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Article

Hydrogeochemical Processes and Sustainability Challenges of Arsenic- and Fluoride-Contaminated Groundwater in Arid Regions: Evidence from the Tarim Basin, China

by
Yunfei Chen
1,
Jun Hou
1,
Jinlong Zhou
2,*,
Jiawen Yu
3,
Jie Zhang
4 and
Jiangtao Zhao
5
1
School of Intelligent Built Environment and Architectural Engineering, Neijiang Normal University, Neijiang 641000, China
2
College of Hydraulic and Civil Engineering, Xinjiang Agricultural University, Urumqi 830052, China
3
College of Management and Economics, Tianjin University, Tianjin 300072, China
4
School of Geographic Sciences and Geomatics, Neijiang Normal University, Neijiang 641000, China
5
College of Water Conservancy and Hydropower Engineering, Sichuan Agricultural University, Ya’an 625014, China
*
Author to whom correspondence should be addressed.
Sustainability 2025, 17(17), 7971; https://doi.org/10.3390/su17177971
Submission received: 7 July 2025 / Revised: 12 August 2025 / Accepted: 2 September 2025 / Published: 4 September 2025
(This article belongs to the Special Issue (Re)Designing Processes for Improving Supply Chain Sustainability)
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Abstract

The anomalous enrichment of arsenic (As) and fluoride (F) in groundwater in the oasis area at the southern margin of the Tarim Basin has become a critical environmental and sustainability challenge. It poses not only potential health risks but also profound socio-economic impacts on local communities, threatening the long-term security of water resources in arid regions. Therefore, an in-depth investigation of the hydrochemical characteristics of groundwater and the co-enrichment mechanism of As and F is essential for advancing sustainable groundwater management. In this study, 110 phreatic water samples and 50 confined water samples were collected, and mathematical and statistical methods were applied to analyze the hydrochemical characteristics, sources, and co-enrichment mechanisms of As and F. The results show that (1) the groundwater chemistry types are mainly Cl·SO4-Na, SO4·Cl-Na·Mg, Cl·SO4-Na·Mg, and Cl-Na, and the chemistry is primarily controlled by evaporation and concentration processes, with additional influence from human activities and cation exchange; (2) As and F mainly originate from soils and minerals, and are released through dissolution; (3) As and F enrichment is positively correlated with pH, Na+, and HCO3−, but negatively correlated with Ca2+, Mg2+, and SO42−, indicating that a weakly alkaline hydrochemical environment with high HCO3− and Na+, and low Ca2+ promotes their enrichment; (4) strong evaporative concentration in retention zones, combined with artificial groundwater extraction, further intensifies As and F accumulation. This study not only provides an innovative theoretical and methodological framework for exploring trace element enrichment mechanisms in groundwater under arid conditions but also delivers critical scientific evidence for developing sustainable water resource management strategies, mitigating water-related health risks, and supporting regional socio-economic resilience under global climate change.

