Multi-isotopes and hydrochemistry combined to reveal the major factors affecting Carboniferous groundwater evolution in the Huaibei coalfield, North China

Self-Organizing Map provides new insights into the MixSIAR model for calculating the source contributions of sulfate contamination in groundwater

The concentration of sulfate in global groundwater has been observed a significant upward trend in recent years. Excessive sulfate levels contribute to increased groundwater salinity and acidification, thereby posing a threat to human health and ecological balance. For effective groundwater pollution management and control, accurately quantifying the sources of sulfate pollution remains a challenge. This research integrates the Self-Organizing Map (SOM) clustering method to enhance the accuracy of the Bayesian isotope mixing model (MixSIAR) in quantifying the contribution rate of groundwater sulfate. During the dry season, sulfate (SO42-) primarily originates from the oxidation of pyrite, whereas SO42- sources include both pyrite oxidation and the co-dissolution of carbonate rocks and gypsum during the normal and wet seasons. Incorporating SOM, the MixSIAR model demonstrates reduced values of Leave-One-Out Information Criterion (LOOIC), and Widely Applicable Information Criterion (WAIC) (LOOIC = 82.5, and WAIC = 82.3). Overall, in the study area, coal mines (accounting for 34.3% - 48.4%) are identified as the primary pollution sources, particularly in Clusters 3, 4 and 5. Clusters 1, 2, and 5 are more significantly affected by other pollution sources, with fertilizers contributing 32.7%, evaporite dissolution contributing 24.1% and 24.2%, respectively. This study supports the development of regional pollution control strategies.

Impacts of mining on the diversity of benthic macroinvertebrates - A case study of molybdenum mining area in Luanchuan county

Mineral exploitation is one of the human activities that seriously affect freshwater ecosystems. It is of great significance to study the impact of mining on the α and β diversity of macroinvertebrates. This study reveals the response of taxonomic and functional α and β diversity of macroinvertebrates to mining activities in the Luanchuan molybdenum mining area. A total of 40 sets of macroinvertebrates, sediment and water samples in the Taowan North River (TR), Yu River (UR) and Hongluo River (HR) in the molybdenum mining area were collected. The results show that: 1) the mining activities led to obvious differences in the environmental factors of the three rivers. The heavy metals in the sediments and water bodies of TR and UR showed different degrees of exceedance, while there was no exceedance of heavy metals in HR; 2) The taxonomic and functional α diversity was much lower in the TR and the UR than in the HR. The concentrations of heavy metals in sediments and water bodies were significantly negatively correlated with the taxonomic and functional α diversity; 3) Mineral extraction resulted in significant differences in macroinvertebrate β diversity among the three rivers. The taxonomic and functional β diversity of the macroinvertebrate communities in TR and UR was much higher than that in HR. The turnover and nestedness of functional β diversity showed significant differences. Functional β diversity was more obviously affected by heavy metal exceedance than taxonomic β diversity. Nestedness were more sensitive to exceedance of heavy metals than turnover. The results of this study can provide a theoretical basis for ecological restoration and protection of rivers in mining areas.

Remediation of AMD based on hydrogeochemical zonation: A typical metal mine in China

Mining activities disrupt the natural oxidative balance underground, increasing the oxidation of metal sulfides like pyrite. This process leads to the formation of highly acidic mine drainage (AMD) with elevated concentrations of iron (Fe) and sulfate (SO42-). However, generic plugging and backfilling methods, when applied without considering the specific post-mining oxidative environments of different metal mines, often yields minimal results. To clarify the distribution of the underground redox environment after mining of a metal mine in Dexing, China, fifteen water samples from flood and dry periods, as well as fifteen borehole samples, were collected for hydrogeological and chemical analysis. For the first time, the study proposed that the redox zone could be identified and delineated through vertical analysis of water storage media, mineral composition, and hydrochemical characteristics. A hydrogeochemical cause model was constructed, revealing that AMD formation primarily occurs in oxidative and transition zones. Based on the redox zone characteristics of the study area, actual engineering sealing was performed on the oxidation and transition zones of cavity No. 23. As a result, the pH increased from 2.5 before remediation to 4.5, indicating a reduction in acidity. The concentrations of SO42- and Fe significantly decreased, reducing from 1360.0 mg/L and 147.0 mg/L before treatment to 726.0 mg/L and 23.6 mg/L after treatment; the total decrease amounting to 46.6 % and 84.0 %, respectively. The concentrations of Mn and Cu similarly, decreased by 10.7 % and 15.6 %, respectively. This study provides a novel approach and valuable reference for the refined identification and classification of redox zones after metal mine exploitation, as well as for the targeted plugging and treatment of cavities that produce AMD.

