Effects of mining activities on evolution of water chemistry in coal-bearing aquifers in karst region of Midwestern Guizhou, China: evidences from δ13C of dissolved inorganic carbon and δ34S of sulfate

Regional characteristics of groundwater sulfate source and evolution in the multi-layer aquifer system of the northern Shaanxi coal mine base, northwestern China: Evidence from geochemical and isotopic fingerprints

Groundwater sulfate contamination in mining areas has attracted widespread attention. However, deciphering the source and evolution of sulfate in large-scale mining areas remains a challenge due to intense anthropogenic influences and complex hydrogeological conditions. In this study, 94 groundwater samples were analyzed by a combination of self-organizing maps, MixSiar model, multi-isotope analyses (δ34S, δ18OSO4, δD and δ18Owater) and hydrogeochemical methods to investigate the regional characteristics of groundwater sulfate source and evolution in China's largest coalfield (the Shenfuyu Coalfield). The results showed that the source and evolution of groundwater sulfate were controlled by human activities (mining and agricultural activities) and hydrogeological conditions. The groundwater sulfate primarily originated from pyrite oxidation, gypsum dissolution and human inputs. For the mining districts with shallow mining depths, pyrite oxidation and fertilizer contributed to groundwater sulfate. In addition, the ground cracks and abandoned mines controlled the BSR and pyrite oxidation processes. In contrast, the gypsum dissolution and cation exchange dominated the sulfate evolution in the mining districts with deep mining depths due to slow groundwater circulation. This study provided new insights into the source and evolution of groundwater sulfate in large coalfields, as well as references for regional water resource utilization and protection.

Construction and application of a composite model for acid mine drainage quality evaluation based on analytic hierarchy process, factor analysis and fuzzy comprehensive evaluation: Guizhou Province, China, as a case

The process of mining activities often causes the formation of acid mine drainage (AMD). Through rock fractures and underground rivers, AMD can easily enter the groundwater environment near mines and cause serious pollution to water quality. In order to effectively evaluate the quality of polluted mine water and to understand its threat to the ecosystem around the mine. In this study, four AMD pollution distribution areas, Guiyang City, Bijie City, Qianxinan Prefecture, and Qiandongnan Prefecture in Guizhou Province, were used as the study area. A composite model for mine water quality evaluation was constructed using factor analysis (FA), analytic hierarchy process (AHP), and fuzzy comprehensive evaluation (FCE). Furthermore, by introducing the weighted average method and the level characteristic value (J), the water quality type and the water body environmental quality were evaluated comprehensively, respectively. Compared with the traditional evaluation model, the AHP-FA-FCE model has obvious advantages in the selection of evaluation indicators, the determination of indicator weights, and the comprehensive evaluation of water quality types, and the evaluation results obtained are more reasonable and accurate. Three common factors mainly controlled by mineral oxidation factor, human activity factor, and mineral dissolution factor were extracted by dimension reduction of the original hydrochemical data by FA. The water quality of the mine water samples was evaluated using SO4 2- , Fe, Al, Mn, Na, and F- as evaluation indicators, and the results showed that the mine water samples in the study area as a whole were dominated by class V water, which accounted for 77.78% of the total. Based on the statistical analysis of the original data, it was found that influenced by the water-rock interactions in the study area and the AMD pollution components, the hydrochemical type of the mine water is mainly SO4 2- -Ca-Mg type. The water body environmental quality of mine water in four areas, Guiyang City, Qianxinan Prefecture, Bijie City, and Qiandongnan Prefecture, is from excellent to poor. The average level characteristic value of all the areas is more than 3, and the overall environmental quality of the water body is poor. The strong water-rock interaction and mining activities in the study area may be the main cause of AMD pollution. The results of this study may provide some theoretical reference for the water quality evaluation of AMD-polluted areas. PRACTITIONER POINTS: A composite model for mine water quality evaluation was constructed. A factor analysis-based evaluation indicator selection method is proposed. This study improved the weighting process of the traditional fuzzy comprehensive evaluation. A water quality discriminant based on the weighted average method is proposed. The water environmental quality of various types of mine water was evaluated.

