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Article

Hydrochemical Characteristics and Controlling Factors of Shallow and Deep Groundwater in the Heilongdong Spring Basin, Northern China

by
Ming Gao
1,2,3,
Xiangquan Li
1,3,
Jiazhong Qian
2,
Zhenxing Wang
1,3,*,
Xinwei Hou
1,3,
Chunlei Gui
1,3,
Zhanxue Bai
1,3,
Changchang Fu
1,3,*,
Jinqiu Li
1,3 and
Xuefeng Zuo
1,3
1
Institute of Hydrogeology and Environmental Geology, Chinese Academy of Geological Sciences, Shijiazhuang 050061, China
2
School of Resources and Environmental Engineering, Hefei University of Technology, Hefei 230009, China
3
Key Laboratory of Groundwater Sciences and Engineering, Ministry of Natural Resources, Shijiazhuang 050061, China
*
Authors to whom correspondence should be addressed.
Sustainability 2023, 15(21), 15447; https://doi.org/10.3390/su152115447
Submission received: 19 September 2023 / Revised: 23 October 2023 / Accepted: 27 October 2023 / Published: 30 October 2023
(This article belongs to the Special Issue Sustainable Groundwater Management Adapted to the Global Challenges)
Browse Figures
Versions Notes

Abstract

:
Heilongdong Spring Basin (HSB) is located in a semi-arid region in northern China. In the past few decades, it has been influenced by anthropogenic activities. Currently, there is a lack of information about the impact on the hydrogeochemistry and groundwater quality of different aquifers. To address this concern, the present study used conventional hydrochemical diagrams, stable isotope analysis and multivariate statistical techniques to analyze hydrochemical characteristics and controlling factors of shallow and deep groundwater in the study area. The results showed that all groundwater samples were weakly alkaline. The shallow groundwater (SGW) was mainly composed of SO4-Ca and SO4·HCO3-Ca·Mg water types with high TDS values. However, the predominant water types of deep groundwater (DGW) were HCO3-Ca·Mg and HCO3·SO4-Ca·Mg types with relatively low TDS. The large majority of shallow groundwater had poor water quality, which was influenced by natural factors and anthropogenic activities, characterized by high concentrations of SO42−, NO3−, and Cl−. In contrast, the overall water quality of deep groundwater was good, mainly controlled by the natural background. Nonetheless, a few karst groundwater samples of DGW in runoff areas exhibited close hydraulic connections with SGW samples and presented contamination to a certain degree. Our research results provide a scientific basis for the utilization and protection of groundwater in different aquifers in northern China.

1. Introduction

Groundwater is a major component of global water resources and plays an important role in the maintenance of water and ecological cycles [1]. Due to its wide distribution, reliability and accessibility, groundwater has become an essential drinking water resource for people [2]. It provides nearly half of the agricultural irrigation water, and approximately one-third of the global population depends on groundwater as a source of drinking water [3,4,5]. It is even the only source of drinking water in some arid areas [6]. However, in recent decades, with the changes in natural conditions and the intensification of anthropogenic activities, many countries and regions are generally facing environmental problems such as groundwater decline, deterioration of water quality, damage to aquifers, and heavy metal pollution in groundwater [7,8,9]. China is facing a crisis of groundwater resources depletion and water quality deterioration [10,11]. Long-term extraction of groundwater resources has led to a significant decrease in the groundwater table and the formation of exploitation depression cones in northern China [12]. Therefore, it is necessary to strengthen research on the chemical characteristics and quality of groundwater to ensure its safety and sustainable utilization.
The chemical composition and water quality of groundwater are co-influenced by natural processes and human inputs [13,14]. Thus, the identification of hydrogeochemistry and hydrological processes is of great significance for the protection of groundwater quality [15]. Hydrochemical indicators and isotope tools can serve as effective methods for solving complex hydrogeological problems [16]. Hydrogeochemical graphs, including the Piper diagram, Durov diagram, Gibbs diagram and ionic ratios diagrams, have been widely used to study the controlling mechanisms of hydrochemical evolution [17,18,19]. Stable isotopes are very useful in providing new insights into the process of hydrogeological evolution. For instance, 2H and 18O isotopes are ideal tracers for investigating the hydrological cycle, which can be applied to determine the origin of groundwater [20,21]. Additionally, multivariate statistical methods, such as factor analysis (FA), principal component analysis (PCA), cluster analysis (CA) and discriminant analysis (DA), are widely used to analyze hydrochemical data through data reduction and classification techniques [22,23].
The Heilongdong Spring Basin (HSB) is located in a typical semi-arid region with relatively scarce water resources in northern China. Many scholars have conducted extensive research on groundwater in the region. These studies mainly focus on the hydrodynamic and hydrochemical characteristics of karst aquifers, groundwater pollution, and water resource management [24,25,26,27,28,29,30]. However, most previous studies focused in the central and eastern part of the basin, where industry, agriculture and coal mining were concentrated, lacking systematic analysis of the hydrochemical characteristics of the entire basin [31]. In addition, there was almost no information on the quality of groundwater in different aquifers throughout the entire basin. Understanding the impact of anthropogenic activities on groundwater quality in different aquifers is a prerequisite for groundwater protection and management. Therefore, it is urgent to understand the degree of impacts and controlling factors of anthropogenic activities on different types of groundwater.
Consequently, the primary objectives of this article are to: (1) clarify the hydrogeochemical and isotopic characteristics of groundwater from different depths of aquifers; and (2) reveal the natural processes and anthropogenic factors that affect the hydrochemical evolution of shallow and deep groundwater. The result is a necessary supplement for research on groundwater at different depths from aquifers in the HSB, and has a certain guiding significance for promoting the protection and utilization of local groundwater resources.

