Exploring the Hydrogeochemical Formation and Evolution of the Karst Aquifer System in the Yufu River Based on Hydrochemistry and Isotopes

Abstract

Jinan, renowned as the “Spring City” in China, relies significantly on karst groundwater as an indispensable resource for socio-economic development, playing a crucial role in ecological regulation, tourism, and historical and cultural aspects. The Yufu River basin, situated within Jinan’s karst region, represents a vital riverine leakage zone. Therefore, investigating the evolutionary characteristics and causative mechanisms of surface water and groundwater at different aquifer levels in the Yufu River basin can provide a scientific foundation for the protection of Jinan’s springs. This study, based on hydrogeochemical and isotopic data from the river water, shallow groundwater, deep groundwater, and springs in the Yufu River basin, explored the hydrogeochemical evolution in this region. The findings revealed significant spatial variations in the hydrochemical parameters of the Yufu River basin. Groundwater received contributions from surface water, while springs represented a mixture from both surface water and various recharge aquifers. Dominant ions include Ca²⁺ and HCO₃⁻, with prevailing hydrochemical types being HCO₃·SO₄-Ca and HCO₃-Ca. Atmospheric precipitation served as the primary source of recharge for surface water and groundwater in the Yufu River basin, albeit influenced by pronounced evaporation processes. The hydrochemical composition in the Yufu River basin was primarily attributed to water–rock interactions, mainly driven by the combined effects of carbonate rock, silicate rock, and gypsum weathering and dissolution. Among these, the weathering and dissolution of carbonate rocks played a dominant role, with human activities exerting a relatively minor influence on the hydrochemistry of the Yufu River basin.

1. Introduction

Karst water constitutes a crucial water source for urban supply, industrial, and agricultural activities, playing a pivotal role in safeguarding residents’ livelihoods and supporting sustainable economic and social development [1,2,3]. Jinan, renowned as China’s “City of Springs”, boasts abundant karst underground water resources, integral to Jinan’s socio-economic development [2,3,4]. These resources hold indispensable value in regional ecological regulation, tourism, and historical and cultural domains [5]. Since the 1970s, in tandem with population growth and rapid socio-economic development, the extraction of karst underground water in Jinan has steadily increased. This has resulted in a continuous decline in the groundwater table, water quality degradation, cessation of spring flow, and a substantial impact on the distinctive landscape of the “Spring City” [5,6,7]. It was not until the early 21st century, following the widespread implementation of ecological regulatory measures, that the four major spring groups experienced a resurgence [8]. The Yufu River basin, a crucial river-type leakage zone within the Jinan Spring area, has been targeted for a comprehensive multi-source recharge initiative [9]. Leveraging the natural advantages of the Yufu River’s channel, water conditions, and groundwater infiltration, the government has undertaken extensive recharge projects in the Yufu River basin [10]. Spring water protection has remained a focal and challenging aspect of hydrogeological efforts in Jinan’s spring area. Current research predominantly focuses on hydraulic connectivity [11,12,13]. Given the significant variations in water quality from diverse sources in the Yufu River, understanding the hydrochemical characteristics and formation mechanisms of river water and groundwater post-artificial recharge holds paramount significance for the sustainable utilization and conservation of karst water resources.

Groundwater chemical composition is a crucial component of the groundwater environment, influenced not only by precipitation composition and quantity but also by aquifer minerals, geological structures, groundwater dynamics, and human activities [14]. Specifically, the primary chemical characteristics of groundwater are closely associated with the processes of dissolution and precipitation of minerals in the aquifer, and the type of water–rock interaction predominantly governs the evolution of groundwater hydrochemistry [15,16]. These interactions serve as “fingerprints” for recording the geochemical history and current status of different aquifers, crucially providing key evidence for hydrogeochemical interactions occurring during groundwater mixing processes. Additionally, variations in groundwater flow conditions and hydrochemical sources and sinks induced by human activities significantly influence groundwater chemistry [15]. Therefore, investigating the chemical characteristics and formation mechanisms of groundwater is essential for analyzing the evolutionary processes of the groundwater environment, understanding the sources of ions, groundwater circulation, water–rock interactions, and identifying the main controlling factors of groundwater chemical components [17,18,19]. Additionally, hydrogen and oxygen stable isotopes are considered conservative tracers to describe the conceptual hydrogeological model features of karst aquifers, such as determining recharge areas, estimating recharge amounts, revealing responses to rainfall events, separating groundwater flow components, and estimating aquifer storage [20,21,22,23]. Hydrogen and oxygen isotopes have been successfully applied to reveal interactions between surface water and groundwater, identify sources of springs, and elucidate relationships among rainfall, surface water, and groundwater [16]. The integrated application of stable isotopes and hydrochemistry not only helps clarify the formation mechanisms of groundwater chemistry but also provides crucial information for assessing interactions between surface water and groundwater, particularly in understanding the evolution of karst hydrological systems with significant spatial and temporal variability.

