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.

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 ${\mathrm{CO}}_3^{2-}$ to form the highly soluble UO₂(CO₃${)}_3^{4-}$ 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 UO₂(s). Belli et al. [31] thought that a certain number of OM and/or competing cations of iron (such as Ca²⁺ and Mg²⁺) 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.

2. Materials and Methods

2.1. Sampling Locations

There are many lakes in the Qaidam Basin and most of them are saline lakes. The water samples in this study were collected from four lakes of different salinities in the northeast of Qaidam Basin: Gahai lake (GH), Keluke lake (KLK), Tuosu lake (TS), and Xiligou lake (XLG). In order to further explore the influence of water sources on the uranium isotope of KLK lake, we obtained some water samples from the Bayin river (BY). These sampling locations are shown in Figure 1. Two sampling sites were established in XLG: one (XLG 1) in the center of the lake, and the other (XLG 2) in a shallow pool next to the lake, with the pool water mainly originating from natural precipitation with water quality indicators similar to those of KLK. The XLG 2 pool samples can therefore be regarded as freshwater lake samples. All five sampling sites could be roughly divided into two categories: freshwater lake and saline lakes, with the former consisting of KLK and XLG 2, and the latter GH, TS, and XLG 1. The study area belongs to the typical alpine continental climate, characterized by a large temperature difference between day and night, low precipitation, and high drought throughout the year. The main vegetation types are alpine meadows and alpine steppes. The overall elevation of the area is about 3000 m.

According to data from the National Meteorological Station of Delhi city, Han [32] determined that the mean annual precipitation over the period of 1961–2017 was 185.1 mm, and the mean annual evaporation ranged between 2242.8–2439.4 mm. Precipitation is uneven during the year, showing a unimodal distribution. Maximum precipitation occurs in June–July, and the minimum precipitation occurs in December. The total area of GH is about 37 km² with a mean water depth of 8 m and a maximum water depth of 15 m. The lake basin has no perennial surface rivers, relying on atmospheric precipitation and subsurface runoff to supply water. TS has a total area of about 180 km² and a maximum water depth of 25.7 m, with the water mainly originating from the drainage of the TS waters, atmospheric precipitation, and groundwater [21,24].

The Bayin river originates from the northern foot of the ZongWu Long Mountain which has a total length of 188 km and a drainage area of 7281 km². The main water supply source in the upper reaches of the Bayin river is natural precipitation, and in lower reaches is groundwater. After the Bayin river flows out of the ZongWu Long Mountain, a great deal of river water infiltrates into the ground water because the sediments on the riverbed in the Gobi Desert are mostly gravel. Then, the river passes through Delhi city. The infiltration process of river water was prevented by the hardening of the riverbed in Delhi city. After the river flows out of the city, the infiltration process of the river recommences. When the groundwater flows near a tree, a large amount of groundwater overflows into a pool of springs and becomes the main supply water source of the Bayin river, because of the lithological thinning and the blocking effect of the Denan hills. Then, the river water and the groundwater flow into the Keluke lake. Eventually, the Bayin river leaves Keluke lake and flows into Tuosu lake [33,34].

2.2. Sampling Design and Methods

The samples were collected in three batches. The analysis of the first batch of samples showed no significant differences in the horizontal or vertical distribution of aqueous ²³⁸U within the same lake, indicating a homogenous distribution of ²³⁸U in the water column, such kind of phenomenon has also been showed in the published Qinghai Lake data [25]. Based on this feature, we suggest that one single sample was deemed representative of the whole water column. Therefore, we collected only limited samples in the second and third of batch samplings. The sampling information of each sample was presented in

Table 1. The sampling information of each sample.

