https://orcid.org/0000-0002-6414-8384
Figures
Abstract
This study focuses on mineral groundwater in alpine regions and its sustainable exploitation. The Tongde basin on Tibetan Plateau was investigated to reveal the hydrochemistry and formation of mineral groundwater in alpine basins and its sustainable development under anthropogenic disturbances. The results show that groundwater there is characterized by enriched strontium, with concentrations in the range of 0.29–2.03 mg/L, exceeding the mineral water threshold of 0.20 mg/L. Groundwater has large salinity variation but is predominantly fresh, with an average TDS of 456.72 mg/L and a dominant hydrochemical facies of HCO3-Ca. Groundwater chemistry is primarily governed by water-rock interactions, particularly silicates weathering, carbonates dissolution and cation exchange. The enriched Sr in groundwater primarily originate from the dissolution of silicate minerals and carbonate cements in arkosic sandstone and argillaceous siltstone, with albite, calcite, and strontianite being the primary minerals involved. Groundwater quality in the basin is mostly excellent to good, with EWQI values below 100. While agricultural practices have introduced nitrogen contaminants (NH4+, NO2- and NO3-) into groundwater. Among them, NO3- would pose potential health risks to various populations and should be addressed in the development of alpine basins with the endowment of mineral water resources.
Citation: Xiao S, Wang J, Xiao Y, Yang S, Zhang Y, Wang J, et al. (2025) Characterization of strontium-rich groundwater in a typical alpine basin on Tibetan Plateau: Implications for sustainable exploitation and development. PLoS One 20(9): e0331449. https://doi.org/10.1371/journal.pone.0331449
Editor: Abdur Rashid, Institute of Urban Environment Chinese Academy of Sciences, CHINA
Received: May 1, 2025; Accepted: August 15, 2025; Published: September 10, 2025
Copyright: © 2025 Xiao et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Data Availability: Data cannot be shared publicly because of institutional confidentiality requirements. Data are available from Ethics Committee of Faculty of Geosciences and Engineering, Southwest Jiaotong University (contact via geos@swjtu.edu.cn) for researchers who meet the criteria for access to confidential data.
Funding: This study was financially supported by the Key Lab of Environmental Geology of Qinghai Province in the form of grants (2023-KJ-02 and 2024-KJ-06) received by SY. This study was also financially supported by the Applied Basic Research Project of Science and Technology Program of Qinghai Province in the form of a grant (2024-ZJ-771) received by YX. This study was also financially supported by Fundamental Research Funds for the Central Universities in the form of a grant (2682025ZTZD007) received by YX. This study was also financially supported by the National Natural Science Foundation of China in the form of a grant (42477059) received by YX. This study was also financially supported by the Student Research Training Program of Southwest Jiaotong University in the form of a grant (2025169) received by JW.
Competing interests: The authors have declared that no competing interests exist.
1. Introduction
Mineral water, a natural resource with unique health benefits, has garnered significant attention in recent years for its potential applications in health and industry [1–5]. The presence of essential trace elements such as strontium, magnesium, and calcium in mineral water has been widely recognized for their positive impacts on human health, including bone health, cardiovascular function, and overall well-being [6–9]. Among these, strontium-rich mineral water has emerged as a particularly valuable resource due to its potential therapeutic properties [4,10–12]. The study of such specialized mineral waters is crucial not only for understanding their geological origins but also for assessing their sustainable exploitation and development [13,14].
Over the past few decades, research on mineral water has made significant progress across the world [6,15–21]. Studies have shown that high-altitude regions, particularly those with complex geological structures and unique hydrological conditions, are favorable for the formation of mineral water [4,17,22]. These regions often exhibit high levels of trace elements due to enhanced rock-water interactions and natural filtration processes [15,23]. Alpine regions in various parts of the world have been identified as key areas for the occurrence of high-quality mineral water [13,15–17,24]. However, despite these advancements, there remains a gap in understanding the specific mechanisms that contribute to the formation and enrichment of strontium in groundwater, especially in high-altitude source areas.
The Tibetan Plateau, often referred to as the “Asia Water Tower” [25,26], is an ideal region for exploring the formation and characteristics of mineral water in alpine areas due to its high altitude, low temperature, and pristine environment. This plateau is home to numerous alpine basins that host a variety of mineral water resources [22,27], including those rich in strontium [4]. However, with increasing human activities such as urbanization, tourism, and resource exploitation, the Tibetan Plateau is facing growing environmental pressures [28–33]. These activities may impact the quality and sustainability of mineral water resources. Therefore, it is imperative to clarify the hydrogeochemical mechanisms that govern the formation of mineral water and to assess the suitability of these waters for sustainable development and exploitation.