1. Introduction

The problem of water scarcity in arid and semi-arid regions is becoming increasingly serious, and the chemical characteristics and quality safety of groundwater, as an important source of drinking water and agricultural irrigation water, are directly related to human health and sustainable social development [1,2]. In recent years, the wide distribution of high arsenic (As) and high fluorine (F) groundwater in Asia and South America has triggered international concern [3,4,5,6]. According to the World Health Organization (WHO), at least 150 million people worldwide are at risk of excessive arsenic or fluoride in drinking water, and long-term exposure can lead to public health events such as endemic arsenicosis and fluorosis [7,8]. In China, high arsenic (As) and fluorine (F) in groundwater (As > 10 μg/L and F > 1.0 mg/L) show significant regional distribution characteristics, mainly concentrated in inland basins and alluvial plains in arid and semi-arid climatic zones [9]. Groundwater in the Songnen Plain in Northeast China [10,11], Datong Basin and Yuncheng Basin in Shanxi [12,13], and Tarim Basin and Hetao Plain in Northwest China [14,15,16] all showed different degrees of exceedance of As and F content. In Xinjiang, high As and high F groundwaters are mainly distributed in the oasis zone of the Tarim Basin and the southern edge of the Junggar Basin [17], with high F groundwaters occupying a larger area than high As groundwaters [9,18].
Geological background and sediment characteristics are the basis for the source of arsenic and fluorine in groundwater, and the weathering process of primary arsenic- and fluorine-bearing minerals provides the initial source of arsenic release, while less adsorbent sedimentary mineral components, e.g., iron oxides, clay minerals, and nitrate minerals, accelerate the enrichment of arsenic and fluorine in groundwater [19,20,21]. The migration and enrichment of arsenic and fluorine in groundwater are also controlled by several drivers such as hydrogeochemical environment, microbial action, climate and hydrological dynamics, tectonic movements and earthquakes, and human activities [22]. Reductive dissolution, alkaline desorption, and anthropogenic activities are the three core mechanisms for arsenic enrichment, while alkaline desorption, evaporation concentration alkaline desorption, evaporation concentration, and groundwater over-exploitation are the three core mechanisms of fluorine enrichment. In recent years, the enrichment process of a single hazardous element in groundwater has been studied in detail; however, the key scientific issues such as the synergistic release, transport, and enrichment of arsenic and fluorine in groundwater systems still need to be systematically studied.
As a typical arid inland area, groundwater is an important source of drinking water in Xinjiang, but high arsenic and high fluoride groundwater has been found in the Aksu River Basin, the Manas River Basin, the Kuitun River Basin, and the southern margin of the Tarim Basin [14,23,24]. Lei et al. used statistical and graphical methods to analyze the spatial distribution and form of arsenic in the groundwater of the Aksu River and discussed the influencing factors of high As groundwater enrichment in typical profiles [14]. Liu et al. analyzed the migration processes of arsenic, fluorine, and iodine in the groundwater of the Manas River basin through correlation analysis and hydrogeochemical modeling [23]. In some rural areas along the southern margin of the Tarim Basin, the primary drinking water source relies on groundwater. Long-term consumption of groundwater with high arsenic and fluoride levels may lead to diseases such as black skin disease, keratosis, and dental fluorosis. Additionally, previous studies have found that soil and crop arsenic levels exceed standards, and arsenic in groundwater may pose a threat to public health through the food chain [25]. Currently, studies on high-arsenic and high-fluorine groundwater in the southern margin of the Tarim Basin have focused on the spatial distribution pattern of arsenic and fluorine in groundwater, water quality evaluation, hydrochemical characterization, and health risk assessment [15,25], while the mechanism of high-arsenic and high-fluorine groundwater formation and enrichment by multifactorial coupling needs to be further researched and clarified. Therefore, this paper selects the oasis zone at the southern edge of the Tarim Basin, where human activities are frequent, as the study area, and analyzes the characteristics of groundwater chemical types using mathematical statistics, and explores the formation process and enrichment mechanism of arsenic and fluorine in groundwater by combining the information of natural geographic conditions, geological conditions, and hydrogeochemical processes. This study is intended to provide a reference for the study of arsenic and fluorine in groundwater systems in inland arid zones, as well as a theoretical basis for the rational development of groundwater resources and the prevention of disease and water reclamation in the local area.

2. Materials and Methods

2.1. Study Area

The geomorphology of the study area from south to north is pre-mountain alluvial gravelly plains, alluvial fine soil plains, and wind-accumulated deserts, and the overall elevation is about 1310−1650 m. The overall topography is high in the south and low in the north, with a flat terrain, and the administrative areas involved include Yutian County, Minfeng County, and Ruoqiang County [26]. The study area has a population of approximately 476,000 people, accounting for 2% of the total population of the Xinjiang Uygur Autonomous Region. The regional gross domestic product (GDP) of the study area is approximately 23.592 billion yuan, accounting for 1.1% of the regional GDP of the Xinjiang Uygur Autonomous Region. The agricultural system is oasis irrigation agriculture, with farmland concentrated in the alluvial fans at the foot of the mountains and along riverbanks. Specialty crops include tomatoes, chili peppers, goji berries, pears, apples, grapes, jujubes, peaches, apricots, and walnuts. The study area has a warm-temperate continental desert arid climate, with an average annual precipitation of 52.4 mm and an average annual evapotranspiration of 2423.1 mm. The main rivers in the region include the Kriya River, the Chelchen River, the Milan River, and the Ruoqiang River, which are mainly recharged by ice and snowmelt in the southern Kunlun Mountains, and there are almost no exceedances of the quality of arsenic and fluorine in the surface water bodies [27].
The exposed strata in the study area mainly include the Quaternary Upper Pleistocene-Holocene floodplain (Q3-4apl), Quaternary Holocene alluvium (Q4al), Quaternary Holocene windlog (Q4eol), and Quaternary Holocene chemosyncline (Q4ch). From the mountain front to the plains, there is a clear zoning pattern, mainly flood, alluvial, and wind accumulation, and the lithologic structure varies from coarse to fine, from single to complex structure, and from monolayer to multilayer structure in zoning [28].
The study area is located in the southern edge of the Tarim Basin, which is a typical inland early basin with the general distribution of aquifers, i.e., a single submersible aquifer in the pre-mountain zone, and the transition from submersible aquifers to confined aquifers in the gently sloping plains and desert plains. The lithology of the submersible aquifer is mainly pebble gravel and sand gravel, locally sandy soil and clay lenses. The thickness of the confined aquifer is uneven, and its lithology is sand gravel, medium and fine sand, silt, powder, and fine sand. The hydrogeological unit of the study area is relatively complete, with the pre-mountain alluvial floodplain as the groundwater recharge area and the desert area as the groundwater runoff and discharge area. Due to the aridity and low precipitation in the study area, atmospheric precipitation has little significance for groundwater recharge in the study area. Groundwater recharge in the study area mainly consists of infiltration recharge from rivers, canals, reservoirs and field irrigation water, and lateral recharge from bedrock fissure water in mountainous areas. Groundwater discharge mainly includes phreatic water overflow to the surface or being consumed by evaporation and plant transpiration, and a small portion of groundwater is discharged into the deep desert in the form of submerged flow. The direction of groundwater runoff is from the south to the north in the mountain front belt, the aquifer particles are coarse, the hydraulic gradient is larger, the groundwater transportation is smooth; further north, the aquifer particles become finer, the hydraulic gradient becomes smaller, the seepage rate is very slow, from southwest to northeast, along the way due to ground evaporation and gradually consumed. For the alluvial fan margin, phreatic water is close to the surface and will be dissipated in the form of springs [24].