Patterns of spatiotemporal variations in the hydrochemistry and controlling factors of bedrock aquifers in the northern region of the Linhuan mining area

Systematically studying the hydrochemical evolution of bedrock groundwater in mining areas during mining process is crucial for effective groundwater resource management and coal mine production. The spatiotemporal characteristics and hydrochemical evolution patterns of the Permian fractured sandstone aquifer (PA) and the Carboniferous Taiyuan Formation limestone aquifer (CTA), both of which are directly associated with coal mining in the northern Linhuan mining area, China, were investigated using multivariate statistical analyses, hydrochemical graphical methods, ion ratio analysis, and a conceptual model. 72 groundwater samples, collected before and after mining, were classified into four groups by hierarchical cluster analysis (HCA). Principal component analysis (PCA) and ion ratio analysis indicated that water-rock interactions involve mineral dissolution (carbonates, gypsum, dolomite, silicates), cation exchange, and common ion effects. Hydrochemical evolution is influenced by bedrock paleotopography, aquifer hydraulic conductivity, and mining drainage. Paletopographic differences significantly influence water-rock interactions and spatial variability in hydrochemistry, with ion concentrations in groundwater increasing as paleotopographic elevation decreases. The pattern of hydraulic conductivity reflects the control exerted by variations in aquifer characteristics on mineral dissolution, leading to minor changes in hydrochemical characteristics. Mining activities disrupt the aquifer's reducing environment, resulting in a significant increase in groundwater SO42- concentration. These findings provide insights and a solid theoretical foundation for studying the hydrochemical variations patterns of groundwater and these control mechanisms in the hidden coal fields of North China.

Investigation into factors controlling groundwater evolution in mining areas with an integrated approach

The study of groundwater evolution is of great significance for water resource protection and management, groundwater pollution control, and ecological environment protection. Experts and scholars have found that the hydrochemical processes and evolutionary patterns of groundwater are determined by both natural processes and human activities. However, there is relatively little research on the evolution of groundwater in mining areas where human activities have a significant impact. Therefore, to study the main controlling factors affecting the hydrogeochemical evolution of groundwater in mining areas, this paper proposes a method combining mixed ratio calculation and multivariate statistical analysis. Firstly, a total of 40 groundwater samples are classified into six clusters via hierarchical cluster analysis. By comprehensively analyzing the spatial location of the samples, it was found that there was no obvious distribution pattern of groundwater in space. Furthermore, the rationality of the cluster analysis is evaluated via principal component analysis. Next, hydrochemical and isotopic analyses were conducted to determine the source of groundwater in the mining area, and a three terminal element mixing model was established to identify the source of pollutants and calculate the terminal element mixing ratio. The research results indicate that groundwater in mining areas is formed by a mixture of shallow bedrock fissure water, deep bedrock fissure water, and rainwater, and the mixing effect is the main factor affecting the evolution of groundwater in mining areas, with a more significant impact than the depth of groundwater circulation. In addition, different types and degrees of water-rock interaction in different regions have altered the hydrochemical characteristics of groundwater in mining areas, such as the dissolution of multiple minerals, cation exchange, and common ion effects. Based on the above analysis results, a water circulation model for the mining area has been established. The findings of this study not only contribute to the protection of shallow fissure groundwater in the study area, but also provide a basis for investigating the groundwater evolution patterns in other metal mines.

Baseline survey of environmental parameters, radiation, and drip water hydrochemistry in Niah Caves (Sarawak, Malaysia)