To achieve a comprehensive understanding of mineral water, various techniques have been employed in recent studies [1,16,34–37]. Hydrogeochemical analysis helps identify the chemical composition and major processes influencing groundwater quality [1,38–44]. Isotopic tracing provides insights into the sources and pathways of groundwater [4,45–47]. Numerical modeling aids in simulating the dynamics of groundwater flow and solute transport [31,48], but its application in high-altitude regions like the Tibetan Plateau presents unique challenges due to the remote property and insufficient data. Thus, integrating hydrochemical and environmental isotope approaches becomes the optimal solution to unravel the formation of mineral water resources in these remote alpine regions.
This study aims to address the knowledge gaps in the characterization and sustainable exploitation of strontium-rich groundwater in a typical alpine basin on Tibetan Plateau. Specifically, the objectives of this research are (1) to investigate the hydrochemical characteristics of strontium-rich groundwater in present alpine basin, (2) to elucidate the hydrogeochemical mechanisms and sources of strontium enrichment in groundwater, and (3) to evaluate the suitability of strontium-rich groundwater for sustainable development and exploitation. By achieving these goals, this study will provide valuable insights into the sustainable management of mineral water resources in high-altitude regions and contribute to the broader understanding of groundwater dynamics in alpine environments.
2. Study area
The Tongde Basin is located at the overlap of the northeastern part of the Tibetan Plateau and the Yellow River watershed. It is a typical alpine basin on the Tibetan Plateau and an important source area for the Yellow River. The basin’s longitude ranges from 100°12′37″E to 101°5′33″E, and its latitude ranges from 35°5′11″N to 35°25′44″N, covering an area of 3,063 km2 (Fig 1). Mountains surround the basin to the north and south, while the central part is a plain. The overall terrain is oriented east-west, with higher elevations on the north and south sides and lower elevations in the middle. The eastern part is higher than the western part. The downstream area of the basin shows significant fluvial erosion and a large elevation drop. Elevations in the region vary from 2,650 m to 4,531 m. The main river in the basin is the Baqv River, which flows westward through the basin and eventually drains into the Yellow River. The climate type in the region is a continental plateau climate. Annual temperatures range from −0.77 to 1.69°C, with an average of 0.64°C. Over 80% of the annual precipitation occurs from May to September, totaling 688 mm. The annual evaporation rate is 790 mm, slightly higher than the precipitation. The climate in the basin is characterized by cold temperatures and relatively dry conditions.
(The map was created via ArcGIS 10.2 (https://www.esri.com/en-us/arcgis/products/arcgis-desktop/resources) based on the publicly available landsat 8 collection 2 level 2 data from Geospatial Data Cloud site, Computer Network Information Center, Chinese Academy of Sciences (http://www.gscloud.cn)).
The aquifer system in the study area consists of phreatic and confined water in the Quaternary loose deposits, as well as fracture water in the bedrock. The aquifer systems in the mountainous areas beyond the basin to the north and south are primarily composed of bedrock fracture water, with individual spring discharges exceeding 1 L/s. Inside the basin, the plains are characterized by phreatic and confined water in the Quaternary loose deposits. The water-richness of the aquifers decreases from upstream to downstream. At the upstream of the plains, the aquifers have good hydraulic properties, with both phreatic and confined water coexisting. Near the pediments and riverbanks, the water-richness of the aquifers increases. In areas close to the river, the yield of phreatic water from individual wells can exceed 5,000 t/d, while the yield of confined water ranges from 100 to 1,000 t/d. In the downstream plains, the hydraulic connectivity of the aquifers is weaker, with phreatic water being predominant. The yield of phreatic water from individual wells ranges from 100 to 1,000 t/d, and there is a large area of aquifer pore water being drained near the mountain front on the south side. The abundant groundwater resources and the presence of confined water upstream provide excellent material and dynamic conditions for the formation and emergence of mineral water.
