Tracing and quantifying contributions of end members to karst water at a coalfield in southwest China

Pyrite from acid mine drainage promotes the removal of antibiotic resistance genes and mobile genetic elements in karst watershed with abundant calcium carbonate

More information is needed to fully comprehend how acid mine drainage (AMD) affects the phototransformation of antibiotic resistant bacteria (ARB) and antibiotic resistance genes (ARGs) in karst water and sewage-irrigated farmland soil with abundant carbonate rocks (CaCO3) due to increasing pollution of AMD formed from pyrite (FeS2). The results showed FeS2 accelerated the inactivation of ARB with an inactivation of 8.7 log. Notably, extracellular and intracellular ARGs and mobile genetic elements (MGEs) also experienced rapid degradation. Additionally, the pH of the solution buffered by CaCO3 significantly influenced the photo-inactivation of ARB. The Fe2+ in neutral solution was present in Fe(II) coordination with strong reducing potential and played a crucial role in generating •OH (7.0 μM), which caused severe damage to ARB, ARGs, and MGEs. The •OH induced by photo-Fenton of FeS2 posed pressure to ARB, promoting oxidative stress response and increasing generation of reactive oxygen species (ROS), ultimately damaging cell membranes, proteins and DNA. Moreover, FeS2 contributed to a decrease in MIC of ARB from 24 mg/L to 4 mg/L. These findings highlight the importance of AMD in influencing karst water and sewage-irrigated farmland soil ecosystems. They are also critical in advancing the utilization of FeS2 to inactivate pathogenic bacteria.

Practical application for legacy acid mine drainage (AMD) prevention and treatment technologies in karst-dominated regions: A case study

Acid mine drainage (AMD) from abandoned mines in karst-dominated regions in southwestern China was causing contamination of groundwater and surface streams. To avert the unwise decisions of "pollution first before treatment" during pre-mining, mid-mining and post-mining activities, this paper proposes a contaminant migration prevention technical framework covering 4 comprehensive processes. The formation mechanism of spring pollution, engineering remediation processes and contamination treatment effects were described in Longdong Spring. In 2018, the Longdong Spring water had Fe 33.83 mg/L and Mn 3.60 mg/L, exceeding the Chinese surface water standard (0.3 mg/L and 0.1 mg/L in GB 3838-2002) by 112 and 36 times, respectively. In 2020, after grout blocking, in situ treatment and wetland remediation, the highest Fe was 4.5 mg/L in a short period, and the spring water pollution days in this year were 42 days compared with the previous 320 spring water pollution days in 2018. In 2021, two years of remediation with the implementation of terminal remediation wetlands, the Fe was less than 0.03 mg/L compared with the previous 33.83 mg/L, and the water quality reached water standard (less than 0.3 mg/L). At present, Longdong Spring has become one of the most beautiful natural local landscapes.

Multi-isotope identification of key hydrogeochemical processes and pollution pathways of groundwater in abandoned mining areas in Southwest China

Acid mine drainage (AMD) is considered one of the serious environmental issues in the mining area. Understanding the key processes and pathways of hydrogeochemical evolution is critical for the effective control of AMD pollution. Hydrogeochemical analysis along with environmental isotope tracing was utilized to provide information regarding the hydrogeochemical process of groundwater pollution by using the multi-aquifer of abandoned Dashu pyrite in Southwest China as an example. Using the deuterium excess parameter d of groundwater and the results of ²H, ¹⁸O, and T analysis, the water-rock interaction intensity was determined. The distribution characteristics of d-T revealed that the groundwater primarily originated from the Quaternary reservoir platform groundwater and that there was a close hydraulic connection among the aquifers. The results of ion analysis and sulfur isotope tracing indicated that the sulfur in groundwater was primarily derived from gypsum dissolution, whereas the sulfur in mine water was primarily derived from pyrite oxidation. The results of the hydrogeochemical inversion indicated that mining activities altered the water level and flow conditions, promoted water-rock interactions, altered the hydrogeochemical process, and caused aquifer and mine water cross-contamination. The findings provide theoretical guidance for identifying the pollution sources and critical hydrogeochemical processes that affect groundwater in depleted mining areas of multi-aquifers and also provide technical support for developing water source control and prevention techniques.

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.

