Potential impacts of coal mining activities on nitrate sources and transport in a karst river basin in southwest China.

Typical sources of nitrate pollution in the fragile ecological environment of karst areas, such as agricultural production activities and domestic sewage, have long attracted serious concern. However, coal development can play an equally significant role in releasing the nitrogen fixed in coal into surface watersheds in the form of nitrate, nitrite, or ammonia, consequently threatening the water quality of surface water systems in mining areas. In this study, a typical karst surface watershed system affected by coal mining activities was selected for an in-depth investigation with the aim of realistically assessing the potential contribution of coal mining to nitrogen pollution. The results reveal increasingly concerning nitrate pollution from August 2020 to November 2021 in the Huatan River watershed under the influence of anthropogenic activities, especially mining development and agricultural production. Given that the nitrogen and oxygen isotope compositions of nitrate do not support the presence of denitrification, the variation in the NO3-/Cl- ratio and the relatively stable Cl- concentration may be a reflection of nitrification. Although the leaching of atmospheric precipitation on the strata in the basin promoted the release of nitrogen associated with coal mining, the higher rate of nitrogen cycling in the oligotrophic mine water environment limited the contribution of coal mining to nitrogen pollution in the surface watershed. Specifically, the contribution of coal mining activities to nitrogen pollution in surface karst river is mainly NH4+-N, which contributes 10% or less to the nitrate input to the waters of the Huatan River. The findings thus highlight the necessity of further uncovering the geochemical cycling process of nitrogen during the transport of mine water in the coal mining environment.

Inputs and transport of acid mine drainage-derived heavy metals in karst areas of Southwestern China.

Heavy metal pollution caused by acid mine drainage (AMD) is a global environmental concern. The processes of migration and transformation of heavy metals carried by AMD are more complicated in karst areas where carbonate rocks are widely distributed. Water, suspended particulate matter (SPM), and sediments are the crucial media in which heavy metals migrate and it is important to elucidate the geochemical behavior of AMD heavy metals in these environments. This study tracked AMD heavy metals from release to migration and transformation in a natural river system in a karst mining area. AMD directly impacted the hydrochemical composition of the karst water environment, but the carbonate rock naturally neutralized the acidity of the AMD. AMD heavy metal concentrations decreased gradually after the tributaries from the mining area entered the main river, with the metals tending to accumulate in SPM and sediments. The forms in which heavy metals were present were influenced by pH and their relative concentrations. Raman spectroscopy and transmission electron microscopy of sediments from the mining area suggested that the presence of an iron phase plays an important role in the fate of AMD-derived heavy metals. It is, therefore, necessary to elucidate the mechanisms of iron phase precipitation from sediments in order to control AMD-derived heavy metals in karst mining areas. This study improves our understanding of the geochemical behavior of heavy metals in karst environments and provides direction for the prevention and control of AMD in affected areas.

Persistent thallium enrichment and its high ecological risks developed from historical carbonaceous Hg-Tl mining waste.

Thallium (Tl) is a priority pollutant with high biotoxicity and has been of great concern worldwide in recent years. The former Lanmuchang Hg-Tl mining site in southwest China is a hotspot of multiple metal(loid)s pollution that previously caused large-scale chronic Tl poisoning, mainly resulting from carbonaceous Tl-bearing mining waste. However, arable land destroyed by historical mining wastes persists at high ecological risks decades after reclamation, but little is known about the solid phase partitioning and species of Tl during soil formation of underlying mining wastes as potential Tl sources. In this study, a representative reclaimed soil profile (100 cm depth) was selected in the lowlands to explore the geochemical cycling and environmental fate of Tl in mining waste-derived subsoil. The Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) analysis revealed an unexpected enrichment of Mn (2920-7250 mg/kg) and Tl (205-769 mg/kg) in the mining waste-derived subsoil. Results from BCR sequential extraction, X-ray Photoelectron Spectroscopy (XPS), and Electron Probe Microanalyses (EPMA) indicate that high Tl loading Mn oxide particulates (up to 15,712 ppm Tl) dominate the sequestration of Tl in the subsoil via oxidation-complexation and have a high potential for migration to both topsoil and groundwater. In addition, insights from microbial fossils and Fe-metabolizing bacteria closely related to Tl indicated that Fe (hydr)oxide particulates showing high Tl levels (up to 3865 mg/kg) point to biomineralization. Detailed mineralogical investigations revealed that hematite-siderite syngenetic particulates could serve as a promising mineralogical proxy for redox oscillations under periodic flooding and recorded the frequent groundwater level fluctuations experienced in the probed profile. Despite the potential for long-term preservation of high Tl loading Fe/Mn (hydr)oxides under HCO3-rich groundwater conditions in karst areas, the reductive release of Tl will be inevitable during flooding, implying that underlying carbonaceous mining waste will pose persistent and severe hazards to the ecosystem.