2. Study Area

2.1. Climate and Hydrology

The HSB is located in the southwest of Hebei Province, lying within 113°41′–114°20′ E and 36°12′–36°53′ N, and covering an area of 2002 km2 (Figure 1). The western part of the basin is a mountainous area, belonging to the Taihang Mountains and gradually transitioning to hills and plains towards the east. The western region is a high-altitude distribution area, with an altitude of 400–1509 m, while the elevation varies from 119 to 122 m in the eastern region [31].
The study area has a typical semi-arid climate with an annual average temperature of 14.7 °C and annual mean precipitation of 586.2 mm. Three major rivers in the area are the Zhang, Fuyang and Nanming Rivers. Specifically, the Zhang and Fuyang Rivers are perennial rivers, while the Nanming River is a seasonal river. In addition, there are two reservoirs (DWS and YC) in the study area.
The Heilongdong Springs are located at the lowest elevation of the basin, comprising more than 60 karst springs distributed within an area of 2 square kilometers. The discharge of Heilongdong Springs was approximately 7–9 m3/s before the 1980s. After the 1980s, with the intensification of industrial, coal mining and agricultural activities, groundwater extraction and mine drainage increased significantly, leading to a sharp decline in the regional groundwater table. Additionally, the springs have experienced multiple events of depletion. In recent years, with the gradual increase in the groundwater table, the measured spring flow reached 4.76 m3/s in 2019.

2.2. Geological and Hydrogeological Setting

The strata outcropping in the study area include Sinian, Cambrian, Ordovician, Carboniferous, Permian, Triassic and Quaternary in the order from old to new. The exposed areas of Carboniferous, Permian, Triassic strata are very small, mostly covered by the Quaternary strata, while the rest of the strata are well exposed. The bedrock strata strike towards the northeast, with a dip angle of 5–15° SE.
According to lithology and aquifer media features, the groundwater in the study area can be divided into loose rock pore groundwater, bedrock fissure groundwater and carbonate karst groundwater. The loose rock pore aquifer is mainly composed of Quaternary gravel, sand and clay. The bedrock fissure is dominated by sandstone, shale, and multilayer coal seams interbedded, while the carbonate karst aquifer is mainly made up of limestone, dolomite and thin layer gypsum lens. It is worth noting that karst groundwater mainly originates from the Ordovician and Cambrian aquifers, characterized by abundant and good water quality, which makes it the key groundwater type for exploitation in this area.
Naturally, the groundwater in the study area is mainly recharged by the infiltration of atmospheric precipitation and irrigation water, as well as lateral runoff from the surrounding bedrock outcrop, flowing from the western, northern, southwestern, and northeastern parts of the basin to the Heilongdong Springs. Manual extraction, spring discharge, and mine drainage are the main forms of groundwater discharge.
Coal mines are mainly distributed in the central and eastern regions. Coal seams are present in the coal-bearing strata of Permian and Carboniferous with a total thickness of 170 to 250 m. There is usually no hydraulic connection between neighboring aquifers, because of the presence of impermeable layers. However, due to the influence of coal mining and faults, hydraulic connections may occur among different aquifers. Such large-scale coal mining may have a significant impact on surrounding groundwater.

2.3. An Overview of Human Activities

The region has a long history of industrial, agricultural, and coal-mining activities. Specifically, west of the region is a mountainous area, mainly dominated by agricultural production. The central and eastern regions are densely populated urban and farming areas. In addition to agricultural activities, there are some industrial enterprises involving coal, steel, chemistry, building materials, and electric power industries. The main pollution sources of local groundwater include industrial wastewater (including coal mines and related industries), domestic sewage, and agricultural irrigation water. In general, the major pollution sources related to coal mines consist of mine water, coal-washing water, coke wastewater, and power plant wastewater. Although most of these are industrial wastewater and domestic sewage discharge after treatment, there are still some cases of discharge that do not reach the standard. A large amount of agricultural irrigation water brings pesticides and fertilizers into groundwater, exacerbating groundwater pollution.

