Research Hotspots in and Progress of Stable Isotopic Techniques Applied in Tracing Mine Water Pollution and Its Environmental Impact: A Bibliometric and Visualization Analysis from 1998 to 2023

Abstract

Stable isotope techniques have become a critical tool for tracking mine water and identifying its contamination. In order to explore in depth the research hotspots and trends in stable isotope technology in the study of mine water and the environmental pollution it induces, the Web of Science Core Collection (WoSCC) database of the relevant literature in this field from 1998 to 2023 was used for visual bibliometric analysis by applying CiteSpace software (version 5.7R5). The results showed that the periodical literature in this field shows a fluctuating upward trend. In the cooperation network of country and institution, the centrality of the United States was as high as 0.74 and 0.23, much higher than that of other countries, which means that in terms of the institutions, the number of publications, and the status of research, the United States is ahead of other countries. China’s research started later than the United States’s but is developing rapidly. Although its importance and influence in this research field are only slightly lower than those of the United States, China still needs to improve its cooperation with other countries and regions. The research hotspots in this area center around identifying and understanding pollution processes, studying mine water sources and mixing, exploring the evolution of water chemistry and its isotopic composition, and investigating the environmental impacts of mine water. Innovative isotope-tracing methods and techniques, isotope fractionation mechanisms, sources of sulfate, and their impact on the water environment will remain the focus of the forthcoming research phase. This study uses bibliometrics to systematically summarize the research hotspots and trends in stable isotope techniques in mine water problems in terms of their footprint in the academic literature, which is of great significance for the utilization of water resources in mine drainage and pollution control in mines.

1. Introduction

As critical natural resources, mineral resources play an essential role in developing a country’s industrialization. Although new energy is of growing importance, fossil fuels still occupy a pivotal position in the energy structure [1,2,3]. Large-scale mining unavoidably generates significant amounts of mine water. In the face of severe water scarcity and environmental pollution problems, a large amount of mine water drainage will not only cause the pollution and waste of groundwater resources but also seriously affect the balance and stability of the groundwater system. The specificity and complexity of its water quality are also prone to cause a deterioration of the ecological environment in mining areas [4,5,6,7,8,9,10]. It has been calculated that mine water discharges have severely damaged a total of 72,000 hectares of lakes and reservoirs around the world [11]. Therefore, how to effectively monitor and manage mine water pollution has been the focus of research conducted by a wide range of researchers. In this context, stable isotope technology has become an essential tool in mine water research. Stable isotopes have a wide range of applications and significant advantages, which have been highlighted in source tracking, water cycle identification, and environmental impact assessment.

Due to the complexity of the groundwater environment, the various geochemical processes that occur during the formation and discharge of mine water are influenced by a variety of factors, such as hydrology, climate, mining conditions, biological activities, and mineralogy [12]. A solitary approach using only water chemistry and statistical analysis is inadequate for thoroughly investigating this multifaceted process. Stable isotope techniques are effective means of assessing and quantifying mine water problems in combination with traditional water chemistry analysis methods due to their high sensitivity and advantages in source tracking [13,14]. Stable isotope techniques are a hydrogeochemical instrument that has been investigated since the 1960s, finding numerous applications in water, the atmosphere, and soil, among other environments [15,16,17,18]. Isotopes are classified into radioisotopes and stable isotopes based on whether they decay or not, and their application is therefore divided into two main categories. Radioisotopes are mainly used to study the age of groundwater and the mechanisms of groundwater movement through their dating and tracing functions [19,20,21]. Stable isotopes are often used as natural tracers to study groundwater formation mechanisms, sources of contamination, and groundwater hydraulic connections due to their specific compositional ratios in different sources [22,23,24].

Previously, researchers thoroughly investigated the geochemical processes occurring in mine water and identified complex process mechanisms, especially the mechanism of pyrite oxidation reaction and the kinetics of iron and sulfur during the formation of acid mine water. The application of stable isotopes in the mining environment can provide strong support for this research process. For example, stable isotopes such as hydrogen, oxygen, sulfur, carbon, and iron all play important roles in the field of acid mine drainage. Hydrogen and oxygen isotopes can help to determine the source of water for acid mine drainage. In the study of sulfide oxidation mechanisms, the application of sulfur and oxygen isotopes has provided insight into the different oxidation mechanisms of sulfides and assessed the rate and kinetics of sulfide oxidation, among other important research aspects, which has helped to gain a deeper understanding of the mechanism of AMD formation [25,26]. Carbon isotopes can be used to study the sources and transformation processes of organic matter in mine water [27,28]. In addition, iron isotopes have some applications in acid mine water research. Iron isotopes can help researchers to understand the mechanisms of redox reactions during the formation of iron in acid mine water, as well as iron transport and transformation processes [29].

There are many reports on the application of isotope techniques in the field of groundwater environment research. The stable isotope technique, specifically, is a natural tracer which has a specific composition ratio that does not change significantly with the transformation of pollutants during flow, and its application in the research process is safe and efficient. At present, research has progressed from traditional water chemical analysis to water analysis including multi-isotope tracing methods, often combined with mathematical model analysis and isotope-mixing models, thus advancing from qualitative judgment to quantitative results. To date, hundreds of papers have been published on mine water sources and hydrology [30,31,32], acid mine drainage (AMD), discharge and contamination processes [33,34,35], and contaminant enrichment and transport [36,37,38]. Stable isotope techniques have been widely used in groundwater and mine water studies because of their high degree of accuracy, applicability, and ability to identify the contribution of different sources quantitatively. Global environmental changes will further emphasize the urgency of research into mine water pollution, and more effective tools are urgently needed to protect and manage valuable water resources and to mitigate the challenges posed by mine water pollution. As the technology continues to develop, it is anticipated that there will be an increased emphasis on using isotopic techniques in future mine water studies.