The non-coevolution of DIC and alkalinity and the CO2 degassing in a karst river affected by acid mine drainage in Southwest China.

Dissolved inorganic carbon (DIC) in mine water generated during coal mining is a large and potential source of atmospheric CO2, however its geochemical behaviors under the influence of AMD in relation to CO2 degassing and carbonate buffering are not well known. In this study, water temperature, pH, DO, alkalinity, Ca2+ concentration, and the carbon isotope of DIC were measured monthly from November 2020 to November 2021 and carbonate chemistry and CO2 emission flux were calculated to reveal the processes of DIC evolution and CO2 degassing from the Chetian River draining a karst region, which is materially affected by the input of large quantities of AMD. The results showed that carbonate erosion, the mineralization of terrestrial organic matter, and domestic sewage input are all identified to contribute DIC to different degrees to the river. Throughout the year, the Chetian River undergoes high-intensity CO2 degassing, which is dominated by HCO3--neutralized degassing and proton-enhanced degassing in different reaches. The pCO2 in the river under the influence of AMD is as high as 237,482 µatm, while the F-CO2 approaches 316.9 g C m-2 d-1. Meanwhile, the carbonate system in the downstream karst river buffers an average of 85.2 % of DIC release at the river's outlet. The input of AMD significantly altered the carbon cycle of the surface watershed in the headwaters of tributaries, and greatly enhanced the release of CO2 from surface water to the atmosphere; meanwhile, the buffering of carbonates on acidity in the water of main streams causes pCO2 to rapidly reduce over a short distance. Obviously, the carbon emission effect generated by the interaction between AMD and carbonate mainly occurs in the tributary water system. Considering the huge amount of AMD worldwide, this large potential source of atmospheric CO2 requires a specific and precise quantitative analysis based on actual observations.

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.

[Karst Hydrogeochemical Characteristics and Controlling Factors of Carlin-type Gold Mining Area Based on Hydrochemistry and Sulfur Isotope].

Mining activities change the groundwater level and flow conditions through pumping and drainage, which enhances the interaction between groundwater and aquifer rocks; mine drainage is discharged into the surface water system, which affects the whole karst water hydrogeochemical process. Based on hydrogeochemistry and the δ34S isotope, the hydrogeochemical processes, characteristics, and main controlling factors for waste water, karst groundwater, and surface water in a typical Carlin gold mining area and its surrounding areas were revealed. The results showed that:chemical compositions of groundwater and surface water unaffected by gold mining activities were mainly controlled by the weathering of limestone and dolomitic limestone; Ca2+, Mg2+, and HCO3- were main ions; and the water chemical types were Ca-HCO3. The mine wastewater and its downstream receiving water were affected by the dissolution of carbonate and silicate minerals, and cation exchange also played a role; the main ions were Ca2+, Mg2+, Na+, and SO42-, and the hydrochemical type gradually evolved from Ca-HCO3 to Ca-SO4. SO42- was the characteristic component in various water bodies affected by mining, and the concentration of SO42- gradually decreased from top to bottom in the well. The values of δ34S for unaffected groundwater and surface water were positive, and SO42- was mainly derived from realgar oxidation. Conversely, mine wastewater and downstream water were negative, SO42- was mainly influenced by the mixing action of realgar oxidation and meteoric precipitation, and pyrite also contributed to a certain extent. At the same time, NO3- came from agricultural fertilizer and rural domestic sewage discharge directly. Principal component analysis (PCA) further demonstrated:sulfide mineral oxidation and mining activities were the main controlling factors for the water chemical composition of mine wastewater and downstream water, whereas unaffected groundwater and surface water were mainly influenced by water-rock (carbonate rock) interactions. Agricultural fertilizer and rural sewage discharge also had a certain influence. Therefore, the study area should strengthen the interception of surface water, control-block-management of sulfide oxidation, rural domestic sewage treatment, and agricultural fertilizer.

The effects of weathering of coal-bearing stratum on the transport and transformation of DIC in karst watershed.