Batch	Location	Sample No.	Depth (m)	Latitude (°N)	Longitude (°E)	Date
1	Tuosu lake	TS-1-1	0.5	37°08′40″	96°57′24″	9 December 2017
		TS-1-2	10
		TS-1-3	21
		TS-2	4	37°09′11″	96°57′49″
		TS-3-1	0.02	37°09′05″	96°58′18″
		TS-3-2	13
		TS-4-1	0.02	37°09′11″	96°58′58″
		TS-4-2	14
		TS-5	2	37°09′18″	96°59′31″
	Gahai lake	GH-1-1	0.02	37°08′28″	97°31′34″	10 December 2017
		GH-1-2	8
		GH-2-1	0.02	37°08′01″	97°32′34″
		GH-2-2	10
		GH-3-1	0.02	37°08′27″	97°32′29″
		GH-3-2	9.98
		GH-4-1	0.02	37°08′48″	97°32′26″
		GH-4-2	8.98
		GH-5-1	0.02	37°09′19″	97°32′21″
		GH-5-2	7.48
2	Gahai lake	GH	0.2	37°07′51″	97°34′58″	15 June 2018
	Keluke lake	KLK 1-T	0.2	37°17′24″	96°53′34″	16 June 2018
		KLK 1-M	2.5
		KLK 1-B	4.5
		KLK 2-T	0.2	37°17′55″	96°53′42″
		KLK 2-M	1.5
		KLK 2-B	3.5
		KLK 3-T	0.2	37°18′27″	96°53′55″
		KLK 3-M	1
	Tuosu lake	TS-1-T	0.2	37°10′21″	96°58′34″	16 June 2018
		TS-1-M	8
		TS-2-T	0.2	37°10′27″	96°58′59″
		TS-2-M	4.5
		TS-3-T	0.2	37°10′35″	96°58′57″
		TS-3-M	3
	Xiligou lake	XLG 1	0.2	36°49′10″	98°26′52″	17 June 2018
		XLG 2	0.2	36°49′03″	98°26′41″	17 June 2018
3	Keluke lake	KLK-W	0.2	37°17′01″	96°51′35″	22 July 2019
		KLK-N	0.2	37°18′56″	96°54′07″
	Bayin river	Upper reaches	0.2	37°26′33″	97°44′19″	22 July 2019
		Middle reaches	0.2	37°22′37″	97°26′44″
		Delhi city	0.2	37°22′30″	97°21′43″
		Lower reaches	0.2	37°14′09″	97°1′22″
		KLK-TS-River	0.2	37°13′11″	96°49′44″

.

The sampling area is shown in Figure 1. We selected these sampling periods because water inputs northeast of the Qaidam Basin exists in greater variation, and during other seasons are almost covered with ice coverage. Thus, these sampled lake waters could show typical saline lake water uranium distribution characteristics.

The surface water dozens of meters off the shore was collected into HDPE (High Density Polyethylene) plastic bottles (Nalgene 2002-0032) from the depth of 10 cm below surface water by hand with Derma Free^® Vinyl Gloves. We collected other samples from various depths using PMMA (Polymethyl Methacrylate) water collectors and placed into HDPE plastic bottles (Nalgene, 2002-0032). After collection, about 1000 mL of each water sample was filtered through a mixed cellulose filter membrane (0.8-μm pore size, Millipore AAWP04700) with a manual vacuum filter system (Nalgene, 300-4100; Nalgene, 6133-0010), and the filtrate was stored for later analysis.

2.3. Analytical Methods

Samples analysis for water chemistry parameters (anions, cations, salinity, TDS, and pH) were carried out at the Key Laboratory of Surface System and Environmental Carrying Capacity of Shaanxi Province, Northwest University. Anion and cation concentrations of the samples were determined using two Dionex AQUION ICs [35,36]. Conductivity (ms/cm), TDS (g/L), salinity (‰), and pH were determined using a Mettler-Toledo Five Easy Plus pH meter. Uranium isotopes in the samples were measured at the Xi’an Jiaotong University Isotope Laboratory. Methods of investigation of uranium isotopes mainly include chemical separation and instrument analysis. The chemical separation process mainly includes the digestion process, centrifugal coprecipitation, ion exchange resin separation, uranium isotope collection, and purification. The steps involved are as follows. To analyze the U isotopic compositions of the water, the acidified aliquots of each sample were spiked with ²³³U–²³⁶U–²²⁹Th, preconcentrated and digested with HNO₃ and HClO₄, and then co-precipitated with Fe oxyhydroxide. The Fe precipitate was moved to a centrifuge tube for centrifugation and rinsed with deionized H₂O (>18 MΩ) to eliminate the large seawater ions. The precipitate was then dissolved in 14N HNO₃ (1N = 1M) and moved to a Teflon beaker, before being dried and dissolved in 7N HNO₃ for anion-exchange chromatography with AG1-X8, 100–200 mesh size resin and a polyethylene frit. Separation was perfromed on Teflon columns with a 0.5 mL column volume (CV). The first separation was performed on Teflon columns with 4CV 7N HNO₃, 3 CV of 8N HCl (to get rid of Fe and Th fraction), and 3 CV of deionized H₂O (to gather the U fraction). The U fractions were then dried with 2 drops of HClO₄ and brought to volume with 7N HNO₃. The final U fractions were then dried with 2 drops of HClO₄ and dissolved in weak nitric acid for analysis on the mass spectrometer. The concentrations of ²³⁴U, ²³⁵U, and ²³⁸U were calculated by isotope dilution using the nuclide ratios determined on a Thermo-Finnigan Neptune mass spectrometer. All measurements were performed using a peak-jumping routine in ion counting mode on the discreet dynode multiplier behind the retarding potential quadrupole. Each sample measurement was bracketed by the measurement of an aliquot of the run solution, which was utilized to adjust for the instrument background count rates on the measured masses. For U, the measurement uncertainties involved propagated errors from ICP-MS isotope ratio measurements, spike concentrations, and blank corrections. The procedural blanks for chemical and mass spectrometric analyses at the Laboratory of Isotope Geochemistry in Xi’an Jiao Tong University are approximately 830 ag (2.2 × 10⁶ atoms) for ²³⁴U, 29 fg (7.7 × 10⁷ atoms) for ²³⁵U, 2 pg (5.2 × 10⁹ atoms) for ²³⁸U. The methods used here have been fully described by Cheng et al. [1,37] and Shen et al. [38,39].