Behavior of Sb and As in the hydrogeochemistry of adjacent karst underground river systems and the responses of such systems to mining activities

Through the investigation of Qinglong mining area and adjacent karst underground river system, mining activities and water-rock interactions are found to control the hydrogeochemical evolution of karst underground water. Along the flow direction of the karst underground river, the hydro-chemical type is converted from HCO₃-Ca type to SO₄-Ca type. The concentrations of Sb and As also gradually decrease. Using PHREEQC to calculate the SI shows that: in the karst underground river system, both gypsum and fluorite are unsaturated, indicating a high degree of water-rock interaction. LogPCO₂ is negatively correlated with pH, indicating that the karst underground river systems are both open systems. The dissolution of carbonate minerals and the alternate adsorption of ions are the main water-rock interactions that lead to the rapid decline of Sb and As concentrations. This research also applies principal component analysis to identify the types of pollution in adjacent karst underground river systems. The results show that the LongBaiwei underground river was mainly affected by coal mining activities, and Fe was more prominent; the ShuiYa underground river was more significantly affected by the leachate from the antimony tailings yard. This study provides a scientific basis for the evolution of the water environment as well as strategies for pollution prevention and control in typical karst underground river systems owing to the influence of mining activities.

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.

Quantitative identification of nitrate and sulfate sources of a multiple land-use area impacted by mine drainage

The rapid increase in urbanization and intensive coal mining activities have accelerated the deterioration of surface water quality. Environmental problems caused by the accumulation of nitrate and sulfate from natural, urban, and agricultural sources have attracted extensive attention. Information on nitrate and sulfate sources and their transformations is crucial for understanding the nitrogen and sulfur cycles in surface water. In this study, we monitored nitrate and sulfate in three representative rivers in mining cities in northern China. The main pollution sources and biogeochemical processes were identified by using stable isotopes (δD, δ¹⁸O_H2O, δ¹⁵N, δ¹⁸O_NO3, δ³⁴S and δ¹⁸O_SO4) and hydrochemistry. The contribution of natural and anthropogenic sources was quantitatively estimated based on a Bayesian mixed model. The results indicated a large variation in sulfate and nitrate sources between the different rivers. Nitrate in the Tuohe River mainly derived from manure/sewage (57.9%) and soil N (26.9%), while sulfate mainly derived from manure/sewage (41.7%) and evaporite dissolution (26.8%). For the Suihe River, nitrate was primarily sourced from chemical fertilizer (37.9%) and soil nitrogen (34.8%), while sulfate was mainly sourced from manure/sewage (33.1%) and chemical fertilizer (21.4%). For the Huihe River, nitrate mainly derived from mine drainage (56.6%) and manure/sewage (30.6%), while sulfate predominantly originated from mine drainage (58.3%) and evaporite dissolution (12.9%). Microbial nitrification was the major pathway for the migration and transformation of nitrate in the surface water. However, denitrification and bacterial sulfate reduction (BSR) did not play a significant role as aerobic conditions prevailed. In this study, we elucidated the sources and transformation mechanisms of nitrate and sulfate. Additionally, we provided a reference for formulating a comprehensive strategy for effective management and remediation of surface water contaminated with nitrate and sulfate in mining cities.

A hydrochemical and multi-isotopic study of groundwater sulfate origin and contribution in the coal mining area

Coal mining cities are universally confronted with the degradation of groundwater quality, and the sulfate pollution of groundwater has become a widely studied environmental problem. In this study, we combined multi-isotope (δ³⁴S, δ¹⁸O-SO₄^2- and ⁸⁷Sr/⁸⁶Sr) approach with hydrochemical technique and a Bayesian mixed model to clarify sources and transformations and to quantitatively assess the contribution of sulfate from potential sources. The concentrations of SO₄^2- in groundwater ranged from 7.7 mg/L to 172.9 mg/L, and the high-value areas were located in coal mining area and residential area. The total values of δ³⁴S and δ¹⁸O-SO₄^2- varied from 10.6‰ to 26.9‰ and 6.9‰ to 14.1‰, respectively, in the groundwater. Analyses of SO₄^2- and Sr isotopes and water chemistry indicated that SO₄^2- in groundwater originated from various sources, such as atmospheric precipitation, sulfide mineral oxidation, evaporite dissolution, sewage and mine drainage. The oxidation of pyrite and bacterial sulfate reduction (BSR) had no significant impact on the stable isotopes of groundwater. At the same time, the calculation results of the Bayesian mixed model showed that the sources of SO₄^2- in groundwater mainly include evaporite dissolution in aquifer and mine drainage in the mixture of shallow and deep groundwater, with high contribution proportions of 39.8 ± 10.9% and 31.9 ± 5.7%, respectively, while the contributions of sewage (13.9 ± 8.5%), atmospheric precipitation (9.6 ± 8.6%) and the oxidation of sulfide (4.7 ± 3.3%) to SO₄^2- were lower. The research results revealed the source of SO₄^2- pollution in shallow groundwater in the coal mine area and provided an important scientific basis for the effective management and protection of groundwater resources.

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.

