3.3. Influence of Water Sources Supply on the Uranium Isotopes of Lake
In order to further explore the influence of water sources supply on the uranium isotope of lake, we get some water samples from different sections of the Bayin river (BY) and KLK at the same single day, and the Bayin river is the only incharge river of KLK lake. Results of
238U concentration and [
234U/
238U]
AR in the BY river and KLK waters are shown in
Table 5.
We found that the 238U concentration and [234U/238U]AR in different sections of BY river are not the same. The 238U concentration is highest in the upper reaches (6.0 μg/L) and the lowest in the lower reaches (2.8 μg/L) of natural waters. However, [234U/238U]AR shows the opposite of the distribution of 238U concentration. The Bayin river passes through Delhi city. We collected the highest 238U concentration samples (6.2 μg/L) from the watercourse of Delhi city.
The Bayin river flows into the KLK from the northeast and mixes with the original lake water. Two sampling results show that the average 238U concentration of KLK lake water is about 5.5 μg/L, which is slightly lower than the upper reaches of the Bayin River and is significantly higher than the lower reaches. The Bayin river flows out the KLK from the southwest. The 238U concentration of watercourse connecting KLK and TS is 5.6 μg/L. This is basically consistent with that of KLK lake water.
3.4. Seasonal Distribution Characteristics of Uranium Isotopes
We explored the seasonal distribution characteristics of U in these lake waters in the northeast of Qaidam Basin. The results from the first batch of samples (December 2017), with the minimum recharge water volume in winter, indicated that the aqueous
238U concentration of the same lake remained at a relatively stable level. However, it is unclear whether the contribution of exogenous inputs (e.g., atmospheric precipitation and river inputs) could affect the lake water
238U concentration and the [
234U/
238U]
AR and the effects of exogenous inputs should be clarified before interpreting the environmental implications of the above indicators. Therefore, GH and TS were selected for a second round of sampling in summer with the maximum recharge water volume and measurement in order to conduct a seasonal comparison of the aqueous
238U concentration and the [
234U/
238U]
AR and the results are shown in
Table 6.
We found the 238U concentration of GH and TS changed slightly with the season. In winter (December), 238U concentration of GH is 18.4 μg/L, in summer (June), the result is 16.2 μg/L. We also see the same pattern in TS, where 238U concentration of TS decreased from 35.5 μg/L to 34.7 μg/L in from winter to summer. Whether it is GH or TS, the [234U/238U]AR in different seasons is approximative. From winter to summer, the maximum discharge water volume variation, the [234U/238U]AR of GH vary from 1.792 to 1.798, and the result of TS vary from 1.463 to 1.462.
5. Conclusions
Four lakes in the northeast of Qaidam Basin were selected to represent a sequence of waters salinity in order to test the effect of lake salinity on aqueous uranium isotopes. A total of 43 water samples were taken during three different sampling campaigns and analyzed for water chemistry and uranium isotope concentrations. The aqueous 238U concentration of the freshwater lakes was generally low, while it was generally high in these saline lakes, which is a response to the combined effects of TDS and pH.
Furthermore, pH is the dominant factor affecting changes in aqueous 238U concentration of the sampled saline close lakes, as the lake waters pH could directly affected the speciation of 238U, with lower water pH leading to more fractions of 238U into stable forms, and thereby smaller aqueous 238U concentration. With the exception of pH value, the replenishment of water likely exerts a certain effect on the uranium isotope concentration for open KLK fresh lake located in the northeast of Qaidam Basin in the same high evaporation environment background as close saline lakes.
The [234U/238U]AR of each lake was greater than one, ranging roughly from 1.460 to 1.798, which was likely due to the α-recoil of 238U at the water-rock interface and the leaching-precipitation process. The degree of consistency in the 238U and [234U/238U]AR values across various sites in a lake indicated that aqueous 238U and [234U/238U]AR were evenly distributed in the same 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 these saline lakes. If the concentration of 238U in natural lake waters and the [234U/238U]AR remain almost consistent for a certain period of time. The potential environmental impact is that which is the base of the determination of sediments ages in lake and/or of sedimentation rates could be determined by lake sediment and/or biogenic carbonate in future. Further, we can chronologically decipher the environmental change records in them by measuring the U–Th nuclides of these sediments.