Hydrochemical Characteristics, Controlling Factors and Groundwater Sources of Zaozigou Gold Mine

Abstract

The Zaozigou gold deposit is recognized as one of the largest and most significant independent gold deposits in northwest China, representing a colossal orogenic gold-antimony deposit. It is imperative to delve into the hydrochemical characteristics and controlling factors within the mining area to unveil the groundwater circulation evolution process and enhance water resource management. In 2018, a comprehensive collection of 50 groups of groundwater samples was conducted, alongside 17 groups of surface water and underground mine water samples in 2023. Compositional descriptive analysis, correlation analysis, Piper three-plot, Gibbs map, ion ratio method, hydrogeochemical simulation and PCA methods were employed to reveal the chemical characteristics and evolution process of groundwater in the Zaozigou gold mine. Furthermore, employing water isotopes theory allowed for the identification of recharge sources and circulation conditions within the Zaozigou Gold Mine. The findings indicated a transition from HCO₃-Na·Ca type polluted water in 2018 to primarily the SO₄-Ca·Mg type and HCO₃·SO₄-Na·Ca·Mg type groundwater hydrochemistry types by 2023. The hydrochemical characteristics of the study area are closely related to the mining depth and time. The chemical characteristics are influenced by various factors such as rock weathering, mineral dissolution, cation exchange processes, atmospheric precipitation as well as human activities related to pollution from industrial mining activities.

1. Introduction

Groundwater plays a crucial role in the earth’s water resources. Therefore, a comprehensive study of the chemical composition of groundwater is a necessary action to monitor and protect the groundwater environment [1,2,3]. The chemical composition of groundwater is influenced by interactions with the surrounding environment as it permeates through rock pores over extended periods. At the same time, this chemical evolution of groundwater is continuous, with control factors shaping its characteristics and the ability to trace the origins of its components shedding light on environmental impacts [4,5]. Understanding these factors not only aids in water resource protection but also offers insights into mineral resource distribution.

Recently, significant research has been conducted on the controlling factors and sources of chemical components in groundwater. Traditional hydrochemistry methods, such as plotting Piper trilinear diagrams and Gibbs diagrams, are commonly utilized to analyze the types and origins of groundwater chemical components [6,7,8]. In addition, multivariate statistical techniques, including correlation analysis and principal component analysis, are frequently employed in hydrochemistry to analyze ion components systematically and to provide a deeper understanding of hydrochemical evolution [9,10,11,12]. The integration of isotope theory and hydrogeochemistry is extensively used to identify the sources of soil and groundwater recharge, elucidate the influences of natural and anthropogenic factors on hydrogeochemical processes, and more [13,14,15].

Gold, as a fundamental component of global economic activities, finds wide applications across various domains. Gold mining, being a vital mineral resource for national economic advancement, has long garnered the attention of geological experts [16,17]. The extraction of gold mines also leads to specific environmental impacts in mining regions, encompassing both surface and deep-seated gold deposits. Nonetheless, current investigations into groundwater chemistry predominantly concentrate on river basins, coastal areas, and karst regions, with inadequate focus on the controlling factors and groundwater pollution in deep-seated gold mining locales.

The Zaozigou gold mine, unearthed in 1997, commenced open-pit mining operations in 1998 and shifted to underground mining post-2004. Situated in the West Qinling Mountains of China, it represents a classic colossal orogenic gold-antimony ore deposit. Post-discovery, research has predominantly addressed the ore-forming fluid sources, characteristics, evolutionary patterns, tectonic dynamic background, and prospecting trajectories pertinent to the Zaozigou gold mine. Nevertheless, there exists insufficient research on the characteristics of underground water chemistry, controlling factors, and recharge sources, impeding the safeguarding of deep-seated groundwater resources within the Zaozigou mining region [18,19,20,21]. This study initiates by analyzing the surface water and groundwater chemical components in the Zaozigou gold mine area, applying hydrochemistry and isotope theory alongside methodologies, employing pertinent multivariate statistical tools to uncover the primary controlling factors of groundwater chemical components and the evolution of hydrogeochemistry, determining groundwater recharge origins. The ultimate goal is to furnish a scientific foundation for monitoring and safeguarding the groundwater environment in the mining area and to serve as a pivotal reference for the Zaozigou gold mine mining operations.

2. Materials and Methods

2.1. Study Area

The Zhaozigou Gold Mine is situated in Nawu Township, Hezuo City, Gansu Province, at a southwest direction of 260 degrees from the city center, 9 km away. The mining coordinates are 102°48′41″ East longitude and 34°57′56″ North latitude, covering a mining rights area of 2.528 square kilometers. Positioned as a fourth-grade tributary of the Yellow River, the area merges with the Cooperative River at Zhaozigou’s mouth. The lowest erosion base level in the mining region is at Zhaozigou, with an average elevation of 3.045 km. The annual flow ranges from 0.075 to 0.120 m³/s, with four seasonal branches extending north and south, exhibiting significant fluctuations across different seasons. The mining area, situated at the eastern edge of the Qinghai–Tibet Plateau, boasts a typical plateau zhongshan landform characterized by substantial topographic variations. The terrain’s highest points are at the north and south ends, gradually sloping from southwest to northeast, with topographic slopes between 15° and 45°. The local climate features a plateau continental monsoon climate, marked by heavy rain and snow, a prolonged winter, and short summer periods. Neotectonic movements in the area are predominantly uplift-driven, with a seismic fortification intensity reaching 7 degrees.