Bibliometrics is now a powerful tool for identifying established and emerging research themes in a given area, and for helping to identify clusters of research institutions and practitioners [39]. Therefore, to gain a deeper understanding of the application and research of stable isotope techniques in tracing mine water pollution and its environmental impact, this paper provides an in-depth analysis using bibliometrics to provide references and ideas for future researchers.

2. Materials and Methods

2.1. Research Methods

Bibliometric analysis provides an objective and systematic method to explore the evolution and hotspots of specific research areas, as documented in the literature [39,40,41]. To date, various bibliometric software tools have been extensively utilized by researchers during literature reviews, demonstrating their powerful functionalities [42,43,44,45]. Among these tools, CiteSpace, developed by Chaomei Chen at Drexel University (USA), is particularly significant. It serves as a crucial visualization tool in bibliometric research, enabling the detailed analysis and representation of trends and patterns within fields [46].

This study employed CiteSpace (version 5.7R5), which is hereafter referred to as CiteSpace, to analyze various bibliometric aspects such as document types, publication years, countries, institutions, authors, keywords, and co-cited references, forming a social network graph. These visualizations offer insights into the application of current stable isotope techniques in mine water contamination research, providing researchers with a more intuitive understanding of the field.

CiteSpace is a Java-based application designed to analyze and visualize co-citation networks, utilizing co-citation analysis theory and Pathfinder network algorithms [46,47]. The workflow in CiteSpace includes several procedural steps: time slicing, thresholding, modeling, pruning, merging, and mapping. Initially, a set of bibliographic data files is imported into CiteSpace. Subsequently, time slices and thresholds are established. The entire time interval is divided into equal-length segments, ranging from one year to one interval. In CiteSpace, users can select node types and criteria. Node types, such as the author, country, subject, abstract, keywords, and references, define the network type. Selection criteria may include the g-index, Top N, and Top N%, among others. The k value is a parameter used for setting the g-index, which is defined as the highest order g such that the top g papers, when sorted by citation frequency, have at least g² cumulative citations [48]. In CiteSpace, the desired g-value can be adjusted using a scale factor k, allowing for the inclusion or exclusion of more nodes. During the modeling phase, co-citation counts within each time-sliced segment are computed and normalized as cosine coefficients.

$$cccosine={\displaystyle \frac{cc\left(i,j\right)}{\sqrt{c\left(i\right)\times c\left(j\right)}}}$$

(1)

where cc(i, j) is the co-citation count between documents i and j, and c(i) and c(j) are their citation counts, respectively. The user can specify a selection threshold for co-citation coefficients; the default value is 0.15.

Although pruning is not always essential, effective pruning can enhance the clarity of the final network visualization. CiteSpace incorporates two prevalent network pruning algorithms: Pathfinder and Minimum Spanning Tree. In this study, the Pathfinder algorithm has been selected for network pruning. The underlying principle of this algorithm is that in a network, paths with a high number of nodes are likely to represent major pathways due to strong interactions between the nodes on these paths, indicating their significance within the network. This algorithm effectively simplifies the network and accentuates its critical structural features, offering a unique solution in comparison to other algorithms. Ultimately, the sequence of time-sliced networks is consolidated into a synthesized network. The layout for each network is generated using Kamada and Kawai’s algorithm [49].

CiteSpace employs a hard clustering approach to segment a co-citation network into multiple non-overlapping clusters, grouping strongly connected nodes together while assigning loosely connected nodes to different clusters. In this process, co-citation similarities between nodes i and j are quantified using cosine coefficients [50]. Cluster label candidates are subsequently derived from the titles, abstracts, and index terms of the articles citing each cluster. CiteSpace utilizes three term-ranking algorithms: tf*idf [51], log-likelihood ratio (LLR) tests [52], and mutual information (MI). This paper adopts the LLR algorithm, which assesses the strength of association between two events by calculating the difference between the joint occurrence probability of the events and the probability of each event occurring independently. In text analysis, this algorithm is typically used to determine the co-occurrence strength of words. Additionally, metrics such as burstiness, centrality, modularity, and silhouette are implemented and integrated into various visualizations within CiteSpace. Burst detection, as introduced by Kleinberg [53], assesses statistically significant fluctuations in frequency functions over short intervals within the overall period and can also identify notable enhancements in specific connections over brief periods. Burst strength quantifies the abrupt increase in the frequency of occurrence of a keyword or topic within a designated period. Betweenness Centrality, referred to here as centrality, is a pivotal metric in social network analysis, measuring a node’s importance by determining how frequently it appears on the shortest path between all node pairs within the network. Nodes with higher occurrence rates exhibit greater centrality [54]. The modularity Q and silhouette S are metrics used to evaluate clustering effectiveness [50]. The modularity Q assesses the degree to which a network can be divided into distinct modules, with higher modularity suggesting a well-structured network.

$$Q={\displaystyle \frac{1}{2m}}\sum _{ij}\left(A_{ij}-{\displaystyle \frac{k_{ij}}{2m}}\right)\delta \left(c_i,c_j\right)$$

(2)

where A_ij is the weight of the connection between nodes i and j. k_i and k_j are the number of edges connected to nodes i and j, respectively, m is the total connection weight of the network, c_i and c_j are the communities in which nodes i and j are located, and the δ function δ (u, v) is 1 if u = v and 0 otherwise [55].

