Hydrogeochemical Characteristics and Controlling Factors of Groundwater in a Typical Oasis in Arid Regions

doi:10.21203/rs.3.rs-8100581/v1

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Hydrogeochemical Characteristics and Controlling Factors of Groundwater in a Typical Oasis in Arid Regions

https://doi.org/10.21203/rs.3.rs-8100581/v1

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In arid regions, oases are critical to human survival and development, relying heavily on groundwater to sustain ecological balance and support socio-economic growth. This study focuses on a representative oasis city, where 198 groundwater samples were systematically collected in September 2022. A suite of analytical methods—including statistical analysis, Piper and Gibbs diagrams, ion-ratio analysis, and hydrogeochemical modeling—was used to characterize groundwater chemistry and identify its controlling factors. The results indicate that groundwater in the area is slightly alkaline. Unconfined groundwater is dominated by HCO₃·SO₄–Ca and HCO₃·SO₄–Na·Ca types, whereas confined groundwater is mainly HCO₃·SO₄–Na·Ca and HCO₃·SO₄·Cl–Na·Ca. Groundwater ion composition is influenced by water–rock interaction, with major ions primarily derived from the dissolution of halite, evaporites, and carbonates; cation exchange processes further affect confined groundwater. Halite and gypsum dissolve, while calcite and dolomite are near dissolution–precipitation equilibrium; overall dissolution propensity is higher in unconfined than in confined groundwater. Hydrogeochemical modeling quantitatively corroborates the primary water–rock interactions inferred from qualitative analyses of groundwater flow. These findings improve understanding of hydrogeochemical evolution in oasis groundwater within arid regions.

Arid Region Oasis

Groundwater

Hydrogeochemistry

Hydrogeochemical Modeling

Controlling Factors

Arid regions cover a substantial portion of global land, and their unique climatic conditions and fragile ecosystems have attracted considerable interdisciplinary attention (Yao et al., 2024; Tang et al., 2024). Within these environments, oases function as relatively humid refugia that support human survival and development, playing an essential regional role (Liu et al., 2023; Ju et al., 2023). Their existence relies on multiple natural resources, with groundwater being the most critical. Groundwater provides indispensable water for vegetation growth and agricultural irrigation and is central to maintaining ecological balance (Sheng et al., 2022; Gobashy et al., 2021; Ullah et al., 2024). Yet groundwater in arid regions is particularly vulnerable to environmental change because of limited recharge and high evaporative losses, making its sustainable management a global challenge (Yan et al., 2025).

In recent years, climate change and intensifying human activities have caused groundwater degradation in more than 20% of oasis irrigated areas worldwide (Mao et al., 2020). Problems such as persistent groundwater-level decline and progressive water-quality deterioration have emerged (Palma et al., 2023), placing unprecedented pressure on aquifer systems. For instance, excessive irrigation pumping has driven salinization and nitrate pollution in many arid oases, threatening ecosystem health and water security. As a crucial component of water resources in oasis systems, the hydrogeochemical characteristics of groundwater reveal its formation and evolution; however, the relative contributions of controlling factors to groundwater chemistry are often poorly quantified (Wang et al., 2024; Liu et al., 2020; Koncsos et al., 2024). Recent studies highlight the value of integrating hydrochemical and isotopic techniques to disentangle complex interactions between natural processes (e.g., rock weathering, evaporation) and anthropogenic influences (e.g., irrigation return flow, wastewater discharge) on groundwater quality (Preethi et al., 2025). A comprehensive understanding of groundwater hydrogeochemistry and its controlling factors in typical arid-region oases is therefore of great theoretical and practical significance for sustainable use and protection of groundwater resources, maintaining ecological balance, and ensuring safe water supplies.

Despite substantial progress, several knowledge gaps remain. First, the relative roles of natural processes (e.g., mineral dissolution, evaporation) and human activities (e.g., agricultural practices, urbanization) in shaping groundwater chemistry are still not well constrained (Li et al., 2024). Second, the spatial and temporal variability of groundwater quality in oasis systems—particularly under a changing climate—remains insufficiently understood (Guo et al., 2019). Third, there is a lack of integrated frameworks that couple traditional hydrochemical methods with advanced modeling to predict future groundwater evolution (Kumar et al., 2017).