Relict caves in fenglin karst may typically have numerous entrances and openings. Hence, they host a variety of environments in which parameters such as light, airflow, humidity, and temperature may vary significantly over short distances. Similarly, drip water hydrochemistry, including isotopic values, may vary due to different contributions of various sources and residence time in the karst. This study investigated environmental parameters, including radiation, using hand-held instruments, along a transect within several major caves in the Niah karst of Sarawak (Malaysia). This has led to a baseline data set which showed an inverse relationship between humidity and temperature, gamma radiation levels that are about 25% of that in surrounding non-karst region, and high percentages of twilight zones in the studied caves. Airflow was found to be variable, with high values of 530 m3/s in Painted Cave and 122 m2/s in parts of Gan Kira passage, with flow towards the southeast at the time of the study. The hydrochemistry of the drip water and surface water was also analyzed and found to be dominantly Ca-Mg-Cl water type which indicates dissolution of minerals through water-rock interaction. In addition, the cave environment, particularly air temperature, humidity, and ventilation, also influences the drip water composition and isotopic values. Three different origins (precipitation, evaporation, and paleo-recharge) of drip water have been identified through the δ18O-δ2H diagram. Enriched isotopic values were observed in the cave entrance due to increased evaporation caused by lower humidity and higher air temperature. Factor analysis identifies the key geochemical processes responsible for the drip water chemistry. The outcome of this study provides the first baseline environmental data for the Niah caves, which could support future initiatives for sustainable management of this famous archeological site in southeast Asia.

Effects of deep coal mining on groundwater hydrodynamic and hydrochemical processes in a multi-aquifer system: Insights from a long-term study of mining areas in ecologically fragile western China

The groundwater hydrodynamic and hydrochemical process of the multi-aquifer system will experience complicated and serious influence under deep coal mining disturbance. There is relatively little research that has integrated hydrodynamic and hydrochemical properties of groundwater to investigate the spatiotemporal distribution characteristics and evolution patterns of hydrogeochemistry and hydrodynamic information in deep multi-aquifer systems. The study of the groundwater hydrodynamic and hydrochemical spatiotemporal coupling response of multi-aquifer systems under the deep and special thick coal seam mining-motivated effect in ecologically fragile western mining areas is of great significance for the safe mining of coal resources and ecological environment protection. In this research, the hydrochemical analysis data composed of 218 groundwater samples from Tangjiahui coalfield, Northwest China with 1526 measurements and a 6-year (2016-2021) sampling period were collected for studying the hydrogeochemical spatiotemporal evolution process and governing mechanism of the multi-aquifer system using hierarchical cluster analysis, ion-ratio method, saturation index and multidimensional statistical analysis. Additionally, wavelet analysis and cross-wavelet coherence analysis were implemented to quantitatively recognize the spatiotemporal variation characteristics of hydrodynamic information and analyze the coherence relationships between time series. The results demonstrate that the hydrochemical characteristics exhibit significant spatial differences, while the temporal variation of hydrochemical characteristics in the Permian Shanxi Formation fractured sandstone aquifer (PSFFA), mine water (MW), and Ordovician karst limestone aquifer (OKA) is not significant. The water-rock interaction is the predominant control mechanism for the spatial evolution of hydrogeochemistry in the research area. Moreover, the large-scale mining of deep coal seams controls the type and degree of water-rock interactions by damaging the structure of aquifers and altering the hydrodynamic conditions of groundwater. The period from 2016 to 2021 exhibits multi-time scale characteristics in time series of precipitation, mine water discharge, and the water level of PSFFA and OKA. The mine water discharge has a positive correlation with the water level of PSFFA and OKA, whereas the significant period of precipitation and the water level of PSFFA coherence is not obvious. The research findings not only provide in-depth insights to protect the groundwater resources in water-shortage mining areas but also promote the secure mining of deep coal resources.

Origin and stability of pit lake water in Baiyinhua, Inner Mongolia, based on hydrochemistry and stable isotopes

Isotope technology is widely used in geochemical mechanisms analysis; however, studies on the origin of pit lake water by isotopes in coal concentration areas in grassland are rare. In this study, 20 groups of water samples were collected, which were subjected to chemical analysis to determine the hydrogeochemical characteristics of pit lake water. The mechanisms of pit lake water formation and recharge-evaporation were ascertained through principal component analysis and the Rayleigh fractionation model. The results indicate that the phreatic water is least affected by evaporation, followed by confined water, surface water and pit lake water. The ionic composition of surface water, phreatic water and most of the confined water is mainly affected by leaching, some confined water can be recharged by surface or phreatic water; while the ionic composition of pit lake water is dominantly affected by evaporation (69.4 %) and is less affected by groundwater recharge (17.1 %) and human activities (11.5 %). The pit lake water is recharged by precipitation, phreatic water and the lateral runoff of confined water; however, the proportion of phreatic and confined water recharge is small. The evaporative loss of the pit lake water is 40-61 % of the initial water body.