The introduction of nitrogen from coal into the surface watershed nitrogen cycle due to coal mining activity

Human activity has doubled the turnover rate of the terrestrial nitrogen cycle, leading to a series of environmental problems. A little-studied nitrogen source in terrestrial and aquatic environments is the nitrogen release associated with rock strata. Southwest China features the largest continuous karsts in the world, featuring a fragile ecological environment but abundant coal resources. The current study selected a typical coal mining area to evaluate the migration and transformation of nitrogen related to coal mining in surface watershed. The findings reveal that the total nitrogen in coal seams was as high as 10,162.3 mg/kg, mainly in the form of organic nitrogen, followed by NH4+-N, while the content of NO3--N was negligible. Based on the isotope fractionation and the co-evolution between Δ15NNO3-NH4 and δ15N-NO3-/δ15N-NH4+, coal mining changed the coal seams' oxidation-reduction state, resulting in the mineralization of organic nitrogen to NH4+-N. Next, NH4+-N gradually oxidized to NO3--N. Various forms of coal-origin nitrogen may be leached out by acid mine drainage (AMD), potentially contributing >10 % of NO3--N and 90 % of NH4+-N to the surface river. Another nitrogen source that requires serious consideration is the wide use of ammonium nitrate explosives in coal mining, as blasting residues may contribute about another 10 % to NO3--N in surface water. Since organic nitrogen accounts for >90 % of extractable nitrogen, the release of coal-origin nitrogen may contribute much more to the total nitrogen in surface water than to NO3--N. Based on the fractionation of nitrogen and oxygen isotopes of nitrate, low-pH AMD promotes the volatilization of nitrate in the form of nitric acid. The conversion of different forms of nitrogen in AMD will be the focus of future attention.

A multiple isotope (S, H, O and C) approach to estimate sulfate increasing mechanism of groundwater in coal mine area

Groundwater in North China type coal mine area is an important source of domestic, industrial and agricultural water. To explore the sulfate increasing mechanism of groundwater in mining area and identify key influencing factors. In this paper, hydrochemistry and multi-isotope tracer techniques such as δ34SSO4, δ18OSO4, δ2HH2O, δ18OH2O and δ13CDIC were used to study the groundwater circulation law and the migration and transformation mechanism of sulfate and carbonate in coal mine area. The results show that: the hydrochemical types of groundwater in the coal mine area are mainly HCO3- and SO42- anions, while the cations are mainly Ca2+ and Mg2+. The sulfate content is significantly increased, and the pH shows weak alkalinity; the relationship between δ18OH2O and δ18HH2O shows that the dynamic field of groundwater changes significantly after coal mining or closure, and limestone water mainly comes from surface water recharge through 'skylight' infiltration. The relationships between δ18OSO4 and δ18OH2O, δ34SSO4 and δ18OSO4 show that the sulfate in groundwater of coal mine area is mainly derived from sulfide oxidation. The ∆δ18OSO4-H2O value of groundwater in coal mine area is greater than 8 ‰, and the oxygen content in sulfate is 25 %-75 % from oxygen in water, indicating that coal mining has disturbed the groundwater in the study area from reducing environment to oxidizing environment, promoted sulfide oxidation, and accelerated the dissolution of carbonate minerals. The δ13CDIC value and δ34SSO4 value in the coal mine area are inversely proportional. The δ13CDIC of groundwater in the coal mine area is affected by the δ34SSO4 value to a certain extent. Sulfuric acid participates in the dissolution of carbonate minerals, making the pH value weak and alkaline as a whole. This paper expounds the migration and transformation law of sulfate in groundwater in coal mine area, which has practical significance for groundwater quality management. The research results can provide theoretical support for the rational development and utilization of groundwater resources in coal mine areas.

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.

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.

Hydrogeochemical Characteristics and Quality Assessment of Mine Water in Coalfield Area, Guizhou Province, Southwest China

Coal resources are widely distributed in Guizhou province, China and environmental pollution caused by coal mining is becoming increasingly serious, especially mine drainage. A total of 120 mine water samples collected from different coalfields were analyzed to investigate the hydrogeochemical characteristics and assess the water quality for drinking, domestic, and irrigative purposes. Water samples had a pH of 1.90-9.12 and most of them were acidic or weakly acidic. Total dissolved solids (TDS) ranged from 254 to 13,944 mg/L and correlated closely with the electrical conductivity (EC). The coal mine drainage of Guizhou was characterized typically by low pH value, high Fe and SO₄^2- concentration, which were mainly attributed to oxidative weathering of pyrite. The most dominant type for abandoned coal-mine drainage was Ca-Mg-SO₄, while that of the underground drainage for active mine were mainly Na-SO₄ and Na-HCO₃ because of high concentration of Na⁺ from the dissolution of evaporites and clastic rocks. High concentration of the TDS, SO₄^2-, Fe, Mn, et al. made it unsuitable for drinking and domestic use, but part of the coal mine drainage could be used for irrigation at some sites.