Evaluating water-yield property of karst aquifer based on the AHP and CV

In order to ensure the safety of mine production, it is of great practical significance to make a reasonable evaluation of the water-yield property (WYP) of a karst aquifer. In this paper, we selected fault-lines distribution, fault-scale index, aquifer thickness, water pressure, consumption of rinsing liquid, and hydraulic conductivity as the evaluation indexes to analyze the WYP of a karst aquifer. Meanwhile, the analytic hierarchy process (AHP) is used to calculate the subjective weight of indexes, and the coefficient of variation (CV) is used to calculate the objective weight of indexes. Combined with GIS, a multi-factor composite superposition is carried out to evaluate the WYP of a karst aquifer. The reliability of the research results is verified by the specific yield. Besides, for improving the reliability of evaluation results, the chemical composition of karst water was discussed. The results show that the selection of indexes is reasonable and the AHP–CV method is effective to evaluate the WYP of a karst aquifer. Therefore, on the premise of reasonable index selection, the evaluation models of AHP and CV can be used to evaluate the WYP of a karst aquifer and provide reference for coal mine water control measures.

Impacts of acid mine drainage on karst aquifers: Evidence from hydrogeochemistry, stable sulfur and oxygen isotopes

The pollution of karst aquifers by acid mine drainage (AMD) waters is increasing. Major and minor ions (Ca²⁺, Mg²⁺, HCO₃^-, SO₄^2-, F^-, and Fe), stable sulfur and oxygen isotopes of dissolved sulfates (δ³⁴S_SO4 and δ¹⁸O_SO4) and oxygen isotope of water (δ¹⁸O_H2O), were analyzed in rainwater, surface water, groundwater, and AMD water sampled from the Babu subterranean stream watershed, in Southwest China. The principal aim of this study was to explore the impact of AMD waters on the evolution of karst aquifers. Based on hydrogeochemistry and stable isotopes (δ¹⁸O_H2O, δ¹⁸O_SO4 and δ³⁴S_SO4): (1) the chemistry of AMD waters was primarily controlled by pyrite oxidation, karst conduit water by AMD waters and mixing with calcite and dolomite dissolution, and spring water by atmospheric precipitation and carbonate dissolution; (2) contamination of the karst conduit water was mainly attributed to the input of AMD waters, resulting in a shift of δ³⁴S_SO4 towards more negative values (from 3.4‰ to -13.2‰); (3) the quality of karst conduit water changed from suitable to unsuitable for irrigation and drinking, particularly due to the increase in total Fe, SO₄^2-, and F^- concentrations, reflecting the cumulative effect of AMD waters derived from tailings dumps; this influence was enhanced during rainstorm/drought and anthropogenic activities; and (4) the flow of contaminated groundwater through the conduit promoted the dissolution of carbonates, especially during the dry season due to the greater proportion of AMD in the groundwater. This released more CO₂ to the atmosphere. We believe that analysis of stable isotopes (δ¹⁸O_H2O, δ¹⁸O_SO4, and δ³⁴S_SO4), combined with hydrogeochemistry, is effective for exploring the impact of AMD on karst aquifers. Therefore, reasonable treatment methods should be taken to reduce the negative impacts of tailings dumps on karst aquifers.

Identification and quantification of contributions to karst groundwater using a triple stable isotope labeling and mass balance model

Although karst groundwater systems provide critical ecosystem services in many regions worldwide, anthropogenic contamination has seriously degraded groundwater quality. Properly elucidating geochemical processes, quantifying contributions of natural and anthropogenic end members, and then protecting karst aquifer systems remain challenging from scientific and engineering aspects. To identify the hydrochemical processes and quantifying contributions of end members (especially, contamination end members), 49 samples were collected from cave waters (CW), artesian springs (AS), and gravity springs (GS) in a karst watershed in Guiyang, China. With increased anthropogenic contamination, the CW, AS, and GS characterized by a Ca-Mg-SO₄^2--HCO₃^- composition often had pH and SO₄^2- concentrations exceeding USEPA secondary drinking water standards. That is attributed to the influence of water-rock interaction, rainfall, and anthropogenic sources (mainly, sewage and fertilizers), in agreement with the compositions of δ³⁴S_SO4, δ¹⁸O_SO4, and ⁸⁷Sr/⁸⁶Sr as well as the results of principal component analysis and correlation coefficients. Based on an end-member mixing model, contributions of rainfall and anthropogenic sources were 47% and 33% of GS, 52% and 41% of CW, and 58% and 35% of AS, respectively. It suggests that the karst groundwater quality is predominantly controlled by rainfall and anthropogenic sources (especially, land use). Results may be applied to properly evaluate the impacts of natural and anthropogenic sources on karst aquifers, coupled with actions to efficiently control potential contamination end members.