Use of Hydrogeochemistry and Isotopes for Evaluation of Groundwater in Qilian Coal Base of China

The Jiangcang Basin is an important mining area of the former Qilian Mountain large coal base in Qinghai Province, and understanding the groundwater circulation mechanism is the basis for studying the hydrological effects of permafrost degradation in alpine regions. In this study, hydrogeochemical and multiple isotope tracer analysis methods are used to understand the chemical evolution and circulation mechanisms of the groundwater in the typical alpine region of the Jiangcang Basin. The diversity of the groundwater hydrochemistry in the study area reflects the complexity of the hydrogeochemical environment in which it is located. The suprapermafrost water and intrapermafrost water are recharged by modern meteoric water. The groundwater is closely hydraulically connected to the surface water with weak evaporation overall. The high δ34S value of deep groundwater is due to SO4 reduction, and SO4 2--rich snow recharge with lixiviated sulfate minerals are the main controlling factor for the high SO4 2- concentration in groundwater. According to the multivariate water conversion relationships, it reveals that the river receives more groundwater recharge, suprapermafrost water is recharged by the proportion of meteoric water, which is closely related to the mountainous area at the edge of the basin, while intrapermafrost water is mainly recharged by the shallow groundwater. This study provides a data-driven approach to understanding groundwater recharge and evolution in alpine regions, in addition to having significant implications for water resource management and ecological environmental protection in coal bases of the Tibetan Plateau.

Multiple stable isotopes and geochemical approaches to elucidate groundwater reactive transport paths in mining cities: A case from the northern Anhui, China

Mining cities are ecotone areas where human and natural components interact. Indeed, the negative effects of mining activities on drinking water quality have become a serious public concern worldwide. To elucidate groundwater genesis and reactive transport path controlling the water pollution, a multi-bodies system in the Sunan Mine area in China was considered in this study. The results of the mineral phase characterizations, hydrochemical analysis, and multiple stable isotopes (δ2H/δ18O, δ34S and 87Sr/86Sr) indicated that calcite, dolomite, gypsum, quartz, halite, organic carbon, and gases (O2, CO2 and H2O) were the primary reactants in the aquifer system, accompanied by dissolution and precipitation of minerals, cation exchange, desulfation, and evaporation. An inverse hydrogeochemical model was employed to identify three paths, Path 1 demonstrated that mine water mainly originated from the Quaternary loose aquifer water (QLA), Permian fractured sandstone aquifer water (PFA), and Carbonifer fractured limestone aquifer water (CFA), accompanied by high K++Na+ and HCO3- concentrations due to the carbonate dissolution, halite dissolution, and cation exchange processes. Path 2 showed that the recharge of the CFA and Ordovician fractured limestone aquifer (OFA) occurred from the shallow recharge zone to the deeper OFA water through faults and fractures, mainly involving halite dissolution, carbonate dissolution, and gypsum dissolution. Path 3 demonstrated the interaction between the Hui River, collapsed pond water, and QLA, accompanied by gypsum dissolution, calcite dissolution, and cation exchange. Although the shallow QLA quality met the WHO drinking water standards, the pollution risk from the surface collapse pit water cannot be ignored. Therefore, effective approaches need to be considered in the study area to reduce the connection between the collapse pit water and QLA. The study results can help decision makers to predict water quality of complex water systems in ecotone areas and other similar regions worldwide.

Identification of nitrate accumulation mechanism of surface water in a mining-rural-urban agglomeration area based on multiple isotopic evidence

The accumulation of nitrate (NO3-) in surface waters resulting from mining activities and rapid urbanization has raised widespread concerns. Therefore, it is crucial to develop a nitrate transformation information system to elucidate the nitrogen cycle and ensure sustainable water quality management. In this study, we focused on the main river and subsidence area of the Huaibei mining region to monitor the temporal and spatial variations in the NO3- content. Multiple isotopes (δD, δ18O-H2O, δ15N-NO3-, δ18O-NO3-, and δ15N-NH4+) along with water chemistry indicators were employed to identify the key mechanisms responsible for nitrate accumulation (e.g., nitrification and denitrification). The NO3- concentrations in surface water ranged from 0.28 to 7.50 mg/L, with NO3- being the predominant form of nitrogen pollution. Moreover, the average NO3- levels were higher during the dry season than during the wet season. Nitrification was identified as the primary process driving NO3- accumulation in rivers and subsidence areas, which was further supported by the linear relationship between δ15N-NO3- and δ15N-NH4+. The redox conditions and the relationship between δ15N-NO3- and δ18O-NO3-, and lower isotope enrichment factor of denitrification indicated that denitrification was weakened. Phytoplankton preferentially utilized available NH4+ sources while inhibiting NO3- assimilation because of their abundance. These findings provide direct evidence regarding the mechanism underlying nitrate accumulation in mining areas, while aiding in formulating improved measures for effectively managing water environments to prevent further deterioration.