Therefore, this study collected hydrochemistry and isotope samples of surface water, groundwater of different aquifers and spring water in Yufu River basin, and analyzed the characteristics and genetic mechanism of hydrochemistry evolution, in order to solve the following problems: (i) characteristics of hydrochemistry changes in surface water, shallow groundwater, deep groundwater and spring water in Yufu River basin; (ii) isotopic variation characteristics of surface water, shallow groundwater, deep groundwater and spring water in Yufu River basin; (iii) characteristics and genetic mechanism of hydrochemical evolution in Yufu River basin. This study can provide a scientific basis for better understanding the complexity and connectivity of karst groundwater in different layers in Jinan, and provide guidance for the protection and spring preservation of karst water resources in Jinan.

2. Materials and Methods

2.1. Study Area

Jinan Spring area is located in the transition zone between the middle Shandong mountains and the sloping plain in front of the mountains. The bottom layer is a slightly northward sloping monoclinal structure, which is a typical karst development area in northern China [10,11,24]. The strata, from top to bottom, consist of the Quaternary, Ordovician Beianzhuang Formation, Donghuangshan Formation, Sanshanzi Formation, Cambrian Chaomidian Formation, Zhangxia Formation, and the Yanshanian igneous rock body [8,11,25]. The aquifer system is composed of the Cambrian Zhangxia Formation limestone, Upper Fengshan Formation, Cambrian–Ordovician Sanshanzi Formation, Chaomidian Formation, and the Ordovician aquifer rock layers. The lithology mainly includes limestone, dolomite, dolomitic limestone, argillaceous limestone, and limy dolomite [8,24,25]. Karst fractures are well developed within the spring area, facilitating good connectivity, which is advantageous for the infiltration of atmospheric precipitation and the runoff, convergence, and discharge of groundwater [8,26]. The spring city is divided from south to north into an indirect recharge zone, a direct recharge zone, and a discharge zone [9].

The Yufu River is located in the western part of the Jinan Spring area, with the Ma Shan Fault on its west side. The riverbed is covered with cobblestones and gravel, serving as the primary conduit for surface leakage replenishing groundwater [27]. The exposed strata in the Yufu River basin mainly consist of the Lower, Middle, and Upper Cambrian series of the Paleozoic era, the Lower Ordovician series, and a 10-meter-thick Quaternary sediment layer composed of sand, gravel, and cobblestones [7,10,27]. A strong leakage zone has formed in the Xikema area, where the riverbank and valley are covered with Quaternary strata, and karst fissures are well developed [7,27].

2.2. Sampling and Analysis Methods

In the period from December 2021 to January 2022, a total of 19 samples were collected in the Yufu River basin. This comprised 5 samples of surface water, 9 samples of shallow groundwater, 4 samples of deep groundwater, and 1 sample of water from the Heihu Spring. The distribution of sampling points is illustrated in Figure 1. On-site procedures involved the use of a GPS device to record the latitude and longitude coordinates of sampling points. Water levels and well diameters were measured using a water level gauge and meter, respectively.

The DDS-307A conductivity meter was employed in the field to assess temperature (T), electrical conductivity (EC), and total dissolved solids (TDS) with resolutions of 0.1 °C, 1 μS/cm, and 1 ppm, respectively. pH and oxidation reduction potential (ORP) were gauged using a pHS-3G pH meter with resolutions of 0.01 pH and 1 mV, respectively. Alkalinity was determined via titration using the Aquamerck^® universal titration kit (Newbury Township, OH, USA), with a resolution of 0.05 mmol/L, and each sample underwent titration 2–3 times for enhanced accuracy.