The surface covering layer of the Zaozigou gold deposit is mainly Quaternary residual material, little bedrock is exposed, and there is also magmatic rock mass distribution. The mining area is located in the northern fault-fold belt of the West Qinling fold belt, which presents an anticlinal structure. The Zaozigou gold deposit is controlled by the NWW-trending fault zone, and the proven gold deposits are largely affected by the regional fault zone and its secondary faults. The fault tendency is mainly NE direction, and the overall trend is NW direction. Secondly, the fault also plays a significant role in regulating the distribution of gold mineralization points in the region.

The groundwater in this area is composed of shallow Quaternary loose rock pore water, clastic rock fracture pore water and bedrock fracture water with the widest and deepest distribution (Figure 1). The bedrock fissure water in this area mainly exists in magmatic rocks and various rock fissures before the Triassic period. According to the formation reason of fissure water, it can be divided into two forms: weathering fissure water and tectonic fissure water. This area is located in a strong compression zone, and most of the faults formed are pressure-type. The regional and secondary fracture zones produced by tectonic changes often become the storage space and water channel of groundwater. In the process of migration to the gully and valley, the surface runoff formed by precipitation in the mountain area will permeate down to supplement the bedrock fissure water, and the rest will be discharged out of the area.

The depth of the formation is mainly influenced by structural cracks ranging in width from 1 to 2 mm, with a maximum width of 30 to 40 mm. These cracks are filled with argillaceous, siliceous, carbonate, pyrite, and limonized materials. Mining activities at greater depths gradually diminish the influence of structural fracture water on weathering fracture water, making the water conduction of the structural fracture zone the primary factor governing deep fracture water.

2.2. Sample Collection and Analysis

In this study, 40 water samples were collected from exploration drilling holes situated at an elevation of 2660 m above sea level in 2018. The samples were obtained at various depths in February, May, and September 2023, resulting in a total of 17 water samples comprising 5 surface water samples and 12 groundwater samples (Figure 1 and Figure 2,

Table 1. Location of water sampling points in the Zaozigou gold deposit area.

Time	Sample Type	Altitude/m	Sample Number	Sampling Position
2018	GW	2660	40	Exploration boreholes of exploration line 74–100
2023	GW	2660	1	12 middle section
		2610	3	13 middle section
		2560	5	14 middle section
		2510	1	15 middle section
	SW	3100	5	By the Zaozi River

). The surface water samples were gathered sporadically along the Zaozi River, with some taken in areas where groundwater visibly infiltrated and replenished the river. Specifically, the surface water samples were collected near grasslands devoid of dense farmland, with a dispersed distribution in pastoral regions, and the main principle is to choose the side outlet of groundwater, the upper spring of the river, and the part that has not been in contact with too much human activity.

All water samples collected in 2023 were obtained onsite. Water samples were collected and processed in strict accordance with the Geological Exploration Code for Geothermal Resources (GB/T11615-2010) [22]. White polyethylene kettles with a 2.5 L capacity were cleaned and filled thrice with the specific water sample, sealed with plastic film, and secured with medical tape. The samples underwent analysis and testing at the Hydrology and Water Resources Investigation and Evaluation Center of the Third Institute of Geology and Mineral Resources Exploration and Development of the Gansu Provincial Bureau of Geology and Mineral Resources. The anions and cations were measured using Inductively Coupled Plasma Mass Spectrometry (X2ICPMS Thermo Fisher Scientific, Waltham, MA, USA) and Inductively Coupled Plasma Optical Emission Spectrometry (ICAP6300 Thermo Fisher Scientific, Waltham, MA, USA). The pH value was gauged using a pH meter, and δD and δ18O were examined using a water isotope analyzer (Picarro L2130i, Santa Clara, CA, USA) based on the V-SMOW standard, with an accuracy of ±1‰ and ±0.1‰, respectively. Tritium (3H) was detected using an ultralow background liquid scintillation spectrometer named Quantulus 1220 (Waltham, MA, USA) boasting a measuring accuracy of ±0.1 TU.

2.3. Thoughts and Methods

The study initially involved using Excel 2019 to summarize and organize the measured water chemical indices. In addition, SPSS 27 (International Business Machines Corporation, Armonk, NY, USA) and Python 3.9.2 (Python Software Foundation, Nanakht, NY, USA) software were utilized to generate thermal maps showcasing the chemical components of the effluent.

We used the Shukarev hydrochemical classification method and Piper trilinear diagram method to classify the hydrochemical types of water samples in 2018 and 2023 in the mining area, and determined the dominant ion components. Considering that the 2018 water sample type is too simple and not time-sensitive, the follow-up study only used water samples taken in 2023. We used the Gibbs diagram, ion ratio method and principal component analysis to evaluate the hydrochemical characteristics of 2023 water samples from Zaozigou, and obtained the correlation between each hydrochemical index. According to the experience of determining the control factors of forming various ion ratio combinations, the influencing factors and ion sources of groundwater and surface water in mining areas are discussed in many aspects, and the groundwater is judged to be affected by cation exchange and human activities. At the same time, we use the saturation index calculated by hydrogeochemical simulation to judge the dissolution state of various minerals in the water to explore the specific situation of each ion by rock dissolution.