Combining hydrochemistry and 13C analysis to reveal the sources and contributions of dissolved inorganic carbon in the groundwater of coal mining areas, in East China

East China is a highly aggregated coal-grain composite area where coal mining and agricultural production activities are both flourishing. At present, the geochemical characteristics of dissolved inorganic carbon (DIC) in groundwater in coal mining areas are still unclear. This study combined hydrochemical and carbon isotope methods to explore the sources and factors influencing DIC in the groundwater of different active areas in coal mining areas. Moreover, the 13C isotope method was used to calculate the contribution rates of various sources to DIC in groundwater. The results showed that the hydrochemical types of groundwater were HCO3-Ca·Na and HCO3-Na. The main water‒rock interactions were silicate and carbonate rock weathering. Agricultural areas were mainly affected by the participation of HNO3 produced by chemical fertilizer in the weathering of carbonate rocks. Soil CO2 and carbonate rock weathering were the major sources of DIC in the groundwater. Groundwater in residential areas was primarily affected by CO2 from the degradation of organic matter from anthropogenic inputs. Sulfate produced by gypsum dissolution, coal gangue accumulation leaching and mine drainage participated in carbonate weathering under acidic conditions, which was an important factor controlling the DIC and isotopic composition of groundwater in coal production areas. The contribution rates of groundwater carbonate weathering to groundwater DIC in agricultural areas and coal production areas ranged from 57.46 to 66.18% and from 54.29 to 62.16%, respectively. In residential areas, the contribution rates of soil CO2 to groundwater DIC ranged from 51.48 to 61.84%. The results will help clarify the sources and circulation of DIC in groundwater under the influence of anthropogenic activities and provide a theoretical reference for water resource management.

Environmental risk of tailings pond leachate pollution: Traceable strategy for leakage channel and influence range of leachate

Identifying the leakage channel and the influencing range is essential for controlling the environmental risks of leachate from the tailings pond. The investigation of leachate pollution in tailings pond has the defect of focusing only on the scope of tailings pond in recent studies. This study innovatively built a comprehensive investigation and accurate verification system for leachate leakage of tailings pond integrated with the aeromagnetic survey, ground penetrating radar, hydrochemistry and isotope coupling methods. Geophysical exploration found that among the four fault zones, and the F1 was the channel for leachate to recharge the groundwater 2.53 km away from the tailings pond. The fissures inside the tailings pond were connected with the natural fissures outside, forming a leachate migration channel. The hydrochemistry and isotope characteristics showed that the groundwater far away from the tailings pond were polluted by arsenic containing leachate, which verified the geophysical exploration results. The significant correlation between arsenic and SO2-4 concentration indicated that arsenic in leachate originated from the oxidation release of sulfide minerals (i.e., arsenopyrite). This study sheds light on the comprehensive investigation of leachate leakage in the tailings pond. This development method also provides guidance for environmental risk identification of other contaminated sites.

Characterizing spatial dependence of boron, arsenic, and other trace elements for Permian groundwater in Northern Anhui plain coal mining area, China, using spatial autocorrelation index and geostatistics

Anthropogenic and geological factors play an essential role in the variability of groundwater quality, resulting in a weak spatial dependence of groundwater trace elements. Thus, it is an essential study to investigate the factors affecting groundwater quality and its spatial abundance of trace elements (including As, B, and other metalloids). In this study, samples are obtained from a Permian sandstone fracture aquifer in a coal mining area. A multivariate statistical analysis, hydrogeochemistry modeling, and spatial autocorrelation analysis were used to analyze the data. The results showed that Moran index was positive for all trace elements, which had good spatial autocorrelation. The Local indicators of spatial association (LISA) indicated that trace elements were clustered. The hydrogeochemical modeling results indicated that the precipitation and stability of iron-phase minerals, such as rhodochrosite and arsenic (As) absorption on the surface of iron-phase minerals in the aquifer, may limit concentrations in the southern region. The spatial autocorrelations of both As and Boron (B) were positive (high-high) in the western areas, indicating that As contamination occurred from both natural geological causes and human coal mining activities. In contrast, B contamination was mainly linked to the influence of human agricultural or industrial activities. Over 96% of the groundwater concentrations of As (10 μg/L) and B (300 μg/L) in the study area exceeded World Health Organization (WHO) limits. Overall, the results of this work could help decision-makers involved in regional water quality management visualize disperse zones where specific anthropogenic and geological processes may threaten groundwater quality.