3. Material and methods
3.1. Water sampling and analyses
A total of 26 phreatic groundwater samples were collected across the basin during July-August 2024. The samples in the upstream area of the basin are relatively evenly distributed, mainly along the south bank of the Baqv River and the mountains and pediments on the west bank of the Cihawuqv River. In the downstream area, most samples are distributed along the Baqv River, with a few located in the mountains on its south bank. These samples represent the condition of mineral water in the basin. During sampling, each well was pumped for over 15 minutes to eliminate the influence of standing water. All target groundwater samples were tested in situ for parameters such as pH before sampling. The samples were collected in 500 mL high-density polyethylene bottles that had been pre-rinsed 3–5 times with the target groundwater. After collection, the samples were stored at 4°C and transported to the laboratory for analysis within 48 hours.
In-situ parameter measurement (HACH HQ4D, Californian, USA) was conducted using portable measurement devices. In the laboratory, major cations (Na+, K+, Ca2+, Mg2+) and Sr were analyzed using an inductively coupled plasma mass spectrometer (Agilent 7500ce ICP – MS, Tokyo, Japan). An ion chromatography (Shimadzu LC-10ADVP, Kyoto, Japan) was employed to analyze major anions (SO42-, Cl-, NO3-, NO2-) and NH4+. Acid-base titration and gravimetric analysis were used to measure HCO3- and TDS, respectively. Sr was separated from the matrix using Sr-Spec resin and then analyzed for isotopes using thermal ionization mass spectrometry (Thermo Fisher Scientific) [49]. To ensure accuracy, each sample was tested three times with standard and blank controls interspersed. The results showed that the ionic charge balance error percentage of the groundwater samples was within ±5%, indicating reliable results.
3.2. Entropy-weighted water quality index
The Entropy -weighted Water Quality Index (EWQI) represents an enhancement of the traditional Water Quality Index (WQI) [50]. As the predominant water quality assessment tool, EWQI incorporates entropy to minimize human subjectivity [51]. It assigns specific weights to each factor. It serves as a reliable standard for evaluating groundwater quality [52–54]. The specific calculation formula for EWQI is shown in Eq. 1.
Where, ωj is the entropy weight of the jth water chemical parameter involved in water quality analysis; qij is the quality rating scale for different parameters. The calculation formulas for ωj and qij are Eqs. 2 and 3, respectively.
Where, ej denotes the information entropy of the jth water chemical parameter. Sj represents the acceptable value of the jth water chemical parameter as per the Chinese drinking water standards and the World Health Organization’s recommendations. X is the feature matrix of each groundwater sample for each water chemical parameter. Xij is the feature value of the jth water chemical parameter for the ith groundwater sample. The calculation formula for the information entropy of each water chemical parameter is shown in Eq. 5.
Where, pij is the proportion of the ith groundwater sample in the jth water chemical parameter. Its calculation formula is given in Eq. 6.
Where, yij represents the value of xij after normalization. The normalization process is shown in Eq. 7.
3.3. Human health risk assessment
This study employs the Human Health Risk Assessment (HHRA) model developed by the USEPA [55] to assess the potential health risks to the public from contaminated groundwater. This model provides a practical approach for ensuring safe water supply by quantitatively characterizing the health impacts of exposure to polluted groundwater [56–58]. The primary exposure pathways are oral intake and dermal contact [59]. Consequently, the study focuses on estimating health risks through these two main pathways. The population is divided into four groups: infants, children, adult females, and adult males, considering physiological differences [60]. Parameters used in the HHRA model for various populations are listed in S1 Table. The process of determining the potential health risks of pollutants to humans using the HHRA model is as follows:
Step 1: Calculate the estimated Chronic Daily Intake (CDI) and Dermal Absorption Dose (DAD) of specific pollutants. The calculation formulas are given in Eqs. 8 and 9.
Where, Ci denotes the concentration of each toxic ion involved in the health risk assessment. IR is the daily water intake. EF represents the exposure frequency. ED is the exposure duration. BW stands for the body weight of the assessed population. AT is the average time of exposure occurrence. Kp is the dermal permeability coefficient. SA represents the skin contact surface area. T is the contact duration. EV is the average frequency of skin contact. CF is the unit conversion factor.
Step 2: Use CDI and DAD to calculate the non-carcinogenic risk (HQi) respectively. The specific calculation formulas are given in Eqs. 10–12.
Where, HQIi and HQDi represent the non-carcinogenic risks of the ith pollutant under ingestion and contact conditions, respectively. RfDi is the reference value corresponding to the ingestion dose of the ith pollutant.
