Hydrochemical Formation Mechanisms and Source Apportionment in Multi-Aquifer Systems of Coastal Cities: A Case Study of Qingdao City, China

Abstract

This study systematically unravels the hydrochemical evolution mechanisms and driving forces in multi-aquifer systems of Qingdao, a coastal economic hub. Integrated hydrochemical analysis of porous, fissured, and karst water, combined with PHREEQC modeling and Positive Matrix Factorization (PMF), deciphers water–rock interactions and anthropogenic perturbations. Groundwater exhibits weak alkalinity (pH 7.2–8.4), with porous aquifers showing markedly higher TDS (161.1–8203.5 mg/L) than fissured (147.7–1224.8 mg/L) and karst systems (361.1–4551.5 mg/L). Spatial heterogeneity reveals progressive hydrochemical transitions (HCO₃-Ca → SO₄-Ca·Mg → Cl-Na) in porous aquifers across the Dagu River Basin. While carbonate (calcite) and silicate weathering govern natural hydrochemistry, evaporite dissolution and seawater intrusion drive severe groundwater salinization in the western Pingdu City and the Dagu River Estuary (localized TDS up to 8203.5 mg/L). PMF source apportionment identifies acid deposition-enhanced dissolution of carbonate/silicate minerals, with nitrate contamination predominantly sourced from agricultural runoff and domestic sewage. Landfill leachate exerts pronounced impacts in Laixi and adjacent regions. This study offering actionable strategies for salinity mitigation and contaminant source regulation, thereby providing a scientific framework for sustainable groundwater management in rapidly urbanizing coastal zones.

1. Introduction

As a critical freshwater resource, groundwater quality and sustainability are directly linked to regional ecological security and socio-economic development. In recent years, accelerated urbanization and industrialization have intensified anthropogenic impacts on groundwater hydrochemistry, particularly in coastal areas where seawater intrusion and complex geological settings synergistically drive significant spatial heterogeneity and complexity in groundwater chemical composition [1]. Existing studies indicate that groundwater chemical evolution is jointly controlled by natural processes (e.g., water–rock interactions, cation exchange, evaporation–concentration) and anthropogenic activities (e.g., agricultural fertilization, sewage discharge, industrial pollution) [2]. However, current research predominantly focuses on single driving factors, with limited systematic analysis of multi-factor interactions, especially regarding the differential responses of multi-aquifer systems (porous, fractured, and karst aquifers) in coastal cities [3].

As a coastal economic hub on China’s Jiaodong Peninsula, Qingdao exemplifies these challenges. Its groundwater system integrates natural geogenic signatures with multi-stress anthropogenic inputs. While the widespread karst landscapes and granite weathering in the Jiaodong Peninsula provide abundant Ca²⁺, Mg²⁺, and HCO₃⁻, forming a natural HCO₃-Ca-type hydrochemical signature [1], intensive agriculture in the Dagu River Basin introduces nitrate pollution (NO₃⁻ > 50 mg/L in localized areas) [4], industrial zones along Jiaozhou Bay discharge heavy metals [5], and over-extraction-induced seawater intrusion (marked by elevated Cl⁻ and Na⁺ concentrations) [6]. Despite these documented pressures, prior research has focused narrowly on single aquifer types or pollution sources, failing to address how lithologically distinct aquifers modulate the spatial divergence of multi-factor interactions—a critical gap for managing this interconnected resource [7,8,9,10].

To address these limitations, this study advances a novel comparative framework to unravel aquifer-specific hydrochemical controls in coastal multi-aquifer systems. We hypothesize that (1) porous aquifers exhibit salinization dominated by seawater intrusion and evaporative concentration; (2) fractured aquifers demonstrate heightened vulnerability to anthropogenic pollutants through preferential flow paths; (3) karst aquifers maintain hydrochemical homogeneity via carbonate weathering dominance. Through systematic sampling integrated with PHREEQC inverse modeling and PMF receptor modeling, we quantify the contribution ratios of natural processes (carbonate/silicate weathering) versus anthropogenic sources (agricultural/industrial/sewage inputs), while elucidating aquifer-specific mechanisms controlling contaminant migration and natural attenuation.

2. Materials and Methods

2.1. Study Area

Qingdao City (Figure 1) is situated in the southeastern part of the Shandong Peninsula, China, bordering the Yellow Sea to the east. Characterized by a temperate monsoon climate with maritime influences, the region experiences four distinct seasons, moderate humidity, and an annual precipitation of approximately 662.1 mm. The Dagu River, the longest inland watercourse in Qingdao, spans a vast catchment area along a north–south axis, serving as a critical ecological corridor and primary water resource for the region. Geologically, Qingdao lies at the intersection of the Jiaonan Uplift (northeastern margin) and the Jiaolai Depression (central-southern section) within the Neocathaysian tectonic system, belonging to the Ludong Uplift Belt of the Sino-Korean Paraplatform. NE-trending faults dominate regional magmatic activities and tectonic evolution. Cenozoic tectonic movements have shaped a horst-graben landscape, exhibiting a “U-shaped” topography with elevated eastern and western margins flanking a central depression. Stratigraphically, Paleozoic strata are absent, while Mesozoic Cretaceous volcanic rocks prevail, overlain by Late Yanshanian Laoshan-type granites [11]. The structural stability and permeable lithology provide optimal hydrogeological conditions for groundwater storage and exploitation [12].