3. Results

The water chemistry results of each sampled lake are presented in

Table 2. Water chemistry parameters and isotope measurement results for all lake samples.

Sample	pH	Conductivity mS/cm	Cl− mg/L	${\mathbf{SO}}_4^{2-}$ mg/L	${\mathbf{HCO}}_3^{-}$b mg/L	${\mathbf{CO}}_{2-3}$b mg/L	NO mg/L	${\mathbf{NO}}_3^{-}$ mg/L	Na+ mg/L	K+ mg/L	Mg2+ mg/L	Ca2+ mg/L	TDS (g/L)	Salinity (‰)	238U (μg/L)	[234U/238U]AR
GH	7.84	78.62	31,865.59	9436.10	19.38	0.06	296.70	18.07	18,830.66	2102.96	2852.06	199.02	39.31	72.36a	16.2	1.798
KLK 1-T	7.73	1.09	205.23	164.40	15.04	0.04	11.17	0.50	117.45	22.75	44.86	30.69	0.54	0.54	5.5	1.505
KLK 1-M	7.73	1.10	204.37	163.91	15.04	0.04	11.15	0.86	117.35	23.09	44.85	30.78	0.55	0.55	5.5	1.506
KLK 1-B	7.52	1.14	204.90	164.79	9.28	0.01	10.98	1.29	117.49	22.87	44.45	30.70	0.57	0.57	5.6	1.503
KLK 2-T	7.69	1.11	203.59	162.11	13.72	0.03	10.85	0.75	115.90	22.90	44.34	28.74	0.55	0.55	5.5	1.504
KLK 2-M	7.71	1.12	203.09	162.05	14.37	0.03	10.90	0.66	116.22	23.04	44.51	28.85	0.56	0.55	5.4	1.506
KLK 2-B	7.77	1.07	198.50	159.99	16.49	0.04	10.96	0.45	116.53	23.64	44.71	28.92	0.54	0.53	5.5	1.504
KLK 3-T	7.76	1.22	217.37	161.77	16.12	0.04	11.95	1.08	127.04	22.98	48.29	34.47	0.61	0.60	5.4	1.502
KLK 3-M	7.77	1.21	225.53	166.76	16.49	0.04	12.31	1.35	126.77	23.50	48.18	34.24	0.60	0.60	5.4	1.504
TS-1-T	8.83	33.51	13,212.45	7187.91	189.38	5.89	192.07	13.61	8294.33	848.16	2027.81	30.91	16.54	20.91	34.4	1.462
TS-1-M	8.87	33.20	10,942.59	5936.21	207.65	7.08	164.27	7.29	6809.05	801.47	1675.98	29.30	16.60	20.68	34.6	1.462
TS-2-T	8.87	33.08	10,802.30	5880.24	207.65	7.08	160.38	5.76	6758.64	776.27	1659.81	23.62	16.54	20.60	34.6	1.462
TS-2-M	8.88	33.10	11,206.53	6075.39	212.49	7.42	173.93	11.48	6944.53	855.72	1713.14	27.32	16.55	20.62	35.0	1.461
TS-3-T	8.88	33.29	14,840.03	8097.02	212.49	7.42	208.02	7.03	9666.54	894.58	2353.41	29.93	16.66	20.75	34.9	1.460
TS-3-M	8.88	33.99	11,192.01	6104.67	212.49	7.42	168.63	5.87	7037.38	801.03	1723.72	23.51	16.46	20.50	34.9	1.462
XLG 1	8.51	14.95	4030.92	1741.98	90.64	1.35	64.35	3.73	2129.13	361.96	385.49	62.99	7.20	8.67	19.8	1.565
XLG 2	7.53	1.25	242.26	163.88	9.49	0.01	12.94	3.37	196.65	23.60	24.53	32.25	0.63	0.63	12.6	1.673