2.2. Sample Collection and Testing

In this study, groundwater samples were collected from local agricultural wells in the oasis zone at the southern margin of the Tarim Basin from 2015 to 2017, including 110 groups of submersible samples and 50 groups of confined water samples, totaling 160 groups (Figure 1). Before sampling, the sampling bottles were moistened with the groundwater to be collected for three times, and in order to obtain fresh groundwater, the groundwater samples were collected after the pump was turned on for 10 min, filtered through 0.45 μm acetate filter membrane, sealed, and preserved in cold storage at 4 °C for testing.
The groundwater samples were tested by the Mineral Water Testing Center of the Institute of Hydrogeology and Environmental Geology, Chinese Academy of Geological Sciences. The testing indexes include pH, total dissolved solids (TDS), total hardness (TH), K+, Na+, Ca2+, Mg2+, HCO3−, SO42−, Cl−, CO32−, NO3−, F, and As. Among them, the glass electrode method is used for pH, the dry weight method at 105 °C is used for TDS, the volumetric method of EDTA is used for TH, the turbidimetric method of barium sulfate is used for SO42−, the titration method of hydrochloric acid is used for HCO3− and CO32−, the silver nitrate volumetric method is used for Cl−, the flame atomic absorption spectrophotometry is used for K+ and Na+, the ethylenediaminetetraacetic acid disodium salt titration method is used for Mg2+ and Ca2+ [10], the ultraviolet spectrophotometry is used for NO3−, the ion selective electrode method is used for F, and the hydride atomic fluorescence method is used for As [14]. The limit of detection (LOD) was 0.01 mg/L for all tested ionic indicators except As, which was 0.0005 mg/L.
In this study, the anion–cation balance test was used to check the reliability of the water sample data, and the calculated anion–cation balance error range of 160 groups of groundwater samples was −1.03–4.28%, which indicated that the data were reliable.
This paper uses Excel 2012 to process the test data, Origin 2018 to draw relevant figures, and SPSS 26 to perform linear regression analysis and correlation analysis, and calculates the R2 and p-values for each group of data.

3. Results

3.1. Groundwater Chemistry

Statistical analysis of groundwater chemical indicators in the oasis area at the southern edge of the Tarim Basin (Table 1) shows that the range and standard deviation of each component in the groundwater are relatively large, indicating that groundwater concentrations are spatially non-uniform. The pH value of the aquifer ranges from 7.11 to 8.90, with an average of 8.05, while the pH value of the confined aquifer ranges from 6.30 to 8.34, with an average of 7.68. Overall, the groundwater is slightly alkaline. The TDS values for groundwater and confined water ranged from 285.89 to 41,282.73 mg/L and 536.17 to 358,693.99 mg/L, respectively, with average values of 4027.42 mg/L and 57,782.18 mg/L. In the samples of groundwater (n = 110) and artesian water (n = 50), 37 and 6 groups of water samples, respectively, had TDS values below 1000 mg/L, indicating that the overall natural mineral content of groundwater is high. The TH values for groundwater and confined water ranged from 81.06 to 6549.30 mg/L and 202.20 to 106,956.45 mg/L, respectively, with average values of 911.12 mg/L and 7584.70 mg/L. The groundwater is predominantly hard water and extremely hard water.
The Piper three-line diagram can intuitively display the composition ratio and relative content of major anions and cations in groundwater, providing an important basis for the classification of groundwater chemical types. It plays a key role in studying the hydrogeochemical characteristics and evolutionary patterns of the research area [29]. According to the Piper diagram (Figure 2) analysis, the water sample points mainly fall in the 2/3/4 zone of the Piper diagram, indicating that non-carbonate hardness and alkali metals exceed 50%. Na+ is the dominant cation in groundwater, with concentration ranges of 9.76% to 91.97% and 19.92% to 91.10% for groundwater and confined water, respectively, followed by Mg2+ and Ca2+. In groundwater, the ion concentrations in milligram equivalents per cent are 3.20% to 69.70% and 0.37% to 55.42%, respectively, while in confined water, the ion concentrations in milligram equivalents per cent are 7.03% to 74.19% and 0.33% to 44.34%, respectively. Cl− is the dominant anion in groundwater, with ion concentrations in milligram equivalents ranging from 16.75% to 79.86% in phreatic water and from 16.37% to 82.48% in confined water, followed by SO42− and HCO3−, with ion concentrations in milligram equivalents as a percentage ranging from 12.88% to 71.72% and 1.90% to 52.51% in groundwater, respectively, and from 17.27% to 79.30% and 1.17% to 32.68% in confined water, respectively.
Based on cation classification, the groundwater chemical types in the study area are mainly sodium-type water and sodium–magnesium-type water, with the proportions of sampling points for these two types accounting for 59.4% and 40.6% of the total number of sampling points, respectively. Based on anion classification, the groundwater chemical types in the study area are mainly chloride–sulfate type water, sulfate–chloride type water, and chloride-type water. The proportions of sampling points for these three types account for 42.5%, 26.3%, and 9.4% of the total number of sampling points, respectively. Overall, the groundwater chemical types in the study area are mainly Cl·SO4-Na, SO4·Cl-Na·Mg, Cl·SO4-Na·Mg, and Cl-Na. The sampling points for these four groundwater chemical types account for 25.00%, 22.50%, 17.50%, and 8.75% of the total number of sampling points, respectively.
The coefficient of variation (Cv) values of water chemical elements such as Na+, K+, Cl−, SO42−, and TDS in groundwater are relatively high, indicating significant spatial variability. Among these, high concentrations of Na+ and K+ are present throughout the entire region, which may be related to the dissolution of evaporite and silicate rocks as well as cation exchange processes. High concentrations of Cl− and SO42− are concentrated in the northern part of Ruoqiang County and the eastern part of Qiemo County, which may be related to evaporation processes and wastewater discharge from nearby treatment plants. The elevated TDS values in groundwater may be attributed to a combination of factors, including scarce precipitation, intense groundwater evaporation, and poor groundwater runoff conditions.