Step 3: Sum the non-carcinogenic risks of all pollutants to obtain the total non-carcinogenic health risk index (HI). The calculation formula is given in Eq. 13.
4. Results and discussion
4.1. General hydrochemical characteristics
The hydrochemical parameters of groundwater sampled in the basin are listed in Table 1, along with the permissible limits set by World Health Organization (WHO) [61] and Chinese guidelines [62], as well as the standards for Natural Drinking Mineral Water [63] for comparison. The pH of groundwater ranges from 6.82 to 8.10, with an average of 7.78, and all samples meet the WHO guideline. The Total Hardness (TH) and Total Dissolved Solids (TDS) of groundwater range from 205.84 to 485.39 mg/L (mean 284.11 mg/L) and 282.00 to 1,263.83 mg/L (mean 456.72 mg/L), respectively. Only 3.85% of sampled groundwater exceeds the permissible limits of 450 mg/L for TH and 1000 mg/L for TDS set by the WHO guideline, indicating that most groundwater in the Tongde Basin is soft and fresh, suitable for drinking.
Among major ions, Na+ and Ca2+ are the most abundant cations in the groundwater, with concentrations ranging from 9.42 to 275.00 mg/L (mean 74.24 mg/L) and 44.90 to 110.22 mg/L (mean 46.34 mg/L), respectively. The concentration of Mg2+ ranks second, ranging from 9.72 to 69.26 mg/L with a mean of 23.99 mg/L. K+ is the lowest major cation, with a range of 0.56 to 18.20 mg/L. Overall, except for K+, all major cations exceed the WHO guideline limits in 3.85% to 38.46% of the samples. The anions in groundwater are predominantly HCO3-, with contents ranging from 246.00 to 537.00 mg/L (mean 326.11 mg/L), followed by SO42- (9.61 to 384.24 mg/L, mean 50.32 mg/L) and Cl⁻ (4.76 to 187.89 mg/L, mean 39.33 mg/L). Only SO42- exceeds the WHO guideline of 250 mg/L in 3.85% of the samples.
Nitrogen concentrations were also determined for groundwater in the Tongde Basin to assess potential anthropogenic influences. As shown in Table 1, most groundwater samples exhibit very low nitrogen concentrations. However, NH4+ exceeds the Chinese guideline limits of 0.2 mg/L [62] in 21.05% of the samples, and NO2- exceeds the Chinese guideline limit of 0.02 mg/L [62] in 8.00% of the samples. No groundwater has been observed with NO3- concentration beyond the permissible limit of 50 mg/L for drinking recommended by WHO [61]. This indicates that only certain areas in the Tongde Basin are slightly affected by anthropogenic inputs, ensuring the overall suitability of groundwater for drinking purposes.
The special component contents of groundwater were determined. The concentration of Sr ranges from 0.29 to 2.03 mg/L, which is the most abundant and the only minor element meeting the minimum threshold (0.20 mg/L) of Drinking Natural Mineral Water [63], with a compliance rate of 100%. Additionally, to further analyze the source of Sr, the 87Sr/86Sr ratio was measured, with results averaging around 0.71.
4.2. Groundwater hydrochemical facies
Piper trilinear diagram is an effective tool for identifying groundwater hydrochemical facies and providing valuable insights into the potential evolution of groundwater hydrochemistry [64–66]. As shown in Fig 2, Ca2+ and HCO3- are the dominant cations and anions, respectively. The majority of groundwater samples are classified as HCO3-Ca type. Only two samples are HCO3-Na·Ca type, and one sample is Cl-Na type, which may be associated with potential evaporation effects in some shallow locations. Overall, groundwater in the Tongde Basin exhibits a fresh hydrochemical facies of HCO3-Ca type, with no significant trend toward evolution into saltwater.
4.3. Mechanisms governing groundwater chemistry
4.3.1. Major hydrochemistry.
Hydrochemical diagrams, correlation analysis, and the chlor-alkali index are used to identify the mechanisms governing the chemistry of strontium-rich groundwater in the Tongde Basin. Generally, the hydrochemical components of groundwater are primarily controlled by natural factors such as recharge water (usually precipitation), water-rock interactions, and evaporation [43,67–69]. Human activities further shape the hydrochemical composition of groundwater beyond these natural processes [70,71].