, 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:

yNaCl = 1.3888x − 0.02478xt − 6171.9

(1)

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 ${\mathrm{HCO}}_3^{-}$ and ${\mathrm{CO}}_3^{2-}$ have been calculated by these two formulas [40].

$$\lg \left[{\mathrm{HCO}}_3^{-}\right]=-11.338+\mathrm{pH}$$

(2)

$$\lg \left[{\mathrm{CO}}_3^{2-}\right]=-21.668+2\mathrm{pH}$$

(3)

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 Mg²⁺ with the second highest concentration, while Ca²⁺ had the lowest cation concentrations measured. Of note, the XLG 2 waters had a slightly higher concentration of Ca²⁺ than Mg²⁺. 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 ${\mathrm{SO}}_4^{2-}$ with the second highest concentration, while ${\mathrm{HCO}}_3^{-}$, ${\mathrm{CO}}_3^{2-}$, ${\mathrm{NO}}_2^{-}$, and ${\mathrm{NO}}_3^{-}$ 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. Horizontal distribution characteristics of the ²³⁸U concentration and the [²³⁴U/²³⁸U]_AR in the water of KLK and TS.

Sample		238U (μg/L)	Average Value	[234U/238U]AR	Average Value
KLK-T	1	5.5	5.4	1.505	1.504
	2	5.5		1.504
	3	5.4		1.502
KLK-M	1	5.5	5.5	1.506	1.505
	2	5.4		1.506
	3	5.4		1.504
KLK-B	1	5.6	5.5	1.503	1.503
	2	5.5		1.504
TS-T	1	34.4	34.6	1.462	1.461
	2	34.6		1.462
	3	34.9		1.460
TS-M	1	34.6	34.8	1.462	1.462
	2	35.0		1.461
	3	34.9		1.462

.

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. Vertical distribution characteristics of the ²³⁸U concentration and the [²³⁴U/²³⁸U]_AR in the water of KLK and TS.

Sample		Depth (m)	238U (μg/L)	Average Value	[234U/238U] AR	Average Value
KLK-1	T	0.2	5.5	5.5	1.505	1.505
	M	2.5	5.5		1.506
	B	4.5	5.6		1.503
KLK-2	T	0.2	5.5	5.5	1.504	1.505
	M	1.5	5.4		1.506
	B	3.5	5.5		1.504
KLK-3	T	0.2	5.4	5.4	1.502	1.503
	M	1	5.4		1.504
TS-1	T	0.2	34.4	34.5	1.462	1.462
	M	8	34.6		1.462
TS-2	T	0.2	34.6	34.8	1.462	1.462
	M	4.5	35.0		1.461
TS-3	T	0.2	34.9	34.9	1.460	1.461
	M	3	34.9		1.462

.

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. ²³⁸U concentration and [²³⁴U/²³⁸U]_AR in different sections of BY river and KLK lake waters.

Sample		238U (μg/L)	[234U/238U]AR
BY-River	upper reaches	6.0	1.392
	middle reaches	5.9	1.402
	Delhi city	6.2	1.408
	lower reaches	2.8	1.464
	KLK-TS-River	5.6	1.482
KLK	KLK-W	5.6	1.476
	KLK-N	5.3	1.451

.

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. Seasonal distribution characteristics of lake waters uranium isotopes using GH and TS as representative lakes.

Season	Sample	Depth (m)	238U (μg/L)	Average Value	[234U/238U]AR	Average Value
December (Winter)	GH-1	0.02	18.5	18.4	1.791	1.792
		8.0	18.5		1.792
	GH-2	0.02	18.3		1.792
		10.0	18.3		1.793
	GH-3	0.02	18.2		1.793
		9.98	18.4		1.791
	GH-4	0.02	18.3		1.796
		8.98	18.4		1.794
	GH-5	0.02	18.3		1.793
		7.48	18.3		1.794
June (Summer)	GH	0.2	16.2	16.2	1.798	1.798
December (Winter)	TS-1	0.5	35.4	35.5	1.464	1.463
		10.0	35.8		1.462
		21.0	35.6		1.462
	TS-2	4.0	35.2		1.462
	TS-3	0.02	35.2		1.463
		13.0	35.3		1.46
	TS-4	0.02	35.5		1.464
		14.0	35.7		1.460
	TS-5	2.0	35.8		1.461
June (Summer)	TS-1	0.2	34.4	34.7	1.462	1.462
		8.0	34.6		1.462
	TS-2	0.2	34.6		1.462
		4.5	35.0		1.461
	TS-3	0.2	34.9		1.460
		3.0	34.9		1.462

.