3.2. Co-Occurrence of As and F in the Groundwater

Among the 110 phreatic water samples collected, the concentration of arsenic ranged from <0.001 to 0.091 mg/L, with an average value of 0.0174 mg/L (Table 1). Among these, nine samples had As concentrations exceeding 0.01 mg/L, with the number of samples exceeding the drinking water limit (As > 0.01 mg/L) accounting for 8.20% of the total groundwater samples (Figure 3). Among 50 samples of confined water, the concentration of arsenic ranged from <0.001 to 0.068 mg/L, with an average value of 0.021 mg/L. Among these, four samples had arsenic concentrations exceeding 0.01 mg/L, and the number of samples exceeding the drinking water limit accounted for 8.00% of the total number of confined water samples.
In the phreatic water samples, the concentration of F ranged from 0.10 to 28.31 mg/L, with an average value of 2.78 mg/L. Among these, 60 samples had F concentrations exceeding 1.00 mg/L, accounting for 54.54% of the total number of phreatic water samples. In the confined water samples, the concentration of F ranged from 0.07 to 8.27 mg/L, with an average value of 1.45 mg/L. Among these, 25 samples had As concentrations exceeding 0.01 mg/L, accounting for 50.00% of the total number of confined water samples.
According to the above statistical results, the number of groundwater samples in the Tarim Basin oasis area with F concentrations exceeding drinking water limits is greater than that of As, and the multiple of F concentrations exceeding drinking water limits is higher than that of As. The issue of F treatment in groundwater requires high attention and should be the focus of further research.
From a regional perspective, samples with F content exceeding 1 mg/L were distributed across Yutian County, Minfeng County, Qiemo County, and Ruoqiang County (Figure 1). Samples with As content exceeding 0.01 mg/L and both As and F exceeding drinking water limits were concentrated in Minfeng County. There were seven samples with both As and F exceeding drinking water limits. Samples with F content exceeding 1 mg/L in confined aquifers were mainly concentrated in Ruoqiang County and Qiemo County, while samples with As content exceeding 0.01 mg/L were all found in Ruoqiang County. A total of three samples exceeded the drinking water limits for both As and F, all of which were concentrated in the lower reaches of the Cheerchen River in Ruoqiang County.

3.3. Relationship Between As and F in Groundwater and Other Chemical Components

The groundwater in the study area is predominantly weakly alkaline. Except for a few exceptional points, there is a gradual increase in the concentrations of As and F as pH increases (Figure 4a and Figure 5a). This phenomenon may be related to the competitive adsorption of anions under alkaline conditions. At higher pH levels, the adsorption capacity of mineral surfaces for As and F decreases, leading to an increase in their solubility in water. As in groundwater exhibits a negative correlation with alkaline earth metal cations (Ca2+, Mg2+) (Figure 4b,c), while F also shows a negative correlation with Ca2+ and Mg2+ (Figure 4b,c). However, As and F in groundwater exhibit a positive correlation with alkali metal ions (Na+) (Figure 4d and Figure 5d). As and F show a positive correlation with HCO3− (Figure 4e and Figure 5e) but a negative correlation with SO42− (Figure 4f and Figure 5f). These results indicate that a weakly alkaline water chemistry environment with high HCO3− and Na+, and low Ca2+ and Mg2+, is the key controlling factor for the synergistic enrichment of As and F. Alkaline earth metal ions (Ca2+, Mg2+) inhibit the migration of As and F through precipitation and adsorption. Alkali metal ions (Na+) promote their release through cation exchange and complexation. HCO3− enhances solubility, while SO42− may inhibit enrichment through redox reactions or competitive adsorption.