The analysis of cation and anion samples was conducted at the Shandong Provincial Institute of Water Science. The EDTA titration method was employed for testing Ca²⁺ and Mg²⁺; Na⁺ and K⁺ were measured using atomic absorption spectrometry; NO₃⁻, Cl⁻, and SO₄²⁻ were determined using ion chromatography, and HCO₃⁻ and CO₃²⁻ were quantified through titration. Samples for dissolved organic carbon (DOC), particulate organic carbon (POC), δ¹⁸O, and δ²H were dispatched to the Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences. DOC analysis was performed using a total organic carbon analyzer (TOC-LCPH, Shimadzu, Japan), POC measurement utilized an elemental analyzer (Vario MACRO cube, Elementar, Germany), and δ¹⁸O and δ²H assessments were conducted using an LGR liquid water isotope analyzer (Los Gatos Research, San Jose, CA, USA).

3. Results and Discussion

3.1. Hydrochemical Characteristics of Yufu River Basin

The results revealed significant variations in hydrochemical parameters in the Yufu River basin (Figure 2). The pH values ranged from 7.02 to 8.19, with an average of 7.565, indicating a moderately alkaline to weakly alkaline water. Specifically, the river water’s pH ranged from 7.91 to 8.19, with an average of 8.096, the shallow groundwater pH varied from 7.06 to 7.63, with an average of 7.334, and the deep groundwater pH fluctuated from 7.02 to 7.60, with an average of 7.365. The pH of river water was significantly higher than that of groundwater, possibly due to the utilization of dissolved inorganic carbon (DIC) by aquatic photosynthetic organisms in river water, as well as the precipitation of calcite and the degassing of CO₂, which reduced [HCO₃⁻] and led to higher dissolved carbon dioxide concentrations [28,29,30]. Similarly, variations in DO, DIC concentration, and ρCO₂ further support these two reasons. The mean DO concentrations in surface water, shallow groundwater, and deep groundwater were 5.542 mg/L, 3.679 mg/L, and 3.27 mg/L, respectively. The mean DIC concentrations were 3.98 mmol/L, 5.4 mmol/L, and 4.475 mmol/L, and the mean ρCO₂ concentrations were 1507.93 ppmv, 14,447.87 ppmv, and 10,366.35 ppmv, respectively. Surface water exhibited higher DO concentrations than groundwater, while ρCO₂ and DIC concentrations were significantly lower in surface water than in groundwater, indicating the influence of both biological processes and water–rock interactions on the hydrochemistry of the study area. Additionally, the clear interaction between surface water and groundwater in the strong leakage zone results in elevated pH and DO levels in the groundwater compared to other regions, while ρCO₂ and DIC concentrations were notably lower.

Additionally, the concentrations of DOC and POC in surface water were significantly higher than those in groundwater (Figure 2). Generally, the sources of TOC and POC can be classified into autochthonous and allochthonous components. (1) Allochthonous sources primarily originate from the surrounding land (such as plants or soil) or the upstream watershed ecosystem; (2) autochthonous sources stem from the degradation of organic organisms in the water column (including photodegradation and bacterial degradation) and the leakage of planktonic bacterial cells [31,32,33]. Typically, the origin of organic carbon can be determined by the C/N ratio of POC, where a C/N > 12 indicates allochthonous sources, and a C/N < 8 indicates autochthonous sources [34,35,36]. It can be observed that the C/N ratios of river water in the Yufu River basin range from 2.19 to 5.13, with an average of 3.49; shallow groundwater C/N ranges from 0.94 to 5.77, with an average of 3.18; deep groundwater varies from 1.03 to 2.22, with an average of 1.46; and spring water has a C/N of 5.09, all of which were less than 8 (Figure 2). Consequently, the organic carbon at the sampling points was predominantly of autochthonous sources, aligning with Zhang et al. [36]. This may be attributed to low rainfall and weak hydrodynamics during the winter, resulting in minimal allochthonous input and emphasizing autochthonous organic carbon sources in the study area [36]. Simultaneously, bacterial C/N ratios were typically around 2 to 3, while algal ratios were around 6 [35]. It is evident that only the C/N ratio of deep groundwater fell within the range of 2 to 3, indicating minimal influence from river water, while shallow groundwater and spring water were influenced by river water.

