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.

Monitoring of flow field based on stable isotope geochemical characteristics in deep groundwater.

The water circulation in deep aquifers controls not only chemical composition of the groundwater, but also stable isotope composition. In order to analyze the flow field in the process of the deep groundwater circulation in different aquifers, specimens belonging to the fourth aquifer in the Quaternary (the fourth aquifer for short), the coal and sandstone cranny aquifer in the Permian, and Carboniferous (the coal catena aquifer for short), the Taiyuan group limestone aquifer in the Carboniferous (the Taiyuan limestone aquifer for short), and the limestone aquifer in the Ordovician (the Ordovician limestone aquifer for short) were gained from the top down in Renlou colliery and local Linhuan coalmine district, northern Anhui, China, in the study. δD, δ(18)O, and the content of tall dissolve solids (TDS for short) of these specimens were tested. The experimental results had revealed that the groundwater in the fourth aquifer and the Taiyuan limestone aquifer takes on (18)O excursion and the coal catena aquifer takes on D excursion in Linhuan coalmine district, while excursion characteristic in the Ordovician limestone aquifer is not evident in the coalmine district. By analysis, δ(18)O and the content of TDS are in negative relationship in the groundwater of the fourth aquifer and the Taiyuan limestone aquifer in Linhuan coalmine district, yet δD and the content of TDS are in positive relationship in the coal catena aquifer. Mining greatly influences the fourth aquifer and the coal catena aquifer so the groundwater in the fourth aquifer flows from northwest and southeast to mining areas and the groundwater in the coal catena aquifer flows from around to mining areas. However, mining does not influence the Taiyuan limestone aquifer evidently so the groundwater flows from east to west still.

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 δ13CDIC 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 SO42- and δ34SSO4 indicate that a small portion of SO42- 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.

Metal-free catalysts of graphitic carbon nitride-covalent organic frameworks for efficient pollutant destruction in water.

Novel metal-free catalysts via integration of covalent organic framework (COF) and graphitic carbon nitride (g-C3N4@COF) with a high graphitization degree and nitrogen content were fabricated and exhibited an outstanding activity in a wide pH range for peroxymonosulfate (PMS)-driven oxidation of refractory organic pollutants in water. Scanning electron microscopy images showed many aggregated COFs crystals anchored on the irregular g-C3N4 surface to form 3D structures. The precursors (urea, melamine, and dicyandiamide) of g-C3N4 determined the porous structures and properties of the g-C3N4@COF materials. The hybrids possessed superior reactivity in Orange II removal (100%) compared to pristine g-C3N4 (10%) and COF (5%), benefiting from high-temperature pyrolysis to generate crystal carbon and modulate nitrogen doping. Besides, removal efficiency of target pollutants depended on the oxidant dosages (0.33-1.30 mM), initial concentrations of organics (10-40 mg/L), temperatures (5-45 °C), pHs (1.72-10.3), and anions (Cl-, SO42-, NO3-, HCO3-, CO32-, and HCOO-). Quenching experiments and electron paramagnetic resonance demonstrated that non-radical singlet oxygen (1O2) was the dominant species for the oxidation of organic pollutants via electron transfer in the g-C3N4@COF/PMS system. It was inferred that the good balance between graphitization degree and nitrogen content benefited to enhancing the catalytic performance for the refractory pollutant degradation. The present investigation provides a new avenue for the design and construction of metal-free hybrid composites for environmental remediation.