Based on the analysis of deuterium (²H), tritium (³H) and δ¹⁸O in the water sample of 2023, the source and circulation pattern of groundwater in the mining area are clarified. Specifically, deuterium (²H) and δ¹⁸O were used to calculate the evaporation fitting line of the water sample collected, and then compared with the local atmospheric precipitation line to obtain the correlation between groundwater and atmospheric precipitation in the mining area and the influence of evaporation fractionation. At the same time, we analyze the tritium (³H) content to distinguish whether the groundwater in the mining area belongs to the modern water range, and then judge the rate of groundwater recharge and renewal capacity.

3. Results and Discussion

3.1. Hydrochemical Characteristics

3.1.1. Hydrochemical Composition

Since the water chemical composition of the groundwater samples collected in 2018 has a single characteristic, a specific analysis was not produced by us.

Table 2. Descriptive statistics of surface water and groundwater hydrochemistry (unit: mg/L, except pH).

Sample Type	Item	pH	TDS	K+	Na+	Ca2+	Mg2+	Cl−	${\mathbf{SO}}_4^{2\mathit{-}}$	${\mathbf{HCO}}_3^{\mathit{-}}$	${\mathbf{NO}}_3^{\mathit{-}}$
groundwater	Mean	8.04	1043.8	7.46	111.6	118.8	80	13.74	514	367.5	1.93
	Max	8.35	2080	11.8	189	260	212	25.1	1344	820	4.61
	Min	7.42	348	3.04	63.8	31.5	23.6	5.17	38.5	254	0.67
	SD	0.24	647.9	3.15	40.48	82.41	65.7	6.85	485.8	165.6	1.19
	CV	3	62	42	36	69	82	50	95	45	62
Surface water	Mean	8.19	606.2	5.2	44.72	92.12	32.3	8.5	245.2	237.2	5.35
	Max	8.31	1198	9.11	144	158	43.8	16.9	794	346	9.86
	Min	7.94	276	1.36	6.89	49.9	27.6	2.72	10.9	33	3.5
	SD	0.16	390.9	3.65	58.72	48.26	6.54	7	349.5	136.4	3.04
	CV	2	64	70	131	52	20	82	143	58	57

TDS is the total dissolved solid, the unit is mg·L⁻¹; pH is dimensionless; The unit of coefficient of variation is %; Other indicators are mg·L⁻¹; SD: standard deviation; CV: coefficient of variation.

reflects the hydrochemical composition of groundwater and surface water in the study area for 2023. The groundwater pH ranges from 7.42 to 8.35, averaging 8.04, indicating weak alkalinity. The total dissolved solids (TDS) mass concentration in groundwater varies from 348 to 2080 mg·L⁻¹, with an average of 1043.8 mg·L⁻¹. Similarly, the pH of surface water falls between 7.94 and 8.31, averaging 8.19, also indicative of weak alkalinity. TDS of Surface water ranges from 276 to 1198 mg·L⁻¹, with an average of 606.2 mg·L⁻¹. The variation coefficients of TDS in both groundwater and surface water suggest substantial and similar spatial variability.

Among the cations in groundwater, Na⁺, Ca²⁺, and Mg²⁺ prevail, with Ca²⁺ content being the highest and K⁺ being the lowest. As for anions, ${\mathrm{SO}}_4^{2-}$ and ${\mathrm{HCO}}_3^{-}$ dominate, with ${\mathrm{SO}}_4^{2-}$ content being the highest and ${\mathrm{NO}}_3^{-}$ being the lowest. Surface water exhibits the analogous cation and anion composition characteristics as groundwater, with comparatively lower ion concentrations, indicative of a consistent source from surface water and bedrock fissure water. Due to direct recharge by atmospheric precipitation, surface water shows somewhat diluted ion concentrations. We detected many ions in surface water. Their levels are very low, generally below 0.001 mg·L⁻¹, so there is no risk of contamination by other ions.

Analyzing the variation dimension of ion concentrations, ${\mathrm{SO}}_4^{2-}$ and Mg²⁺ in groundwater exhibit high coefficients at 95% and 82%, indicating significant spatial differences. Ca²⁺ and ${\mathrm{NO}}_3^{-}$ ions also show pronounced spatial variations, primarily attributed to the coexistence and uneven distribution of underground silicic rock and carbonate karst decomposition. Na⁺, Cl⁻, and ${\mathrm{SO}}_4^{2-}$ ions in surface water exhibit high coefficients at 131%, 82%, and 143%, respectively, demonstrating robust spatial discrepancies. With high variation coefficients of other ions excluding Mg²⁺, the presence of strong spatial differences is associated with diverse industrial and mining activities in the mining area.