Gibbs diagrams, constructed using Na ⁺ /(Na⁺+Ca²⁺) or Cl-/(Cl- + HCO₃-) versus TDS [72], are effective tools for visually identifying the major natural mechanisms governing the hydrochemical composition of natural waters [73,74]. As shown in Fig 3a, b, all sampled groundwaters fall within the rock-dominated zone of the Gibbs diagrams, indicating that water-rock interactions are the primary drivers of the hydrochemical composition of strontium-rich groundwaters in the Tongde Basin. Additionally, a few samples show a trend towards the evaporation-dominated zone, suggesting that evaporation also plays a role in shaping the hydrochemical characteristics to some extent.
The concentration ratios of various ions are used to further identify the principal sources of the hydrochemical constituents in groundwater. The ion ratio relationship between Na+ and Cl- shows that most sampled groundwaters cluster close to the 1:1 ratio line, indicating that halite dissolution is the primary potential source of Na+ and Cl- in the groundwaters (Fig 3c). However, some groundwaters significantly deviate from this line, showing anomalously high Na+ levels. This deviation suggests that these waters are influenced by cation exchange or silicate rock dissolution rather than halite dissolution. The weathering and dissolution of albite can increase both Na+ and HCO3- concentrations in groundwater (Eq. 12). In Tongde basin, a positive correlation (correlation coefficient 0.39) between Na+ and HCO3- in strontium-rich groundwater indicates potential silicate rock weathering (Fig 4a). Additionally, a negative correlation (correlation coefficient −0.39) between Na+ and Ca2+ suggests cation exchange may also affect Na+ concentrations in groundwater.
In general, freshwater typically has low levels of SO42-, which originates from the dissolution of gypsum and anhydrite. The ratio plot of SO42- versus Ca2+ shows that almost all sampled groundwaters in Tongde basin have higher Ca2+ milligram-equivalent concentrations compared to SO42- (Fig 3d). This indicates that gypsum is not the primary mineral determining the SO42- and Ca2+ contents in groundwater. Furthermore, a weak negative correlation exists between SO42- and Ca2+ (correlation coefficient −0.18, Fig 4a), suggesting these ions may come from other sources. In contrast, a stronger correlation between SO42- and Mg2+ (correlation coefficient 0.7, Fig 4a) implies that their concentrations in groundwater are likely controlled by similar hydrochemical processes. The study area has well-developed fracture structures, and pyrite and magnesite in these zones may be potential sources of SO42- and Mg2+ in groundwater.
All sampled groundwaters in Tongde basin fall between the calcite and dolomite dissolution lines on the scatter plot of HCO3- versus Ca2+ (Fig 3e), indicating that the weathering and dissolution of calcite and dolomite contribute to the enrichment of Ca2+ and HCO3- in groundwater. During dolomite dissolution, HCO3- and equivalent amounts of Ca2+ and Mg2+ are released into the groundwater (Eq. 13). However, the groundwater shows a low negative correlation between Ca2+ and Mg2+ (correlation coefficient −0.14), inconsistent with dolomite dissolution (Fig 4a). Therefore, the Ca2+ in groundwater mainly originates from calcite, while the excess HCO3- relative to Ca2+ results from silicate weathering and dissolution rather than dolomite. End-member diagrams based on Mg2+/Na+ and HCO3-/Na+ versus Ca2+/Na+ were constructed to identify the major rocks (including evaporites, silicates, and carbonates) involved in water-rock interactions in the aquifer [75]. As shown in Fig 4b, c, most sampled groundwaters plot in the silicate-dominated zone, with only a few in the evaporite-dominated zone. This confirms that silicate dissolution is the primary process governing the hydrochemical composition of strontium-rich groundwaters in the Tongde Basin, while evaporite dissolution also plays a significant role in groundwater hydrochemistry in some locations.
As mentioned above, cation exchange is a potential process controlling the hydrogeochemical evolution of groundwater. The chloro-alkaline indices (CAI-I and CAI-II) (Eqs. 14 and 15) based on the ratio of milligram equivalent concentrations of ions are introduced to further substantiate this potential process in the aquifer. When both CAI-I and CAI-II results are negative, it indicates the occurrence of positive cation exchange interactions in the groundwater (Eq. 16). Conversely, if there is a reverse cation exchange process, both CAI-I and CAI-II results will be positive (Eq. 17). Nearly 92% of the sampled groundwaters in the study area exhibit negative CAI results (Fig 3f), indicating that a positive cation exchange process occurs in the aquifer. Furthermore, the excess of Na+ and the deficiency of Ca2+ and Mg2+ in the groundwater also signify that a positive cation exchange process has occurred in the aquifer, leading to the replacement of Ca2+ and Mg2+ by Na+ (Fig 3g).