The variations in the ionic composition in the Yufu River basin are depicted in Figure 3. The average concentrations of cations and anions in the Yufu River water, groundwater, and spring water were consistent. The mean concentrations of major cations were Ca²⁺ > Na⁺ > Mg²⁺ > K⁺, while the mean concentrations of major anions were HCO₃⁻ > SO₄²⁻ > Cl⁻ > NO₃⁻. The coefficient of variation was employed to indicate the complexity of factors influencing the formation and evolution of groundwater chemical components. The variation coefficients for surface water were relatively small, with all values except for Cl⁻ (0.66) being less than 0.5. For groundwater, the coefficients were larger, particularly for SO₄²⁻ in deep groundwater, reaching a high value of 1.7. This indicated significant spatial variability and complexity in the sources of parameters in groundwater. In contrast, the spatial variability of parameters in surface water was lower, suggesting a more stable ion source and relatively minor overall spatial concentration changes.

3.2. Stable Isotope Characterization

The δ¹⁸O values of Yufu River water ranged from −8.12‰ to −7.38‰, with an average of −7.66‰ and a coefficient of variation of 0.04. The δ¹⁸O values of shallow groundwater vary from −8.43‰ to −6.49‰, with an average of −7.75‰ and a coefficient of variation of 0.10. In deep groundwater, the δ¹⁸O values ranged from −9.55‰ to −7.61‰, with an average of −8.71‰ and a coefficient of variation of 0.10. The δ¹⁸O value for spring water is −7.55‰. The δ²H values of Yufu River water ranged from −63.37‰ to −60.23‰, with an average of −62.92‰ and a coefficient of variation of 0.02. Shallow groundwater δ²H values varied from −67.71‰ to −57.96‰, with an average of −61.51‰ and a coefficient of variation of 0.06. In deep groundwater, δ²H values ranged from −74.53‰ to −63.70‰, with an average of −69.39‰ and a coefficient of variation of 0.08. The δ²H value for spring water was −62.24‰. Surface water showed more positive δ¹⁸O and δ²H values, and spatially, there was no significant difference in the δ¹⁸O and δ²H values of surface water. This was attributed to surface water being an open system, and most surface water has undergone intense evaporation [37]. Additionally, the δ¹⁸O and δ²H values of groundwater near surface water sampling points were more positive than in other areas, while deep groundwater had more negative values. This suggested that in the strong leakage zone, groundwater may undergo evaporation along the river channel or undergo a transformation between surface water and groundwater (Figure 2).

In general, δ¹⁸O and δ²H were effective tracers for identifying groundwater recharge sources [22,23,25]. As depicted in Figure 4, most samples fall below the atmospheric precipitation line (GMWL: δ²H = 8δ¹⁸O + 10) [38], indicating that atmospheric precipitation is the source of recharge for both groundwater and surface water. According to the local atmospheric precipitation line obtained by Guo et al. [39], which is δ²H = 8.26δ¹⁸O + 13.38, the stable isotope regression lines for surface water, shallow groundwater, and deep groundwater, based on the least squares method, were δ²H = 3.53δ¹⁸O − 34.44, δ²H = 4.19δ¹⁸O − 30.59, and δ²H = 5.81δ¹⁸O − 18.76, respectively (Figure 4). Surface water exhibited a smaller slope, indicating exposure to a highly evaporative environment, while the slopes of shallow and deep groundwater successively increase, suggesting a mixture with surface water or an evaporation process [37,40] (Figure 4). The intersection of the evaporation line and the atmospheric precipitation line represents the initial stable isotope composition of precipitation. Most samples have isotopes more positive than the precipitation’s isotopic composition, further confirming the occurrence of evaporation in precipitation. Springwater samples were located near surface water samples but fell on the stable isotope regression lines of shallow and deep groundwater, indicating that spring water was a mixture of surface water and different recharge aquifers (Figure 4).