4. Discussion

4.1. Main Controlling Factors of Water Chemistry

The chemical composition of groundwater is influenced by a variety of factors. Gibbs diagrams can be used to determine the influence of rock leaching, atmospheric precipitation, and evaporation and concentration on groundwater evolution [30]. Most of the groundwater sampling points are located within the evaporation zone, indicating that the groundwater and confined water in the study area are primarily controlled by evaporation–concentration processes. A small portion of the sampling points is influenced by the combined effects of rock leaching and evaporation (Figure 6). Some sampling points are located outside the Gibbs diagram model range, indicating that the study area may be affected by human activities and cation exchange in addition to evaporation and concentration [31].
Through Gibbs diagrams, it can be preliminarily determined that natural factors affecting water chemistry are related to evaporation and concentration effects and rock leaching effects. The relationship characteristics between Ca2+/Na+ and HCO3−/Na+ can be further used to determine the types of rocks involved in leaching effects [32]. Most groundwater sampling points are located in the interval between evaporite and silicate rocks (Figure 7). The exposed strata in the oasis belt at the southern margin of the Tarim Basin are mainly Quaternary loose sediments [24], indicating that the formation of groundwater chemical characteristics is mainly related to evaporite and silicate rocks.
The proportional coefficient relationship between major water chemistry ions is the primary means of analyzing hydrogeochemical processes [33]. The ionic ratio between Na+ and Cl− is used to determine the sources of Na+ and Cl− in groundwater. Most of the groundwater in the study area is located near the 1:1 line (Figure 8a), indicating that Na+ and Cl− mainly originate from atmospheric precipitation or the dissolution of rock salt. Some confined water samples are located above the 1:1 line (Figure 8a), indicating that Na+ may originate from cation exchange. The milliequivalent ratio of (Ca2+Mg2+)/(SO42−+HCO3−) was used to determine the types of rock minerals dissolved in the groundwater within the study area. Most of the water samples were located near or below the 1:1 line (Figure 8b), indicating that the groundwater and confined water mainly originated from the dissolution of rock salt, gypsum, hard gypsum, and other minerals.
Based on the aforementioned analysis, cation exchange adsorption may occur in groundwater. The direction of cation exchange adsorption in groundwater can be further determined through the chlor–alkali index relationship [32]. According to the chloride index of groundwater in the study area (Figure 8c), most sampling points are located in areas where both CAI1 and CAI2 are negative, indicating that Na+ in the rock and soil has undergone positive cation exchange with Ca2+ and Mg2+ in the groundwater. Human activities are increasingly becoming an important factor affecting the chemical characteristics of water bodies. Generally speaking, the characteristic factor of industrial activities is mainly SO42−, the characteristic factor of domestic sewage is mainly Cl−, and the characteristic factor of agricultural activities is mainly NO3− [34]. Due to frequent groundwater extraction activities in the oasis area, both phreatic water and confined water are affected by human life, industry, and water use (Figure 8d), with confined water being more sensitive to external factors.