The shallow depths of the phreatic groundwater in the study area and the open groundwater system make the hydrochemical components of groundwater susceptible to a range of anthropogenic activities in addition to natural processes. NO3- contamination from various sources is prevalent in groundwater environments worldwide, and its concentration has become a crucial indicator for assessing whether human activities have impacted the hydrochemical quality of groundwater [76]. When NO3- concentrations are below 10 mg/L, their levels are primarily controlled by geological factors [71]. Conversely, concentrations exceeding 10 mg/L indicate the presence of anthropogenic inputs of NO3- pollution [77]. In the study area, 32% of the sampled groundwater exhibited NO3- concentrations exceeding 10 mg/L (Fig 3h), indicating the involvement of anthropogenic activities in shaping the hydrochemical components of groundwater in the study area. The extensive agriculture and animal husbandry within the basin may be the major anthropogenic sources of nitrate contaminants in the groundwater environment.
Overall, the hydrochemical composition of strontium-rich groundwater in the Tongde Basin is controlled by both natural processes and anthropogenic factors. The ion composition of groundwater is primarily dominated by the weathering of silicates, such as feldspar in the sedimentary sandstones within the basin, and cation exchange processes. Additionally, the dissolution of magnesite, pyrite, and calcite also contributes to groundwater hydrochemical composition. The development of agriculture and animal husbandry are the main reasons for the enrichment of nitrates in some groundwaters.
4.3.2. Beneficial components.
In the Tongde Basin, the strontium (Sr) concentration in groundwater ranges from 0.29 to 2.03 mg/L, with relatively high overall content, making it a dominant component in groundwater. Groundwater samples with higher Sr concentrations also exhibit relatively higher total dissolved solids (TDS), showing a certain linear positive correlation (Fig 5a). This suggests that the increase in Sr concentration in groundwater may be related to evaporation or water–rock interactions processes [78].
The relationship between Sr isotopes (87Sr/86Sr) and other physicochemical characteristics of groundwater is an effective means to explore the formation mechanism of dominant components in groundwater. The relationship between 87Sr/86Sr and 1/Sr reveals the source of dominant components in groundwater of the Tongde Basin. The values of 87Sr/86Sr in the basin range from 0.7108 to 0.7116. These values are higher than those in carbonate leaching solutions (0.709) [79] but lower than those in silicate leaching solutions (0.715) [80]. All samples are distributed between the silicate and marine carbonate end-members (Fig 5b), indicating that the groundwater within the Tongde Basin is affected by a mixing process from the dissolution of silicates and carbonates. Therefore, Sr, as the beneficial component in the basin groundwater, likely originates from water–rock interactions with silicate and carbonate rocks, rather than from evaporative processes.
Strontium (Sr) and calcium (Ca2+) share similar hydrogeochemical properties and are commonly found in abundance in carbonate and silicate rocks. The relationship between Sr and Ca2+ can indicate the source of Sr. An strong positive correlation between Sr and Ca2+ suggests that Sr originates from the dissolution of carbonate minerals [81]. As shown in Fig 5c, there is a weak positive correlation between Sr and Ca2+ for groundwater within the basin. This indicates that while the dissolution of carbonate minerals does provide a portion of the Sr in groundwater, the remaining Sr is derived from other sources. Given that the primary source of Ca²⁺ in groundwater within the basin is the dissolution of calcite, the Sr from carbonate rocks is likely derived from the dissolution of strontianite and the substitution of Sr for Ca2+ in calcite [82].
The other source of Sr in groundwater of the Tongde Basin is revealed through the relationship between Sr/ Na+ and Cl-. As shown in Fig 5d, the groundwater within the basin exhibits a negative correlation between Sr/ Na+ and Cl-, indicating that another source of Sr is sodium silicate minerals, primarily dominated by albite [83]. These Sr atoms enter the mineral lattice by substituting for Na+ positions within the mineral structure and are subsequently released into groundwater through water–rock interactions. Similarly, if Sr in groundwater had a single source, the ratios of ⁸⁷Sr/⁸⁶Sr to Sr/ Na+ and Sr/ Cl- should show a clear linear positive or negative correlation [84]. However, no significant linear relationship is observed in Fig 5e and 5f, indicating the diversity of Sr sources in the groundwater. The rock formations exposed in the peripheral mountainous areas of the basin are mainly arkosic sandstone and argillaceous siltstone. These rocks contain significant amounts of feldspar, quartz, and other silicate minerals, as well as carbonate minerals present as cementing materials [85]. Overall, the Sr in the Tongde Basin originates from the dissolution of silicate minerals and carbonate cements in arkosic sandstone and argillaceous siltstone, with the minerals involved in dissolution including albite, calcite, and strontianite.