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.

4. Discussion

4.1. Factors Influencing the Concentration Distribution Characteristics of ²³⁸U in These Lake Waters in the Northeast of Qaidam Basin

Based on the existing literature, we hypothesized that TDS, salinity, and pH may account for the differences in the spatial distribution of aqueous ²³⁸U concentration. The factors that are likely to affect the distribution of ²³⁸U concentration in these lake waters in the northeast of Qaidam Basin are addressed in more detail as follows. For the convenience of discussion, we will use the parameters measured in KLK and TS as representative of the freshwater and saline lakes of the region, respectively.

4.1.1. Response Relationship of ²³⁸U Concentration to TDS and Salinity in These Lake Waters in the Northeast of Qaidam Basin

Water Chemistry parameters of these lake waters show that the major cation is mainly Na⁺ and the major anion is mainly Cl⁻. This means that TDS and salinity may be similar in the saline lake from the northeast of Qaidam Basin. As shown in Figure 3, the relationship between TDS and salinity of these lake waters clarifies this perspective.

Therefore, we selected TDS as the research object to investigate the response relationship of ²³⁸U concentration in these lake waters to TDS. The relationship of ²³⁸U concentration to TDS in these lake waters in the northeast of Qaidam Basin is shown in Figure 4.

Where TDS increases as TDS _(KLK): 0.56 g/L < TDS _{(XLG 2)}: 0.63 g/L < TDS _{(XLG 1)}: 7.2 g/L < TDS _(TS): 16.56 g/L. For the ²³⁸U concentration, we found TDS increased as ²³⁸U _(KLK): 5.5 μg/L < ²³⁸U _{(XLG 2)}: 12.6 μg/L < ²³⁸U _{(XLG 1)}: 19.8 μg/L < ²³⁸U _(TS): 34.7 μg/L. When it comes to the relationship of ²³⁸U concentration to TDS in these lake waters in the northeast of Qaidam Basin, in the absence of interference from exogenous factors, we found a higher lake water TDS in a certain range would lead to a higher concentration of ²³⁸U. This indicates that the enrichment process of uranium involves a series of evaporation and concentration steps. We could speculate that the migration and the ability of uranium to transform in water increases with an increase of the TDS. Shi [41] suggested the main reason for this phenomenon is that ²³⁸U is difficult to be adsorbed in an environment with high TDS. The mobility of U is largely controlled by their ability to form complexes both with inorganic (F⁻, Cl⁻, ${\mathrm{PO}}_4^{3-}$, ${\mathrm{CO}}_3^{2-}$) and organic ligands. This has been described in detail for U [42,43]. Studies in the ocean [44] and the Jiu long River Estuary [45] also showed that the aqueous ²³⁸U concentration increased linearly with an increase in TDS.

The results of the KLK and TS samples are compared in Figure 5.

The TDS varied over a small range within same lake. TDS _(KLK) from 0.54 g/L to 0.61 g/L, TDS _(TS) from 16.46 g/L to 16.66 g/L. The results showing the ²³⁸U concentration was basically at the same level within same lake. The change range of ²³⁸U concentration is 3.64% in KLK and 1.73% for TS.

In addition, the TDS of GH samples far exceeded the TS samples (TDS _(TS): 16.56 g/L < TDS _(GH): 39.31 g/L; However, mean aqueous ²³⁸U concentrations were less for the GH samples than for the TS samples (²³⁸U _(GH): 16.2 μg/L < ²³⁸U _(TS): 34.7 μg/L). These results indicate that TDS are not the only factors affecting ²³⁸U concentration in the waters of terrestrial lakes. The other possibility is there is a certain threshold. When above a TDS range, the ²³⁸U concentration will decrease with the increase of TDS [22]. This should be the reason for GH with high TDS but low ²³⁸U concentration. In summary, the positive relationship of aqueous ²³⁸U concentrations to the factors of TDS may exist only when TDS are lower than certain thresholds, and if the impacts of other factors are deemed negligible. In other words, we speculate that, within a certain range of TDS, the aqueous ²³⁸U concentration increases with the increase of TDS, but this will decrease rather than increase once beyond the range. More research is required to determine the veracity of this speculation, and to understand the underlying mechanism of thresholds for TDS that affect aqueous ²³⁸U concentrations.

4.1.2. Response Relationship of ²³⁸U Concentration to pH in These Lake Waters in the Northeast of Qaidam Basin

Most of these lakes in the northeast of Qaidam Basin are saline lakes with a pH generally above 7.0 (i.e., basic). As shown in Figure 6, the ²³⁸U concentration in these lake waters in the northeast of Qaidam Basin increased with pH.