The silhouette score S is utilized to assess the uncertainty associated with the determination of clustering characteristics.

$$S={\displaystyle \frac{b-a}{\max \left(a,b\right)}}$$

(3)

where a is the average distance from the object to all other objects in its own cluster and b is the average distance from the object to all other objects in some other cluster. b is the smallest one [56].

2.2. Data Source and Search Criteria

The data for the statistical analyses presented in this paper were sourced from the Web of Science Core Collection (WoSCC) database. This study utilized an advanced search strategy to conduct a comprehensive database search using the following terms: TS = (mine-water OR mine-drainage OR mining-drainage OR mining-water) AND TS = isotope. This search criterion was designed to identify articles where terms like “mine-water” or “mine-drainage” appear in the title, abstract, or keywords and are associated with the term “isotope”. The use of hyphens in search terms ensures the appearance of phrases as combinations, such as “mine-water”, to exclusively retrieve records containing the exact phrases without including unrelated records. The literature search covered the period from 1998 to 1 January 2023, without restrictions on language, document type, or data category. The initial search yielded 448 records, with articles and review articles comprising the largest proportion, amounting to 95.76%. This significant proportion reflects the development trends and changes in isotope techniques in mine water pollution research. Consequently, the focus was narrowed to these two types of documents, totaling 429 papers, by performing a secondary manual screening based primarily on the titles, abstracts, and keywords of the papers. Due to the broad scope of the search, some documents of little relevance were initially retrieved, necessitating manual screening to ensure relevance. Articles that mentioned only “mine water” or “isotopes” without relevance to the research topic, as well as those retrieved because search terms appeared in “keywords plus” but were unrelated to the topic, were excluded. Following this secondary screening, 300 records remained. Figure 1 illustrates this research selection process in a flowchart.

3. Results and Discussion

3.1. Quantity of Published Articles and Research Area

According to the records from the Web of Science Core Collection (WoSCC), the 300 papers related to the field of this study accrued a total of 7506 citations between 1998 and 2023. Figure 2 illustrates the annual number of publications and citations in this research field over the same period. As depicted in Figure 2, despite some fluctuations in the number of articles and citation frequency in certain years, there is an overall increasing trend. The growth in citation frequency has lagged behind the number of publications, with notably slow or stagnant growth in citation frequency prior to 2006, which correlates with a lower number of publications in those years. This trend may be attributed to changes in keywords and themes over the course of the study, as well as the inability to retrieve some of the earlier literature in a machine-readable format, leading to its omission. Post-2006, there was a significant increase in citation frequency, peaking at 992 citations in 2022.

The research areas highlighted in Figure S1 predominantly focus on environmental sciences. Additionally, this field is interdisciplinary; the formation and discharge of mine water and its related pollution to the surrounding environment represent complex interactions among surface and subsurface elements, hydrogeological structures, natural processes, and human activities. The application of isotope techniques in studying mine water processes spans multiple environmental disciplines, including groundwater resources, general geosciences, process engineering, chemical engineering, and civil engineering, with a particular emphasis on hydrogeochemistry and hydrology. Consequently, it is advantageous to integrate knowledge and technical methods from a broad spectrum of research fields.

3.2. Analysis of Countries, Institutions, Authors, and Co-Reference Articles

3.2.1. Countries

An analysis of literature from various regions highlights the contributions and impact of different countries in specific fields. Table S1 presents the top 10 countries by the total number of published articles. In the realm of isotope techniques applied to mine water research, the United States leads with the highest number of publications, accounting for 33.00% with 99 articles. It is followed by China with 62 articles (20.67%), Canada with 23 articles (7.67%), Germany with 22 articles (7.33%), Spain with 16 articles (5.33%), and Poland also contributing significantly with 16 articles (5.33%). The countries listed predominantly feature rich mineral resources and demonstrate strong scientific research capabilities.

To ascertain the general trend of interrelationships among countries in this field, this study designated a time slice of five years, selected countries as the nodes for analysis, and set the data selection criterion at k = 25. Concurrently, to enhance the intuitiveness of the visualization, nodes with lower frequencies were hidden. This approach facilitated the mapping of international collaborations (Figure 3).

In the visualization, the nodes consist of “tree rings” in various colors, representing six distinct time periods from 1998 to 2023, each denoted by a specific color ranging from purple to red. The thickness of these “tree rings” correlates with the frequency of occurrence of each country within a given period. The color of the lines connecting the nodes denotes the time period during which the cooperation occurred, while the thickness of these lines indicates the intensity of the cooperation between the countries. The centrality is represented by violet rings on the outer edges of the nodes [42,57]. Figure 3 illustrates that the United States holds a prominent position, with an intermediary centrality score of 0.74, highlighting its extensive publication record and frequent collaborations with other nations. The United States was also one of the pioneers in researching this field. Although China’s research commenced later, as indicated by the prominent yellow circle, its publication volume has quickly surpassed that of all other countries except for the United States, reflecting China’s rapid development and expansion in this field since 2018. However, in terms of collaboration, China primarily engages more intensively with the United States, Japan, and Spain, and less so with other leading countries.

3.2.2. Institutions

In the CiteSpace analysis, when setting the node to represent institutions, other settings, nodes, and link color thicknesses maintain the same interpretative value as in the national network mapping. Figure 4 illustrates a map of publications from research institutes and their collaborative relationships over the past nearly 25 years. Concerning these nodes and their connections, a total of 194 organizations have employed isotope techniques to address issues related to mine water, engaging in a total of 225 collaborative interactions.