In China, Xinjiang is a typical arid region with numerous oases, among which the Changji Hui Autonomous Prefecture stands out as a representative area. The unique geographical location and geological conditions of this region contribute to the complexity and distinctiveness of its oasis groundwater system. For example, the interaction between surface water and groundwater, coupled with the high evaporation rates, creates a dynamic hydrogeochemical environment that is highly sensitive to external perturbations. In recent years, the growing reliance on groundwater has resulted in excessive extraction, significantly altering its chemical composition. Therefore, this study focuses on a typical oasis in this arid region. Using field surveys, water sample collection, and analytical testing, a comprehensive approach was adopted, integrating statistical analysis, Piper diagrams, Gibbs diagrams, ion ratios, and hydrogeochemical modeling to reveal the groundwater’s chemical characteristics and controlling factors in the area. Insights into groundwater circulation processes from this study provide scientific evidence and references for the sustainable utilization of groundwater resources and environmental protection in arid-region oases.

2.1. Study Area

The Changji Hui Autonomous Prefecture is situated on the northern slope of the Tianshan Mountains and the southeastern edge of the Junggar Basin, between latitudes 43°20′–45°00′ N and longitudes 85°17′–91°32′ E, covering a total area of 73,900 square kilometers (Fig. 1). The topography gradually descends from higher altitudes in the southern region to lower areas in the north, with a southeast-to-northwest gradient. The region's landforms can be divided into four types: the Tianshan Mountains, sloping plains, desert basins, and the Beita Mountains. The central plains lie between the southern mountains and northern desert, forming a flat, fertile oasis zone with favorable water and soil conditions created by alluvial and fluvial deposits. The study area has a typical continental arid climate, marked by low precipitation and high evaporation. Annual precipitation ranges from 160 to 190 mm, while evaporation reaches 1,900 to 2,700 mm. The average annual temperature is 6 to 7.5°C, indicating a typical arid oasis environment.

Geologically, the study area is divided from south to north into the Bogda Anticline, the Junggar Central Depression, the Qitai Uplift, and the Almant-Karamay Mountain Uplift. The exposed strata in this area consist primarily of the Quaternary, Carboniferous, and Devonian systems, with lithology becoming progressively finer from south to north, characterized by cobble, gravel, sand, and silty sand. The piedmont sloping plain serves as the main groundwater recharge zone, with replenishment primarily through river valley underflow, infiltration from storm runoff, and return flow from irrigation canals and fields. This process forms an unconfined aquifer with a single-layer structure, where groundwater depth exceeds 5 meters. The general groundwater flow direction is from south to north, with a hydraulic gradient of 3–4‰. In the alluvial and aeolian plains, the aquifer medium is composed of fine-grained or weakly permeable layers, resulting in poor groundwater flow conditions where vertical alternating movement predominates. This creates an upper unconfined-lower confined multilayered aquifer structure, with groundwater depths ranging from 5 to 200 meters. Discharge occurs mainly through spring outflow, evaporation, lateral runoff, and artificial extraction.

2.2. Sample Collection and Chemical Analysis of Groundwater

In September 2022, groundwater samples were methodically collected from boreholes across the study area, comprising a total of 198 samples—133 from unconfined aquifers and 65 from confined aquifers (Fig. 1). Samples were collected in 500 mL polyethylene bottles, which were rinsed three times with deionized water before sampling and then rinsed three times with the water to be collected. All water samples were filtered using a 0.45 µm microporous membrane. For anion analysis, samples were sealed immediately after filtration, while for cation analysis, high-purity nitric acid was added to acidify the samples to a pH of ≤ 2. GPS was used to record the latitude, longitude, and elevation of each sampling location.