3.3. Hydrochemical Origin and Evolution Process

3.3.1. Hydrogeochemical Type

The Piper trilinear diagram is widely employed in the study of groundwater hydrochemical characteristics, commonly used for the classification of groundwater hydrochemical components and the analysis of water quality evolution characteristics [41,42]. As depicted in Figure 5, sampling points for surface water, groundwater, and springs in the Yufu River basin were all situated in the upper-left region of the diamond, indicating that the concentration of alkaline earth metal ions (Ca²⁺ and Mg²⁺) was greater than that of alkali metal ions (Na⁺ and K⁺). This suggested that the chemical nature of groundwater was dominated by alkaline earth metals and weak acids. Additionally, the distribution of spring water was in the middle of the sampling points, indicating that the spring water receives contributions from multiple sources. From Figure 3 and Figure 5, the main cations and anions in the water of the Yufu River basin were Ca²⁺ and HCO₃⁻, respectively. Using the Schöeller classification method to categorize the hydrochemical types, three types were identified in the study area: HCO₃·SO₄-Ca type, HCO₃-Ca type, and SO₄-Ca type. Surface water, G4, G6, G5, G13, and the spring were all of the HCO₃·SO₄-Ca types, while the rest of the sampling points exhibited the HCO₃-Ca type. Additionally, from the distribution map of the sampling points, it is evident that G4, G6, and G5 are located near surface water, indicating that the hydrochemical type of groundwater was influenced by surface water (Figure 5).

3.3.2. Water–Rock Interaction

(1)

The Gibbs model

The Gibbs model provides a clearer understanding of the origin and evolution patterns of groundwater chemical components, characterizing whether various ions in groundwater have undergone processes such as precipitation, water–rock interaction, and evaporation concentration [42,43,44]. It analyzes the main controlling factors in the hydrochemical evolution process. The Gibbs model classifies the controlling factors of hydrochemical components into three types: precipitation control, water–rock interaction, and evaporation concentration [42,45]. According to the Gibbs model diagram (Figure 6), the water bodies in the Yufu River basin were concentrated in the water–rock control zone, with TDS concentrations ranging from 100 to 1000 mg/L. The Na⁺/Na⁺+Ca²⁺ and Cl⁺/Cl⁺+HCO₃⁻ ratios were all less than 0.5, indicating that the hydrochemical composition in the Yufu River basin was primarily influenced by water–rock interactions. This suggested that while atmospheric precipitation was the main source of surface water and groundwater in the Yufu River basin (Figure 4), water–rock interactions, ion exchange, adsorption, and other processes during runoff and infiltration reduced the impact of atmospheric precipitation on hydrochemical components.

(2)

The mineral saturation index and ion ratio

The characteristics of groundwater chemical components result from the combined influence of various factors. For instance, water–rock reactions are a crucial factor affecting the majority of groundwater characteristics and are controlled by the hydrodynamics of groundwater flow, the physicochemical environment, and the mineral types within the aquifer [14,25,46]. Therefore, the compounds and their relationships in groundwater can explain the origin of chemical components and the hydrogeochemical evolution process. The mineral saturation index describes the dissolution/precipitation tendency of minerals in groundwater [14,39]. The saturation indices (SI) for calcite, dolomite, and gypsum are shown in Figure 7. The calcite saturation index (SIc) ranged from 0.47 to 0.68 for surface water, 0.16 to 0.75 for shallow groundwater, 0.19 to 0.31 for deep groundwater, and 0.47 for spring water. Overall, calcite minerals in the Yufu River basin were generally oversaturated, indicating that groundwater has undergone sufficient reactions with carbonate rocks during the infiltration of atmospheric precipitation and runoff processes, leading to oversaturation of SIc (Figure 7a). The dolomite saturation index (SId) ranged from 0.36 to 0.78 for surface water, −1.02 to 0.94 for shallow groundwater, −0.32 to 0.20 for deep groundwater, and was 0.37 for spring water. Some groundwater samples did not reach saturation for dolomite minerals (Figure 7b). The gypsum saturation index (SIg) ranged from −1.65 to −1.28 for surface water, −1.69 to −1.38 for shallow groundwater, −1.92 to −0.02 for deep groundwater, and was −1.32 for spring water. All gypsum minerals were undersaturated (Figure 7c), which indicated that gypsum dissolution was not the main source of ions in the surface and groundwater of the Yufu River basin.