3. Materials and Methods

3.1. Field Sampling

In this study, a total of 32 groundwater samples from wells were collected, including 11 shallow groundwater (SGW) samples composed of loose pore water and bedrock fissure water, with a well depth of 5–120 m (depth to groundwater table inferior to 50 m). All the 21 deep groundwater (DGW) samples were carbonate karst groundwater, with a well depth greater than 170 m and groundwater table burial depth > 80 m (Figure 1).
In addition, all the shallow groundwater samples (A1–A5, E1–E6) were obtained from the runoff area, where intensive mining activities were developed simultaneously with industrial and agricultural activities. Only 6 samples (C1–C6) of deep groundwater were taken from the recharge area; the other samples (C7–C21) were all collected from the runoff area.
All samples were analyzed for major ions, trace element and stable isotope (δ18O and δ2H) tests, except for 3 samples that did not pass the δ18O and δ2H isotope analyses due to damage during transportation. To obtain a representative groundwater sample, at least three well volumes of groundwater were pumped before sampling, and the sampling bottle was cleaned three times with the sample water. Then, pH, conductivity (EC), and water temperature were measured in situ using a portable multi-parameter water quality meter (HI98194, Hanna, Limena, Italy), with an accuracy of ±0.02 for pH, ±1 μS/cm for EC, and ±0.15 °C for temperature.
All samples were filtered through a 0.45 μm membrane filter, then collected in 1.5 L and 0.25 L high-density polyethylene bottles for major ions and trace elements tests, respectively. Among them, samples of trace elements were acidified with double distilled nitric acid until pH was less than 2. Then, all samples were stored at 4 °C until further analysis.

3.2. Laboratory Analyses

Laboratory testing was carried out in the Groundwater Mineral Water and Environmental Monitoring Center of the Institute of Hydrogeology and Environmental Geology, Chinese Academy of Geological Sciences. Major cations and minor elements were detected by an Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES, Optima 8000, PerkinElmer, Waltham, MA, USA; precision, ±1%). The anions SO42−, Cl−, and NO3− were measured by ion chromatography (ICS-900, Dionex, Sunnyvale, CA, USA; precision, ±1%). The HCO3− and CO32− concentrations were determined within 8 h of sampling by acid titration method, and TDS was analyzed using the gravimetric method.
The hydrogen and oxygen stable isotopes of water were analyzed using isotopic mass spectrometry (L2130-I, Picarro, Santa Clara, CA, USA). All the δ18O and δ2H values were expressed in standard δ-notation (‰) relative to Vienna Standard Mean Ocean Water (VSMOW), with a precision of ±0.1‰ for δ18O, and ±1‰ for δ2H.
In this study, Quality Control (QC) and Quality Assurance (QA) measures were implemented from sampling, transportation, storage and analytical procedure. Each groundwater sample was performed in triplicate in order to ensure data quality, samples from each batch were interspersed with standards and blanks, and all data were checked through instrument drift. The relative errors of all parameters were lower than 10%.

4. Results and Discussion

4.1. Hydrochemical Characteristics and Groundwater Quality Assessment

The hydrochemical parameters of groundwater in the study area were compared with the Standard for Groundwater Quality of China [32], and the results are shown in Table 1. Both shallow and deep groundwater were weakly alkaline. Moreover, the average TDS and TH concentrations in shallow groundwater were 2.5 and 2.2 times higher, respectively, than those in deep groundwater. In addition, 81.8% of TDS and 90.9% of TH concentrations in shallow groundwater exceeded the standard, whereas TDS concentrations in deep groundwater were within the normal range, with only 19.0% of TH concentrations exceeding the standard.
The relative abundance of major cations in shallow and deep groundwater samples was Ca2+ > Na+ > Mg2+ > K+ and Ca2+ > Mg2+ > Na+ > K+, respectively. Although the concentration of Na+ was within the normal range, the average concentration of Na+ in shallow groundwater was 3.7 times that of deep groundwater. For the major anions, the abundance of these anions in shallow groundwater and deep groundwater was arranged in the following order: SO42− > HCO3− > Cl− and HCO3− > SO42− > Cl−, respectively. The Cl− concentration of all groundwater samples was within the permissible limit, but it was significantly higher in shallow groundwater than that in deep groundwater. The concentrations of SO42− in the shallow and deep groundwater were in the ranges of 120.5–694.9 and 35.94–313.80 mg/L, with a mean value of 421.45 mg/L and 125.86 mg/L, respectively. An amount of 81.8% of the SO42− concentration exceeded the standard range in shallow groundwater, while 85.7% of the SO42− concentration was within the permissible limit in deep groundwater.
Nitrates and nitrites are naturally occurring ions in groundwater, but in most cases high concentrations are often the result of human pollution. The average concentration of NO3− was 92.97 mg/L and 23.13 mg/L in shallow and deep groundwater, respectively. Four samples in shallow groundwater exceeded Chinese standards, and all deep groundwater samples were within the range of Chinese standards. NO2− in all groundwater samples was within the permissible limit. In addition, the concentrations of I− and Pb in some shallow groundwater samples also exceeded the permissible limit, while the concentrations of F−, Fe, Mn, As, Cr6+ and Hg in all groundwater samples were lower than the Chinese standard (Table 1).
In conclusion, shallow groundwater was more vulnerable to contamination due to its shallow water table and the lack of a continuous impermeable layer. This was consistent with the fact that shallow groundwater quality was poor. In contrast, the water quality of deep aquifers was relatively good due to the presence of impermeable layers. However, deep groundwater in carbonate-exposed areas and coal-mining areas could also be contaminated, as evidenced by the high concentrations of NO3− and SO42− in some DGW samples.
The Piper diagram is frequently used for reflecting hydrochemical characteristics and groundwater types [33,34]. Figure 2 shows that all shallow groundwater samples fall into zone III of the upper diamond except one sample. In zone III, Ca2+, Mg2+ and SO42− concentrations were relatively high, indicating that these shallow groundwater samples were mainly composed of SO4−Ca and SO4·HCO3-Ca·Mg types. In contrast, most of the deep groundwater samples were distributed in Zone I, characterized by Ca2+, Mg2+ and HCO3−, showing that HCO3-Ca·Mg and HCO3·SO4-Ca·Mg were the dominant hydrochemical facies in deep groundwater.
In addition, it is worth noting that three DGW samples from the runoff area were located in Zone III, close to the SGW samples, with high concentrations of NO3− and SO42−, suggesting that there was probably a close hydraulic connection between shallow and deep aquifers in some areas. This was consistent with the actual situation, and the above three DGW samples were contaminated to varying degrees, with peculiar smell or discoloration, and were no longer used as drinking water sources by local residents.