The groundwater system in Qingdao demonstrates pronounced spatial heterogeneity (Figure 1). Quaternary porous aquifers are predominantly distributed in the Dagu River Basin, covering Pingdu City, southern Laixi, and western Jimo District. Specific yields exceed 3000 m³/day in southern and northern Pingdu, constituting the primary extraction zones, whereas yields decline below 500 m³/day in central Pingdu, eastern Laixi, and northern Jimo. Fractured groundwater systems are extensively developed in hilly terrains of northeastern Pingdu, northern Laixi, Jimo, Laoshan, and Huangdao Districts, controlled by fault networks. Their productivity varies significantly, with specific yields ranging from 100–500 m³/day in northeastern Pingdu, southwestern Laixi, and southern Huangdao to less than 100 m³/day in parts of Jimo. Karst aquifers are locally confined to carbonate formations in Laixi and Pingdu, exhibiting high yields (exceeding 500 m³/day) but challenging exploitation conditions. Notably, groundwater discharge zones in southwestern Pingdu (characterized by a cone of depression) and the Dagu River estuary suffer severe salinization due to seawater intrusion.

2.2. Sample Collection and Testing Analysis

This study systematically established 62 monitoring points (36 porous water, 20 fractured water, and 6 karst water) across Qingdao’s plain area, considering aquifer type distribution, township demographics, and geomorphic zoning (Figure 1). In situ measurements of temperature, pH, and total dissolved solids (TDSs) were performed using a multiparameter water quality probe (SMARTROLL; accuracy: pH ± 0.01, TDS ± 1%). Filtered samples (0.45 μm Whatman GF/C membrane) were acidified to pH < 2 with ultrapure nitric acid, stored in pre-cleaned polyethylene bottles (electrical conductivity < 2 μS/cm), and transported at 4 °C to Qingdao Geological Survey Laboratory. Analyses followed China’s Groundwater Quality Standard (GB/T 14848-2017) [13]: Ca²⁺, Mg²⁺, Na⁺, and K⁺ were determined by flame atomic absorption spectroscopy (contrAA300, Jena; detection limit 0.01 mg/L); HCO₃⁻ was measured via hydrochloric acid titration (±0.1 mmol/L accuracy); Cl⁻, NO₃⁻, and SO₄²⁻ were analyzed by ion chromatography (IC883, Metrohm; Metrosep A Supp 5 column, eluent 2.0 mM Na₂CO₃/0.75 mM NaHCO₃, flow rate 0.7 mL/min). Charge balance errors (CBEs) were maintained within ±5%, with outliers reanalyzed.

2.3. Research Methods

Hydrochemical datasets were statistically analyzed (mean, standard deviation, coefficient of variation) and classified using Durov diagrams. Ionic ratios (e.g., Ca²⁺ + Mg²⁺ vs. HCO₃⁻ + SO₄²⁻) and Gibbs plots qualitatively identified dominant processes (evaporation, rock weathering, cation exchange). Hydrogeochemical evolution mechanisms were deciphered by integrating Cl⁻/NO₃⁻ background values, groundwater level monitoring (pressure transducers, ±0.1 m accuracy). Spatial distribution maps of hydrochemical parameters were generated using interpolation analysis in ArcGIS 10.8. Mineral saturation indices (SI) were computed using PHREEQC 3.7 (USGS geochemical modeling software, phreeqc.dat database, temperature set to mean ± 2 °C). Positive Matrix Factorization (EPA PMF 5.0) was implemented with input parameters including eight ion concentrations (Ca²⁺, Mg²⁺, Na⁺, K⁺, HCO₃⁻, SO₄²⁻, Cl⁻, NO₃⁻) and uncertainties (instrumental error +10% detection limit). Factor numbers were optimized via Q-minimization and residual analysis. Ion ratio diagrams and model outputs were visualized in Origin 2023, with statistical significance at p < 0.05.

3. Results and Discussion

3.1. Hydrochemical Characteristics of Groundwater

3.1.1. Characteristics of the Hydrochemical Composition of Groundwater

Statistical results of key hydrochemical parameters for porous, fractured, and karst groundwater are summarized in

Table 1. Mathematical statistics of hydrochemical characteristics of groundwater in the study area.