On-site measurements of pH, temperature, conductivity, TDS, and redox potential were conducted using a pH meter (pH818), a portable conductivity/TDS meter (AR8011), and an oxidation-reduction potential meter (AZ8552), with accuracies of 0.01, 0.1°C, 1 µS•cm⁻¹, 1 mg•L⁻¹, and 1 mV, respectively. Ion analysis was performed by the Changji Prefecture Institute of Water Science and Technology, testing for pH, K⁺, Na⁺, Ca²⁺, Mg²⁺, Cl⁻, SO₄²⁻, HCO₃⁻, total hardness (TH), and TDS, along with other indicators. Concentrations of K⁺, Na⁺, Ca²⁺, and Mg²⁺ were measured using an inductively coupled plasma optical emission spectrometer (iCAP6300); Cl⁻, SO₄²⁻, and NO₃⁻ were determined using an ion chromatograph (ICS1100); and HCO₃⁻ was measured by acid-base titration. The detection limit for all ions was 0.01 mg•L⁻¹. The cation-anion charge balance error for all water samples was maintained within ± 5%.

2.3. Analytical Method

Using the hydrogeological conditions and groundwater chemistry data from the study area, the chemical characteristics of groundwater and their controlling factors were analyzed through a combination of statistical analysis, Piper trilinear diagrams, Gibbs diagrams, ion ratios, and hydrogeochemical modeling. Excel was used for statistical analysis of hydrochemical parameters, while Grapher 10.0 was employed to generate Piper trilinear diagrams, Gibbs diagrams, and ion relationship plots. The saturation index and inverse hydrogeochemical modeling were conducted using PHREEQC 2.8, and other figures were created with Photoshop and ArcGIS 10.2.

3.1. Statistical Results

The statistical results of groundwater hydrochemical parameters in the study area are presented in Table 1. The pH of unconfined groundwater ranges from 7.08 to 8.52, with an average of 7.69, while the pH of confined groundwater ranges from 7.19 to 8.30, with an average of 7.81, indicating weak alkalinity in both cases. The TDS of unconfined groundwater varies from 182.45 to 3255.81 mg/L, with an average of 551.44 mg/L, whereas TDS in confined groundwater ranges from 241.20 to 3886.37 mg/L, with an average of 640.02 mg/L. The total hardness of unconfined groundwater ranges from 88.12 to 1261.76 mg/L, with an average of 273.64 mg/L, while in confined groundwater, it ranges from 79.11 to 1472.06 mg/L, with an average of 276.62 mg/L. Ion concentrations of both cations and anions are higher in confined groundwater than in unconfined groundwater.

The average cation concentration in unconfined groundwater follows the trend Ca²⁺ >Na⁺ + K⁺ >Mg²⁺, while the average anion concentration follows HCO₃⁻ >SO₄²⁻ >Cl⁻. In confined groundwater, the average cation concentration trend is Na⁺ + K⁺ >Ca²⁺ >Mg²⁺, and the anion concentration follows SO₄²⁻ >HCO₃⁻ >Cl⁻.

Table 1
Statistical results of hydrochemical parameters of groundwater
Parameters	Unconfined groundwater (n = 133)					Confined groundwater (n = 65)
Parameters	Max	Min	Mean	SD	CV	Max	Min	Mean	SD	CV
pH	8.52	7.08	7.69	0.27	0.03	8.30	7.19	7.81	0.20	0.03
TDS	3255.81	182.45	551.44	378.00	0.69	3886.37	241.20	640.02	556.46	0.87
TH	1261.76	88.12	273.64	182.59	0.67	1472.06	79.11	276.62	231.12	0.84
Na⁺+K⁺	587.86	1.75	51.71	66.00	1.28	941.64	12.61	84.89	124.18	1.46
Ca²⁺	566.00	21.10	83.63	64.71	0.77	447.16	27.27	73.63	59.51	0.81
Mg²⁺	125.00	0.24	17.61	19.28	1.09	170.03	1.22	22.68	27.00	1.19
Cl^-	830.57	8.51	71.07	92.70	1.30	830.57	12.00	86.31	121.44	1.41
HCO^3-	440.05	90.45	180.78	78.34	0.43	510.94	100.23	184.30	76.07	0.41
SO₄^2-	989.05	30.45	149.67	142.49	0.95	1545.29	28.53	188.20	231.26	1.23
Note: In the table, SD represents standard deviation; VC represents coefficient of variation; pH represents a dimensionless quantity; the rest of the water chemical indicators are in mg/L, and the coefficient of variation is in %.