The mining of medium- to high­sulfur coal in karst areas has led to serious acidification problems in surface water, thus encouraging a re-evaluation of DIC transformation and CO2 source-sink relationships in karst watersheds. The weathering of limestone and sulfide-rich coal measures jointly influence the pH of the Huatan River in karst areas in Southwest China, which is lower in the rainy season and higher in the dry season. Due to CO2 degassing, DIC concentration tends to decrease along the flow direction, while δ13C-DIC gradually becomes heavier. In general, DIC transformation in the Huatan River is controlled by AMD input, CO2 degassing, organic matter (OM) degradation, and the dissolution and precipitation balance of carbonate minerals in different seasons. In spring, the mineralization of OM from terrestrial and domestic sewage gradually enhances and replenishes DIC in the water. As the pH increases in this season, the capacity for buffering CO2 increases. Meanwhile, OM degradation generates a large amount of CO2 in summer, and carbonic acid begins to dissolve limestone. In autumn, the pH decreases due to the enhanced weathering of sulfide-rich coal measures and the mass input of AMD. Thus, the river shows the ability to drive CO2 outgassing. In winter, CO2 degassing gradually weakens, DIC concentration is at its lowest, and δ13C-DIC reaches the heaviest value.

EESANet: edge-enhanced self-attention network for two-dimensional phase unwrapping.

In this paper, we first propose a quantitative indicator to measure the amount of prior information contained in the wrapped phase map. Then, Edge-Enhanced Self-Attention Network is proposed for two-dimensional phase unwrapping. EESANet adopts a symmetrical en-decoder architecture and uses self-designed Serried Residual Blocks as its basic block. We add Atrous Spatial Pyramid Pooling and Positional Self-Attention to the network to obtain the long-distance dependency in phase unwrapping, and we further propose Edge-Enhanced Block to enhance the effective edge features of the wrapped phase map. In addition, weighted cross-entropy loss function is employed to overcome the category imbalance problem. Experiments show that our method has higher precision, stronger robustness and better generalization than the state-of-the-art.

Biogeochemical transformation of sulfur and its effects on arsenic mobility in paddy fields polluted by acid mine drainage.

This study aimed to study the biogeochemical behaviour of sulfur and its potential impact on arsenic in farmland polluted by acid mine drainage (AMD). Soil samples were collected from paddy fields that were near arsenic-rich coal mines. Different geochemical analysis technologies, combined with the study of microbial populations, were used to analyse the enrichment and transformation characteristics of exogenous sulfur in the soil profile as well as their coupling effect with arsenic transformation. The results showed that sulfur, iron, and arsenic were obviously enriched in the soil, and sulfur easily migrated to deeper layers. The content of arsenic in contaminated soil was about 1.30-3.14 times higher compared to standard Chinese soil quality. Polluted soil had a higher content of bioavailable sulfur and adsorbed sulfur accounted for about 29% of the total sulfur. This promoted an increased microbial population abundance of Desulfosporosinus. The δ34S values of the polluted soil were lower than the background, ranging from -0.69‰ to 11.44‰, but were higher than the theoretical values. There was evidence of dissimilatory sulfur reduction and enrichment in 34S. A significant positive correlation was observed between the contents of adsorbed sulfur and total arsenic. The biogeochemical transformation of sulfur was conducive to the transformation of iron oxide-bound arsenic into sulfide minerals, which enhanced its stability. These results provide theoretical support for understanding the cycling transformation and environmental impact of sulfur in paddy fields polluted by AMD.

Diel variation of CH4 emission fluxes in a small artificial lake: Toward more accurate methods of observation.

Lakes are significant sources in global methane (CH4) budgets. However, estimations of the magnitude of global CH4 emissions from lakes may be highly biased owing to the uncertainties in data originating from observation times, methods, and parameterizations of the gas transfer velocity (k). Here, we conducted continuous 48-hour measurements of CH4 fluxes using the floating chamber method seasonally at Lake Baihua, a small reservoir in southwestern China, and compared the results with estimates derived from boundary layer models. Results showed that there was a weak dependency of k on wind speed, indicating that wind speed was not the major factor regulating gas exchange in such small lakes. It is thus concluded that the wind speed-dependent boundary layer model method is not suitable for CH4 flux observations in small and medium-sized lake, and that the floating chamber method is recommended for use instead. The measured CH4 fluxes displayed remarkably diurnal patterns, therefore the use of single observations to represent daily average values comes with unacceptably large uncertainties. A reasonable alternative is averaging observations made at sunrise and at sunset to represent daily values, which has a much smaller uncertainty (ranging from 0.8% to 13.6%). The coincident peaks of CH4 and chlorophyll concentrations in the subsurface indicate that CH4 originated mainly from aerobic methanogenesis. Solar radiation is likely one of the major factors regulating CH4 production and emissions in the lake through enhancing CH4 production, inhibiting CH4 oxidation, and probably changing hydrodynamics conditions. Therefore, irradiation should be taken into consideration as a key factor in observing CH4 fluxes in lakes. As sampling times are limited, observations during both sunny and cloudy weather should be proportionally included. This is the first time, to the best of our knowledge, that solar radiation has been proposed as a key driver of CH4 emissions from lakes.