3.1.2. Hydrochemical Correlation

The groundwater’s chemical indexes can be utilized to trace characteristic ions [23,24]. In the study area in 2023, SPSS and Python software facilitated Pearson correlation analysis on the general indicators of groundwater hydrochemistry, enabling the creation of heat maps. The heat mapping revealed high correlations between Ca²⁺, Mg²⁺, and TDS (total hardness) with correlation coefficients of 0.98 and 0.95, respectively. Additionally, a robust correlation of 0.98 was observed between ${\mathrm{SO}}_4^{2-}$ and the TDS index. These findings indicate that Ca²⁺, Mg²⁺, and ${\mathrm{SO}}_4^{2-}$ predominantly contribute to the total hardness in water, while other ions (in 2023) make relatively modest contributions (Figure 3).

The analysis of ${\mathrm{SO}}_4^{2-}$ ion correlation was conducted through a heat map (Figure 3), revealing a strong correlation and deep thermal color for Ca²⁺ and Mg²⁺, with coefficients as high as 0.95 and 0.98. Further examination of the thermal color associated with Ca²⁺ demonstrated a significant correlation with Mg²⁺, indicated by a coefficient of 0.9. This suggests a shared source, likely arising from the dissolution of evaporite minerals like gypsum or anhydrite.

To sum up, TDS in the water samples in 2023 are mainly contributed by Ca²⁺, Mg²⁺ and ${\mathrm{SO}}_4^{2-}$, which means that the main dominant ions in the groundwater of each sublevel in 2023 are these three types. Some of the sources of Ca²⁺, Mg²⁺ and ${\mathrm{SO}}_4^{2-}$ are consistent, which is presumed to be contributed by human activities. The remaining Ca²⁺, Mg²⁺, and ${\mathrm{SO}}_4^{2-}$ are derived from the dissolution of evaporite minerals such as gypsum or anhydrite.

3.1.3. Hydrochemical Type

The Piper trilinear diagram HCO₃·SO₄-Na·Ca·Mg is a direct indicator of water body hydrochemical types, while the Shukarev and trilinear diagram methods are frequently employed to categorize distinct hydrochemical water types [25,26].

The underground water samples from exploration drilling in 2018, as depicted in Figure 4, consistently converge towards the (${\mathrm{CO}}_3^{2-}$ + ${\mathrm{HCO}}_3^{-}$) endpoint in the anion diagram and the Na⁺ endpoint in the cation diagram, with a partial presence of Ca²⁺ and Mg²⁺ ions. Out of 50 groundwater chemical types, 49 are classified as HCO₃-Na·Ca type while only one falls under the HCO₃-Na·Ca·Mg category. This chemical composition shift is a result of introducing quicklime and flocculant into the circulating water during mine development, where quicklime raises the water body’s pH, leading to the formation of ${\mathrm{HCO}}_3^{-}$ through the interaction of CO₂ with OH⁻ in an alkaline setting.

In the anion diagram at the lower right of Figure 5, the groundwater samples from the mining area in 2023 tend towards the endpoints of ${\mathrm{CO}}_3^{2-}$ + ${\mathrm{HCO}}_3^{-}$) and ${\mathrm{SO}}_4^{2-}$, inversely suggesting low Cl⁻ levels, indicating that the anions ${\mathrm{HCO}}_3^{-}$ and ${\mathrm{SO}}_4^{2-}$ are predominant, with minimal Cl⁻ content. On the cation diagram at the lower left, Ca²⁺ ion endpoints are common in the groundwater samples, with substantial proportions of Ca²⁺, Mg²⁺, and Na⁺ cations. These cations, particularly Ca²⁺ and Mg²⁺, are primarily attributable to the dissolution of carbonate rocks, silicate rocks, and some evaporative salt rocks. In the groundwater of the mining area, five hydrochemical types exist, mainly the SO₄-Ca·Mg type and the HCO₃·SO₄-Na·Ca·Mg type, comprising 50% and 16.7% of the total water samples, respectively. Additionally, there are three additional types: HCO₃-Na·Ca·Mg, HCO₃-Na·Ca, and SO₄-Na·Ca·Mg. The groundwater’s hydrochemical characteristics are closely tied to the lithology of calcareous, siliceous slate, and feldspar sandstone in the mining area, which represent the primary mineral-bearing host rocks of the Zaozigou mining site. In contrast, compared to groundwater, surface water in the mining area showcases simpler compositions, predominantly featuring the HCO₃-Ca·Mg and SO₄-Ca types only.

From the internal comparison of the water chemical types of the water samples (

Table 3. Summary of hydrochemical types of all water samples collected in the Zaozigou gold deposit area.