Performance and Mechanisms of PropS-SH/HA Coatings in the Inhibition of Pyrite Oxidation

Acid mine drainage (AMD) entering the environment will cause long-term environmental pollution and ecological damage, the treatment or remediation for which has become a difficult worldwide problem. To control AMD at the source, a novel composite coating, hydroxyapatite (HA) as the filler embedded in a γ-mercaptopropyltrimethoxysilane (PropS-SH) coating, was introduced in this study. The performance and mechanisms of PropS-SH/HA coatings in the inhibition of pyrite oxidation were investigated by chemical leaching testing and material structure characterization. The results of the investigations revealed that the addition of an appropriate amount of HA can enhance the passivation efficiency of the PropS-SH coating. The best coating was obtained from 3% (v/v) of PropS-SH solution with 16 wt % HA, as this coating decreased pyrite oxidation by 78.7% (based on total Fe release). The main mechanism of PropS-SH/HA for the inhibition of pyrite oxidation involved the generation of a PropS-SH network through a polycondensation reaction. The addition of HA increased the stability of the passivation film composed of PropS-SH as well as the combining capacity of PropS-SH/HA through the formation of Si-O-Si and Fe-O-Si bonds, respectively.

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

Both natural processes and anthropogenic activities have significant effects on groundwater evolution in coal mining regions. In this study, the primary controlling mechanism of the groundwater chemistry evolution for the Carboniferous groundwater in the Huaibei coalfield, North China was proposed based on the hydrogeochemical indicators combining with multiple isotope tracers. The diversity of hydrochemical types indicates the complexity of the hydrogeochemical environment in the groundwater, which is recharged by precipitation infiltration with minimal evaporation according to the distributions of δD and δ¹⁸O. Additionally, ion correlation analysis suggests that minerals dissolution and cation exchange between Na⁺ and Ca²⁺ are the dominant processes within that groundwater. The hydrochemical and δ¹³C_DIC characteristics of the groundwater demonstrate that HCO₃^- is mainly controlled by the dissolution of carbonate minerals and soil CO₂, and the proportion of the latter is believed to be dominated by the hydrogeologic conditions. Similarly, the values of SO₄^2- and δ³⁴S_SO4 indicate that a small portion of SO₄^2- in the groundwater in the northern part originates from the meteoric precipitation, while it is mainly derived from the dissolution of gypsum in the southern part. Furthermore, mining activities also alter the groundwater level and flow conditions through pumping and drainage, which enhances the interaction between groundwater and aquifer lithologies, thereby affects the hydrogeochemical processes. The findings of this work are of great significance for promoting the safe exploitation of deep coal resources and the sustainable utilization of groundwater in the Huaibei coalfield, as well as the most of other coalfields in North China.

Evaluation of the Geochemical Characteristics and Exploitation Potential of Produced Water from Coalbed Methane Wells in Eastern Yunnan, China

Based on the analysis of the geochemical characteristics of the produced water from coalbed methane wells in eastern Yunnan, the effects of the water-rock interactions on the produced water were discussed, and the mining potential of each of the four wells was evaluated. The results show that with the increase in drainage time, the Na⁺ and Cl^- concentrations decrease while the HCO^-₃ concentration increases. The produced water from the two wells in the Enhong Block shows D drift characteristics, while that of the two wells in the Laochang Block shows O drift characteristics. The order of the produced water affected by the surrounding rock is D-1 well > M-1 well > D-2 well > M-2 well, and the order of the produced water influenced by the coal seam is D-1 well > D-2 well > M-1 well > M-2 well. According to the variation in the As trace element concentration, it is inferred that in the four coalbed methane wells, the D-1 and D-2 wells have a greater exploitation potential than that of the M-1 and M-2 wells. On this basis, the influencing characterization parameters of the water-rock interactions on the produced water are established.