Classification	Project	pH	TDS	K+	Na+	Ca2+	Mg2+	Cl−	SO42−	HCO3−	NO3−-N
Pore water n = 36	Max	8.4	8203.5	49.6	1805.1	769.2	342.7	4049.7	1089.2	842.2	101.7
	Min	6.9	161.1	0.3	12.6	31.7	4.7	3.3	21.1	23.2	0.1
	Mean	7.7	716.6	4.1	69.9	134.4	27.7	123.0	122.4	256.1	23.2
	Std	0.4	1660.9	10.5	413.3	138.9	77.1	826.3	245.2	167.5	21.9
	CV	0.06	1.29	1.6	1.99	0.82	1.41	2.08	1.16	0.58	1.06
Fracture water = 20	Max	8.3	1224.8	7.1	139.3	261.1	50.7	186.1	221.7	444.3	70.1
	Min	7.0	147.7	0.5	16.7	14.7	0.7	19.3	30.0	0.0	1.4
	Mean	7.6	562.2	2.9	61.4	99.7	18.6	90.5	106.7	167.0	20.4
	Std	0.7	275.2	1.8	33.4	65.6	12.7	50.8	55.3	126.5	19.2
	CV	0.1	0.49	0.61	0.54	0.66	0.66	0.56	0.52	0.76	0.94
Karst water = 6	Max	7.7	4551.5	30.3	84.6	1143.5	220.4	1934.1	382.2	475.5	121.0
	Min	7.1	361.1	0.4	14.6	83.3	13.9	27.5	94.9	142.6	12.1
	Mean	7.5	743.5	10.1	45.9	314.9	68.2	392.5	182.5	335.3	44.7
	Std	0.2	1431.3	10.1	20.9	372.0	69.4	690.6	97.8	117.8	35.4
	CV	0.02	1.04	1.00	0.46	1.18	1.02	1.76	0.54	0.35	0.79

Max represents the maximum value; represents the standard deviation; CV represents the coefficient of variation; pH is dimensionless, and the units for other components are mg/L.

. The pH of groundwater ranges from 6.9 to 7.4, indicating weakly alkaline conditions (mean 7.5–7.7). Porous water exhibits a mean total dissolved solids (TDS) of 716.6 mg/L, with 61.11% of samples classified as freshwater (TDS < 1000 mg/L). Notably, three sites (P32, P33, P34) near Pingdu City and the Dagu River estuary show significantly elevated TDS (>3977 mg/L, saline water), while others are brackish. Fractured and karst groundwater have lower TDS values (mean 562.2 mg/L and 743.5 mg/L, respectively), with 95% and 83.33% of samples classified as freshwater. All parameters except pH display coefficients of variation (CV) >0.2, with porous water showing higher spatial heterogeneity (e.g., Cl⁻ CV = 2.08 vs. 0.56 for fractured water). Nitrate distribution further highlights anthropogenic impacts: porous water has a mean NO₃⁻-N concentration of 23.2 mg/L (maximum 101.7 mg/L), with 10% of samples classified as nitrate-type water, compared to 20.4 mg/L and 44.7 mg/L for fractured and karst water, respectively.

From the boxplot (Figure 2) and Durov diagram (Figure 3), it can be observed that cations in the groundwater are dominated by calcium ions, followed by sodium and magnesium ions. The anions are primarily bicarbonate, followed by sulfate and chloride ions. Most of the pore water samples show a gradual transition from the HCO₃-Ca type to SO₄-Ca type, with a small portion exhibiting a transition toward the Cl-Na type. This indicates that the hydrochemical type of the pore water is influenced by mineral dissolution and evaporation–concentration processes, resulting in a diverse range of hydrochemical types, with relatively high TDS and significant dispersion. Fractured water is predominantly of the HCO₃-Ca type and HCO₃·SO₄-Ca·Na type, with some samples leaning towards the SO₄-Ca type, showing diverse hydrochemical characteristics. In karst water, the dominant anion is bicarbonate, and the cation is primarily calcium, with hydrochemical types being HCO₃-Ca and HCO₃·SO₄-Ca, whose chemical composition is controlled by the dissolution of carbonate rocks, leading to a more concentrated TDS distribution.

As shown in Figure 2c, the distribution of nitrate nitrogen in pore water, fractured water, and karst water differs significantly. The median of nitrate nitrogen in pore water is relatively low; however, it exhibits a high frequency of outliers and a broad distribution range, and 10% of the pore water monitoring points are classified as nitrate-type water, reflecting a significant influence from human activities such as agricultural fertilizer application and domestic wastewater. In contrast, fractured water, while subject to a lower pollution load than pore water, has a higher median nitrate nitrogen concentration, with a more concentrated distribution. Notably, 5% of fractured water monitoring points are classified as nitrate-type water, indicating that fractured aquifers are more vulnerable. The nitrate nitrogen concentration in karst water is the lowest and most stable, reflecting a strong self-purification ability of the karst water system with minimal interference from human activities. Pore water is widely distributed with active replenishment, and its quality is significantly affected by human activities. Fractured water, due to preferential flow paths and bedrock fractures, is more susceptible to surface pollution sources [14]. Karst water, with its rapid circulation and dilution processes, experiences the least pollution [15].