Time	Sample Type	Altitude/m	Sample Number	Hydrochemical Type
2018	GW	2660	49	HCO3-Na·Ca
			1	HCO3-Na·Ca·Mg
2023	GW	2660	3	SO4-Ca·Mg
		2610	3	SO4-Ca·Mg
		2560	5	SO4-Ca·Mg SO4-Na·Ca·Mg HCO3·SO4-Na·Ca·Mg HCO3-Na·Ca·Mg
		2510	1	HCO3-Na·Ca
	SW	3100	5	HCO3-Ca·Mg SO4-Ca

) collected in 2023, there are certain similarities between the water chemical types of surface water and groundwater, indicating that there is a certain hydraulic connection between surface water and groundwater. In the water samples collected in 2023, the hydrochemical type of the water samples in the middle section of 2660 m is SO₄-Ca·Mg type, which is different from the HCO₃-Na·Ca type of the water samples in 2018, indicating that the groundwater polluted by the pre-development circulation of the mine in the middle section of 2660 m has gradually recovered and become jointly controlled by various natural factors. Along the mining depth, the water chemical type gradually approaches HCO₃-Na·Ca, and the newly developed water chemical type in the middle section of 2510 m is this type. The middle section of 2550 m developed earlier shows the transition from HCO₃-Na·Ca type to SO₄-Ca·Mg type. Since the water chemical characteristics and origin of the groundwater samples collected in 2018 are relatively simple, the following research only uses the water samples collected in 2023.

3.2. Control Factors of Groundwater Hydrochemistry

3.2.1. Dissolution of Rock Weathering

The vertical axis of the Gibbs diagram represents the TDS value, while the horizontal axis correlates to the ratios of γ(Cl⁻) to γ(Cl⁻ + ${\mathrm{HCO}}_3^{-}$) and γ(Na⁺) to γ(Na⁺ + Ca²⁺). Based on the governing factors of water chemical components, water types are categorized into three control types: precipitation, weathering, and evaporation [27,28]. As depicted in Figure 6, all water sample points within the area fall under weathering control. Consequently, the mining area’s overall hydrochemical composition predominantly aligns with the weathering control classification, indicating robust water–rock interactions where various ions are sourced from the rocks.

3.2.2. Source of Major Ions

When the groundwater moves through the rocks in the aquifer, the ions in the groundwater and the ions in the surrounding rock will be converted by the leaching action, and some chemical components of the rocks will dissolve into the water, resulting in changes in the chemical composition of the water To delve deeper into the origins and chemical transformations of ions within the study area’s water bodies, an ion ratio map was utilized for thorough investigation [29,30]. Displayed in Figure 7, the map delineates the ion ratios specific to the Zaozigou gold mine region.

The ratio of γ(Ca²⁺ + Mg²⁺)/γ(${\mathrm{HCO}}_3^{-}$ + ${\mathrm{SO}}_4^{2-}$) served as a pivotal metric to assess the gypsum dissolution’s contribution to the Ca²⁺ and Mg²⁺ presence in the water samples [31,32]. Proximity to a ratio of 1 indicates that Ca²⁺ and Mg²⁺ are primarily sourced from gypsum dissolution. Notably, the values of γ(Ca²⁺ + Mg²⁺)/γ(${\mathrm{HCO}}_3^{-}$ + ${\mathrm{SO}}_4^{2-}$) across the study area’s water samples consistently hovered around 1, implicating a significant role of gypsum dissolution in contributing to the Ca²⁺ and Mg²⁺ concentrations (Figure 7a). Furthermore, the origins of ${\mathrm{HCO}}_3^{-}$, Ca²⁺, and Mg²⁺ ions were scrutinized using γ(Ca²⁺ + Mg²⁺)/γ(${\mathrm{HCO}}_3^{-}$ + ${\mathrm{SO}}_4^{2-}$). Analysis revealed that a ratio below 1 indicates control by silicate rock dissolution, while a ratio exceeding 1 signifies carbonate karst lysis. Proximation to the value of 1 suggests a joint influence of silicate rock and carbonate karst decomposition on the ionic components. Predominantly, the water samples in the mining area clustered around a γ(Ca²⁺ + Mg²⁺)/γ(${\mathrm{HCO}}_3^{-}$ + ${\mathrm{SO}}_4^{2-}$) ratio of 1:1 (Figure 7a), emphasizing the shared contribution of silicate and carbonate dissolution to the Ca²⁺ ions, Mg²⁺ ions, and ${\mathrm{HCO}}_3^{-}$ ions within the mining area.

The γ(Na⁺)/γ(Cl⁻) ratio is frequently utilized to discern the origins of certain groundwater components, providing insights into Na⁺ and Cl⁻ concentrations in groundwater while reflecting cation exchange intensity [33,34]. Numerous studies have indicated that equimolar Na⁺ and Cl⁻ levels stem from salt rock dissolution. Within the mining area, water samples are widely dispersed above the γ(Na⁺)/γ(Cl⁻) line at a ratio predominantly exceeding 1:10, pointing towards a substantial influence of sodium mineral dissolution as opposed to salt rock dissolution (Figure 7b). It is postulated that leaching processes from silicate minerals rich in sodium and potassium underpin the Na⁺ and K⁺ contributions in the water samples.