3.2. Hydrochemical Types

The Piper diagram is a widely adopted tool for illustrating the composition and evolution of major ions in groundwater chemistry, offering the benefit of being independent of human-induced impacts. (Wang et al., 2023; Yang et al., 2016). As shown in the Piper trilinear diagram for the study area (Fig. 2), in the cation triangle (left triangle), most unconfined and confined groundwater samples are categorized as calcium-type with mixed sodium-type, with a higher proportion of sodium-type in confined groundwater, indicating the influence of ion accumulation in confined water. In the anion triangle (right triangle), both unconfined and confined groundwater are primarily bicarbonate-type and sulfate-type, with a small fraction of unconfined water showing chloride-type characteristics. The Piper diamond-shaped diagram further shows that the hydrochemical types of unconfined groundwater are predominantly HCO₃•SO₄-Ca and HCO₃•SO₄-Na•Ca, while confined groundwater is mainly of the HCO₃•SO₄-Na•Ca and HCO₃•SO₄•Cl-Na•Ca types.

3.3. Mechanisms Controlling Groundwater Geochemistry

3.3.1. Dominant Factors

The Gibbs diagram provides a qualitative framework for evaluating hydrochemical evolution processes, which are influenced by atmospheric precipitation (Marandi&Shand, 2018), evaporative concentration, and rock weathering. Although initially developed for surface water, the Gibbs diagram requires contour adjustments for groundwater analysis, as groundwater has a much longer residence time in aquifers. This extended residence time leads to extensive water-rock interactions, which expand the “rock weathering control” range (Marandi & Shand, 2018). The Gibbs diagram for groundwater in the study area shows that both unconfined and confined groundwater are predominantly located within the rock weathering zone (Fig. 3), indicating that groundwater in this area is primarily influenced by rock dissolution, with minimal influence from evaporation concentration or atmospheric precipitation. Some confined groundwater samples show a distinct clustering in γ(Na⁺)/γ(Na⁺+Ca²⁺), while γ(Cl⁻)/γ(Cl⁻+HCO₃⁻) displays no clear pattern.

3.3.2. Cation Exchange

Cation exchange is typically assessed by examining the relationship between γ (Mg²⁺ + Ca²⁺ − SO₄²⁻ − HCO₃⁻) and γ (Na⁺ − Cl⁻). If cation exchange occurs, the ratio between these two values should be approximately − 1(Xiao et al., 2015). As shown in Fig. 4a, confined groundwater exhibits a strong negative correlation (R² = 0.91) between γ (Mg²⁺ + Ca²⁺ − SO₄²⁻ − HCO₃⁻) and γ (Na⁺ − Cl⁻), with the ratio close to -1, indicating the occurrence of cation exchange in confined groundwater. In contrast, unconfined groundwater shows a ratio that deviates significantly from − 1, with a weaker correlation (R² = 0.29), suggesting that cation exchange is not prominent in unconfined groundwater. This observation is consistent with the findings of Ma et al., who reported cation exchange in confined groundwater within alluvial fan systems (Ma et al., 2022).

The chloro-alkaline indices (CAI-1 and CAI-2) are used to characterize the direction and intensity of ion exchange during groundwater chemical evolution (Singh et al., 2015; Nematollahi et al., 2018). When CAI-1 and CAI-2 are less than 0, it indicates forward cation exchange, where Ca²⁺ or Mg²⁺ in groundwater displaces Na⁺ from aquifer minerals. Conversely, when CAI-1 and CAI-2 are greater than 0, it suggests reverse cation exchange, where Na⁺ in groundwater displaces Ca²⁺ or Mg²⁺ from aquifer minerals (Wang et al., 2015). Figure 4b shows the relationship between the chloro-alkaline indices and TDS for confined groundwater, where most CAI-1 and CAI-2 values are less than 0, indicating forward cation exchange. This suggests that Ca²⁺ and Mg²⁺ in confined groundwater have exchanged with Na⁺ and K⁺ from the surrounding rock, consistent with previous analyses. The larger absolute values of CAI-2 further indicate stronger ion exchange. Along the groundwater flow path, the particle size of confined aquifer media in the alluvial plain gradually becomes finer, with an increase in clay minerals. This enhances cation exchange, as surface-adsorbed Na⁺ is replaced by Ca²⁺ and Mg²⁺ in the water.