3.1.2. Distribution Characteristics of Groundwater Hydrochemistry

The spatial distribution of groundwater hydrochemical characteristics in Qingdao is illustrated in Figure 4. In the carbonate-rich areas of eastern Pingdu and Laixi, prolonged water–rock interactions result in elevated HCO₃⁻ (mean > 250 mg/L) and Ca²⁺ (peak 769.2 mg/L), forming the dominant HCO₃-Ca-type water. However, low-lying zones in Pingdu are identified as pollutant sinks due to agricultural fertilization (local NO₃⁻-N >100 mg/L) and industrial discharges (SO₄²⁻ mean 122.4 mg/L) [16]. In Laixi, domestic sewage inputs (Cl⁻ anomaly 186.1 mg/L) drive hydrochemical evolution toward SO₄-Ca and NO₃-Ca mixed types [17]. Fractured aquifers in Laoshan and Huangdao, characterized by high-altitude recharge (residence time <5 years) and rapid flow (TDS mean 562.2 mg/L, CV = 0.49), maintain HCO₃-Ca-type water with high ion migration rates [18]. Human activities in Jimo significantly alter fractured water chemistry, with some samples shifting to HCO₃·SO₄-Ca·Na types, highlighting low contaminant retention in fissured media.

The Dagu River estuary and western Pingdu exhibit Cl-Na-type water due to seawater intrusion (Cl⁻ > 4000 mg/L, Na⁺ > 65%) and groundwater over-extraction-induced depression cones. Spatial correlation analysis reveals that Cl⁻ and Na⁺ hotspots align with high conductivity zones (TDS > 3000 mg/L), demonstrating synergistic effects of saline intrusion and hydraulic gradients. Overall, the hydrochemical differentiation in Qingdao is governed by a ternary coupling mechanism of “lithology-anthropogenic activities-hydrodynamics”. The spatial segregation of salinization (Cl-Na-type) and pollution (NO₃-Ca-type) hotspots provides critical targets for coastal groundwater management.

3.2. Analysis of Natural Sources of Hydrochemical Components in Different Groundwater Types

3.2.1. Water–Rock Model Analysis

Hydrochemical analysis based on Gibbs diagrams (Figure 5) reveals differential controls of rock weathering and evaporation–concentration on groundwater chemistry. Over 70% of Quaternary porous water samples cluster in the rock weathering domain, supported by high Ca²⁺/Na⁺ (molar ratio mean 3.2) and HCO₃⁻/Na⁺ (mean 4.8), indicating synergistic contributions from silicate and carbonate dissolution. Porous water in localized discharge zones (e.g., southwestern Pingdu) shifts toward the evaporation–concentration endmember due to prolonged residence time (>10 years) and enhanced evaporation (Cl⁻ > 2000 mg/L), with TDS peaking at 8203.5 mg/L. Despite anthropogenic inputs (NO₃⁻ mean 23.2 mg/L, SO₄²⁻ mean 122.4 mg/L), the low hydraulic conductivity in porous media significantly restricts contaminant migration, resulting in localized pollutant accumulation [19,20].

Bedrock fissure water exhibits minimal evaporation–concentration signals (Cl⁻/Na⁺ ≈ 1.1) but significant anthropogenic impacts: 30% of samples exceed the NO₃⁻ drinking water standard (30 mg/L), and preferential flow paths (fracture aperture > 0.5 mm) facilitate vertical contaminant migration (SO₄²⁻ peak 221.7 mg/L in Huangdao industrial zones). Karst water, dominated by carbonate dissolution (Ca²⁺+Mg²⁺ > 85%), shows high homogeneity (HCO₃⁻ CV = 0.35) with negligible evaporite contributions (Na⁺/Cl⁻ < 0.5).

Thus, rock weathering is the primary process controlling the chemical composition of most water bodies. Evaporative–concentration effects mainly influence pore water, particularly in groundwater discharge areas, where evaporation intensifies and leads to the accumulation of dissolved ions. Due to the open flow paths of fractured water, the impact of human activities is more pronounced, with significant inputs from pollution sources. Although pore water is also affected by pollution, its relatively slow movement and isolation make it less susceptible to these influences.

Hydrogeochemical modeling (Figure 6) further delineates the following: most samples align with carbonate-silicate mixed weathering trends (n(Ca²⁺)/n(Na⁺) = 2.5–5.0), while only 12% of porous and 8% of fractured water reflect evaporite dissolution (n(Mg²⁺)/n(Na⁺) < 0.3). The stronger silicate weathering signature in karst water (n(HCO₃⁻)/n(Na⁺) = 8.2 vs. 4.5 in porous water) underscores continuous feldspar dissolution during deep circulation [21,22,23].