Figure 4 depicts the collaboration network among the most frequently mentioned institutions, with U.S. and Chinese university institutions prominently featured and divided into two main research subjects. Specifically, the centrality of U.S. institutions, at 0.23, is significantly higher than that of institutions from other countries. The first research cluster, spearheaded by the United States Geological Survey, began its investigations in the field during the 2003–2008 period. The second cluster, led by the Chinese Academy of Sciences and the China University of Geosciences, demonstrated closer collaboration, albeit with a later start. According to Table S2, this Chinese group of researchers exhibits less prominence and influence in the field compared to the initial U.S.-led group. From a temporal standpoint, the majority of the research activities and collaborative efforts by these institutions were initiated post-2008.

3.2.3. Authors

CiteSpace (version 5.7R5) was utilized to perform a visual analysis of author collaborations in our research field from 1998 to 2023, maintaining all other settings as constant. This analysis generated a knowledge map of author collaborations (Figure 5) and a statistical table featuring authors who appeared with a frequency of five or more (

Table 1. High-frequency authors with a frequency of 5 or higher in this bibliometric study.

Rank	Author	N (%)	Highest Cited Documentation
1	Pan Wu	8 (2.67%)	Stable sulfur and oxygen isotopes as geochemical tracers of sulfate in karst waters
2	Marion Tichomirwa	7 (2.33%)	The isotopic composition of sulfate from anaerobic and low oxygen pyrite oxidation experiments with ferric iron—New insights into oxidation mechanisms
3	Agnieszka Galuszka	6 (2.00%)	Geochemistry and stable sulfur and oxygen isotope ratios of the Podwisniowka pit pond water generated by acid mine drainage (Holy Cross Mountains, South–Central Poland)
4	Chunlu Jiang	6 (2.00%)	Using delta S–34–SO4 and delta O–18–SO4 to trace the sources of sulfate in different types of surface water from the Linhuan coal-mining subsidence area of Huaibei, China
5	Fengshan Ma	6 (2.00%)	Investigating the characteristics of mine water in a subsea mine using groundwater geochemistry and stable isotopes
6	Haijun Zhao	6 (2.00%)	Investigating the characteristics of mine water in a subsea mine using groundwater geochemistry and stable isotopes
7	Jie Guo	6 (2.00%)	Investigating the characteristics of mine water in a subsea mine using groundwater geochemistry and stable isotopes
8	Liugen Zheng	6 (2.00%)	Using δ34S–SO4 and δ18O–SO4 to trace the sources of sulfate in different types of surface water from the Linhuan coal-mining subsidence area of Huaibei, China

Note: The authors in the table are not necessarily the first author of their corresponding highest cited documentation.

).

CiteSpace identified a total of 216 authors to analyze general trends in the interrelationships among them. These authors were linked through 296 collaborative interactions, resulting in a network density of 0.0127. As depicted in Figure 5, most authors engaged in frequent collaborations particularly after 2013, forming numerous “small circles” where they worked closely. However, it is notable that these “circles” were largely isolated, exhibiting minimal interconnections.

Table 1 lists the eight most productive authors in this field. The most prolific, Pan Wu, has authored eight publications, with the most cited work being “Stable sulfur and oxygen isotopes as geochemical tracers of sulfate in karst waters”. This research utilizes hydrochemical variations and stable isotope dynamics in karst waters to trace mining contamination in karst aquifers and to assess the impact of AMD-related waters on karst groundwater. It also aims to elucidate the origins and pathways of dissolved sulfate in low-pH karst waters [58]. Marion Tichomirowa and Agnieszka Galuszka follow in the ranking, each with more than five publications. The aforementioned “small circles” predominantly revolve around these frequently cited authors. In the distribution of these author groups, apart from the close collaboration centered around Pan Wu, there are several tightknit author circles. Researchers including Liugen Zheng and Chunlu Jiang focus on the effects of mining activities, such as mine water pollution, on regional hydrology within mining areas. Their studies primarily address contaminants like sulfate and nitrate among other pollution issues [59,60,61,62]. Strontium and iron isotopes have been extensively analyzed by researchers, notably Rosemary C Capo, to determine the origins of total dissolved solids in mine water and the migration of brines [63,64,65]. Additionally, researchers such as Jie Guo and Haijun Zhao et al. concentrate on employing water chemistry and isotope techniques to explore the connectivity between water bodies, identify sources of mine water, and quantify the contribution of each water source [66,67,68,69].

3.2.4. Analysis of Co-Reference Articles

CiteSpace visual analysis software was utilized to conduct co-citation analysis. To streamline the network and emphasize its core structural features, pruning was applied to reduce the number of links, thereby simplifying the network and ensuring that the final visualization was not too dense to discern structural details [42,47]. All other settings, including the color code and line thickness, remained unchanged. Literature co-citation analysis facilitates the identification of the most highly cited and influential research references within a specific research area [70].

As illustrated in Figure 6 and

Table 2. Highly co-cited papers in this bibliometric study.

Rank	Title	Frequency	First Author	Year
1	Oxygen and sulfur isotope systematics of sulfate produced by bacterial and abiotic oxidation of pyrite	20	Balci N	2007
2	Geochemistry and stable isotope composition of the Berkeley pit lake and surrounding mine waters, Butte, Montana	13	Pellicori DA	2005
3	Stable sulfur and oxygen isotopes as geochemical tracers of sulfate in karst waters	12	Sun J	2017
4	Stable isotope geochemistry of acid mine drainage from the Wisniowka area (South–Central Poland)	12	Migaszewski ZM	2018
5	Using stable isotopes (S, O) of sulfate to track local contamination of the Madison karst aquifer, Montana, from abandoned coal mine drainage	10	Gammons CH	2013
6	Hydrochemical and stable isotope indicators of pyrite oxidation in a carbonate-rich environment; the Hamersley Basin, Western Australia	10	Dogramaci S	2017
7	Pyrite oxidation: A state-of-the-art assessment of the reaction mechanism	10	Rimstidt JD	2003