3.3.3. Dissolution Processes

The primary minerals in the study area’s strata include sulfates, carbonates, silicates, and halite. Sulfate minerals consist mainly of gypsum and mirabilite, carbonate minerals include calcite and dolomite, and silicate minerals are primarily quartz, mica, and feldspar. Through weathering and water-rock interactions, these minerals dissolve into the groundwater, contributing to its chemical composition (Mahamuda et al., 2024; You et al., 2024). Ion ratio relationships are used to analyze these dissolution processes.

Unconfined groundwater sampling points are located near the line γ (Na⁺ + K⁺)/γ (Cl⁻) = 1 (Fig. 5a), indicating that the hydrochemical composition of unconfined groundwater is primarily influenced by halite dissolution. In contrast, most confined groundwater sampling points are positioned above γ (Na⁺ + K⁺)/γ (Cl⁻) = 1, suggesting that, in addition to halite dissolution, the hydrochemical composition of confined groundwater may also be affected by the dissolution of other sodium salts. In the confined groundwater of the river alluvial plain, Na⁺ + K⁺ concentrations deviate significantly from the 1:1 line, likely due to cation exchange, resulting in Na⁺ + K⁺ concentrations exceeding those of Cl⁻.

Both unconfined and confined groundwater sampling points are positioned above the line γ (Ca²⁺ + Mg²⁺)/γ (HCO₃⁻) = 1 (Fig. 5b), indicating that, in addition to the weathering and dissolution of calcite and dolomite, other Ca²⁺-bearing minerals are also dissolving (Wang et al., 2024). A significant linear relationship is observed between γ (Ca²⁺ + Mg²⁺) and γ (HCO₃⁻ + SO₄²⁻) (Fig. 5c), with most groundwater sampling points falling below the line γ (Ca²⁺ + Mg²⁺)/γ (HCO₃⁻ + SO₄²⁻) = 1.

The relationship between γ (SO₄²⁻ + Cl⁻) and γ (HCO₃⁻) can be used to evaluate the contributions of sulfate and carbonate minerals to groundwater ions (Liu et al., 2023). Most unconfined and confined groundwater sampling points are located above the line γ (SO₄²⁻ + Cl⁻)/γ(HCO₃⁻) = 1 (Fig. 5d), indicating that sulfate dissolution is the dominant contributor to ions in the water, with Ca²⁺ primarily sourced from the dissolution of gypsum and mirabilite. A linear relationship between γ(Ca²⁺) and γ(SO₄²⁻) (Fig. 5e) further suggests that gypsum dissolution is the main source of both Ca²⁺ and SO₄²⁻ in the groundwater.

In addition to the increase in Na⁺ concentration in confined groundwater due to cation exchange, the primary sources of Na⁺ in groundwater are the weathering and dissolution of halite and mirabilite, while Ca²⁺ mainly originates from gypsum dissolution and, to a lesser extent, the weathering of carbonates. The parameter γ (Ca²⁺ + Mg²⁺ − HCO₃⁻) represents the Ca²⁺ concentration derived from gypsum dissolution, and γ [SO₄²⁻ − (Na⁺ − Cl⁻)] represents the SO₄²⁻ concentration from gypsum dissolution. If all SO₄²⁻ in the water samples is derived from gypsum dissolution, the value of γ (Ca²⁺ + Mg²⁺ − HCO₃⁻)/γ [SO₄²⁻ − (Na⁺ − Cl⁻)] should equal 1. As shown in Fig. 5f, groundwater sampling points in the study area are located near or below the y = x line, further confirming that SO₄²⁻ in groundwater primarily originates from gypsum dissolution.

PHREEQC software was used to calculate the saturation indices (SI) of various minerals to assess their dissolution and precipitation states (Liu et al., 2021; Kim et al., 2008). Generally, when SI < − 0.5, the mineral is in a dissolution state; when SI is between − 0.5 and 0.5, the mineral is in a dissolution-precipitation equilibrium state; and when SI > 0.5, the mineral is in a saturated state (Rouabhia et al., 2011).