3.2.2. Major Weathering Processes and Hydrochemical Evolution

Ionic ratio analysis (Figure 7a) indicates that 88% of groundwater samples align with the halite dissolution line (γ(Na⁺ + K⁺)/γ(Cl⁻) = 1), with 26.67% of porous water, 66.67% of fractured water, and 20% of karst water plotting below the line (Cl⁻ excess, γ < 1). This suggests significant influences from cation exchange (e.g., Na⁺-Ca²⁺ replacement) and anthropogenic inputs (e.g., Cl⁻ from sewage) alongside silicate weathering (γ > 1 in 12% of samples) [24]. Cl⁻ excess in porous water (e.g., Dagu River estuary, Cl⁻ > 4000 mg/L) correlates spatially with seawater intrusion and agricultural drainage, whereas fractured water (Cl⁻/Na⁺ ≈ 1.1) reflects preferential flow enhancing vertical salt transport.

The γ(Ca²⁺ + Mg²⁺)/γ(HCO₃⁻ + SO₄²⁻) relationship (Figure 7b) reveals that 66% of samples follow the 1:1 line, dominated by carbonate (calcite, dolomite) and evaporite (gypsum) dissolution [25,26,27]. Specifically, 87% of karst and 74% of porous water plot above the carbonate dissolution line (Ca²⁺ + Mg²⁺ excess), while 33.3% of fractured water deviates due to SO₄²⁻ anomalies (industrial zone peak 221.7 mg/L), indicating sulfate-driven mineral dissolution (e.g., pyrite oxidation).

The γ(Ca²⁺)/γ(Mg²⁺) ratios (Figure 7c) further delineate differential contributions from calcite and dolomite: 78% of porous and 65% of karst water cluster near 0.5 (co-dissolution), whereas fractured water ratios center at 0.8–1.2 (mean 1.05), highlighting dolomite weathering dominance [28,29,30]. Additionally, pyroxene dissolution in basaltic fractures (Mg²⁺/Ca²⁺ = 0.3–0.5) elevates Mg²⁺ concentrations in fractured water.

3.2.3. Mineral Dissolution Equilibrium

The mineral saturation index (SI) is a commonly used indicator in hydrogeochemical studies, providing a direct insight into whether specific minerals are in a dissolved or precipitated state in the water. It also reveals the conditions for the formation of hydrochemical components [31,32,33,34]. The SI of relevant minerals in different water bodies of the study area was simulated using the PHREEQC Interactive module, and boxplots of the mineral saturation indices for common minerals (gypsum, calcite, dolomite) in various groundwater types are shown in Figure 8. The boxplot statistically synthesizes SI distributions derived from hydrogeochemical datasets, with the interquartile range (IQR: 25th–75th percentiles) delineating the central 50% of mineral saturation states. The median line represents equilibrium tendencies, while whiskers (1.5 × IQR) define natural variability thresholds; outliers beyond this range signify extreme SI anomalies attributable to localized geochemical perturbations. This analysis presents the chemical states and distribution characteristics of key minerals in fractured water, pore water, and karst water from multiple perspectives. By analyzing the central tendency and dispersion of the SI values, it is possible to not only reveal the dissolution and precipitation behavior of various minerals but also reflect the complexity and variability of the water chemical environment.

The SI values for gypsum exhibit relatively low dispersion across the three types of groundwater. The SI values of fractured water and pore water are concentrated between −3 and −1, indicating that gypsum is in a stable undersaturated state in these water bodies, showing consistent dissolution behavior. The SI values for karst water are more tightly clustered and close to −0.5, suggesting that in karst environments, local influences from external sources (such as evaporative concentration or dissolution of underground rocks) cause some fluctuations in the SI value as it approaches equilibrium. The low dispersion of gypsum reflects its insensitivity to changes in the water chemical conditions.

The SI distribution for calcite is markedly different from gypsum, exhibiting greater dispersion. The SI distribution of fractured water is the broadest, ranging from −1.5 to 1, spanning the boundaries of dissolution, equilibrium, and oversaturation. Karst water also shows some dispersion, which may be related to the CO₂ degassing and calcium ion concentration effects caused by the rapid flow in fractured and karst waters, as well as the impact of related human activities. In contrast, the SI values for pore water and karst water are more concentrated, primarily ranging from 0 to 1, reflecting the stability of calcite’s dissolution-precipitation trend in the pore water environment. Overall, the higher dispersion of calcite in fractured and karst waters is influenced by local hydrodynamics and karst processes, while the lower dispersion in pore water indicates more uniform water chemistry conditions.