4.4. Groundwater exploitation suitability for drinking purpose
4.4.1. Overall groundwater quality based on EWQI assessment.
Parameters of Na+, K+, Ca2+, Mg2+, HCO3-, SO42-, Cl-, TDS, TH, Sr, NH4-, NO2- and NO3- are selected as the basis variables for estimating the overall water quality of groundwater within the Tongde Basin based on the Entropy-weighted Water Quality Index (EWQI). The EWQI value of G24 is the highest at 170.83, while G9 has the lowest value at 43.58 (Fig 6a). The average EWQI value of sampled groundwaters within the basin is 69.12. Among them, 23% of the sampled groundwaters have an EWQI value below 50, falling into the excellent category (Fig 6a). The majority (69%) of the samples are in the good category. There is one sample each in the medium and poor categories, with no samples in the extremely poor category. Overall, the groundwater quality in the Tongde Basin is relatively good, with most samples in the good category. The positive correlation between EWQI and TDS (Fig 6a) further indicates that changes in groundwater quality are closely related to factors causing an increase in TDS.
As shown in Fig 6b, the distribution of EWQI for groundwater in the basin is relatively uniform, generally ranging between 50 and 100. Groundwater samples classified as excellent are primarily found in the upstream area. Agricultural activities in this region can alter the hydrochemical composition of groundwater (e.g., G6), leading to higher EWQI values. The groundwater sample with the highest EWQI (G24) is located in the downstream area. This sample’s hydrochemical characteristics are marked by significantly higher concentrations of certain ions (Mg2+, Na+, K+, HCO3-, SO42- and Cl-) compared to other samples, likely due to prolonged water-rock interactions.
4.4.2. Potential health risk of dissolved harmful substances.
The non-carcinogenic health risk assessment results for the four population groups in the study area show that infants are at the highest risk, with HI values ranging from 0.08 to 1.89 and an average of 0.45 (Fig 7). Following infants are children, adult females, and adult males, with average HI values of 0.26, 0.26, and 0.24, respectively. In comparison, the average HI values for children, adult females, and adult males are relatively close, with only the HI value for infants being significantly higher. This is attributed to the immature immune system of infants, which is less tolerant to toxic substances [42,53]. Additionally, the maximum HI values for all four population groups exceed 1, indicating that some individuals in the study area are facing non-negligible non-carcinogenic risks [86].
As shown in Fig 7, NO3- poses the highest non-carcinogenic risk to all four population groups, with average HQ values of 0.43, 0.25, 0.25, and 0.23 for infants, children, adult females, and adult males, respectively. In contrast, the average HQ values for NH4+ and NO2- do not exceed 0.01 for any group, indicating that these components have a very limited impact on non-carcinogenic risks for local residents. The results are clearly visualized in Fig 7, showing that some HQ values for NO₃ ⁻ exceed the acceptable limit of 1. In summary, NO₃ ⁻ is the primary cause of increased non-carcinogenic risks for residents within the basin.
It can be seen from Fig 8, groundwater with high HI values is mainly distributed in the upstream area of the basin. This is primarily due to large-scale farms in the upstream areas, where the use of fertilizers and pesticides increases the content of specific harmful substances in groundwater, thereby posing a risk to human health. Compared to the high HI values for infants, the distribution of HI values for children, adult females, and adult males is more uniform and generally falls within the acceptable range. Intense human activities have not caused significant non-carcinogenic risks for these three groups. Furthermore, the HI for infants in some downstream areas of the basin remains high. This is due to the presence of a few scattered farmlands around these groundwater samples. Although the agricultural activities are relatively minor compared to those in the upstream area, their impact on groundwater is still significant and cannot be ignored.