4.2. The δ2H and δ18O Isotope Characteristics

Oxygen (δ18O) and hydrogen (δ2H) stable isotopes in water are useful tracers for identifying the origin of different groundwaters, and are widely used in the study of the hydrogeological cycle [35,36]. Nonetheless, since there was no Global Network for Isotopes in Precipitation (GNIP) in the study area, the adjacent Shijiazhuang station was selected.
Table 2 showed that the δ18O and δ2H values of shallow groundwater ranged from −8.8‰ to −7.7‰ and from −65.0‰ to −57.0‰, with mean values of −8.4‰ and −61.7‰, respectively. Meanwhile, the δ18O and δ2H values for deep groundwater from the recharge area (DGW1) varied from −9.5‰ to −9.2‰ and from −68.0‰ to −65.0‰, with mean values of −9.4‰ and −67.4‰, respectively. Similarly, deep groundwater from the runoff area (DGW2) ranged from −9.6‰ to −8.6‰ and from −69.0‰ to −64.0‰, with mean values of −9.3‰ and −67.5‰, respectively. Compared to deep groundwater, shallow groundwater was more enriched in heavy isotopes, indicating that shallow groundwater presumably suffered intensive evaporation processes. However, there was little difference in oxygen and hydrogen isotopes between the deep groundwater in the recharge and runoff areas.
Figure 3 showed that all the samples were close to GMWL and LMWL, suggesting that both shallow and deep groundwater originated from meteoric water. In addition, most of the DGWs were plotted in the lower left of LMWL and were relatively concentrated with relatively negative δ18O and δ2H values, implying that most DGWs were recharged from cold and humid climates or high-altitude areas [16]. SGWs were mainly distributed in the bottom right of LMWL, indicating that the initial recharge sources were mainly meteoric water. Due to the strong evaporation effect, the measured δ18O and δ2H values were relatively heavy.
It is worth noting that some DGWs and SGWs had similar δ18O and δ2H values. Further analysis indicated that these DGWs were located in runoff areas with coal mine distribution. Deep groundwater might flow upwards into shallow aquifers through faults and fractures to achieve hydraulic connections between different aquifers.