Source apportionment and ecological health risks assessment from major ions, metalloids and trace elements in multi-aquifer groundwater near the Sunan mine area, Eastern China

Evaluating the ecological health risks created by major ions, metalloids and trace elements concentrations in groundwater and pollution sources were essential to effectively protect groundwater resources. For this study, A total of 93 samples were collected from multiple aquifers in the Sunan mining area, eastern China. The Positive matrix factorization (PMF) model results revealed the following sources, in percentages. The Quaternary loose aquifer (QLA) water includes CaMg mineral dissolution (30.3 %), salinity (28.2 %), metal industrial wastewater (26.3 %), iron and manganese minerals (8.0 %) and coal gangue (7.2 %). The Permian fractured sandstone aquifer (PFA) water includes CaMg mineral dissolution sources (29.8 %), mine wastewater (28.6 %), aluminosilicate (21.6 %) and pyrite source (20.0 %). The Carbonifer fractured limestone aquifer (CFA) water includes and mine wastewater (34.2 %), CaMg mineral dissolution (25.4 %), pyrite (22.6 %) and aluminosilicate (17.7 %). The Ordovician fractured limestone aquifer (OFA) water includes manganese and aluminum metal minerals (27.9 %), halite dissolution materials (24.9 %), industrial and agricultural waste water (24.0 %) and calcium‑magnesium minerals (23.2 %). A PMF-based assessment of ecological health risk indicates that the concentrations of elements As and Co are the dominant elements impacting non-carcinogenic and carcinogenic risks; and As, Cr, and Cu are the dominant elements impacting potential ecological risks. These mainly originate from geological sources, coal gangue sources, mine drainage sources and agricultural sewage discharge sources. The study showed the sources of groundwater pollution in multiple aquifers and their priority treatment areas, providing a basis for groundwater management and protection.

Source and evolution of sulfate in the multi-layer groundwater system in an abandoned mine-Insight from stable isotopes and Bayesian isotope mixing model

The source and evolution of sulfate (SO₄^2-) in groundwater from abandoned mines are widely concerned environmental issues. Herein, major dissolved ions, multi-isotopes (δ³⁴S, δ¹⁸O_sulfate, δ²H and δ¹⁸O_water), machine learning (Self-organizing maps) and Bayesian isotope mixing model were used to identify the source and evolution of SO₄^2- in an abandoned mine (Fengfeng mine, northern China) with a multi-layer groundwater system. Groundwater in the study area was mainly divided into three clusters (Cluster I, Cluster II and Cluster III), dominated by Na-SO₄, Ca-SO₄ and Ca-HCO₃ types, respectively. According to δ²H and δ¹⁸O_water, groundwater in the study area mainly originated from atmospheric precipitation. δ³⁴S, δ¹⁸O_sulfate and SO₄^2- suggested that bacterial sulfate reduction did not affect the SO₄^2- isotopic composition. Dual SO₄^2- isotopes, and MixSIAR model revealed that the main source of SO₄^2- in the study area was pyrite oxidation/gypsum dissolution, accounting for an average of 57.4 % (gypsum), 71.24 % (pyrite oxidation) and 52.93 % (pyrite oxidation) of SO₄^2- in the samples of Clusters I-III, respectively. Combined with the hydrochemical diagrams, the evolution of SO₄^2- in different clusters of samples was derived. Cluster I was mainly gypsum dissolution; In contrast, Clusters II and III were mainly pyrite oxidation accompanied by carbonate dissolution, and Cluster II was also influenced by cation exchange. These findings will help in developing management strategies for protecting groundwater quality, which will provide a reference for the study of solute sources and S cycling in abandoned mines.