Dolomite’s SI values show the highest dispersion in fractured water, with a quartile range from 0.5 to −4, and several outliers. This suggests that in fractured water environments, the chemical behavior of dolomite is controlled by complex hydrodynamic and reactive conditions, such as local fluid replenishment or differences in flow paths leading to fluctuations in its saturation state. In pore water and karst water, the SI values are relatively concentrated, primarily ranging from −0.5 to 1, with a narrow fluctuation range, indicating that dolomite tends toward an equilibrium state in these waters, with more stable chemical conditions. In general, the instability of dolomite’s SI value in fractured water is closely related to the pronounced alternation of water in fractured zones.

The dispersion of the mineral saturation index not only reflects the dissolution and precipitation trends of various minerals in different water bodies but also reveals differences in the stability of hydrodynamic conditions and chemical environments. The generally higher dispersion in fractured water (especially for calcite and dolomite) indicates complex hydrodynamic conditions influenced significantly by local geological structures and water alternation processes. In contrast, pore water and karst water show lower dispersion, reflecting the more uniform hydrochemical characteristics and more stable water chemical formation conditions of these two types of water.

3.2.4. Cation Exchange Processes

The analysis of γ(HCO₃⁻) + γ(SO₄²⁻) − γ(Ca²⁺) − γ(Mg²⁺) versus γ(Na⁺) + γ(K⁺) − γ(Cl⁻) (Figure 9a) and the chloro-alkaline index (CAI) (Figure 9b) confirms distinct cation exchange behaviors across groundwater types. The formula for calculating the chlor-alkali index is shown in Equations (1) and (2) below. Approximately 4/5 of porous water and all karst water samples exhibited Na⁺/K⁺ depletion and Ca²⁺/Mg²⁺ enrichment, while nearly 2/3 of fractured groundwater samples displayed the opposite trend (Figure 9a). This divergence indicates contrasting cation exchange directions: Na⁺-Ca²⁺ exchange dominates in porous and karst systems, whereas Ca²⁺-Na⁺ exchange prevails in fractured groundwater [35,36].

CAI-1 = γ(Cl⁻ − Na⁺ − K⁺)/γ(Cl⁻)

(1)

CAI-2 = γ(Cl⁻ − Na⁺ − K⁺)/γ(HCO₃⁻ + SO₄²⁻ + CO₃²⁻ + NO₃⁻)

(2)

In Figure 9a, most groundwater samples cluster near the 1:1 line, reflecting strong symmetry between cation concentrations and confirming the significant role of cation exchange (e.g., Na⁺-Ca²⁺) in hydrochemical evolution [7,9,10,37]. Fractured and porous groundwater data show tighter clustering, suggesting consistent exchange mechanisms within each aquifer type. In contrast, karst water samples deviate slightly, particularly at lower values, likely due to unique exchange processes in carbonate-rich environments, such as enhanced Ca²⁺ replacement under high-calcium conditions (Figure 9a).

The CAI results further validate these trends (Figure 9b). CAI > 0 (observed in ~2/3 of porous water and all karst water samples) indicates Na⁺/K⁺ release from groundwater to the aquifer matrix, while CAI < 0 (predominant in fractured groundwater) suggests reverse exchange. Notably, the shift in cation exchange direction in porous water correlates with groundwater overexploitation-induced cone-of-depression and secondary seawater intrusion in the Dagu River estuary. These anthropogenic activities amplify groundwater salinization and soil sodicity, exacerbating Ca²⁺/Mg²⁺ leaching and pore clogging, which further aggravates soil compaction [38]. This data-driven interpretation aligns with the observed hydrochemical anomalies and underscores the interplay between natural processes and human impacts in shaping groundwater quality.

3.3. Analysis of Anthropogenic Sources of Hydrochemical Components in Different Groundwater Types

The presence of nitrate-dominated water samples highlights significant anthropogenic influences on groundwater chemistry in the study area (Figure 10). To further elucidate these impacts, nitrate (NO₃⁻), chloride (Cl⁻), and sulfate (SO₄²⁻)—sensitive indicators of human activities—were analyzed. Acid deposition (e.g., sulfuric and nitric acids) from agriculture, industry, and urbanization has notably enhanced carbonate weathering. As shown in Figure 10a, the ratios γ(Ca²⁺)+γ(Mg²⁺)/γ(HCO₃⁻) and γ(SO₄²⁻)/γ(HCO₃⁻) reveal the involvement of both carbonic and sulfuric acids in carbonate dissolution. Pure carbonic acid-driven dissolution would yield γ(Ca²⁺)+γ(Mg²⁺)/γ(HCO₃⁻) = 1 and γ(SO₄²⁻)/γ(HCO₃⁻) = 0, while sulfuric acid-dominated dissolution would result in γ(SO₄²⁻)/γ(HCO₃⁻) = 1 and γ(Ca²⁺)+γ(Mg²⁺)/γ(HCO₃⁻) = 2 [25,26,39]. Data confirm that both acids contribute to carbonate dissolution in select fractured and porous groundwater samples, underscoring the role of anthropogenic acid inputs in altering natural weathering processes.