Since KLK is an open lake, it was excluded from the following discussion of close lakes. For the close saline lakes, the pH results of these sampled lake waters show that pH _{(XLG 2)}: 7.53 < pH _(GH): 7.84 < pH _{(XLG 1)}: 8.51< pH _(TS): 8.87. The ²³⁸U concentration of different waters samples were ²³⁸U _{(XLG 2)}: 12.6 μg/L < ²³⁸U _(GH): 16.2 μg/L< ²³⁸U _{(XLG 1)}: 19.8 μg/L < ²³⁸U _(TS): 34.7 μg/L. We can see the aqueous concentration of ²³⁸U of these lake waters increased with pH, which is attributed to the fact that weakly basic conditions would enhance the leaching of uranium in rocks and thereby increase the migration of uranium into water [41]). The pH value of lake waters depends mainly on the contents of ${\mathrm{HCO}}_3^{-}$ and ${\mathrm{CO}}_3^{2-}$ [40]. The results show that the pH value of these lakes increases with an increase in the contents of ${\mathrm{HCO}}_3^{-}$ and ${\mathrm{CO}}_3^{2-}$. Under basic conditions, uranium is mainly present in the form of UO₂(CO₃${)}_3^{4-}$. As the decrease of pH, UO₂(CO₃${)}_3^{4-}$ decompose into UO₂(CO₃${)}_2^{2-}$ slowly in water. Two highly mobile forms will eventually become one of the stable forms (UO₂CO₃, UO₂(OH${)}_3^{-}$, and UO₂(OH)⁺) when the pH is low enough. A study by Mochizuki [46] on Lake Biwa in Japan showed that the change of lake waters pH affected the leaching process of uranium at the sediment/water interface, which would directly affect the aqueous ²³⁸U concentration.

Under natural conditions, aqueous ²³⁸U concentrations are affected by TDS, and pH, which jointly make different lakes in the same region have different aqueous ²³⁸U concentrations. In order to identify the major controlling factors of aqueous ²³⁸U concentration of saline lake, we performed a correlation analysis of aqueous ²³⁸U concentration with TDS and pH. The results showed that the ²³⁸U concentration was affected by pH to a far greater extent than by TDS. The ²³⁸U concentration in these sampled lakes vary with the range from 12.6 μg/L to 34.7 μg/L. The pH only changes from 7.53 to 7.88, but TDS changes from 0.63 g/L to 39.31 g/L. Namely, the slightly pH variation can result in the bigger variation of ²³⁸U concentration in the sampled saline lake waters than that of TDS.

4.1.3. Response Relationship of ²³⁸U Concentration to Organic Matter in These Lake Waters in the Northeast of Qaidam Basin

Studies have also confirmed the ²³⁸U concentration could be related with biogeochemical process (i.e., the presence of organic matter (OM) and its interaction with iron and manganese oxides/hydroxides) [28,29,30]. The sorption of uranium complexes on iron hydroxides is significantly reduced in the presence of organic matter and/or competing cations, such as Ca²⁺ and Mg²⁺ [31]. Other studies have shown that the concentration of U(VI) in the aqueous phase drastically decreases during bacterial processes because of the formation of insoluble U(IV) compounds [47,48]. In addition, humic substances (HS), including fulvic acid and humic acid, have a strong complexing ability for ²³⁸U of all types of organic matter. If the bottom sediment of a lake is covered with a large amount of organic matter, ²³⁸U could precipitate after forming organic complexes with HS, thereby reducing the concentration of ²³⁸U in the lake waters [49,50]. Some researches mentioned that Qinghai is one of the few provinces in China where natural Artemia is distributed and GH is the largest saline lake producing Artemia in Qaidam Basin of Qinghai Province [51,52,53]. There could exist a layer of organic matter attached to the bottom of GH. If we speculate that there exists an evident biogeochemical process in GH, then the decomposition of organic matter and release of CO₂ is possible, which will result in the carbon isotope variation of DIC (Dissolved Inorganic Carbon) and/or biological shell in lake. The carbon and oxygen isotope research of DIC and shell in GH, showed that there is a limited effect on lake water chemistry from biogeochemical process with shallow lake water environment [54,55]. Because the limited data about organic matter effect and the Fe/Mn element result of the sampled lakes, here, we will not present much more discussion.