The ratios of Ca²⁺/Na⁺, Mg²⁺/Na⁺, and ${\mathrm{HCO}}_3^{-}$/Na⁺ are generally affected by the physical level and stable at the chemical level, which can represent the specific situation of water–rock interaction [35]. The groundwater sample points of the Zaozigou mining area are concentrated between the control range of silicate rock and carbonate rock: From the perspective of ${\mathrm{HCO}}_3^{-}$, the water sample points are more inclined to be between silicate rock and evaporative rock, and from the perspective of Mg²⁺, the water sample points are more inclined to be between silicate rock and carbonate rock, indicating that the chemical components of groundwater in the mining area accept the contribution of evaporative rock, silicate rock and carbonate rock at the same time, and the contribution of silicate rock to the weathering dissolution is greater. The surface water sample points in this area tend to be controlled by carbonate rocks, which indicates that the surface water in the Zaozigou gold mine area is closely related to the weathering and dissolution of carbonate rocks (Figure 7c,d).

3.2.3. Cation Exchange

Cation exchange is one of the important controlling factors of hydrochemical components. The ratio of (Ca²⁺ + Mg²⁺)–(${\mathrm{HCO}}_3^{-}$ + ${\mathrm{SO}}_4^{2-}$) to (Na⁺–Cl⁻) concentrations in water samples is frequently employed to specifically assess cation exchange dynamics between water bodies and rocks. A ratio nearing −1 signifies significant cation exchange. Notably, the regression lines’ slopes from the surface water and groundwater samples shown in Figure 8a approximate −1, indicative of notable cation exchange between groundwater and surface water within the Zaozigou gold mine area [29,36].

In evaluating the intensity of cation exchange, the chloralkali index (CAI) can be calculated [37] and compared utilizing the formula below:

$$\mathrm{CAI}1={\displaystyle \frac{\mathrm{N}\left({\mathrm{Cl}}^{-}\right)-\mathrm{N}\left({\mathrm{Na}}^{+}+{ \mathrm{K}}^{+}\right)}{\mathrm{N}\left({\mathrm{Cl}}^{-}\right)}}$$

(1)

$$\mathrm{CAI}2={\displaystyle \frac{\mathrm{N}\left({\mathrm{Cl}}^{-}\right)-\mathrm{N}\left({\mathrm{Na}}^{+}+{ \mathrm{K}}^{+}\right)}{\mathrm{N}\left({\mathrm{SO}}_4^{2^{-}}\right)+ \mathrm{N}\left({\mathrm{HCO}}_3^{-}\right)}}$$

(2)

When both CAI1 and CAI2 values are negative concurrently, it denotes an exchange interaction between Ca²⁺ and Mg²⁺ ions in the water and Na⁺ and K⁺ in the rock; conversely, positive values indicate an absence of ion exchange. Moreover, the greater the absolute value, the more pronounced the intensity of the ion exchange process. As depicted in Figure 8b, all water samples exhibit negative CAI values, suggesting that Ca²⁺ and Mg²⁺ in the water body replace Na⁺ and K⁺ across all samples within the mining area. Most groundwater sample points noticeably diverge from the zero point, whereas the surface water samples predominantly cluster near the zero point, indicating a heightened cation exchange adsorption in groundwater compared to surface water overall. This underscores that Na⁺ in groundwater derives not solely from the leaching of silicate and carbonate rock minerals but also from water cation exchange processes.

3.2.4. Influence of Human Activities

The ${\mathrm{NO}}_3^{-}$/Na⁺ and Cl⁻/Na⁺ ratios in water commonly indicate the degree of association with human activities [33,38]. Figure 9a illustrates that groundwater in the mining area distances itself from the influence of human activities, signifying limited impact from agricultural practices and domestic sewage. Groundwater sample points are situated between the weathering of silicate rock and the weathering and dissolution of carbonate rock. In contrast, surface water aligns more closely with human activities, indicating a greater susceptibility to human-induced influences compared to groundwater.

Similarly, the ${\mathrm{SO}}_4^{2-}$/Ca²⁺ and ${\mathrm{NO}}_3^{-}$/Ca²⁺ ratio relationships serve as tools to evaluate the impact of diverse human activities on major ion concentrations in water [39,40]. When ${\mathrm{SO}}_4^{2-}$/Ca²⁺ < ${\mathrm{NO}}_3^{-}$/Ca²⁺, activities are categorized into agricultural practices and domestic sewage control, with the reverse indicative of industrial and mining activities. As depicted in Figure 9b, groundwater in the mining area displays higher ${\mathrm{SO}}_4^{2-}$ ions and lower ${\mathrm{NO}}_3^{-}$ ions. Overall, a minimal correlation is observed with human activities, leaning towards industrial and mining influences. Given that groundwater samples are obtained from deep underground sources within mining activity zones, the impact of agricultural activities and domestic sewage is limited, highlighting a stronger association with mining activities.

3.2.5. Hydrogeochemical Modeling

Employing Phreeqc (1999) software to model the chemical reactions involved in water–rock interactions stands as a conventional methodology within hydrochemistry. By configuring the temperature and pressure settings according to the various water sample locations, the measured ion components and related parameters are inputted into the software to simulate the saturation index (SI) of individual minerals.

The saturation index (SI) is a widely used indicator describing the interaction of specific minerals with groundwater, reflecting the dissolution extent of various minerals in the water [41]. A mineral with an SI value exceeding 0.5 suggests saturation in the groundwater, potentially causing precipitation. Conversely, an SI value between −0.5 and 0.5 signifies a balance in mineral dissolution. An SI below −0.5 implies ongoing mineral dissolution, indicating unsaturation [42].