The impact of weathering from three different rock types, carbonate rocks, evaporites, and silicate rocks, on the chemical composition of groundwater can be discerned through the analysis of relationships such as HCO₃⁻/Na⁺ vs. Ca²⁺/Na⁺ and Mg²⁺/Na⁺ vs. Ca²⁺/Na⁺. As depicted in Figure 7d,e, surface water and groundwater were distributed between silicate and carbonate rocks, indicating that the ion composition in the Yufu River basin is primarily influenced by the joint weathering of carbonate and silicate rock minerals. The predominant dissolved components in rock dissolution in groundwater are Ca²⁺, Mg²⁺, HCO₃⁻, and SO₄²⁻. The weathering of calcite and dolomite is depicted in Figure 7f as 1:1 and 1:2, respectively, from which it can be seen that most of the water samples were located between 1:1 and 1:2, indicating that the water of the Yufu River was affected by a combination of both calcite and dolomite weathering, and that the water samples are closer to 1:1, and therefore, the groundwater ions are more affected by calcite. As the dissolution of gypsum and salts (e.g., NaCl, CaCl₂) does not affect alkalinity, an increase in alkalinity and the total concentration of major cations in water results mainly from the dissolution of carbonate and silicate [37,46]. However, Figure 7g shows that the ratio of total cations to bicarbonate exceeds 1:1, indicating the dissolution of gypsum in the study area. Moreover, if all Ca²⁺, Mg²⁺, and HCO₃⁻ are derived solely from the dissolution of carbonate, the relationship between Ca²⁺+Mg²⁺ and HCO₃⁻ should be close to 1:1. However, samples in the study area lie above the 1:1 line, indicating the presence of gypsum dissolution. Considering the thin layer of gypsum in the geological formation, the dissolution of gypsum needs to be considered. To further assess the influence of calcite, dolomite, and gypsum dissolution on groundwater chemistry, Ca²⁺-SO₄²⁻ was used to represent Ca²⁺ unaffected by gypsum, and Ca²⁺-0.33HCO₃⁻ represented Ca²⁺ unaffected by carbonate rocks. The effects of calcite and dolomite are indicated by 1:2 and 1:4, respectively (Figure 7j), with the majority of the samples distributed in the middle of the 1:2 line and the 1:4 line and closer to 1:2, suggesting that calcite dissolution has a greater effect on the chemical composition of the groundwater. In Figure 7k, the 1:1 line represents gypsum dissolution, and most samples deviated from this line, suggesting a minimal impact of gypsum on the hydrochemistry of the study area. Additionally, the Ca²⁺/Mg²⁺ ratio can be used to analyze the proportion of calcite and dolomite dissolution. Lines denoted as 10:1, 3:1, and 1:1 represent calcite dissolution, mixed dissolution of calcite and dolomite, and dolomite dissolution, respectively. Most water samples fell within the 10:1 and 1:1 range, closer to 3:1, indicating that the dissolution of calcite and dolomite is the primary source of Ca²⁺ in the study area.