Spatial distributions of microbial diversity in the contaminated deep groundwater: A case study of the Huaibei coalfield

The impact of coal mining activities on the structure of groundwater microbial communities in coal mining areas has gradually received academic attention. In this study, hydrochemical analysis and sequencing of the V4 region of the 16S rRNA gene were carried out using groundwater samples from the fourth aquifer in the loose layer of Cenozoic, the sandstone fissure aquifer in the coal measure strata of Permian, the Carboniferous Taiyuan Formation limestone aquifer, and the Ordovician limestone aquifer, at depths of 250 m, 600 m, 750 m, and 1000 m in monitoring wells. Results showed that the operational taxonomy units (OTUs) in the deep groundwater ecosystem were clustered distinguishably between the contaminated and the uncontaminated aquifers. The microbial community alpha-diversity of groundwater was significantly correlated with depth, and the richness of microbial community composition decreased with increasing depth. Proteobacteria (34.41%-97.41%), was found to be the dominant phylum, Gammaproteobacteria (10.05%-92.06%) was the dominant class and "Unassigned" (4.12%-64.72%) was dominant at the genus level. The number of endemic bacteria in the four aquifers was 1, 33, 99 and 11, respectively. It was also found that F^-, oxidation-reduction potential (ORP), and TOC were the main environmental variables affecting the groundwater all OTUs, abundant OTUs, and rare OTUs, respectively. These results indicate that the activity of rare OTU subcommunities increases gradually with increasing aquifer depth and that mining significantly enriched Thiovirga in deep groundwater. In addition, it was found that with the increase of depth, the effect of ORP on microbial community abundance decreased. This study deepens our understanding of the evolution characteristics of microbial communities in deep groundwater in coal mining areas. The unique characteristics of microbial communities characteristics of four aquifers with different depths provide a microbial perspective for understanding the characteristics of deep aquifers.

Hydrogeochemical evolution induced by long-term mining activities in a multi-aquifer system in the mining area

The hydrogeochemical evolution of groundwater is related to and affected by long-term mining activities, which may deteriorate the quality of groundwater. The Fengfeng mine in Handan, North China has a 30-y history of coal mining with long-term mining activities and complex geological conditions, resulting in a complex hydrogeochemical environment in the mining region. In this study, the hydrogeochemical evolution mechanism of groundwater in a multi-aquifer system in the Fengfeng Mining Area was investigated using machine learning (self-organizing maps combined with K-means clustering) and sulfur and oxygen isotopes (δ34SSO4 and δ18OSO4). The hydrogeochemical characteristics of different aquifers in the mining area changed to different degrees after mining compared with the characteristics before mining. The spatiotemporal variations in groundwater components were found to be controlled by pyrite oxidation, gypsum dissolution, and carbonate dissolution, which are affected by mining activities. Pyrite oxidation primarily occurred in the Carboniferous thin-layer limestone aquifer (CLA) and Permian sandstone aquifer (PSA). The hydrogeochemical evolution in the Ordovician limestone aquifer (OLA), the main aquifer in the study area, was affected by leakage recharge from CLA and PSA caused by mining activities. The results showed that owing to the effects of long-term mining, the altered groundwater flow system affected the evolution of groundwater components in each aquifer, particularly the sulfate concentration. This study reveals a distinct hydrogeochemical evolution induced by mining activities, which can provide a basis for groundwater resource management in mining areas.

Study on the Influence of Mining Activities on the Quality of Deep Karst Groundwater Based on Multivariate Statistical Analysis and Hydrochemical Analysis

Long-term mining activities have changed the hydrogeochemical evolution process of groundwater and threatened the safe use of groundwater. By using the methods of hydrochemistry and multivariate statistical analysis, this study determined the hydrogeochemical evolution mechanism affecting the quality of karst groundwater by analyzing the conventional hydrochemistry data of the karst groundwater of the Carboniferous Taiyuan Formation in Hengyuan Coal Mine in the recent 12 years. The results show that, under the disturbance of mining, the quality of karst groundwater in Taiyuan Formation is poor, mainly because the contents of Na⁺+K⁺ and SO₄^2- are too high to allow usage as drinking water. The reason for the high content of SO₄^2- in karst groundwater lies in the dissolution of gypsum and the oxidation of pyrite, and the high content of Na⁺+K⁺ lies in the cation exchange. Influenced by the stratum grouting, the circulation of karst groundwater is improved, the cation exchange is weakened, and the desulfurization is enhanced. In the future, it is predicted that the hydrochemical type of karst groundwater in Taiyuan Formation in the study area will evolve from SO₄-Ca·Mg type to HCO₃-Ca Mg type.