In the groundwater environment of the study area, the saturation index (SI) of calcite ranges from − 0.69 to 1.43(Fig. 6), with an average of 0.25, indicating an overall state of equilibrium. The SI of dolomite ranges from − 2.13 to 2.52, averaging 0.09; most dolomite in unconfined groundwater is in equilibrium, with a small portion in a dissolution state, while dolomite in confined groundwater is generally in a saturated or equilibrium state. The SI of gypsum ranges from − 2.29 to − 0.49, with an average of − 1.60, indicating a general dissolution state. Halite has an SI range of − 8.54 to − 4.81, with an average of − 7.27, also indicating dissolution. Overall, these minerals show a higher dissolution capacity in unconfined groundwater than in confined groundwater. The saturation indices of minerals gradually increase from upstream to downstream, trending toward saturation.

3.4. Hydrogeochemical Modeling

PHREEQC software enables quantitative simulation of water-rock interactions influenced by various natural processes and human activities (You et al., 2024). Along the groundwater flow direction, the unconfined groundwater pathway Q45 → Q49 → Q50 and the confined groundwater pathway C49 → C45 → C40 were selected for hydrogeochemical modeling.

In the model, primary minerals in the Quaternary aquifer of the study area—quartz, halite, calcite, dolomite, and gypsum—are designated as “potential mineral phases.” Since unconfined groundwater is in an open system, CO₂ is also included as a “potential mineral phase” along the unconfined groundwater pathway. In contrast, confined groundwater, being relatively deep and in a semi-closed system, does not involve CO₂ in hydrogeochemical processes. However, cation exchange occurs in confined groundwater, and this process is added as an additional “potential mineral phase” along the confined groundwater pathway. The model’s uncertainty factor is set at 0.05.

The simulation results (Table 2) indicate that along the unconfined groundwater pathway Q45 → Q49 → Q50, halite, dolomite, and gypsum undergo dissolution. As gypsum dissolves, the Ca²⁺ concentration in unconfined groundwater increases, leading to calcite precipitation (with a precipitation amount of 5.280 × 10⁻⁴ mmol/L). Along the confined groundwater pathway C49 → C45 → C40, calcite precipitates while halite, dolomite, and gypsum remain in a dissolved state. Forward cation exchange occurs in confined groundwater, with the intensity of this exchange increasing along the flow path (from 1.268 × 10⁻³ mmol/L to 1.872 × 10⁻³ mmol/L), maintaining a relatively low Ca²⁺ concentration. Consequently, the hydrochemical type evolves from HCO₃•SO₄-Ca•Na to HCO₃•SO₄•Cl-Na•Ca.

Table 2
Reverse simulation results of unconfined groundwater and confined groundwater flow paths
Molar Transfer Amount /mmol·L^-1		Unconfined Groundwater Pathway		Confined Groundwater Pathway
Mineral Phase	Chemical Formula	Q45→Q49	Mineral Phase	Chemical Formula	Q45→Q49
Halite	NaCl	2.402×10^− 4	Halite	NaCl	2.402×10^− 4
Calcite	CaCO₃	2.427×10^− 4	Calcite	CaCO₃	2.427×10^− 4
Dolomite	CaMg(CO₃)₂	1.001×10^− 5	Dolomite	CaMg(CO₃)₂	1.001×10^− 5
Gypsum	CaSO₄·2H₂O	1.177×10^− 4	Gypsum	CaSO₄·2H₂O	1.177×10^− 4
CO₂	CO₂	3.355×10^− 4	CO₂	CO₂	3.355×10^− 4
Cation Exchange	CaX₂	—	Cation Exchange	CaX₂	—
Cation Exchange	NaX	—	—	NaX	—
Note: In the table, a positive value indicates dissolution; a negative value indicates precipitation; and “—” indicates that the mineral did not participate in the reaction.