Cl⁻, a stable component in natural water systems, serves as a reliable tracer for sewage/fecal contamination or dilution effects, as its concentration changes only through mixing with other Cl⁻-enriched sources [40,41,42,43]. Similarly, SO₄²⁻ and NO₃⁻ act as markers for industrial discharges and agricultural/livestock activities, respectively [44,45,46]. Figure 10a demonstrates that NO₃⁻/Ca²⁺ and SO₄²⁻/Ca²⁺ ratios in most groundwater samples align with agricultural and domestic wastewater influences, except for a few outliers linked to industrial pollution. Furthermore, the NO₃⁻/Cl^— versus-Cl⁻ molar relationship (Figure 10b) reveals that low NO₃⁻/Cl⁻ ratios coupled with high Cl⁻ concentrations dominate the dataset, indicating mixed contamination from sewage and fecal sources [47,48,49,50,51,52]. Only 5.35% of samples exhibit nitrate levels associated with natural soil nitrogen, while the majority cluster between agricultural and domestic wastewater endmembers, with porous and fractured groundwater samples leaning toward sewage/fecal inputs (Figure 10c).

These findings underscore the dual impact of agricultural and domestic wastewater on groundwater quality. Nitrate accumulation, particularly in porous and fractured aquifers, not only degrades water quality but also poses long-term health risks, including elevated cancer incidence [53,54,55]. This data-driven analysis confirms that human activities—notably unsustainable agricultural practices and inadequate wastewater management—are the primary drivers of groundwater contamination in the study area, aligning with the “natural-anthropogenic” coupling model proposed in the conclusions.

3.4. Source Apportionment of Hydrochemical Components Through PMF Analysis

Positive Matrix Factorization (PMF) was employed to identify pollution sources in pore water, fissure water, and karst water (Figure 11). Concentration data and uncertainty values for all eight hydrochemical components were incorporated, with signal-to-noise ratios (S/N) classifying all species as “strong” (S/N > 2), ensuring high data quality. The model was iterated 20 times with factor numbers optimized between 4 and 6, guided by Q-minimization and residual analysis. Scaled residuals for all elements fell within [−3, 3] and followed a normal distribution, confirming stable solutions. High explanatory power was demonstrated by R² > 0.9 for all factors, aligning with PMF best practices.

For pore water, Factor 1 (dominated by NO₃⁻ and Cl⁻) reflects agricultural nitrogen fertilizer application and domestic sewage inputs; Factor 2 (characterized by HCO₃⁻, Ca²⁺, Mg²⁺) represents natural carbonate weathering; Factor 3 (K⁺) indicates agricultural potassium fertilizer use; Factors 4–5 (SO₄²⁻, HCO₃⁻, Ca²⁺, Mg²⁺) suggest gypsum dissolution processes; Factor 6 (co-variation of Cl⁻ and Na⁺) demonstrates seawater intrusion and evaporative concentration effects.

In fissure water, Factor 1 (HCO₃⁻) predominantly reflects carbonate weathering; Factor 2 (Cl⁻, Na⁺, SO₄²⁻) corresponds to industrial contamination; Factor 3 (association of SO₄²⁻ with Cl⁻/Na⁺) indicates domestic wastewater discharge; Factor 4 (K⁺, Mg²⁺, SO₄²⁻) suggests feldspar weathering; Factor 5 (NO₃⁻ involvement) confirms bedrock dissolution; Factor 6 (Na⁺ and K⁺ dominance) represents cation exchange processes. Karst water analysis revealed the following: Factor 1 (Cl⁻, SO₄²⁻, NO₃⁻) aligns with leachate contamination from the Laixi Municipal Landfill; Factor 2 (K⁺ as an independent contributor) characterizes a unique Ca²⁺/Mg²⁺ ↔ Na⁺/K⁺ substitution mechanism; Factor 3 (HCO₃⁻ with multi-ion correlations) reflects combined carbonate dissolution and cation exchange processes; Factor 4 (HCO₃⁻ dominance) indicates nitrate-involved mineral weathering pathways.

These findings show strong consistency with regional pollution source distributions and hydrogeochemical characteristics. For instance, seawater intrusion in pore water (Factor 6) is validated by elevated Cl⁻/Na⁺ ratios and historical salinity data, while industrial contamination in fissure water (Factor 2) corresponds to nearby factory distributions. The pollution signature of karst water Factor 1 matches the geographical location of the Laixi landfill. The PMF analysis not only quantifies contributions from natural processes (e.g., carbonate weathering, gypsum dissolution) but also clarifies the relative weights of anthropogenic impacts (agricultural activities, domestic sewage, and industrial emissions), providing critical data support for establishing a “natural-anthropogenic” coupled model. This study confirms that groundwater chemical evolution results from synergistic multi-factor interactions, with human activities significantly altering the pathways and intensity of natural water–rock interactions.