4.2. Hydrogeochemical Processes of Groundwater Containing As and F

High As/Cl and F/Cl ratios were found in both the unconfined aquifers and confined aquifers of the oasis zone at the southern margin of the Tarim Basin (Figure 9), indicating the occurrence of As and F enrichment in the regional groundwater. Groundwater samples with high As/Cl and F/Cl ratios indicate that the enrichment of As and F is unrelated to evaporation [35]. However, some water samples with relatively low As/Cl ratios were also found in unconfined aquifers and confined aquifers, indicating that transpiration and evaporation play a certain role in As enrichment [35]. Water samples with relatively low F/Cl ratios were only found in submerged samples, indicating that transpiration evaporation only occurs in aquifers.
There is no correlation between As, F, and NO3− concentrations and anthropogenic groundwater pollution (Figure 10a,b), which also indicates that human agricultural activities are not the main source of As and F in groundwater in the oasis area of the Tarim Basin. NO3− concentrations in groundwater with high As and F levels are generally low. This may be because groundwater with high As and F levels is mostly found in arid inland areas, where groundwater remains for long periods of time and there is no continuous source of NO3− replenishment [36]. In the study area, both groundwater and confined water showed varying degrees of NO3− exceeding drinking water limits (NO3− > 20 mg/L), which may be related to the excessive use of fertilizers and chemicals in agricultural production [37].
The changes in the concentrations of As and F in the groundwater of the study area have significant chemical characteristics, and pH is an important factor affecting the existence state of As and F in groundwater [38]. Arsenate or arsenous acid is the form in which arsenic exists in groundwater. When the pH changes, AsO43− and AsO33− adsorbed by substances such as iron, aluminum oxides, and goethite combine with different concentrations of H+, altering the adsorption state [39,40]. As the pH value increases, OH− increases, and colloidal substances with positive charges that adsorb arsenic acid and arsenous acid change their own charged state, thereby reducing the total amount of adsorption. In a weakly alkaline environment, F− is highly mobile, and colloidal sediments reduce the adsorption of fluoride. In groundwater, OH− competes with F− for adsorption, replacing F− in fluoride minerals, thereby increasing the F− content in groundwater [39].
As and F in groundwater are positively correlated with Na+ and HCO3−, and negatively correlated with Ca2+, Mg2+, and SO42−. Ca2+ and Mg2+ precipitate and fix F- in groundwater to form CaF2 and MgF2, thereby reducing the activity of F- in water. At the same time, Ca2+ and Mg2+ can form insoluble calcium arsenate Ca3(AsO4)2 and magnesium arsenate Mg3(AsO4)2 with AsO43−, directly fixing dissolved arsenic [41]. When microorganisms reduce arsenic-containing iron oxides, the symbiotic carbonate reduction reaction consumes Ca2+ and Mg2+, and the precipitation further reduces the Ca2+ concentration, promoting the release of As [42]. Na+ adsorbs onto clay minerals through cation exchange, displacing Ca2+ from groundwater, thereby weakening CaF2 precipitation and promoting F- dissolution. The low Ca2+ environment formed by increased Na+ content reduces calcium coverage on the surface of iron oxides, weakening their adsorption capacity for arsenic [41]. When the concentration of SO42− in groundwater is high, it promotes the activity of sulfate-reducing bacteria (SRB), which consume SO42− to produce S2−. S2− then combines with Fe2+ to form pyrite (FeS2), thereby fixing a portion of the arsenic [43,44]. Concurrently, Ca2+ preferentially forms gypsum precipitation (CaSO4·2H2O) with SO42−, and the consumed Ca2+ weakens calcium fluoride (CaF2) precipitation, thereby promoting an increase in F concentration. HCO3− in groundwater can compete with As and F for adsorption, competing with F− and AsO43− for adsorption sites, thereby exacerbating the release of As and F [45].

4.3. Sources and Enrichment Mechanisms of As and F in Groundwater

The Quaternary lacustrine sediments in the oasis belt along the southern margin of the Tarim Basin are primarily composed of silt and sub-clay. The arsenic (As) content in these sediments ranges from 3.1 to 39.6 mg/kg, slightly higher than the As content in Chinese soils (2.5–33.5 mg/kg) [25]. The organic-rich clay layer formed by the deposition of riverine biogenic residues is enriched with arsenic sulfides (such as arsenopyrite), which release arsenic through long-term water–rock interaction. In closed hydrological systems, microbial decomposition of organic matter triggers the reductive dissolution of iron oxides (such as hematite and goethite), a process that simultaneously generates dissolved organic carbon (DOC) and HCO3− [46]. With the destruction of the iron oxide structure, the As elements that were originally adsorbed or coprecipitated on the mineral surface are released from the sediments and enter the groundwater [46]. The southern part of the study area is characterized by bedrock mountainous regions containing metamorphic and igneous rocks with relatively high fluorine content. All samples from the study area are concentrated along the fluorite solubility equilibrium line (Ksp = 10.6) [35] (Figure 11), indicating that the fluoride content in groundwater within the region is not only derived from fluorite dissolution but may also be influenced by the dissolution of fluoride-rich minerals such as mica (F content: 2.1–5.2%), tourmaline, and apatite (F content: 1.23%) [47]. All groundwater samples were located above the fluorite dissolution path line (Figure 11), indicating that mineral components containing Ca2+ (such as calcite) were less active. The concentrations of F and Ca2+ in groundwater were negatively correlated, reflecting the limiting effect of the dissolution of minerals such as calcite on F concentrations.
Hydrogeological conditions are one of the key factors influencing the migration and enrichment of arsenic (As) and fluoride (F) in groundwater [4,38]. In the oasis zone of the Tarim Basin, the terrain gradually becomes less steep from north to south, and the lithology of the aquifer transitions from gravel and cobble layers to gravel layers interbedded with silt, and finally to fine sand interbedded with silt and clay layers. Slow groundwater flow makes evaporation the main way groundwater leaves the area. The depth of groundwater also becomes shallower, which makes evaporation stronger in the area. Evaporation and concentration cause As and F to build up in shallow groundwater. In areas where groundwater evaporates, there is a noticeable buildup of groundwater, which lets microbes do their thing and keep releasing As and F into the groundwater.
Due to limited surface water resources in the region, agricultural irrigation and residential water use rely on groundwater. However, artificial extraction of groundwater has disrupted the stable clay layer structure between the original aquifers, altering the migration pathways of As and F [48]. Excessive pumping can cause mixing between aquifers, which may activate geochemical processes such as mineral dissolution (e.g., fluorite), ion exchange (Na+ and Ca2+), and adsorption and desorption, further promoting the enrichment of As and F.
The accumulation of As and F in groundwater severely impacts its development and utilization. Removing As and F from groundwater is an urgent research priority. Currently, the primary methods for removing As and F from water include adsorption, coagulation precipitation, ion exchange, and biological remediation [49]. This paper suggests using lanthanum-loaded activated carbon as an adsorbent for removing As and F from groundwater. Lanthanum-loaded activated carbon is suitable for co-removal of arsenic and fluoride under weakly acidic conditions. When the pH value is between 4 and 6, the adsorption efficiency of lanthanum-loaded activated carbon for both arsenic and fluoride is optimal [50].