Multivariate statistical evaluation of dissolved heavy metals and a water quality assessment in the Lake Aha watershed, Southwest China

Heavy metals are of public concern in aquatic ecosystems due to their growing release from industries and mining activities. This study investigated the sources, temporal-spatial distributions and water quality of dissolved heavy metals (Mn, Co, Al, Ni, Ba, V, Sb, Fe, Sr) in the Lake Aha watershed, an area under the influence of sewage and acid mining drainage. These heavy metals displayed significant spatial and temporal variabilities. The water quality index results (WQI values ranged from 3.21 to 15.64) and health risk assessment (all hazard indexes are below 1) indicated that dissolved heavy metals in this study pose a low risk for human health. Correlation analysis and principal component analysis indicated that Fe and Sr mainly presented a natural geological feature in the study area, and Mn, Co, Al and Ni were influenced by the acid coal mine drainage, whereas Ba, V and Sb were under the impact of local industrial or medical activities. This study provides new insights into the risk assessment of heavy metals in small watersheds.

Arsenic oxidation and immobilization in acid mine drainage in karst areas

High concentrations of arsenic (As) occur in acid mine drainage (AMD), while the mechanisms governing its distribution along the flow of AMD are not fully understood. In this study, As species distribution was surveyed along the flow of an AMD in Jiaole coal mine in a typical kast area, in which length of creek is about 1100 m. AMD from the discharging source contained 1754.2 μg/L As (1570.0 μg/L in As (III)) and 644.1 mg/L Fe (all in Fe (II)) at pH 3.45. Both As and Fe concentrations decreased drastically to trace levels along the flow in the creek. As(III) oxidation to As(V) and Fe(II) oxidation to Fe(III) were discovered in a short distance from the discharging source. Lab experiments were performed to unveil the mechanisms governing As and Fe species distribution. Biological mechanism governed As(III) and Fe(II) oxidation in the AMD phase without contact with solid matrix, while different mechanisms governed the oxidation in the presence of solid matrix at different stages of AMD flow. At the beginning of AMD discharge, its contact with the soil matrix in rich of carbonate minerals in the karst area facilitated Fe(II) oxidation by O₂ due to pH rise, which generated reactive oxidants for As(III) oxidation and iron oxyhydroxides for As adsorption or co-precipitation. Along the AMD flow, bacteria in the underlying sediments profoundly accelerated the biological oxidation of As(III) and Fe(II) as well as the co-precipitation into the sediments. Findings of this study deepen the understanding of As transport and transformation along the AMD flow, particularly in karst areas.

Hydrochemistry of flowback water from Changning shale gas field and associated shallow groundwater in Southern Sichuan Basin, China: Implications for the possible impact of shale gas development on groundwater quality

The worldwide expansion of shale gas production and increased use of hydraulic fracturing have raised public concerns about safety and risks of groundwater resources in shale gas extraction areas. China has the largest shale gas resources in the world, most of which are located in the Sichuan Basin. Shale gas extraction in the Sichuan Basin has been increasing rapidly in recent years. However, the potential impact on shallow groundwater quality has not yet been systematically investigated. In order to evaluate the possible impact of shale gas extraction on groundwater quality, we present, for the first time, the hydrochemistry and Sr isotopic data of shallow groundwater, as well as flowback and produced water (FP water) in the Changning shale gas field in Sichuan Basin, one of the major shale gas fields in China. The Changning FP water is characterized by high salinity (TDS of 13,100-53,500 mg/L), Br/Cl (2.76 × 10^-3) and ⁸⁷Sr/⁸⁶Sr (0.71849), which are distinguished from the produced waters from nearby conventional gas fields with higher Br/Cl (4.5 × 10^-3) and lower ⁸⁷Sr/⁸⁶Sr (0.70830-0.71235). The shallow groundwater samples were collected from a Triassic karst aquifer in both active and nonactive shale gas extraction areas. They are dominated by low salinity (TDS of 145-1100 mg/L), Ca-HCO₃ and Ca-Mg-HCO₃ types water, which are common in carbonate karst aquifers. No statistical difference of the groundwater quality was observed between samples collected in active versus nonactive shale gas extraction areas. Out of 66 analyzed groundwater, three groundwater samples showed relatively higher salinity above the background level, with low ⁸⁷Sr/⁸⁶Sr (0.70824-0.7110) and Br/Cl (0.5-1.8 × 10^-3) ratios relatively to FP water, excluding the possibility of contamination from FP water. None of the groundwater samples had detected volatile organic compounds (VOCs). The integration of geochemical and statistical analysis shows no direct evidence of groundwater contamination caused by shale gas development.