, the most cited article by Balci et al. (2007) [71], published in Geochimica et Cosmochimica Acta, offers insights into the reaction pathways of pyrite oxidation and the influence of biogeochemical controls on the δ¹⁸O and δ³⁴S values of sulfate in acid mine drainage (AMD) and other natural environments. The subsequent article, by Pellicori et al. (2005) in Applied Geochemistry [72], provides a comprehensive analysis of the hydrological relationships in mine water samples from pit lakes, submerged underground mines, and acidic springs, examining the sources and formation processes of sulfate in the evolution of pit lakes. This paper is followed by the studies of Sun et al. (2017) [58] and Migaszewski et al. (2018) [73]. Sun et al. (2017) discussed the hydrochemical and isotopic effects of acid mine water on karst aquifers. Migaszewski et al. (2018) evaluated the potential environmental impacts of pyrite mineralization and AMD-contaminated water in the western part of the Main Range of the Holy Cross Mountains (South–Central Poland) on adjacent rivers and local perched aquifers.

In addition to co-citation frequency, the intensity and centrality of co-citations provide crucial insights into content similarity and the strength of connections between two co-cited articles. Centrality indicates that a specific article has established co-citation links with multiple documents, acting as a central “traffic hub”. An article characterized by both high centrality and frequency highlights the prevalent research themes during that time. Analysis of data from 1998 to 2002 shows a low count of co-cited documents with most centralities at 0, peaking at only 0.09 (

Table 3. Key literature by period in this bibliometric study.

Period	Title	Centrality	Frequency	First Author
1998–2002	Oxygen and sulfur isotope fractionation during anaerobic bacterial disproportionation of elemental sulfur	0.09	2	Bottcher ME
2003–2007	Geochemistry and stable isotope composition of the Berkeley pit lake and surrounding mine waters, Butte, Montana	0.22	13	Pellicori DA
2008–2012	Effect of progressive acidification on stable carbon isotopes of dissolved inorganic carbon in surface waters	0.44	4	Ali HN Atekwana EA
2013–2017	Using stable isotopes (S, O) of sulfate to track local contamination of the Madison karst aquifer, Montana, from abandoned coal mine drainage	0.68	10	Gammons CH
	Strontium isotope quantification of siderite, brine, and acid mine drainage contributions to abandoned gas well discharges in the Appalachian Plateau	0.64	9	Chapman EC
2018–2023	Using multi-isotopes (S–34, O–2, H–2) to track local contamination of the groundwater from Hongshan-Zhaili abandoned coal mine, Zibo City, Shandong Province	0.23	6	Zhou JW

). However, from 2003 to 2007, there was a notable increase in the number and frequency of co-citations, with centralities reaching up to 0.45. During this phase, the study by Pellicori et al. (2005) [72] demonstrated both high centrality and frequency. From 2008 to 2012, several significant studies emerged, such as Ali and Atekwana (2009) [74], which recorded the highest centrality of 0.44. This study explored how acid mine drainage affects the carbonate balance in surface waters and its subsequent impact on the isotope ratios of dissolved inorganic carbon (DIC). It is closely associated with other high-centrality works like Hamel et al. (2010) [75] (centrality = 0.42). The period 2013 to 2017 saw two publications with centralities over 0.6; Gammons et al. (2013) [76] utilized the isotopic composition of dissolved sulfate to investigate AMD-related water sources and trace their pollution impacts, and Chapman et al. (2013) [63] employed strontium isotopes to examine contaminants and their interactions within complex aqueous systems. In the most recent period, 2018 to 2023, although the number of co-cited documents remained high, those with significant centrality were fewer, with the maximum centrality recorded at 0.23. A key work during this time, Zhou et al. (2018) [77], used stable isotopes of water and dissolved sulfate to analyze sulfate sources in aquifers associated with coal mine discharges.

The analysis of collaboration patterns among countries, institutions, and authors reveals a limited extent of cooperative exchanges across geographic regions, contributing to weaker partnerships. This lack of collaboration can lead to resource dispersion and data fragmentation, thereby hindering the production of systematic and comprehensive research findings. Moreover, insufficient communication among researchers may restrict the spread and adoption of innovative technologies and methods. Furthermore, the absence of close collaboration often results in redundant research efforts and resource wastage. Promoting cooperation and communication among researchers is thus crucial for advancing the development of isotope techniques in mine water studies. The literature that receives high citations typically focuses on isotope tracing of mine pollution and hydraulic linkages in mine water, which is not coincidental. On one hand, the formation of mine water is directly linked to complex geological conditions and mining activities, which significantly impact the surrounding environmental and water resource safety. From both governmental policy and corporate responsibility perspectives, there is a mandate for companies to implement effective strategies to mitigate and manage environmental issues arising from mine water. On the other hand, stable isotope tracer technology has emerged as a potent tool for investigating these issues, offering the distinct advantage of precisely identifying pollution sources and delineating the migration paths of pollutants.

3.3. Analysis of Research Themes and Hotspots

3.3.1. Research Themes

Keywords succinctly encapsulate the core content and themes of an article, mirroring specific areas of research. By examining the co-occurrence, frequency, and the strength of connections among these keywords, researchers can identify the prevailing hotspots and discern emerging trends within the field [78,79]. In this study, CiteSpace was employed to generate visualizations including a keyword co-occurrence map (Figure 7a), a time-zone map (Figure 7b), and a timeline map (Figure 8). The configurations for these visualizations were consistent with those outlined in the previous section, with the modification that the node type was set to “keyword”. The centrality of nodes with frequencies below 5 are less than 0.05, and shadowing them does not affect the overall accuracy of the network. Therefore, in order to improve the readability of the graph, setting the frequency greater than 5.

