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Lithium (Li) isotopes have shown large potential in tracing weathering in various water bodies, but there is limited study on Li isotopes in subsurface conditions where CO 2 has been largely consumed. In this study, we use a thick sandstone aquifer in the Ordos Basin, NW China, as a natural setting to investigate the behaviors of Li isotopes in hydrogeochemical conditions with different concentrations of dissolved CO 2. For young groundwater in the recharge area (group R) where CO 2 is abundant (mean P CO2 = 10-2.5 atm), clay formation accompanying with weathering leads to the enrichment of 7Li in groundwater. The four deep samples in the recharge area have uniform Li/Na ratios (with a mean of 2.52 μmol/mmol) and δ7Li (with a mean of 25.0‰), corresponding to a mean Li removal rate of 81.2% compared with the sandstone leachate. For groundwater in the shallow part of the discharge area (group D1), Li was firstly removed by clay formation during weathering in the recharge area and was later removed by physisorption when CO 2 becomes much lower (mean P CO2 = 10-3.1 atm). Different degrees of weathering lead to a wide range of δ7Li varying from 19.7‰ in the deepest well to 33.0‰ in the shallowest well. The proportion of Li removal caused by physisorption is found to increase with groundwater age. After the stage of Li removal by adsorption, Li was released in the deeper part of the discharge area (group D2), and the positive correlation of δ7Li versus Li/Na is explained by a ternary mixing model. The endmember of water brought by cation exchange is inferred to have a heavier δ7Li than sandstone leachate, demonstrating that cation exchange could cause an enrichment of 7Li in water. This study enhances our understanding of the controlling factors of Li isotopes in deep groundwater with low dissolved CO 2 , which have implications for the application of Li isotopes in subsurface water. [ABSTRACT FROM AUTHOR]

Major ion concentrations were measured in 22 groundwater samples collected from a coal bearing aquifer in Zhuxianzhuang coal mine, northern Anhui Province, and the ion source and geochemical characteristics were discussed by the methods such as traditional graphic and multivariate statistic analysis. The results showed the presence of three water types, i.e., HCO3-Na, Cl-Na and SO4-Na. The surrounding rock weathering and evaporation, both controlled the chemical characteristics of groundwater, evidenced by the Gibb's diagrams. In detail, the features of ion ratios such as Ca/Na, HCO3/Na and Mg/Na show that the evaporate dissolution and silicate weathering were the dominant water-rock interactions. Multivariate analysis, such as correlation analysis, cluster analysis and FA analysis were also conducted to reveal the ion source in groundwater, and the Ca2+ and Mg2+ were mainly released from the dissolution of gypsum and/or magnesium and/or anorthite; while the Na+ is influenced by the weathering of feldspar and halite, with the cation exchange in a certain extent. In contrast, the Cl-, HCO3 - and SO42- are also controlled by the halite dissolution, silicate mineral weathering and sulphate mineral dissolution, respectively. In addition, the SO42- in groundwater could be influenced by the oxidation of pyrite, which is abundant in the coal bearing aquifers. [ABSTRACT FROM AUTHOR]

Groundwater chemistry is diverse and complicated and is regulated by both natural hydrogeochemical and anthropogenic processes. Determining the governing processes and their influence on groundwater chemistry is very important to understand groundwater quality evolution and establish reasonable water management strategies. Main cations (Ca2+, Mg2+, Na+, K+, and Sr2+), anions (Cl−, SO42−, HCO−3, NO−3, and F−), and SiO2 and UV254 of 50 shallow groundwater samples were treated and analyzed. Factor analysis combined with ionic ratio and correlation analysis was used to identify the major hydrogeochemical processes responsible for the variation of hydrochemical components. Approximately 76% of the total variance of the data set can be explained by the four factors identified. Composing of Sr2+, Mg2+, Ca2+, and electrical conductivity (EC), Factor 1 accounted for 25.67% of the total variances, and represented groundwater formation background and fundamental water-soil/rock interaction. Factor 2 with high loadings on NO−3, U(Cl−, SO42−, HCO−3, NO−3, and F−) was related to anthropogenic activities, especially, was related to anthropogenic activities, especially the release of domestic sewage and industrial effluents. Factor 3 composed of Na+, HCO3− and EC was interpreted as cation exchange process. Factor 4 explained 15.75% of the total variance, and was attributed to the influence of agricultural activities, especially chemical fertilizer application. [ABSTRACT FROM AUTHOR]

• Geothermal fluid from Fengshun Tangkeng area for power generation is recharged by precipitation from surrounding mountains. • Silicate minerals dissolution, cation exchange in the reservoir and volatile migration from mantle are dominant geochemical processes for geothermal fluid evolution. • About 5.6–12.2% He and Ne are from mantle through regional scale faults systems with orientations of NE and NW. • Mixing with shallow groundwater occurs during geothermal fluid ascending process. This paper aims to determine the recharge sources, geochemical evolution and secondary reactions during ascending process of geothermal fluid with wellhead temperature of 92 °C in the Dengwu power plant of Fengshun area through analysis of hydrogeochemical and isotopic compositions of both geothermal water and gases. Results show that geothermal water for power generation is slightly alkaline HCO 3 -Na type with TDS of 291–369 mg/L and it is N 2 dominated. It is originally recharged by precipitation from surrounding mountainous areas with altitude of 475–683 m and circulates in the granite reservoir for about 7 ka to 9 ka years as evidenced by 14C and tritium dating after infiltration through faults systems of NE, NW and EW orientations. Under reservoir conditions, dissolution of silicate minerals like albite, microcline, muscovite and plagioclase leading to alkaline metal elements of K, Na, Ca and Mg and trace elements of Li, Sr and Rb releasing into the fluid. In addition, previous dissolved Ca2+and Mg2+ by eluviations exchange with Na+ and K+ absorbed on silicate reservoir and this process greatly improved concentrations of Na+ and K+, resulting in the dominant proportions of Na+ (91.3–95.8 meq%) in the geothermal water. And dissolution of CO 2 coming from air-saturated precipitation and organic matter or biogenesis promotes the water-rock interactions in granite reservoir. Helium and neon isotope analysis indicate that up to 12.2% of volatile of mantle migrate into the deep geothermal fluid through the regional scale faults system, e.g., Locus Mountain fault. During the ascending process, mixing with shallow groundwater and RongJiang River water make the geothermal water with lower temperature and TDS outflow along the NW orientated fault in forms of springs and artificial boreholes. It is speculated that the 5760 m3/day of geothermal fluid for power generation from fractured media of granite reservoir are mixture of original geothermal fluid in deep, shallow groundwater and Rongjiang River water. [ABSTRACT FROM AUTHOR]