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.

The generation of acid mine drainage (AMD) may accelerate watershed erosion and promote the migration of heavy metals, then threaten local ecosystems such as aquatic life and even human health. Previous studies have focused primarily on influence of AMD in surface environment. In order to reveal the acidizing processes in karst high-sulfur coalfield in Southwest China, this study, by contrast, focused on the hydrogeochemical evolution process and acidification mechanism of mine water in Zhijin coalfield, western Guizhou Province. The oxidation of pyrite and other sulfides induced strong acidification of mine water according to the water chemical analysis. As a result, a series of geochemical processes such as dissolution of carbonates and silicates, hydrolysis of metal ions, and degassing of CO2 complicated water chemical evolution. The dissolution of silicates controlled the chemical composition of mine water, but more carbonates might be dissolved during the acidification of mine water. The sources of sulfate are quite different in water samples collected from the two selected mine. According to sulfur isotope analysis, the dissolution of gypsum is the primary source of sulfate in samples from Hongfa mine, whereas sulfide oxidation contributed a large amount of sulfate to the mine water in Fenghuangshan mine. The dissolution of carbonates should be an important source of DIC in mine water and CO2 originating from organic mineralization might also have a certain contribution. This study elucidated the groundwater chemical evolution processes in high-sulfur coal-bearing strata and provided a foundation for further study of carbonates erosion and carbon emission during acidification of mine water.

Investigation of the disposal of dead pigs by pig farmers in mainland China by simulation experiment.

Dead pigs are a major waste by-product of pig farming. Thus, safe disposal of dead pigs is important to the protection of consumer health and the ecological environment by preventing marketing of slaughtered and processed dead pigs and improper dumping of dead pigs. In this study, a probability model was constructed for the disposal of dead pigs by pig farmers by selecting factors affecting disposal. To that end, we drew on the definition and meaning of behavior probability based on survey data collected from 654 pig farmers in Funing County, Jiangsu Province, China. Moreover, the role of influencing factors in pig farmers' behavioral choices regarding the disposal of dead pigs was simulated by simulation experiment. The results indicated that years of farming had a positive impact on pig farmers' choice of negative disposal of dead pigs. Moreover, there was not a simple linear relationship between scale of farming and pig farmers' behavioral choices related to the disposal of dead pigs. The probability for farmers to choose the safe disposal of dead pigs increased with the improvement of their knowledge of government policies and relevant laws and regulations. Pig farmers' behavioral choice about the disposal of dead pigs was also affected by government subsidy policies, regulation, and punishment. Government regulation and punishment were more effective than subsidy. The findings of our simulation experiment provide important decision-making support for the governance in preventing the marketing of dead pigs at the source.

A new approach to estimate fugitive methane emissions from coal mining in China.

Developing a more accurate greenhouse gas (GHG) emissions inventory draws too much attention. Because of its resource endowment and technical status, China has made coal-related GHG emissions a big part of its inventory. Lacking a stoichiometric carbon conversion coefficient and influenced by geological conditions and mining technologies, previous efforts to estimate fugitive methane emissions from coal mining in China has led to disagreeing results. This paper proposes a new calculation methodology to determine fugitive methane emissions from coal mining based on the domestic analysis of gas geology, gas emission features, and the merits and demerits of existing estimation methods. This new approach involves four main parameters: in-situ original gas content, gas remaining post-desorption, raw coal production, and mining influence coefficient. The case studies in Huaibei-Huainan Coalfield and Jincheng Coalfield show that the new method obtains the smallest error, +9.59% and 7.01% respectively compared with other methods, Tier 1 and Tier 2 (with two samples) in this study, which resulted in +140.34%, +138.90%, and -18.67%, in Huaibei-Huainan Coalfield, while +64.36%, +47.07%, and -14.91% in Jincheng Coalfield. Compared with the predominantly used methods, this new one possesses the characteristics of not only being a comparably more simple process and lower uncertainty than the "emission factor method" (IPCC recommended Tier 1 and Tier 2), but also having easier data accessibility, similar uncertainty, and additional post-mining emissions compared to the "absolute gas emission method" (IPCC recommended Tier 3). Therefore, methane emissions dissipated from most of the producing coal mines worldwide could be more accurately and more easily estimated.