As depicted in Figure 7, keywords such as “acid mine drainage”, “geochemistry”, “groundwater”, and “water” are prominent due to their high frequency of occurrence, highlighting the significant application of isotope technology in mine water research and associated environmental pollution. This research spans four main topics: First, the environmental impact of mine water, particularly focusing on acid mine drainage (AMD) and its influence on the hydrology and hydrochemistry of mining areas, with key terms including “acid mine drainage”, “groundwater”, “geochemistry”, “aquifer”, “surface water”, and “pollution”. Second, the hydraulic connections between mine water and the natural environment, emphasizing the sources, behaviors, and predictive models for pollutants like sulfate and transition metals, with important keywords being “precipitation”, “sulfate”, “sulfur”, “metal”, and “transport”. Third, the oxidation mechanisms of sulfide minerals, mainly pyrite, and the evolution of their isotopic signatures during oxidation, highlighted by terms such as “pyrite”, “pyrite oxidation”, and “isotope”. Lastly, the study of isotope characterization, fractionation, and biogeochemical processes in the environment, focusing on “stable isotope”, “isotope”, and “fractionation”. Collectively, these topics aim to assess the environmental impacts of mine water using stable isotope techniques and delve into the detailed characterization of isotopes within the mining environment, addressing both practical and theoretical aspects of isotope application in environmental studies.

Based on the keyword co-occurrence map, CiteSpace functionality was utilized to identify clusters and label them using indexing terms. Concurrently, the LLR algorithm facilitated cluster analysis and generated the timeline map illustrated in Figure 8. The LLR is a prevalent algorithm in CiteSpace for analyzing clusters, primarily aimed at identifying themes, keywords, hotspots, and extracting cluster labels from the literature. The modularity (Q) and silhouette (S) values of the clusters are 0.8011 and 0.902, respectively. A Q-value greater than 0.3 suggests that the clustering structure is significant, an S-value above 0.5 indicates reasonable clustering, and values exceeding 0.7 denote highly efficient and convincing clustering. In this analysis, eight major clusters were deemed significant and selected for further examination. The clusters are numbered from 0 to 7, with a decreasing number of keywords as the numbers increase. Detailed cluster information can be found in Supplementary Table S3.

Cluster #0, which pertains to formation water, primarily utilizes stable isotopes to analyze the effects of mine water and its components, notably transition metals, on groundwater and adjacent waters during mining operations. It also focuses on developing recommendations for the remediation and management of groundwater. For instance, Raudsepp et al. [80] employed stable carbon and hydrogen isotopes from methane, along with gene amplicon sequencing techniques, to examine microbial communities in mine water and other coal seam environments. This study particularly addressed the changes in microbial communities in formation water due to mine dewatering. Additionally, Borrok et al. [36] and Batsaikhan et al. [81], together with other researchers, used hydrochemical assessments and isotope techniques to investigate transition metal contamination arising from mine drainage and mining activities in neighboring watersheds. These investigations provide crucial insights into the management of local mining operations and the application of isotope techniques.

Cluster #1, labeled as “environmental isotopes”, explores the extensive applications of these isotopes in mine water research. This cluster encompasses most of the high-frequency keywords and broadly summarizes the research found in several other clusters. The research is primarily dedicated to examining hydrological and biogeochemical processes in the water environments of mining areas [73,82,83], and to analyzing the sources, transport, and transformation of pollutants [84,85]. Additionally, investigations have focused on the origins of mine water, its hydrochemical characteristics, and its environmental impacts [37,86,87]. Further studies have addressed the oxidation pathways and isotopic properties of sulfide minerals, specifically pyrite, during the formation of mine water [88,89,90]. These findings align closely with those derived from the keyword co-occurrence analysis previously described.

Cluster #2, focusing on copper, and Cluster #6, concentrating on nitrogen, employ multiple geochemical tools, including isotope-mixing models, to investigate the origins, movement, and alteration of transition metals and sulfates in acidic mine water. These studies also examine water–rock interactions and the isotopic evolution of mine water during its formation. For instance, Masbou et al. [91] analyzed the export and mobility of copper in acid mine drainage by examining the relative abundance of dissolved copper isotopes and assessing the variations in copper sources under different hydrological conditions. Similarly, Ali and Atekwana [92] undertook a comprehensive study on the impact of dissolved inorganic carbon (DIC) and CO₂ (g) resulting from acidification and neutralization reactions on the evolution of carbonate and stable carbon isotopes in groundwater. This included measuring the concentrations of major ions, DIC, and stable carbon isotope ratios in mine water.

Cluster #3, focusing on hydrology, and Cluster #4, concerning mine water, utilize water chemistry and isotopic analyses to study the thermal energy generation of mine water and its impact on hydrogeology. Additionally, these studies explore the isotopic composition of local aquifers to trace the geochemical origins of mine water. For instance, Burnside et al. [93] and Loredo et al. [94] researched geothermal energy resources in abandoned and flooded mines. Ren et al. [95] investigated the impact of acidic mine water on water quality evolution within karst aquifers. Li et al. [96] examined hydraulic connections between mine adits and adjacent aquifers and quantified the sources of recharge for mine water. Sun et al. [97] evaluated the hydrochemical characteristics of streams in catchments both affected and unaffected by acid mine drainage.