5. Conclusions

The groundwater chemical types in the oasis area at the southern edge of the Tarim Basin are mainly Cl·SO4-Na, SO4·Cl-Na·Mg, Cl·SO4-Na·Mg, and Cl-Na types. The water chemistry is primarily influenced by evaporation and concentration, and may also be affected by human activities and cation exchange. Phreatic and confined water mainly originate from the dissolution of rock salt, gypsum, hard gypsum, and other rock salt and silicate rocks. In the oasis belt along the southern margin of the Tarim Basin, samples with As > 0.01 mg/L and F > 1 mg/L in the unconfined aquifer accounted for 8.20% and 54.54% of the total number of samples, respectively. In the confined aquifer, samples with As > 0.01 mg/L and F > 1 mg/L accounted for 8.00% and 50.00% of the total number of samples, respectively. The number of groundwater samples in the Tarim Basin oasis region exceeding the drinking water limit for F exceeds that for As. How to treat and eliminate F in groundwater is an issue that requires serious attention and should be a focus of further research.
Through dissolution, As and F are released from soil and loose sediments into groundwater. As primarily originates from the dissolution of arsenic sulfides. F not only originates from the dissolution of fluorite but may also be influenced by the dissolution of fluorine-rich minerals such as mica, tourmaline, and apatite. In closed environments, microbial activity drives redox reactions, organic matter degradation, and mineral dissolution and precipitation processes, thereby increasing the solubility of As and F in groundwater.
The migration and enrichment of As and F in groundwater are influenced by both hydrogeological conditions and hydrogeochemical processes. In groundwater retention areas, under the strong evaporation and concentration effect, a weakly alkaline environment rich in Na+ and low in Ca2+ is conducive to the enrichment of As and F. At the same time, the hydrolysis process of HCO3− in groundwater and the adsorption process of As and F attached to mineral surfaces can increase the arsenic and fluoride content in groundwater. Due to cation exchange adsorption, the process of Ca2+ replacing Na+ in groundwater will increase the arsenic and fluoride content in groundwater. Additionally, the mixing of aquifers caused by excessive pumping may lead to further enrichment of arsenic and fluoride in groundwater.
This paper investigates the hydrogeochemical characteristics of As and F in groundwater along the southern margin of the Tarim Basin. Further research should focus on enhancing the use of advanced physical, chemical, and biological technologies to treat and remove As and F from groundwater. Additionally, systematic monitoring of groundwater quality dynamics and the rational development of As and F containing groundwater are critical research priorities for the future.

Author Contributions

Conceptualization, J.Y.; methodology, Y.C.; software, Y.C. and J.Z. (Jiangtao Zhao); validation, J.H.; formal analysis, J.Z. (Jinlong Zhou); investigation, J.H.; J.Z. (Jie Zhang); resources, J.Z. (Jinlong Zhou) and J.H.; data curation, J.Z. (Jie Zhang); writing—original draft preparation, Y.C.; writing—review and editing, J.Z. (Jie Zhang) and Y.C.; visualization, J.Z. (Jie Zhang); supervision, J.Y. and J.Z. (Jie Zhang); project administration, J.Z. (Jinlong Zhou); funding acquisition, J.Z. (Jinlong Zhou) and Y.C. All authors have read and agreed to the published version of the manuscript.

Funding

This work was financially funded by the National Key R&D Program of China (Grant No. 2023YFC3706903, 2023YFC3706901), the Key Research Base of Social Sciences in Sichuan Province–Tuojiang River Basin High-quality Development Research Center Project (Grant No. TJGZL2023-15) and the University-level General Projects of Neijiang Normal University (Grant No. 2023QN07).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study is available on request from the corresponding author.