Tracing Sulfate Source and Transformation in the Groundwater of the Linhuan Coal Mining Area, Huaibei Coalfield, China

Mining activities cause surface sulfate enrichment, which has negative impacts on human health and ecosystems. These high concentrations of sulfate may enter groundwater through the unsaturated zone (UZ), threatening groundwater quality. Therefore, we combined hydrochemical and dual isotopic analyses of sulfate in surface water, soil water and groundwater with evaluations of the UZ to identify the groundwater sulfate source and transformation in the coal mining area. Soil profile samples were collected near gangue heaps (UZ-1, UZ-2) and the mean sulfate concentrations of the UZ-1 profile and UZ-2 profile were 35.4 mg/L and 69.63 mg/L, respectively. The shallow groundwater sulfate was mainly from dissolution of evaporite, sulfide oxidation and sewage. Different sulfate contaminated areas showed different characteristics of sulfate sources. The sulfate source to groundwater near the coal gangue heaps was sulfide oxidation. The groundwater sulfate near the gangue heaps and industrial park compound contamination area was mainly derived from industrial and domestic sewage and sulfide oxidation. In addition, the role of bacterial sulfate reduction (BSR) in the groundwater was not obvious. This research result is of great significance for promoting the safe mining of coal resources and sustainable utilization of groundwater in the Huaibei coal mining area and other coal mining areas in China.

Combining hydrochemistry and hydrogen and oxygen stable isotopes to reveal the influence of human activities on surface water quality in Chaohu Lake Basin

The input of pollutants caused by human activities induces the deterioration of surface water quality. To reveal the characteristics of surface water quality in Chaohu Lake Basin and the influence of human activities, the hydrochemistry and stable isotope composition of hydrogen and oxygen in lake water and inflow river water were analyzed. The results show that the hydrochemical type of lake water is the Na-Cl type,while river water is the Na-Cl, Ca-Cl and mixed types. The ion proportional coefficient method and principal component analysis show that surface water is controlled by weathering of evaporated salt rocks and silicate rocks, in which Cl^- and SO₄^2- are affected by fertilizers and sewage to some extent. There is a strong correlation between conventional ions and nutrient indexes, which indicates that dissolved ions are affected not only by rock weathering but also by human activities (such as the discharge of domestic sewage or nitrogen-containing wastewater and the use of fertilizers). The stable isotope values of hydrogen and oxygen in surface water are distributed at the lower right portion of the local precipitation line and are close to it, indicating that surface water mainly originates from precipitation. The high value of d-excess values in surface water indicates that evaporation is weak. As pollution indicators, EC, Cl^- and NO₃^- indicates that the Nanfei River, Dianbu River, Shiwuli River and Pai River flow in northwestern of Chaohu Lake Basin through Hefei urban city are severely polluted, NO₃^- originates from manure and sewage. Rivers flowing through farmland areas are less polluted, and the use of agricultural fertilizer contributes greatly to NO₃^-.

Effects of multi-factors on the spatiotemporal variations of deep confined groundwater in coal mining regions, North China

Natural processes and anthropogenic activities simultaneously control the long-term spatial and temporal variations of groundwater hydrogeochemistry in coalfields. In this study, the spatiotemporal variations and primary controlling factors of deep groundwater hydrogeochemistry in the Carboniferous limestone aquifer of the Huaibei coalfield, North China were investigated using cluster analysis combined with geological conditions, water-rock interactions and mining activities. The analysis data of 176 groundwater samples collected over five years from 20 monitoring wells were subdivided into six clusters through hierarchical cluster analysis. Moreover, principal component analysis, box plots and Piper and Stiff diagrams were employed to analyze the statistical and hydrogeochemical characteristics of each cluster, and to reveal the differences and connections between the clusters. The results show that there are significantly spatial variations in groundwater hydrogeochemistry, while the temporal variations are not evident with only a few notable exceptions. Geological conditions dominate the groundwater hydrogeochemistry by controlling the hydraulic connection between groundwater and meteoric water and the flow conditions of groundwater. Moreover, the types and degrees of diverse water-rock interactions in different regions are another important factor controlling the spatial variations of groundwater hydrogeochemistry. Anthropogenic activities are mainly pumping and drainage, which has led to the overall decline in groundwater levels and the temporal variations of hydrogeochemistry in some zones. The findings of this study not only have important implications for deep groundwater resources management in the Huaibei coalfield, but also provide a research template for other highly exploited coalfields in North China.