4.3. Multivariate Statistical Analysis

Correlation analysis can reveal the sources and affecting factors of hydrochemical parameters in groundwater [37,38]. Pearson correlation coefficients of different indicators in shallow and deep groundwater were calculated using SPSS 21.0 software, and the results are shown in Table 3.
In shallow groundwater, TDS had a significant positive correlation (r ≥ 0.80, p = 0.01) with Ca2+, Mg2+, and SO42− showing the above ions were the major factors affecting TDS value in shallow groundwater. In deep groundwater, TDS showed a high positive correlation (r ≥ 0.80, p = 0.01) with Ca2+, Mg2+, SO42−, HCO3− and Cl−. Compared with shallow groundwater, the TDS content in deep groundwater was affected by the dissolution of carbonate rock and rock salt.
In addition, the correlations of NO3− and Cl− were also significant in shallow groundwater, with a correlation coefficient of 0.85 (p = 0.01). However, there was no correlation between NO3− and other ions in deep groundwater. To sum up, the nitrate concentration was relatively high and showed a significant positive correlation with Cl− in shallow groundwater, suggesting that the excessive nitrate in shallow groundwater might be related to various anthropogenic activities such as domestic pollution and agricultural irrigation.
Principal component analysis (PCA) is a common statistical method that can be used to compress a large dataset of variables into a small number of unrelated variables. It is broadly used in the study of hydrochemical evolution characteristics [39,40].
In the present study, all the shallow and deep groundwater samples were subjected to PCA of 10 hydrochemical parameters (pH, Na+, K+, Ca2+, Mg2+, HCO3−, SO42−, Cl−, NO3− and TDS). Prior to PCA, the original data were standardized to eliminate the influence of data dimension and order of magnitude. Finally, two principal components were obtained using the Kaiser criterion and varimax rotation method; the results are shown in Table 4 and Figure 4.
For shallow groundwater, the cumulative variance of the two principal components accounted for 76.87% of the total variance, with a weight of 52.17% for PC1 and 24.7% for PC2. The high positive loadings in PC1 were TDS, Ca2+, Mg2+, SO42− and HCO3−, indicating that the dissolution of these ions under natural conditions promoted an increase in TDS content. NO3−, Cl−, and Na+ exhibited relatively strong loadings in PC2, and the high loadings of NO3− and Cl− were considered indicators of pollution sources related to anthropogenic activities.
For deep groundwater, PC1 showed 67.06% of the total variance, with an eigenvalue of 6.71, and was characterized by high positive loadings in Ca2+, Mg2+, Na+, SO42−, Cl−, HCO3− and TDS. This indicated that there were natural processes such as the dissolution of carbonate, gypsum and halite minerals in the deep groundwater of the study area. PC2 represented 15.61% of the total variation, with an eigenvalue of 1.56, and had negative loadings with NO3− and positive loadings with K+ and pH. Possible sources of K+ include natural mineral dissolution and anthropogenic pollution [41]. Similarly, the NO3− content in groundwater reflects the anthropogenic input of pollutants [42].
In summary, whether it was shallow or deep groundwater, the hydrochemical composition of groundwater was influenced by both natural conditions and human pollution. Moreover, shallow groundwater was more severely affected by anthropogenic activities. Compared to deep groundwater, the weight and eigenvalues of PC2 in shallow groundwater were significantly greater.

4.4. Natural Factors Affecting Groundwater Chemistry

4.4.1. The Origin of Groundwater in Different Aquifers

Generally, solutes in groundwater are controlled by various hydrogeochemical factors, such as the interaction between water and rocks, the hydrochemical composition of the recharge source, and mixing and evaporation processes that occur in aquifers [43]. The Gibbs diagram is an effective tool to determine the origin of hydrogeochemistry. According to the relationship between ρ(Na+)/ρ(Na+ + Ca2+), ρ(Cl−)/ρ(Cl− + HCO3−) and TDS, the dominant factors affecting the hydrochemical composition of groundwater under natural conditions can be qualitatively judged [44].
Figure 5 shows that all the deep groundwater samples fell into the rock dominance zone, indicating that the interaction between water and rocks was the dominant factor controlling the chemistry of deep groundwater. The shallow groundwater samples in the study area were influenced by both rock-weathering and evaporation. Due to the shallow groundwater table and strong evaporation, the values of ρ(Na+)/ρ(Na+ + Ca2+) and ρ(Cl−)/ρ(Cl− + HCO3−) were relatively high in shallow groundwater.
In addition, compared with deep groundwater samples from the recharge areas (DGW1), the values of ρ(Na+)/ρ(Na+ + Ca2+) and ρ(Cl−)/ρ(Cl− + HCO3−) in deep groundwater samples from the runoff areas (DGW2) significantly showed a gradually increasing trend with an increase in TDS. Moreover, some deep groundwater was close to shallow groundwater samples, indicating that in addition to water–rock interactions, a small amount of deep groundwater samples from runoff areas might also be affected by other factors.