Within the study area, the SI values for dolomite in groundwater samples consistently exceed 0.5, indicating a saturation point with discontinued dissolution. Calcite’s SI value near 0.5 indicates dissolution near the saturation threshold. Most water bodies exhibit gypsum and anhydrite SI values below −0.5, indicating ongoing dissolution, thus contributing to Ca²⁺ and ${\mathrm{SO}}_4^{2-}$ ions in groundwater. A few water bodies display equilibrium in dissolution. Potassium salt SI values below −0.5 indicate ongoing dissolution providing K⁺ to the water bodies, while the CO₂ SI considerably below −0.5 suggests unsaturation, possibly due to reactions with gypsum, calcite, and other minerals, indicating continued dissolution of carbonate rocks in the water bodies (Figure 10).

3.2.6. Control Factor of Hydrochemistry

Principal component analysis (PCA) was conducted on groundwater samples collected in 2023 from the Zaozigou gold mine to identify the influential factors behind the groundwater’s chemical composition [43,44,45]. A Kaiser–Meyer–Olkin (KMO) value of 0.73 and Bartlett’s sphericity test p-value of 0.00 validate the dataset’s suitability for PCA analysis. The groundwater in the mining area aligns with a three-factor model identified by PCA (Figure 11), where the cumulative variance of these factors accounts for 92.8%. PC1 encompasses notable cations such as Ca²⁺, Mg²⁺, and Na⁺, along with anions ${\mathrm{SO}}_{2-4}$, Cl⁻, NO, and TDS, indicating control by rock weathering, cation exchange, and human activities. In PC2, the predominant contents are Na⁺ and ${\mathrm{HCO}}_3^{-}$, with respective load coefficients of 0.793 and 0.973, consistent with characteristics of contaminated circulating water during mine development. Notably, the higher load coefficient of ${\mathrm{HCO}}_3^{-}$ compared to Na⁺ suggests pollution from industrial and mining activities as the primary ${\mathrm{HCO}}_3^{-}$ source, while Na⁺ also stems from PC1 besides pollution sources. PC3 primarily exhibits elevated K+ content alongside positive ${\mathrm{NO}}_3^{-}$ levels, signifying potentially KNO₃-contaminated groundwater from mine blast remnants, albeit to a lesser extent (

Table 4. Factor loadings of chemical variables on principal components, eigenvalues and variances for groundwater dataset of the Zaozigou gold deposit area.

Variable	Principal Components
	PC1	PC2	PC3
Ca2+	0.976	−0.171	−0.053
Mg2+	0.863	−0.4	−0.211
Na+	0.5	0.793	−0.052
K+	0.623	0.153	0.73
Cl−	0.877	0.25	−0.153
${\mathrm{SO}}_4^{2-}$	0.913	−0.375	−0.123
${\mathrm{HCO}}_3^{-}$	0.04	0.973	−0.066
${\mathrm{NO}}_3^{-}$	0.871	0.035	0.238
Eigenvalues	5.715	2.841	0.727
Variance(%)	57.1	28.4	7.3
Cumulative(%)	57.1	85.5	92.8

Bold values represent strong (>0.60) and positive factor loadings.

).

3.3. Groundwater Recharge Source

Hydrogen and Oxygen Isotope Characteristics

In the investigation of groundwater chemistry, the isotopes δD and δ¹⁸O of hydrogen and oxygen exhibit stable chemical properties and are minimally impacted by environmental influences, making them valuable tools for studying groundwater recharge and runoff processes [46,47,48,49].

The surface water in the study area shows limited variation in δ¹⁸O and δD concentrations, with concentrations clustering around specific values. The δ18O content of surface water ranges from −10.7 to −9.6‰, with an average value of −10.15‰, while the δD content ranges from −74 to −65‰, with an average of −69.5‰. In groundwater, the δ¹⁸O content varies from −12.8 to −8.9‰, with an average of −11.17‰, and the δD content ranges from −61 to −91‰, with an average of −78‰. These results suggest that δD and δ¹⁸O concentrations in groundwater are comparatively lesser than in surface water. Significant variations in stable isotopes of δD and δ¹⁸O are observed across different seasons; overall, from the dry period to the wet period, there is an increase in δD and δ¹⁸O concentrations, an observation consistent with findings in the Dianbu River Basin by Zheng Tao et al., indicating a control by temperature fluctuations. The lower temperatures during the dry period reduce the relative abundance of heavy isotopes due to water evaporation fractionation.

Zheng Shuhui et al. determined the atmospheric precipitation line in China as δD = 7.9δ¹⁸O + 8.2. In the mining area, the equation for the atmospheric precipitation line (LMWL) is δD = 8.1δ¹⁸O + 14.6, with both slope and intercept values exceeding those of the global atmospheric precipitation line, implying a predominantly humid environment in the mining area. All water samples in the study area cluster near the national and local atmospheric precipitation lines, forming a linear distribution pattern, indicating that atmospheric precipitation serves as the primary source for groundwater and surface water in the study area.