Acknowledgments

The authors thank Yanyan Zeng, Yi Xiao, Ying Sun, and Sibo Gu for their fieldwork support.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Distribution of groundwater sampling sites in the oasis belt in the southern margin of Tarim Basin.
Figure 1. Distribution of groundwater sampling sites in the oasis belt in the southern margin of Tarim Basin.
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Figure 2. Piper diagram of hydrochemical types.
Figure 2. Piper diagram of hydrochemical types.
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Figure 3. As and F co-contamination relationship diagram.
Figure 3. As and F co-contamination relationship diagram.
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Figure 4. Correlation of the As concentrations with other measured hydrogeochemical parameters. (a) pH; (b) Ca2+; (c) Mg2+; (d) Na+;(e) HCO3−; (f) SO42−.
Figure 4. Correlation of the As concentrations with other measured hydrogeochemical parameters. (a) pH; (b) Ca2+; (c) Mg2+; (d) Na+;(e) HCO3−; (f) SO42−.
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Figure 5. Correlation of the F concentrations with other measured hydrogeochemical parameters. (a) pH; (b) Ca2+; (c) Mg2+; (d) Na+;(e) HCO3−; (f) SO42−.
Figure 5. Correlation of the F concentrations with other measured hydrogeochemical parameters. (a) pH; (b) Ca2+; (c) Mg2+; (d) Na+;(e) HCO3−; (f) SO42−.
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Figure 6. Gibbs plots of groundwater.
Figure 6. Gibbs plots of groundwater.
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Figure 7. Relationship between Ca2+/Na+ and HCO3−/Na+.
Figure 7. Relationship between Ca2+/Na+ and HCO3−/Na+.
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Figure 8. Ion ratio relationships in groundwater. (a) Cl− and Na+; (b) (HCO3− + SO42−) and (Ca2+ + Mg2+); (c) CAI1 and CAI2; (d) (NO3−/Na+) and (SO42−-/Na+).
Figure 8. Ion ratio relationships in groundwater. (a) Cl− and Na+; (b) (HCO3− + SO42−) and (Ca2+ + Mg2+); (c) CAI1 and CAI2; (d) (NO3−/Na+) and (SO42−-/Na+).
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Figure 9. The relationship between As, F, and Cl. (a) As and As/Cl; (b) F and F/Cl.
Figure 9. The relationship between As, F, and Cl. (a) As and As/Cl; (b) F and F/Cl.
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Figure 10. The relationship between As, F, and NO3−. (a) As and NO3−; (b) F and NO3−.
Figure 10. The relationship between As, F, and NO3−. (a) As and NO3−; (b) F and NO3−.
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Figure 11. The relationship between the activities of F and Ca2+.
Figure 11. The relationship between the activities of F and Ca2+.
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Table 1. Statistical results of the groundwater chemical parameters.
Table 1. Statistical results of the groundwater chemical parameters.
IndicatorsPhreatic Water Samples (n = 110)Confined Water Samples (n = 50)
MinMaxMeanCvMinMaxMeanCv
pH7.11 8.90 8.05 0.05 6.30 8.34 7.68 0.06
K+4.20 776.41 75.56 2.21 8.37 17,898.07 790.57 3.28
Na+27.58 13,582.08 1024.13 2.19 108.94 115,620.81 17,491.58 1.80
Ca2+15.88 702.10 118.27 1.09 32.07 1102.46 315.68 0.96
Mg2+3.66 1307.97 150.78 1.71 27.48 25,366.14 1593.87 2.45
Cl−35.43 14,348.64 1377.51 2.18 127.65 172,922.44 23,967.61 1.86
SO42−55.48 9889.67 1080.88 1.69 140.68 54,147.68 9876.22 1.58
HCO3−36.62 3954.03 365.88 1.43 96.47 1428.15 302.69 0.79
NO3−0.06 78.60 8.74 1.26 0.06 138.44 15.03 1.48
F0.10 28.31 2.78 1.76 0.07 8.27 1.45 0.99
As0.001 0.09 0.18 1.33 0.001 0.07 0.02 1.25
TH81.06 6549.30 911.12 1.47 202.20 106,956.45 7584.70 2.18
TDS285.89 41,282.73 4027.42 1.89 536.17 358,693.98 57,782.18 1.69
pH is dimensionless, the rest of the units are mg/L.
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Chen, Y.; Hou, J.; Zhou, J.; Yu, J.; Zhang, J.; Zhao, J. Hydrogeochemical Processes and Sustainability Challenges of Arsenic- and Fluoride-Contaminated Groundwater in Arid Regions: Evidence from the Tarim Basin, China. Sustainability 2025, 17, 7971. https://doi.org/10.3390/su17177971

AMA Style

Chen Y, Hou J, Zhou J, Yu J, Zhang J, Zhao J. Hydrogeochemical Processes and Sustainability Challenges of Arsenic- and Fluoride-Contaminated Groundwater in Arid Regions: Evidence from the Tarim Basin, China. Sustainability. 2025; 17(17):7971. https://doi.org/10.3390/su17177971

Chicago/Turabian Style

Chen, Yunfei, Jun Hou, Jinlong Zhou, Jiawen Yu, Jie Zhang, and Jiangtao Zhao. 2025. "Hydrogeochemical Processes and Sustainability Challenges of Arsenic- and Fluoride-Contaminated Groundwater in Arid Regions: Evidence from the Tarim Basin, China" Sustainability 17, no. 17: 7971. https://doi.org/10.3390/su17177971

APA Style

Chen, Y., Hou, J., Zhou, J., Yu, J., Zhang, J., & Zhao, J. (2025). Hydrogeochemical Processes and Sustainability Challenges of Arsenic- and Fluoride-Contaminated Groundwater in Arid Regions: Evidence from the Tarim Basin, China. Sustainability, 17(17), 7971. https://doi.org/10.3390/su17177971

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