4.4.2. Quantitative Relationships between Major Ions

The ion ratio relationship can further reveal the hydrogeochemical process of water–rock interactions. Previous studies have shown that if the dissolution of carbonate and gypsum is the major chemical reaction in the groundwater, the ratio of (HCO3− + SO42−) to (Ca2+ + Mg) will follow the 1:1 trend line. As shown in Figure 6a, almost all DGW1 and DGW2 samples were along the 1:1 line, indicating that the weathering of carbonate and gypsum affected the deep groundwater. In contrast, most of the SGW samples fell below the 1:1 line due to an excess of (Ca2+ + Mg2+) over (HCO3− + SO42−), which might be attributed to silicate weathering and/or reverse cation exchange.
The Ca2+ and Mg2+ in groundwater were mainly derived from the dissolution of calcite and dolomite. Figure 6b shows the milligram equivalent relationships between Ca2+ and Mg2+. It can be seen that all deep groundwater samples were located between the 1:2 and 1:3 lines, that is, 1/3 < γMg2+/γCa2+ < 1/2, indicating that the Ca2+ in deep groundwater mainly comes from the dissolution of calcite. Meanwhile, the vast majority of shallow groundwater samples deviated significantly from the 1:3 line, with a higher Ca2+ content (Ca2+ > Mg2+), indicating that the concentration of calcium ions in shallow groundwater was not only controlled by the dissolution of calcite, but also influenced by silicate weathering or anthropogenic activities.
The relative contribution of different weathering processes to the hydrogeochemical evolution of groundwater can be represented by the ratio of HCO3−/Na+ and Ca2+/Na+ [45,46]. It can be seen from Figure 6c that the dissolution of carbonate rocks was the predominant process affecting deep groundwater chemistry in the recharge area (DGW1). Shallow groundwater (SGW) was mainly controlled by silicate weathering, while deep groundwater in the runoff zone (DGW2) was jointly influenced by both carbonate rocks dissolution and silicate weathering. The result is consistent with Figure 6a,b, which further illustrates that silicate weathering was involved in the hydrogeochemical evolution of groundwater in the region.
The plot of (Na+ + K+−Cl− − NO3−) and (Ca2+ + Mg2+ − HCO3− − SO42−) reflects the possibility of cation exchange occurring in groundwater. If the ratio is close to 1 or −1, it means that significant cation exchange may have occurred in the groundwater. As shown in Figure 6d, most of the groundwater samples followed the −1:1 line suggesting the presence of cation exchange in both deep and shallow groundwater.

4.5. Effects of Anthropogenic Factors

Nitrate content is commonly used to evaluate the impact of anthropogenic activities on groundwater. Generally, nitrate concentrations remain very low, usually less than 10 mg/L under natural environmental conditions [47]. Compared to natural factors, the high concentrations of NO3−, Cl−, and SO42− in groundwater are often closely related to anthropogenic activities such as fertilizer use, industrial wastewater and domestic sewage discharge [48,49,50].
Figure 7a shows that 72.7% of SGW samples and 6.7% of DGW2 samples exceeded the nitrate concentration permissible limit (50 mg/L) proposed by the WHO, and the nitrate concentration of the DGW1 sample was within the normal range. This suggests that shallow groundwater was more vulnerable to contamination. In addition, most of the SGW samples were characterized by fairly high TDS and Cl− values, and NO3− showed a significant correlation with TDS and Cl− in shallow groundwater, indicating that NO3− and Cl− might be related to the same anthropogenic sources (Figure 7b,c).
Theoretically, an area with shallower groundwater table is more susceptible to evaporation. As shown in Figure 7d,e, Cl−, NO3− and δ18O showed a positive correlation in shallow groundwater. In contrast, there was no significant correlation between Cl−, NO3− and δ18O in deep groundwater. This was consistent with the actual situation. The shallow aquifer is mainly composed of gravel, sand, and weathered sandstone, with good permeability and strong evaporation. Therefore, it is also more susceptible to anthropogenic activities, characterized by high concentrations of NO3− and Cl− in shallow groundwater.
The NO3−/Cl−~Cl− ratio can further illustrate the source of NO3− in groundwater [51]. As shown in Figure 7f, most of the DGW and SGW samples were mainly distributed between domestic/municipal inputs and agricultural inputs. Additionally, the concentration of Cl− in shallow groundwater was significantly higher than that in deep groundwater. It was noteworthy that DGW1 samples were closer to agricultural inputs, while DGW2 and SGW samples mainly fall in the dominance of domestic/municipal inputs. The results are consistent with the actual situation. DGW1 samples were mainly collected from agricultural areas, with the most likely pollutants originating from fertilizers and pesticides. In contrast, DWG2 and SGW samples were mainly taken from industrial and intensive human activity areas, which implies that the discharge of industrial wastewater and domestic sewage might cause nitrate enrichment in groundwater in the area.

5. Conclusions

This present study used conventional hydrochemical, isotopic, and multivariate statistical methods to analyze the hydrochemical characteristics and control factors of shallow and deep groundwater in the study area. The main conclusions were derived as follows:
(1)
The groundwater in different aquifers was weakly alkaline. Compared to deep groundwater, the concentrations of TDS, TH and major ions were higher, and the hydrochemical types more diverse in shallow groundwater.
(2)
Both shallow and deep groundwater originated from meteoric water. However, shallow groundwater probably underwent strong evaporation, manifested as being richer in heavy isotopes than deep groundwater. In addition, the results of stable isotopes (δ18O and δ2H) indicated that there was a certain hydraulic connection between a small portion of deep groundwater and shallow groundwater in the runoff areas.
(3)
Whether shallow or deep groundwater, the hydrogeochemical composition was influenced by both natural and anthropogenic factors. In addition, shallow groundwater was more severely affected by anthropogenic activities, characterized by high concentrations of SO42−, NO3−, and Cl−, mainly attributed to the use of agricultural fertilizers and pesticides, as well as the discharge of industrial wastewater and domestic sewage.
(4)
Given the increasingly serious problem of groundwater pollution in this region, government agencies should adopt some effective measures to protect local groundwater quality, including reducing the use of agricultural fertilizers and pesticides, strengthening industrial wastewater and domestic sewage discharge monitoring, paying attention to the impact of coal mining and faults on aquifers, and establishing groundwater quality monitoring networks. In addition, it is recommended that further studies are required to identify the hydraulic connections between different aquifers and the sources of pollutants through isotope-tracing experiments in the future.