Evaporation lines for surface water and groundwater during the dry and wet seasons were determined using the least squares method to fit water sample data points. The fitting line for surface water evaporation is δD = 8.1δ¹⁸O + 13.5, with an R² value of 0.96. The slope of 8.1 aligns closely with both the national precipitation line slope (7.9) and the local atmospheric precipitation line slope (8.1), suggesting minimal influence from evaporation fractionation. For groundwater evaporation during the dry season, the fitting line is δD = 7.6δ¹⁸O + 6.8, R² = 0.9. For the wet season, the fitting line is δD = 7.23δ¹⁸O + 2.8, R² = 0.99. The slopes are slightly lower than the slopes of the national precipitation line and mining area atmospheric precipitation line, indicating a minor effect of evaporation fractionation on groundwater in the mining area (Figure 12). However, abnormal isotopic data of groundwater samples were collected near measurement wells, showing a slight enrichment compared to surface water, likely due to earlier excavation times, human activities, and increased surface water recharge. Heavy isotopes are enriched near measurement wells due to minimal water evaporation and fractionation underground.

In the realm of hydrogen and oxygen isotopes, the deuterium excess parameter (d) serves as a crucial indicator for assessing water evaporation imbalances in the study area and plays a significant role in determining the source of groundwater recharge [50,51]. Dansgaard introduced the formula to calculate this parameter in 1964: d = δD-8δ¹⁸O, suggesting that d decreases gradually with increased water evaporation imbalance. During the dry season, groundwater samples in the study area exhibit positive d values ranging from 10.4% to 12.4%, with an average of 11.7%, lower than the local atmospheric precipitation average d value of 13.9%. This discrepancy suggests that groundwater indirectly receives recharge from atmospheric precipitation and undergoes evaporative fractionation in the underground circulation processes.

Tritium (³H), a natural hydrogen isotope, serves as a valuable indicator for assessing groundwater’s regeneration capacity, recharge cycle, and age [52,53]. The tritium content in the mining area’s groundwater ranges from 0.6 to 17.7, with an average of 7.7. Notably, 33% of groundwater samples contain tritium levels between 0.8 and 4.0 TU, indicative of mixed groundwater recharge dating back to the 1950s and recent years. The remaining 66% of samples have tritium content between 5 and 15TU, categorizing them as modern water sources. Overall, the majority of groundwater in the study area falls within the modern water range, highlighting a rapid recharge and discharge process between groundwater and atmospheric precipitation, favorable groundwater circulation runoff conditions, and robust renewal capacity (Figure 13).

4. Conclusions

The hydrochemical type of groundwater samples taken in the Zaozigou gold mine in 2018 is basically HCO₃-Na·Ca type, which is a development type of polluted water. The average pH of the water samples in 2023 is 8.18, which is classified as weakly alkaline water, and the TDS is generally high, ranging from 276 to 3156 mg/L. In groundwater, ${\mathrm{HCO}}_3^{-}$ and ${\mathrm{SO}}_4^{2-}$ are the main components of anions, and Ca²⁺, Mg²⁺, and Na⁺ are the main components of cations. There are five hydrochemical types of groundwater, among which SO₄-Ca·Mg type and HCO₃·SO₄-Na·Ca·Mg type are the main ones. The main cations and anions in surface water are the same as those in groundwater, and the hydrochemical types are only HCO₃-Ca·Mg type and SO₄-Ca type. From the chemical characteristics of water, the deep structural fissure water has a cyclic relationship with the weathered fissure water and surface water.

Analysis of water samples taken in 2023 shows that with the deepening of mining depth, the gradual transition of water chemistry type from SO₄-Ca·Mg to HCO₃-Na·Ca type reflects that the middle part of the newly mined mine is polluted by mining development, which is the same as the groundwater water chemistry type in 2018, while the shallow middle part of the early mined groundwater has gradually returned to the control of natural factors.

In general, the water chemical components of the Zhaozigou gold mine are controlled by water–rock chemistry, mineral dissolution, cation exchange, atmospheric precipitation recharge cycle and human activities. ${\mathrm{SO}}_4^{2-}$ ions, Ca²⁺ ions and Mg²⁺ ions depend partly on the dissolution of gypsum or anhydrite, and the weathering of carbonate rocks and silicate rocks contributes partly to the dissolution of Ca²⁺, Mg²⁺, and ${\mathrm{HCO}}_3^{-}$. ${\mathrm{HCO}}_3^{-}$ and some Na⁺ come from pollution caused by mine development. The leaching and cation exchange of silicate minerals containing sodium and potassium are the main sources of Na⁺ and K⁺ in water. The influence of human activities on the hydrochemistry of mining areas is general, mainly affected by industrial and mining activities, which contribute part of ${\mathrm{SO}}_4^{2-}$ and Na⁺.

The surface water and groundwater in the mining area have received direct or indirect atmospheric precipitation recharge, most of the groundwater belongs to the modern water range, the groundwater circulation runoff condition is relatively good, and the renewal ability is strong. The stable isotopes δD and δ¹⁸O are enriched by stronger evaporative fractionation in water during the wet season, and surface water and groundwater are less affected by evaporative fractionation.