Cluster #5, focusing on bacterial sulfate reduction, and Cluster #7, concentrating on sulfur isotopes, employ sulfur and oxygen isotopes to examine biogeochemical processes including sulfate sources, transformations, and reductions. For instance, M. Yang et al. [98] investigated the factors influencing bacterial sulfate reduction (BSR) and its interaction with organic carbon (OC) mineralization in seasonally hypoxic reservoirs. Their method involved analyzing stable carbon and sulfur isotopes alongside the diversity of microbial communities. Xia et al. [99] integrated isotopic and hydrochemical data with sediment mineralogical composition to explore the mechanisms controlling sulfate levels in a river impacted by AMD, highlighting the significant role of bacterial sulfate reduction in these transformation processes. Knöller et al. [100] introduced glycerol into an AMD-contaminated mine aquifer to stimulate the proliferation and activity of sulfate-reducing bacteria, observing sulfate reduction activity through changes in isotopic composition and fractionation patterns during this intervention.

The keyword and cluster analysis highlights the prominence of topics such as pollutant source identification, transport transformation, and environmental impact assessment. These topics, encompassing a broad range of studies and keywords, align with current environmental protection policies and resource utilization. They are also directly linked to the practical needs for addressing environmental challenges. Conversely, topics related to the characterization of isotopes and biogeochemical processes are less emphasized, tending to focus on fundamental scientific inquiries. The limited number of studies on isotope characterization may stem from their foundational and theoretical nature, where the urgency is not as pronounced as that of applications addressing immediate practical needs. Nevertheless, advancements in understanding isotopic properties are expected to enhance their application in mine water management. Current research predominantly addresses heavy metal and sulfate pollutants, with more limited exploration of other contaminants such as emerging organic pollutants. Moreover, much of the existing research focuses on individual pollutants and lacks a thorough examination of isotope composition and potential isotope fractionation for pollutant identification under complex environmental scenarios involving multiple pollutants. Additionally, most studies on the isotopic composition of mine water and its surrounding water environments have primarily concentrated on spatial variations. There is a noticeable deficiency in comprehensive and multidimensional (temporal and spatial) explorations of isotopic characterization and fractionation. Furthermore, there is insufficient research on isotopic responses to long-term dynamic changes in mine water influenced by climate change.

3.3.2. Research Direction and Hotspots

Burst detection is utilized to identify frequently used, cutting-edge terms known as “hot” words in research fields, primarily based on the vocabulary growth rate in the literature to pinpoint keywords. The intensity of these keywords reflects the significance of the research hotspots [101]. In this study, CiteSpace software was employed to identify 20 keywords with a high burst strength (Figure 9). By combining this with an analysis of the literature related to keywords that exhibit the strongest citation bursts, it is possible to discern the cutting-edge dynamics and development trends in the associated research [102]. The top 11 keywords, each with a burst strength of three or more, effectively represent the predominant research hotspots in the field.

The keywords are now categorized into three phases, illustrating the evolution of research hotspots over the last 20 years more accurately as follows:

(1)

1998 to 2007: The most prolonged keyword burst during this period was “sulfate”, followed by “iron”, “sulfide”, “drainage”, and “geochemistry”, with “sulfate” and “geochemistry” manifesting greater intensity. This trend highlights the focus on isotopic techniques for investigating hydrogeochemical processes involving sulfate and sulfide in mine water. Researchers extensively utilized water isotopes, along with sulfur and oxygen isotopes, coupled with hydrochemistry data, to analyze the geochemical sulfur cycle in mine water and the surrounding areas [33,103]. Furthermore, significant experiments and investigations were conducted to explore isotopic composition and fractionation [89,104].

(2)

2008 to 2012: Keyword bursts in this phase were more intense and sustained over longer periods. Beyond “iron” and “sulfide”, keywords like “metal”, “dissolution”, “Thiobacillus ferrooxidans”, and “Acidithiobacillus ferrooxidans” were prevalent. When linked with keywords such as “kinetics” and pertinent research literature, it becomes evident that the research focus during this stage was primarily on using isotope techniques to examine metal loading pathways and transport mechanisms associated with mine water and their effects on pollution in surrounding natural waters [105,106]. Additionally, the role of biogeochemical controls on isotope fractionation and the mechanisms of dissolution and oxidation of sulfide minerals during mine water formation were significant areas of study [82,107,108].

(3)

2013 to 2023: Long-burst keywords for this phase include “isotope”, “trace element”, “dissolved sulfate”, “surface water”, and “tracer”. Alongside these terms, the integration of “origin”, “hydrochemistry”, and related research literature indicates that the focal points of research during this period are the development of innovative isotope tracer methods and techniques [109,110,111]; investigations into the sources and destinations of dissolved sulfate in mine water and its effects on the surrounding water environment [73,112]; and studies on hydraulic linkages [113,114] and hydrogeochemical characterization [94,97].

During these three phases, the application of isotope techniques to investigate the source of sulfate in mine water and its effects on the surrounding aquatic environment has consistently been a research focus. The sustained burst period for keywords such as “surface water” and “dissolved sulfate” up to the present day indicates that this topic is likely to remain a hotspot in upcoming research phases. Furthermore, the keyword “isotope” has maintained a continuous presence from its initial emergence to the present, suggesting that research into innovative isotope-tracing methods and isotope fractionation mechanisms may also become prominent in the next research phase.