Distribution and origin of potentially toxic elements in a multi-aquifer system

Pollution of the potentially toxic elements (PTEs) is a major concern in the metal ore-mining environment. Active polymetallic industries and mines cause great continuous devastation of both terrestrial and aquatic environments on a local and regional scale. This study investigated the pollution of surface water and groundwater in the area containing six large-scale iron ore mines, which have been in operation for more than a few decades. In order to assess the PTEs pollution, the spatial and temporal distributions of 13 different PTEs (Al, As, Co, Li, Mn, Mo, Ni, Pb, Rb, Se, Si, Sr, and Zn) were measured in 42 water samples collected from the multi-aquifer system including three distinct aquifers (upper alluvial aquifer (UAA), lower alluvial aquifer (LAA), and hard-rock aquifer (HRA)) of the Gohar-Zamin mining area in Iran. The highest concentrations of total dissolved solids (TDS = 164,000 mg/l) and PTEs were measured in HRA. Three trends were identified between the PTE concentration and increasing of TDS based on Spearman correlation analysis: (1) an increasing trend for Al, Co, Li, Mn, Rb, Se, Sr, and Ni; there were strong positive correlations in HRA between TDS and Mn (0.83), Al (0.65), Co (0.74), Li (0.90), Ni (0.83), Rb (0.91), Se (0.82), and Sr (0.84), suggesting a common origin for these elements; (2) no obvious trend for As and Mo, no correlation was founded between As and Mo with other PTEs and TDS, indicating a natural geogenic origin and mutual dependencies of these elements; and (3) a decreasing trend for Si, Zn, and Pb; TDS had a significantly negative correlation with the PTEs and attributing to different chemical properties of infiltrated groundwater. In the principal component analysis (PCA), the first PC that covers 85.09% of the total observed variance is mainly attributed the groundwater salinization. This component is composed of Al, Co, Li, Mn, Rb, Se, Sr, and Ni. The second PC contains elements As and Mo. This PC explain 14.4% of total variance and may be referred to natural origin of PTEs. Si, Zn, and Pb are in the third principal component and cover 9.64% of the variance of the data. Third PC have been attributed to lithogenic and/or primary water chemistry factors. The PTE pollution were evaluated based on heavy metal evaluation index (HEI), heavy metal pollution index (HPI), and degree of contamination (C_d). The results indicated that all of the groundwater samples collected from HRA had HEI, HPI, and C_d values greater than 21, 264, and 14 (highly pollution limits of indices), respectively, and were classified as highly polluted groundwater. HPI values within the UAA, LAA, and salt playa (SP) were lower than the critical level of 100, suggesting a threshold for the drinking water pollution. Moreover, HEI and C_d with values of less than 10 and 7 suggested low-level pollution in UAA, LAA, and SP. However, the contaminated level of PTEs exceeded the WHO standard for drinking water in HRA only. Since groundwater in HRA is a brine with the high values of PTEs, pumping of this water out to the surrounding natural environment may cause harmful impacts on the environment and perhaps living species in Bahram-e-Goor protected area. Graphical abstract.

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

Many studies have been carried out on the water environment in coal-mining subsidence area, which have mainly focused on the two aspects of nitrogen and phosphorus as well as heavy metal pollution in water. However, little attention has been paid to the problem of sulfate pollution. The surrounding conditions of subsidence area in Linhuan are complex, and there are a large number of coal gangue accumulation and coal mining activities. The sulfate pollution in water body is serious, while the specific sulfate source remains unclear. In the present study, the mining subsidence area in Linhuan, Huaibei was selected as the main study area, and 21 water samples were systematically collected from river water, subsidence area water and mine drainage. The conventional hydrochemical indexes were analyzed, and sulfur and oxygen isotopes were used to trace sources of sulfate in river water and subsidence area water. Our results showed that the total dissolved solid content was high in surface water, the hydrochemical type of Huihe River water was mainly Na⁺-Ca²⁺-HCO₃^-, and the hydrochemical type of subsidence area water was mainly Na⁺-Cl^--SO₄^2-. Before flowing through the mining area, sulfate in the main stream of Huihe River was mainly derived from two aspects: the dissolution of evaporative salt rocks in the upper reaches. The sulfate in the Baohe River tributaries was mainly derived from the combined effects of atmospheric precipitation. The two rivers converged and were discharged into the subsidence area, causing serious sulfate pollution and affecting the water quality in the subsidence area. The δ³⁴S_SO4 value and SO₄^2- content showed a good correlation in river water, subsidence area water and mine drainage, indicating that the SO₄^2- content in the subsidence area was mainly affected by double-ended sources. Besides the impact of the Huihe River, the discharge of local mining waste water was also an important sulfate source, reflecting the effect of coal-mining activities on the water environment.