Author Contributions

Conceptualization, M.G. and X.L.; methodology, M.G.; software, M.G. and C.F.; validation, M.G. and C.F.; formal analysis, M.G. and Z.W.; investigation, X.H., C.G., Z.B., J.L. and X.Z.; resources, Z.W.; data curation, Z.W.; writing—original draft preparation, M.G.; writing—review and editing, M.G. and C.F.; visualization, M.G.; supervision, X.L. and J.Q.; project administration, Z.W.; funding acquisition, Z.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by China Geological Survey’s project (Grant Nos. DD20190252, DD20221812, DD20230539), the National Key Research and Development Project of China (Grant No. 2022YFC3003301) and the Fundamental Research Funds for the Chinese Academy of Geosciences (No. SK202310).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The datasets generated and/or analyzed during the current study are not publicly available.

Acknowledgments

We are grateful to the editor and anonymous reviewers for their insightful comments and suggestions.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. The hydrogeological sketch of Heilongdong Spring Basin.
Figure 1. The hydrogeological sketch of Heilongdong Spring Basin.
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Figure 2. Piper diagram of groundwater samples in the study area.
Figure 2. Piper diagram of groundwater samples in the study area.
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Figure 3. Plots of δ18O versus δ2H for all groundwater samples.
Figure 3. Plots of δ18O versus δ2H for all groundwater samples.
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Figure 4. Plot of principal component variables for shallow groundwater (a) and deep groundwater (b) in the study area.
Figure 4. Plot of principal component variables for shallow groundwater (a) and deep groundwater (b) in the study area.
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Figure 5. Gibbs diagram of shallow and deep groundwater in the study area: (a) TDS vs. Na+/(Na+ + Ca2+); (b) TDS vs. Cl−/(Cl− + HCO3−).
Figure 5. Gibbs diagram of shallow and deep groundwater in the study area: (a) TDS vs. Na+/(Na+ + Ca2+); (b) TDS vs. Cl−/(Cl− + HCO3−).
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Figure 6. Relationships among major ions: (a) (HCO3− + SO42−) and (Ca2+ + Mg2+), (b) Mg2+ and Ca2+, (c) HCO3−/Na+ and Ca2+/Na+, (d) (Na+ + K+ − Cl− − NO3−) and (Ca2+ + Mg2+ − HCO3− − SO42−).
Figure 6. Relationships among major ions: (a) (HCO3− + SO42−) and (Ca2+ + Mg2+), (b) Mg2+ and Ca2+, (c) HCO3−/Na+ and Ca2+/Na+, (d) (Na+ + K+ − Cl− − NO3−) and (Ca2+ + Mg2+ − HCO3− − SO42−).
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Figure 7. Scatter plots of (a) well depth versus NO3−, (b) NO3− versus TDS, (c) NO3− versus Cl−, (d) δ18O versus NO3−, (e) δ18O versus Cl−, and (f) NO3−/Cl− versus NO3−.
Figure 7. Scatter plots of (a) well depth versus NO3−, (b) NO3− versus TDS, (c) NO3− versus Cl−, (d) δ18O versus NO3−, (e) δ18O versus Cl−, and (f) NO3−/Cl− versus NO3−.
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Table 1. Comparative analysis of parameters in shallow and deep groundwater with Chinese standards.
Table 1. Comparative analysis of parameters in shallow and deep groundwater with Chinese standards.
ChineseSGW (N = 11)DGW (N = 21)
StandardRangeMeanNECSRangeMeanNECS
pH6.5–8.57.21–7.627.36 07.30–7.747.520
TDS1000563.4–16471167.40 9301.5–798.5466.850
TH450361.3–1187815.45 10250.2–594.0366.234
K+-0.26–2.561.30 -0.92–3.701.63-
Na+20032.53–120.4060.35 05.12–57.7516.320
Ca2+-112.2–358.2254.81 -70.44–170.20103.93-
Mg2+-19.76–71.0743.53 -16.44–46.6925.96-