Based on the above discussion about the relationship between ²³⁸U concentration, TDS, pH, and organic matter, we infer that the dominant factor leading to the low ²³⁸U concentration in GH waters was also pH, although other factors such as TDS and organic matter could have non-negligible effects. There exists a negative correlation between pH value and TDS when TDS is between 50 and 310 g/L for salt lakes [22]. Under natural conditions, when TDS reaches a certain level, pH value will gradually decrease with increasing TDS, and the concentration of aqueous ²³⁸U will be lowered too. Therefore, we conclude that pH is the dominant factor affecting the changes in aqueous ²³⁸U concentration in the sampled saline lakes. The pH values of these saline lakes are not only controlled by the input water, but associated with water–rock interaction during the lake evolution stage, metamorphous degree, and hydrochemistry type [22,56].

4.1.4. Response Relationship of ²³⁸U Concentration to Water Sources Supply in Open Lake Waters

Taking “KLK-Bayin river” as an example to discuss the response relationship of ²³⁸U concentration to water supply sources in an open lake, one of the most important sources of uranium isotopes in water bodies, we note that a large amount of debris is produced by physical erosion and chemical weathering of mountain masses on both sides of rivers.

The upper reaches of the Bayin river are characterized by rolling mountains and abundant rainfall. The leaching effect of the river makes the uranium isotope content of water in the upper reaches generally higher than others. The physical erosion and chemical weathering of mountain masses on both sides of rivers from upper reaches to lower reaches gradually weakened due to the smooth terrain. Because of the large amount of gravel in the riverbed of the Gobi Desert, a large amount of river water supplies groundwater. This makes the uranium content of water in the middle reaches slightly lower than that in the upper reaches

The watercourse hardening of the Bayin river in Delhi city prevents the infiltration process of the river. There are many materials used for watercourse hardening, such as concrete, marble, and so on. This may be one of the main reasons why the uranium isotope content in of Bayin river in Delhi city is higher than others. A large amount of spilt groundwater has become the main source of water for the Bayin river, due to the change of lithology of the watercourse and the obstruction of Denan hills. The dilution of groundwater may result in a significant reduction in uranium isotope content of lower reaches for Bayin river [33,34].

Research [21,24] shows that the evaporation of KLK is greater than precipitation every year under such a high evaporation environment. This means that there is a large amount of replenishing water into KLK every year under such high evaporation environment. The results of samples show that the ²³⁸U concentration of Bayin river near the estuary (2.8 μg/L) is lower than that for KLK (5.5 μg/L). However, the result of the watercourse near the exit of KLK is 5.6 μg/L, which indicates certain effect on aqueous ²³⁸U concentration for KLK.

4.2. Distribution Characteristics and Influencing Factors of the [²³⁴U/²³⁸U]_AR

There is a certain degree of difference between waters and sediment in the radioactivity of ²³⁴U and ²³⁸U under natural conditions. At present, the activity of the number of atoms of a nuclide is the number of decay events per unit of time, referred to as the [²³⁴U/²³⁸U]_AR In theory, the rate at which ²³⁸U decays into ²³⁴U is the same as the decay rate of ²³⁴U. When the so-called radioactive equilibrium is reached, the [²³⁴U/²³⁸U]_AR is equal to 1 [57]. However, the actual [²³⁴U/²³⁸U]_AR in natural waters is usually greater than 1. Scott [58] made a statistical compilation of the data published before 1982 about ²³⁸U concentrations and the [²³⁴U/²³⁸U]_AR in the water of main rivers on five continents and found that the radioactivity ratio of ²³⁴U to ²³⁸U was basically 1.2–1.3, which was indicative of radioactive disequilibrium between uranium isotopes attributed to the properties of the rock formations and soils in the rivers. Chabaux et al. [59] calculated the mean [²³⁴U/²³⁸U]_AR of the world’s major rivers to be 1.17, confirming the above conclusion.

Under natural conditions, the enrichment of ²³⁴ U in water is related to the α-decay of ²³⁸U nuclides and there are two main ways. When ²³⁸U in a mineral undergoes α-decay at the interface between a uranium-bearing mineral and waters, the daughter ²³⁴Th will be ejected out of the crystal lattice due to the recoil effect. ²³⁴Th has a short half-life (24 day) and decays into ²³⁴U, which readily forms complex ions and thereby enters the waters, resulting in an increase in aqueous ²³⁴U concentration. This process is called the “α-recoil process”. When the ²³⁸U radioactive decay occurs, the daughter ²³⁴U will be placed into the crystal lattice destroyed by the ²³⁸U radioactive decay.

The increase in ²³⁴U concentration due to the preferentially dissolution of destroyed crystal lattice. Given the above processes, the [²³⁴U/²³⁸U]_AR in natural waters is ultimately greater than 1 [14,18,60,61,62,63,64].

The measured [²³⁴U/²³⁸U]_AR values of these sampled lakes are shown in Figure 7.