Characteristics and processes of hydrogeochemical evolution induced by long-term mining activities in karst aquifers, southwestern China

Long-term mining activities reshape the hydraulic and hydrochemical field, and threaten the safe use of groundwater and ecosystem balance. Here, we concluded the evolution characteristics and processes of karst groundwater system based on the water level and hydrochemistry data of the Carboniferous and Devonian aquifer in Maoping lead-zinc deposit, a representative in southwestern China. After concentrated mining lasting for nearly three decades, this mining area has generated a huge depression cone coupled with decreased level, changed flowpath, massive drainage, and direct hydraulic connection across the aquiclude. However, these two sets of karst aquifers exhibited distinct hydrochemical evolution patterns in particular with respect to sulfate. SO₄^2- and the total dissolved solids (TDS) increased slightly in the Carboniferous aquifer and maintained the hydrochemical type of HCO₃ and HCO₃·SO₄. While, SO₄^2- and TDS in the Devonian aquifer decreased drastically, and the hydrochemical type was converted from SO₄ and SO₄·HCO₃ to HCO₃ and HCO₃·SO_4. Before concentrated mining, pyrite oxidation and dissolution of carbonate minerals mutually promoted each other in sluggish groundwater flow, then generated abnormally high concentrations of SO₄^2-, Ca²⁺, and Mg²⁺ in Devonian karst aquifer, causing an illusion of saturation state of gypsum dissolution. At present, SO₄^2- is mainly derived from pyrite oxidation indicated by sulfur isotope except deep groundwater contributed by gypsum dissolution. Groundwater quality in the Devonian aquifer was improved together by the dilution of northern shallow groundwater from the external Carboniferous aquifer and upward recharge of southern deep groundwater itself. Results of principal component analysis (PCA) verified the different recharge resources and mixing process resulted from continuous mining activities, which were the driving forces of hydrochemical evolution. Qualitatively speaking, the disturbing degree of the hydrodynamic field was consistent with the variation degree of hydrochemical filed. This research shed light upon the groundwater system and its evolution induced by intensive mining, which will benefit the future mining project.

Multiple isotope geochemistry and hydrochemical monitoring of karst water in a rapidly urbanized region

Karst water is an important resource for drinking water supply. To determine the impacts of urbanization on karst water quality, we performed a case study in the rapidly urbanized Guiyang-Anshun region, Guizhou province, southwestern China. We interpret data from regional reconnaissance and long-term monitoring related to major ion chemistry, stable isotopes (Sr, C, S (for SO₄^2-), and N and O (for NO₃^-)), remote sensing, and socio-economic development. We identify groundwater SO₄^2- and NO₃^- sources by combined use of δ³⁴S and Ca²⁺/Na⁺ molar ratio and δ¹⁵N and NO₃^-/Na⁺ ratio, respectively. We find that carbonate, sulfide, silicate, and gypsum weathering, anthropogenic inputs, and hydrodynamic conditions account for karst water composition and its seasonal variations. Atmospheric N and S deposition, nitrification of soil N, and sulfide oxidation control the background levels of groundwater NO₃^- and SO₄^2-. The elevated concentrations of NO₃^- and SO₄^2- at residential sites in rural and urban areas mainly arise from domestic sewage. Nitrification and fertilizer application are major reasons for the high levels of NO₃^- in regional groundwater systems. Vegetated/forested land area decreases as constructed land area increases, which results in declining biogenic CO₂ production and inputs into the aquifer. Although the local government has attempted to control SO₂ emissions, substantial increases in fossil fuel utilization and fertilizer consumption as well as population growth may have increased atmospheric HNO₃ deposition and induced increasingly severe contamination of groundwater with NO₃^- and SO₄^2-. Our results improve the understanding of urbanization impacts on water quality and are important for water resource management in karst regions.