Cl−25056.1–177.595.94 05.59–54.9725.610
SO42−250120.5–694.9421.45 935.94–313.80125.863
HCO3−-227.3–402.7315.53 -225.8–335.0264.57-
Fe0.3<0.01–0.049-0<0.01–0.14-0
Mn0.1<0.001–0.002-0<0.001–0.02-0
F−10.25–0.500.41 00.21–0.470.290
NO3−-N2012.09–198.0092.97 41.25–64.6223.130
NO2−-N1<0.002–1.0-0<0.002–0.02-0
I−0.08<0.02–0.13-2<0.02-0
Pb0.01<0.001–0.114-2<0.001-0
As0.01<0.001-0<0.001-0
Cr6+0.05<0.004-0<0.004-0
Hg0.001<0.0001-0<0.0001-0
Note: NECS refers to the numbers exceeding the Chinese standards.
Table 2. δ18O and δ2H isotope composition of groundwater samples collected in the study area.
Table 2. δ18O and δ2H isotope composition of groundwater samples collected in the study area.
SGW (n = 11)DGW1 (n = 5)DGW2 (n = 13)
RangeMeanRangeMeanRangeMean
δ18O (‰)−8.8~−7.7−8.4−9.5~−9.2−9.4−9.6~−8.6−9.3
δ2H (‰)−65.0~−57.0−61.7−68.0~−65.0−67.4−69.0~−64.0−67.5
Table 3. Correlation coefficient matrix of groundwater hydrochemical parameters.
Table 3. Correlation coefficient matrix of groundwater hydrochemical parameters.
pHNa+K+Ca2+ Mg2+HCO3−SO42−Cl−NO3−TDS
Shallow
groundwater		pH1.00 −0.28 0.01 −0.76 **−0.47 −0.47 −0.55 −0.40 −0.34 −0.70 *
Na+ 1.00 −0.35 0.55 0.38 0.13 0.47 0.66 *0.64 *0.67 *
K+ 1.00 0.01 0.42 0.35 0.06 −0.28 −0.16 0.01
Ca2+ 1.00 −0.77 **0.64 *0.89 **0.44 0.33 0.98 **
Mg2+ 1.00 0.65 *0.87 **0.18 0.08 0.83 **
HCO3− 1.00 0.68 *−0.02 −0.25 0.60
SO42− 1.00 0.11 0.01 0.90 **
Cl− 1.00 0.85 **0.47
NO3− 1.00 0.38
TDS 1.00
Deep
groundwater		pH1.00 −0.31 0.13 −0.43 *−0.39 −0.33 −0.42 −0.40 −0.16 −0.44 *
Na+ 1.00 0.66 **0.74 **0.82 **0.58 **0.88 **0.91 **−0.38 0.86 *
K+ 1.00 0.28 0.53 *0.26 0.50 *0.43 −0.48 *0.44 *
Ca2+ 1.00 0.89 **0.82 **0.95 **0.88 **−0.10 0.97 **
Mg2+ 1.00 0.89 **0.94 **0.87 **−0.41 0.94 **
HCO3− 1.00 0.78 **0.67 **−0.39 0.81 **
SO42− 1.00 0.93 **−0.27 0.99 **
Cl− 1.00 −0.22 0.94 **
NO3− 1.00 −0.20
TDS 1.00
Note: ** Correlation is significant at the 0.01 level. * Correlation is significant at the 0.05 level.
Table 4. Results of PCA for shallow and deep groundwater in the study area.
Table 4. Results of PCA for shallow and deep groundwater in the study area.
VariablesSGWDGW
PC1PC2PC1PC2
pH−0.74 −0.02 −0.43 0.66
Na+0.67 0.54 0.90 0.19
K+0.04 −0.62 0.53 0.67
Ca2+0.97 −0.06 0.93 −0.26
Mg2+0.83 −0.39 0.97 0.05
HCO3−0.64 −0.59 0.83 −0.05
SO42−0.88 −0.33 0.99 −0.06
Cl−0.52 0.74 0.94 −0.08
NO3−0.41 0.79 −0.35 −0.74
TDS0.99 −0.01 0.98 −0.13
Initial Eigen values5.22 2.47 6.71 1.56
% of variance52.17 24.70 67.06 15.61
Cumulative % of Variance52.17 76.87 67.06 82.67
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Gao, M.; Li, X.; Qian, J.; Wang, Z.; Hou, X.; Gui, C.; Bai, Z.; Fu, C.; Li, J.; Zuo, X. Hydrochemical Characteristics and Controlling Factors of Shallow and Deep Groundwater in the Heilongdong Spring Basin, Northern China. Sustainability 2023, 15, 15447. https://doi.org/10.3390/su152115447

AMA Style

Gao M, Li X, Qian J, Wang Z, Hou X, Gui C, Bai Z, Fu C, Li J, Zuo X. Hydrochemical Characteristics and Controlling Factors of Shallow and Deep Groundwater in the Heilongdong Spring Basin, Northern China. Sustainability. 2023; 15(21):15447. https://doi.org/10.3390/su152115447

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Gao, Ming, Xiangquan Li, Jiazhong Qian, Zhenxing Wang, Xinwei Hou, Chunlei Gui, Zhanxue Bai, Changchang Fu, Jinqiu Li, and Xuefeng Zuo. 2023. "Hydrochemical Characteristics and Controlling Factors of Shallow and Deep Groundwater in the Heilongdong Spring Basin, Northern China" Sustainability 15, no. 21: 15447. https://doi.org/10.3390/su152115447

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