The results are as follows: [²³⁴U/²³⁸U]_{AR (GH)}: 1.798, [²³⁴U/²³⁸U]_{AR (KLK)}: 1.504, [²³⁴U/²³⁸U]_{AR (TS)}: 1.462, [²³⁴U/²³⁸U]_{AR (XLG 1)}: 1.565, and [²³⁴U/²³⁸U]_{AR (XLG 2)}: 1.673. The [²³⁴U/²³⁸U]_AR of each sampled lake clearly deviated from the equilibrium value (i.e., value of [²³⁴U/²³⁸U]_AR ≠ 1), which is due to frequent water–rock interaction (i.e., sediments, atmosphere dust and organic material etc.) that promotes the production and release of ²³⁴U into the waters due to the α-recoil process. This increases the aqueous ²³⁴U concentration and thereby causes the [²³⁴U/²³⁸U]_AR to significantly deviate from the equilibrium.

Based on the current data, we only could show that there are big differences of [²³⁴U/²³⁸U]_AR in the sampled different lakes located in the same regional background, but show consistent [²³⁴U/²³⁸U]_AR in vary sites and depths in any one lake. Further, the [²³⁴U/²³⁸U]_AR in all sampled saline lakes is above 1.5 which is higher than the reported average value (1.17) of river from the world [58], lower than the reported value (~2.5) of lakes in Namshi-Nur and Tsagan-Tyrm [64] and that of rivers in same region [25]. These results mean that the [²³⁴U/²³⁸U]_AR of these saline lakes may be related to the input water and the water–rock interaction during the evolution stage, metamorphous degree, and hydrochemistry type. The input water of saline lakes in the northeast of Qaidam Basin is mainly based on natural precipitation, river water, and groundwater [56,65]. The factors affecting the [²³⁴U/²³⁸U]_AR in water–rock interaction during the evolution stage, metamorphous degree, and hydrochemistry type could be related with the redox conditions, pH of water, and biogeochemical process (i.e., the presence of organic matter (OM) and its interaction with iron and manganese oxides/hydroxides and/or competing cations, such as Ca²⁺ and Mg²⁺) during evolution stage, metamorphous degree, and hydrochemistry of the saline lakes. This will be resolved. By analyzing the major and trace element geochemistry distribution and the [²³⁴U/²³⁸U]_AR of the suspended particles filtered from the water of these sampled lakes in future.

4.3. Seasonal Effects on ²³⁸U Concentration and the [²³⁴U/²³⁸U]_AR in Lakes and Their Potential Environmental Impact

The concentration of ²³⁸U in natural lake waters and the [²³⁴U/²³⁸U]_AR are the best tracers for determining changes in the water environment. The main source of ²³⁸U in lake waters are water–rock interaction and rock weathering, in addition to other sources such as exogenous inputs from rivers, atmospheric precipitation, and groundwater recharge. The radioactive disequilibrium of ²³⁴U/²³⁸U is the best evidence of the water-rock interaction of ²³⁸U. The ²³⁸U concentration and the [²³⁴U/²³⁸U]_AR of various sites in a lake remain almost consistent. However, this will be affected by seasonal variation due to an input of precipitation mainly occurring in the rainy season. The annual precipitation in GH and TS is subject to significant variation, with precipitation mainly concentrated in May–September every year [21,32], which is likely to induce the seasonal variation of aqueous ²³⁸U concentration. However, compared with the total water volume in GH and TS, the effect of seasonal precipitation variation on aqueous ²³⁸U concentration was insignificant. Based on the seasonal effect on the result of ²³⁸U concentration in GH and TS (Table 5), the seasonal change range of ²³⁸U concentration in GH is 11.96%, and the result is 2.25% for TS. Therefore, we conclude that the subtle seasonal difference of ²³⁸U concentration in the lake waters are due to the dilution effect of precipitation and/or groundwater.

In summary, the concentration of ²³⁸U in natural lake waters and the [²³⁴U/²³⁸U]_AR remain almost consistent for a certain period of time. Studies of U-series disequilibria in lakes mainly include the determination of sediment ages and/or of sedimentation rates [66]. Stable U- and Th-series nuclides in lakes is the basis of the determination of sediment ages. The ability of obtaining precise sediment ages is critical to the accuracy of the reconstruction of paleoclimate and paleoenvironment.

U- and Th-series nuclides in lakes are closely related. U- and Th-series nuclides of lake waters may give us some information about geochemical behavior and mixing characteristics of lakes [6]. There are many environmental changes recorded by lake sediments. We can chronologically decipher such records by measuring the U–Th nuclides of these sediments.

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.