In the intricate system of mine water, subtle variations in isotope composition carry significant environmental and geological insights. Consequently, high-precision isotope ratio measurement techniques are essential to accurately interpret these crucial, yet minute, indicators. As research into mine water issues intensifies, the demand for more precise information on isotope composition grows increasingly urgent, necessitating enhancements in the accuracy of isotope ratio measurement methods. Additionally, researchers are focusing more on understanding the evolution mechanisms of mine water quality under the influence of multiple factors. In this context, there is an urgent need to investigate the isotope fractionation mechanisms influenced by complex multifactorial conditions and to elucidate the laws of isotope distribution and transformation under multi-field effects. This research will provide a critical foundation for understanding water quality evolution. Moreover, advanced technologies like big data analysis and numerical simulation are increasingly being integrated into mine water studies. However, the synergy between isotope techniques and such technologies remains insufficiently developed and requires strengthening to enhance the comprehensiveness and precision of research on the sources, migration, and transformation of pollutants in mine water.

4. Conclusions and Outlook

In this study, utilizing WoSCC data from 1998 to 2023, CiteSpace was employed to visualize and analyze the countries, institutions, authors, core literature, and keywords involved in the research and application of isotope technology in mine water and its related environmental pollution. The findings are summarized as follows:

• Over the past two decades, the application and study of isotope technology in mine water and its environmental impacts have spanned numerous disciplines and experienced rapid development, with a noticeable growth in publications post-2006.
• The substantial volume and centrality of publications from the United States highlight its significant contributions and robust international influence in this field. In contrast, despite a high output of articles, China shows a considerable disparity in publication centrality, indicating the need for enhanced impact and influence. Moreover, research institutions in China are dispersed with limited cross-country exchanges and collaborations.
• Author collaborations are marked by low centralization and high decentralization. Active authors and their seminal works are predominantly from the mid-to-late study period, playing a crucial role in advancing the application of isotope techniques for pollution tracing and hydrological linkages. Several pivotal documents have emerged with high centrality and frequent citations, often in studies that integrate theoretical models with empirical pollution research, which tend to receive more citations.
• The application of stable isotopes in research includes studying the origins and pathways of specific contaminants, identifying and quantifying mine water sources and their mixtures, examining hydrological and biogeochemical processes, and elucidating the evolution of water chemistry and isotopic compositions along with their environmental impacts. Keywords like “surface water”, “dissolved sulfate”, and “isotope” have garnered prolonged attention, suggesting that upcoming research will continue to focus on innovative isotope-tracing methods and techniques applicable to mining environments, mechanisms of isotope fractionation, and the sources of sulfate and their environmental effects.
• Presently, the research community predominantly favors the use of stable isotopes, particularly hydrogen, oxygen, and sulfur isotopes, for tracking pollutants and identifying sources. Hydrogen and oxygen isotopes are extensively applied in identifying the sources, transportation, and recharge mechanisms of mine water. In the area of AMD research, which is significant within the mining field, sulfur and oxygen isotopes provide essential data for pinpointing the origins of sulfate in conjunction with sulfur cycling processes and the formation and evolution of AMD. The biogeochemical methodology employing sulfur isotopes to investigate the sulfur cycle offers vital insights into AMD evolution, as it incorporates sulfur redox reactions and sulfur bacterial activities. Concurrently, studies of the carbon cycle involving methane (CH₄) and sulfur isotope studies in resource exploration, such as oil and shale gas, offer crucial guidance for AMD research.

While CiteSpace is an effective tool for bibliometric analysis, its utilization must be mindful of potential limitations, and efforts should be made to minimize the impact of these limitations on research outcomes through enhanced search and analysis strategies. The issues to consider include the following:

• Changes and Evolution of Keywords and Titles: As research fields develop and terminologies evolve, some earlier used keywords may not appear in more recent literature. This evolution can result in the omission of significant studies if searches are confined to contemporary keywords.
• Coverage of Older Literature and “Grey” Literature: Significant early works may not be accessible in machine-readable formats, making them unavailable in electronic databases. This gap can lead to an incomplete representation of the field, especially for historical analyses.
• Exclusion of Non-Academic Publications: Valuable practice-based studies often find their way into reports or internal documents rather than peer-reviewed academic journals. Such sources are typically excluded from bibliometric analyses, potentially omitting practical insights and applications from the analysis.

Through the visualization and analysis of the research landscape and hotspots, it has been determined that, besides the aforementioned characteristics, the research and application of isotopes in the field of mine water exhibit several deficiencies. It is recommended to enhance research in the following three areas:

• Given the complexity of isotope fractionation effects and the environments in which mine water forms, many fractionation effects remain inadequately studied. Current research predominantly addresses the fractionation mechanisms of sulfate–sulfur–oxygen isotopes during pyrite oxidation and bacterial sulfate reduction. However, systematic investigations of other isotope systems, especially non-traditional isotopes like those of transition metals, which are crucial for studying their transport and reactivity, are relatively limited.
• Most studies on isotopic composition in mine water and its surrounding environments have primarily focused on spatial scale differences. There is a need to augment research on the evolution of isotopic composition across temporal scales to better understand the dynamics over time.
• The application and exploration of isotope techniques in studying mine water and its resultant environmental pollution face numerous challenges. On one hand, the isotopic composition and potential isotope fractionation introduce uncertainties in identifying pollutant sources under complex environmental conditions, such as when multiple pollutants are present simultaneously. On the other hand, isotope techniques demand high precision in instrumentation, skilled personnel, and meticulous sample preservation, which can restrict their use in mining environments. Currently, researchers often enhance accuracy by employing a combination of multiple isotopes and complementary techniques. Future improvements in source resolution accuracy could be achieved by intensifying studies on isotope fractionation in mining environments, incorporating a broader range of isotopes, and refining quantitative source resolution models. Furthermore, there is a pressing need to advance research on isotope testing techniques and methods, reduce analysis costs, enhance testing accuracy, and expand the application scope and depth in the field of mine water.