3.4. Impact of Human Activities

The dissolution of calcite, dolomite, and gypsum, along with human activities, constitutes the primary sources of Ca²⁺ and Mg²⁺ in groundwater. SO₄²⁻ is derived from the dissolution of gypsum and human activities. Typically, the correlation between Ca²⁺ + Mg²⁺ and HCO₃⁻ + SO₄²⁻ is used to assess the impact of human activities on ions in water. If all Ca²⁺ and Mg²⁺ originate from the dissolution of calcite, dolomite, and gypsum, all samples should fall on the 1:1 line. As depicted in Figure 8a, the content of Ca²⁺ + Mg²⁺ in the study area exhibited the trend deep groundwater < surface water < shallow groundwater < spring. Most Ca²⁺ + Mg²⁺ and HCO₃⁻ + SO₄²⁻ ion relationships were distributed along the 1:1 line, indicating that Ca²⁺, Mg²⁺, HCO₃⁻, and SO₄²⁻ in the Yufu River basin primarily originated from the dissolution of carbonate minerals such as calcite and dolomite and evaporite minerals such as gypsum. Additionally, some samples deviated from the 1:1 line, suggesting human activity influence in the study area. The Na⁺/Cl⁻ ratio characterizes the enrichment of Na⁺ in groundwater. Under natural conditions, Na⁺ in groundwater primarily originates from atmospheric precipitation, the dissolution of evaporite minerals, and silicate mineral dissolution. Human activities such as industrial production and domestic sewage infiltration can also influence its content. The (K⁺ + Na⁺)/Cl⁻ ratio in the Yufu River basin was distributed both above and below the 1:1 line, indicating the influence of multiple factors such as mineral dissolution, atmospheric precipitation, and human activities. Human activities, such as those from daily life, industrial and agricultural activities, introduced SO₄²⁻ and Cl⁻ into the groundwater system. Under the strong influence of human activities, the total cation value with Cl⁻ + SO₄²⁻ approaches 1:1. However, in this study, all sampling points fell on the 1:1 line, and the correlation between NO₃⁻/Cl⁻ is not significant (r² = 0.077), indicating a minimal impact of human activities on SO₄²⁻ and Cl⁻. Studies suggested that Cl⁻ was a conservative ion in groundwater circulation, unaffected by physical, chemical, and biological processes, making it a reliable indicator of human activities [37]. The assessment of human activity impact on groundwater can be further determined by the relationships NO₃⁻/Cl⁻ and NO₃⁻/Ca²⁺ vs. SO₄²⁻/Ca²⁺. From Figure 8e,f, it can be observed that despite the relatively minor overall impact of human activities on the hydrochemistry of the study area, the infiltration of domestic sewage, among these activities, had a more significant and pronounced effect on the chemistry of the groundwater.

4. Conclusions

This study investigated the hydrochemical and isotopic characteristics of water in the Yufu River basin, Jinan, focusing on understanding the dynamics of karst groundwater across varying depths. The analysis encompassed water samples from river water, shallow groundwater, deep groundwater, and springs, revealing substantial variability in hydrochemical parameters and isotopic compositions. The study aimed to infer factors influencing hydrochemistry, providing crucial insights for a better understanding of the complexity and connectivity of karst groundwater at different depths in Jinan. The results revealed significant variations in hydrochemical parameters among river water, shallow groundwater, deep groundwater, and spring in the Yufu River basin. There was evident interaction between shallow groundwater and surface water, with the spring representing a mixture from both surface water and various recharge aquifers. The predominant ions in the hydrochemical composition of the study area were Ca²⁺ and HCO₃⁻, with the main hydrochemical types being HCO₃·SO₄-Ca and HCO₃-Ca. Stable isotopes (¹⁸O and ²H) served as effective tracers for identifying groundwater recharge sources. Atmospheric precipitation served as the primary source of recharge for surface water and groundwater in the Yufu River basin, undergoing substantial evaporation. Springwater samples were located near surface water samples but fell on the stable isotope regression lines of shallow and deep groundwater, indicating that spring water was a mixture of surface water and different recharge aquifers. Water–rock interactions, primarily dominated by carbonate weathering and dissolution, with secondary effects from silicate and gypsum weathering, controlled the hydrochemical composition. Although human activities had a relatively minor overall impact on groundwater quality, domestic sewage posed a significant potential threat.

Based on the results of this study, we recommend that future research focus on several key aspects. Further isotope studies are necessary to precisely delineate the recharge sources and mixing processes of different aquifers. The seasonal variability of hydrochemical and isotopic parameters should be investigated to understand temporal dynamics and develop effective water management strategies. Additionally, advanced isotopic analysis techniques and numerical models should be utilized to explore the interaction mechanisms between groundwater and surface water and their responses to environmental changes. These insights provide valuable guidance for the sustainable management and protection of karst water resources in the region.