4.1. Hydrochemical Characteristics and Evolution

The weakly alkaline nature of groundwater in the study area is consistent with findings in other arid regions, such as the Tarim Basin and the Yinchuan Plain (Li et al., 2024). The higher TDS and ion concentrations in confined groundwater compared to unconfined groundwater suggest longer residence times and more extensive water-rock interactions, as observed in similar alluvial systems (Marandi & Shand, 2018). The dominance of Ca²⁺ and HCO₃⁻ in unconfined groundwater reflects the influence of carbonate weathering, which is typical in recharge zones where fresh water interacts with carbonate-rich sediments (Katz et al., 2011). In contrast, the shift to Na⁺ and SO₄²⁻ dominance in confined groundwater indicates the increasing role of silicate weathering and cation exchange processes, as reported in the Hexi Corridor. This evolution of hydrochemical types along the flow path is a common feature in arid regions, where groundwater chemistry is strongly influenced by the mineralogy of aquifer materials and the intensity of water-rock interactions.

4.2. Mechanisms Controlling Groundwater Chemistry

The Gibbs diagram results confirm that rock weathering is the primary control on groundwater chemistry, consistent with findings in other arid regions (Gibbs, 1970; Marandi & Shand, 2018). However, the distinct clustering of confined groundwater samples in the γ(Na⁺)/γ(Na⁺+Ca²⁺) plot suggests additional influences from cation exchange, as reported in the North China Plain (Xiao et al., 2015). Cation exchange is particularly significant in confined aquifers due to the presence of clay minerals, which provide abundant exchange sites for Na⁺, Ca²⁺, and Mg²⁺. This process not only alters the ionic composition of groundwater but also affects its suitability for irrigation, as high Na⁺ concentrations can lead to soil sodicity and reduced agricultural productivity (Singh et al., 2015).

The strong negative correlation between γ (Mg²⁺ + Ca²⁺ − SO₄²⁻ − HCO₃⁻) and γ (Na⁺ − Cl⁻) in confined groundwater (R² = 0.91) indicates significant cation exchange, a phenomenon also observed in the Nile Delta (Mostafa & Abdel, 2016). In contrast, the weaker correlation in unconfined groundwater (R² = 0.29) suggests limited cation exchange, likely due to shorter residence times and coarser aquifer materials (Ma et al., 2022). This difference highlights the importance of aquifer lithology and groundwater flow dynamics in controlling hydrochemical processes. For example, the finer-grained sediments in confined aquifers promote more extensive cation exchange, while the coarser materials in unconfined aquifers favor rapid recharge and limited water-rock interactions (Liu et al., 2021).

4.3. Mineral Dissolution and Precipitation

The dissolution of halite and gypsum, along with the equilibrium state of calcite and dolomite, is consistent with findings in the Loess Plateau (Liu et al., 2023) and the Arabian Peninsula (Rouabhia et al., 2011). The higher dissolution capacity in unconfined groundwater compared to confined groundwater reflects the influence of recharge processes and open-system conditions, as observed in the Badain Jaran Desert. In unconfined aquifers, the continuous influx of fresh water enhances the dissolution of soluble minerals such as halite and gypsum, while the semi-closed nature of confined aquifers limits dissolution and promotes mineral saturation (Kim et al., 2008). This spatial variability in mineral dissolution has important implications for groundwater quality, as it influences the concentrations of major ions such as Na⁺, Ca²⁺, and SO₄²⁻, which are critical for assessing water suitability for drinking and irrigation.

4.4. Implications for Groundwater Management

The evolution of hydrochemical types from HCO₃•SO₄-Ca•Na to HCO₃•SO₄•Cl-Na•Ca along the flow path highlights the need for targeted management strategies. For example, the high Na⁺ and SO₄²⁻ concentrations in confined groundwater may pose risks for irrigation and drinking water supplies, as reported in the Indus Basin (Singh et al., 2015). Elevated Na⁺ levels can lead to soil degradation and reduced crop yields, while high SO₄²⁻ concentrations may cause health issues such as gastrointestinal discomfort. To mitigate these risks, it is essential to implement measures such as controlled groundwater extraction, artificial recharge, and the use of alternative water sources for irrigation. Furthermore, the findings of this study have broader implications for sustainable groundwater management in arid regions. The integration of hydrochemical and isotopic techniques, as demonstrated here, provides a robust framework for understanding groundwater evolution and identifying contamination sources (Calvi et al., 2022). Future studies should focus on quantifying the impacts of climate change and human activities on groundwater quality, particularly in vulnerable oasis systems. For example, the increasing intensity of agricultural practices and urbanization in arid regions is likely to exacerbate groundwater degradation, necessitating the development of adaptive management strategies.