3.5. Multi-Source Interaction Mechanisms in Regional Hydrochemical Formation

The hydrochemical composition of groundwater in Qingdao’s multi-aquifer systems arises from the interplay of natural geogenic processes and anthropogenic perturbations, with distinct mechanisms governing porous, fractured, and karst aquifers. Natural processes, including carbonate weathering, silicate dissolution, and seawater intrusion, establish the baseline hydrochemical framework. Carbonate weathering dominates karst aquifers, generating HCO₃-Ca type water with low spatial variability, while silicate weathering and evaporative enrichment contribute to elevated Na⁺ and Cl⁻ in porous aquifers, particularly in coastal discharge zones. Seawater intrusion, driven by over-extraction and hydraulic gradient reversal, further exacerbates salinization in porous aquifers, as evidenced by Cl⁻/Na⁺ ratios > 1.1 and spatial correlation with depression cones. In contrast, fractured aquifers exhibit rapid flow dynamics due to preferential pathways, amplifying anthropogenic impacts such as nitrate infiltration from agricultural runoff and sulfate enrichment from industrial discharges.

Anthropogenic activities alter hydrochemical evolution through dual pathways: direct inputs (e.g., nitrate from fertilizers, chloride from sewage) and indirect modifications to hydrodynamic conditions. PMF source apportionment highlights agricultural activities as the primary contributor to nitrate in porous aquifers, while industrial effluents dominate sulfate anomalies in fractured systems. The synergistic effects of acid deposition (H₂SO₄, HNO₃) enhance carbonate dissolution in karst aquifers, shifting ionic ratios beyond natural weathering thresholds. PHREEQC simulations further differentiate aquifer-specific responses: porous media retain contaminants due to low hydraulic conductivity, whereas fractured systems facilitate vertical pollutant migration, and karst aquifers buffer anthropogenic inputs through rapid dilution and carbonate equilibration.

This integrated analysis underscores how lithological heterogeneity and hydrodynamic conditions modulate multi-source interactions. Porous aquifers act as sinks for salinity and slow-evolving contaminants, fractured aquifers reflect acute vulnerability to point-source pollution, and karst systems demonstrate resilience tied to carbonate buffering. These findings provide a mechanistic basis for tailored management strategies, emphasizing salinity mitigation in porous aquifers, pollution interception in fractured zones, and enhanced monitoring of karst systems to address emerging anthropogenic pressures.

4. Conclusions

(1)

Groundwater in Qingdao is weakly alkaline (pH 7.2–8.4), with Ca²⁺ and Mg²⁺ as dominant cations and HCO₃⁻ as the primary anion. Its chemical composition is co-controlled by natural geogenic processes (carbonate/silicate weathering, evaporation–concentration) and anthropogenic activities, exhibiting marked spatial heterogeneity. Along the Dagu River basin to its estuary, porous groundwater transitions from HCO₃-Ca to SO₄-Ca and Cl-Na types, with a mean TDS of 716.6 mg/L. In coastal areas (e.g., Dagu River estuary) and the Pingdu depression cone, seawater intrusion and evaporation elevate TDS to over 4000 mg/L. Fractured groundwater (mean TDS: 562.2 mg/L) is predominantly HCO₃-Ca type, yet shows HCO₃·SO₄-Ca·Na type in human-impacted Jimo District. Karst groundwater (mean TDS: 743.5 mg/L) remains homogeneous, dominated by HCO₃-Ca type due to carbonate weathering.

(2)

Natural processes govern baseline hydrochemistry: Rock weathering (calcite and dolomite dissolution) serves as the predominant ion source, while cation exchange plays a secondary role in shaping hydrochemical composition Notably, evaporation–concentration significantly elevates TDS in low-lying porous aquifers. Mineral saturation indices (SI) reveal higher dispersion in fractured groundwater (calcite SI: −1.5–1; dolomite SI: 0.5–−4), reflecting complex hydrodynamic controls, whereas porous and karst systems exhibit stable SI distributions (e.g., calcite SI: 0–1), indicating homogeneous chemical conditions.

(3)

Human activities alter hydrochemical evolution through dual pathways: Direct pollutant inputs—agricultural non-point sources and domestic wastewater (Cl⁻, SO₄²⁻) are key anthropogenic contributors. Acid deposition (H₂SO₄, HNO₃) enhances carbonate dissolution, elevating γ(SO₄²⁻)/γ(HCO₃⁻) to 0.3–0.6 in select samples. Hydrodynamic field modification—overexploitation-induced depression cones intensify inland salt accumulation and coastal seawater intrusion (Cl⁻ up to 4049.7 mg/L), reversing cation exchange direction (CAI > 0 in 67% of samples) and triggering Na⁺ enrichment with soil Ca²⁺/Mg²⁺ depletion.