(The map was created via ArcGIS 10.2 (https://www.esri.com/en-us/arcgis/products/arcgis-desktop/resources) based on the publicly available ASTER GDEMV2 data from Geospatial Data Cloud site, Computer Network Information Center, Chinese Academy of Sciences (http://www.gscloud.cn)).
5. Conclusions
The present research focuses on the natural mineral groundwater in alpine regions. The Tongde basin on Tibetan Plateau has been taken as an example to explore the hydrochemical characteristics and formation mechanisms of natural mineral groundwater in alpine basins and its suitability for sustainable development and exploitation under human activities. The main findings are as below:
- (1) Groundwater in the alpine Tongde Basin on Tibetan Plateau belongs to strontium-rich mineral water, with Sr concentration ranging from 0.29 to 2.03 mg/L, exceeding the mineral water threshold of 0.20 mg/L. Groundwater across the basin is neutral to slightly alkaline, with pH values ranging from 6.82 to 7.78 and an average of 7.78. It has a relatively large variation in salinity with TDS varying from 282.00 mg/L to 1,263.83 mg/L, but predominantly fresh with the average TDS of 456.72 mg/L and approximately 96.15% of sampled groundwaters having the TDS below 1,000 mg/L. Groundwater is featured by fresh hydrochemical facies of HCO3-Ca type throughout the basin, but also by saltier types of HCO3-Na·Ca and Cl-Na at some sporadic sites. NH4+ and NO2- exceed the Chinese drinking water permissible limits of 0.2 mg/L and 0.02 mg/L, respectively, in groundwater at 21.05% and 8% of sampling sites. Although the NO3- concentration is within the WHO-recommended drinking water limit of 50 mg/L at all sampling sites, it exceeds the natural NO3- limit of 10 mg/L at many sites, with a maximum concentration of 42.86 mg/L.
- (2) Groundwater in Tongde Basin is primarily governed by natural mechanisms of water-rock interactions. Evaporation also plays a role in shaping the groundwater hydrochemical composition at some sporadic sites, though its influence is limited. Groundwater hydrochemical components are predominantly originated from silicates weathering and cation exchange process, along with the dissolution of magnesite, pyrite, and calcite. The enriched Sr in groundwater derived from the dissolution of silicate minerals and carbonate cements in arkosic sandstone and argillaceous siltstone, with albite, calcite, and strontianite being the primary minerals involved. Agricultural practices and animal husbandry within the basin have significantly altered groundwater chemistry, introducing nitrogen contaminants in the form of NH4+, NO2- and NO3-.
- (3) Groundwater in the basin has a relatively large variation in hydrochemical quality with the EWQI value ranging from 43.58 to 170.83, but is in excellent to good quality in most sites. Only two sampled groundwaters are with the quality varying from medium (site G6) to poor (site G24) categories. Agricultural practices and the prolonged water-rock interaction are primarily responsible for these two medium to poor quality groundwaters. The nitrogen contaminants would pose potential non-carcinogenic health risk with the total non-carcinogenic health risk index (HI) beyond the permissible limit of 1 to all populations. Although levels of NH4+ and NO2- in groundwater exceed Chinese drinking water permissible limits, they generally do not pose significant health hazards to humans. However, the elevated concentrations of NO3- can pose health risks, particularly to the infants. Agricultural contaminants are a significant source of potential health risks for groundwater and must be addressed in the development of alpine basins, especially those with the endowment of mineral water resources. Precision nutrient management, grounded in optimized fertilization strategies, is essential to mitigate nitrogen leaching from agricultural non-point sources into aquifers. Concurrently, proactive surveillance of geogenic trace elements is critical for evidence-based, risk-oriented protection of drinking water quality.
Supporting information
S1 Table. The exposure parameters and RfDoral used in human health risk assessment.
The exposure factors include Exposure Frequency (EF, d/y), Exposure Duration (ED, years), Ingestion Rate (IR, L/d), Body Weight (BW, kg), Skin Surface Area (SA, cm2), Averaging Time (AT, h), Exposure Time (T, h), Event Frequency (EV, day), Volumetric Conversion Factor (CF, L/cm3), and Dermal Permeability Coefficient (Kp, cm/h). Each exposure factor lists the values for four demographic groups (Infants, Children, Adult Females, and Adult Males). The corresponding oral reference doses (RfDoral, mg/(kg × day)) for the contaminants of concern (NO3-, NO2-, NH4+) are listed alongside.
https://doi.org/10.1371/journal.pone.0331449.s001
(DOCX)
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