This study investigated the hydrochemical characteristics and controlling factors of groundwater in a typical oasis city in an arid region using statistical analysis, Piper diagrams, Gibbs diagrams, ion ratios, and hydrogeochemical modeling. The main findings are as follows:

(1) Groundwater in the study area is weakly alkaline, with higher TDS, total hardness, and ion concentrations in confined groundwater compared to unconfined groundwater. The hydrochemical types of unconfined groundwater are predominantly HCO₃•SO₄-Ca and HCO₃•SO₄-Na•Ca, while confined groundwater is mainly HCO₃•SO₄-Na•Ca and HCO₃•SO₄•Cl-Na•Ca.

(2) Rock dissolution is the primary control on groundwater chemistry, with minimal influence from evaporation and atmospheric precipitation. Major ions (Na⁺, Ca²⁺, HCO₃⁻, Cl⁻, and SO₄²⁻) are derived from the dissolution of evaporites (halite, gypsum, and mirabilite) and carbonates (calcite and dolomite). Confined groundwater is significantly influenced by forward cation exchange, where Ca²⁺ or Mg²⁺ in groundwater replaces Na⁺ in aquifer minerals, with exchange intensity increasing along the flow path.

(3) Hydrogeochemical modeling confirms that halite and gypsum are in a dissolved state, while calcite and dolomite are in dissolution-equilibrium. Mineral dissolution capacity is higher in unconfined groundwater than in confined groundwater, with saturation indices increasing downstream. Along the flow path, halite, dolomite, and gypsum dissolve, while calcite precipitates, and forward cation exchange occurs in confined groundwater.

This study enhances the understanding of hydrogeochemical processes in arid oasis systems and provides a scientific basis for sustainable groundwater management. The findings highlight the need to address the risks associated with high Na⁺ and SO₄²⁻ concentrations in confined groundwater, particularly for irrigation and drinking water supplies. Future research should focus on the impacts of climate change and human activities on groundwater quality, as well as the development of adaptive management strategies to ensure the long-term sustainability of groundwater resources in arid regions.

Acknowledgments

The authors would like to thank the anonymous reviewers and editors for their valuable comments, which were crucial in improving the quality of this paper.

Funding

Thisresearch was funded by the Science and Technology Research Program of Chongqing Municipal Education Commission (KJQN202301209), the Science Technology Planning Project of WanZhou District, Chongqing (wzstc20230313, wzstc20230301) and Chongqing Water Conservancy Science and Technology Project (CQSLK-2024023).

Data availability Not applicable. All the data used in this paper are provided herein

Conflict of interests There are no competing interests regarding this work.

Ethical approval Not applicable.

Consent to participate Not applicable.

Consent for publication Not applicable.

Author contributions

Author Contributions: Yulin Zhou as the corresponding author, curated the data and prepared the original draft. Xing Wei was responsible for the conceptualization and methodology of the study. Qingmei Zhang contributed to the visualization and investigation of the study's findings. Libo Ran and Delun Chen provided overall supervision and project administration. Xueting Dou, Yanan Fu and Gang Wu conducted validation and contributed to writing, reviewing, and editing of the manuscript. All authors have read and agreed to the published version of the manuscript.

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You are reading this latest preprint version

Hydrogeochemical Characteristics and Controlling Factors of Groundwater in a Typical Oasis in Arid Regions

Status:

Version 1

Abstract

Figures

1. Introduction

2. Materials and Methods

2.1. Study Area

2.2. Sample Collection and Chemical Analysis of Groundwater

2.3. Analytical Method

3. Results

3.1. Statistical Results

3.2. Hydrochemical Types

3.3. Mechanisms Controlling Groundwater Geochemistry

3.3.1. Dominant Factors

3.3.2. Cation Exchange

3.3.3. Dissolution Processes

3.4. Hydrogeochemical Modeling

4. Discussion

4.1. Hydrochemical Characteristics and Evolution

4.2. Mechanisms Controlling Groundwater Chemistry

4.3. Mineral Dissolution and Precipitation

4.4. Implications for Groundwater Management

5. Conclusions

Declarations

Acknowledgments

Funding

References

Status:

Version 1