Geochemistry of surface and ground water in Guiyang,China: Water/rock interaction and pollution in a karst hydrological system

The chemical compositions of the surface/ground water of Guiyang, the capital city of Guizhou Province, China are dominated by Ca²⁺, Mg²⁺, ${\text{HCO}}_3^{-}\text{and}{\text{SO}}_4^{2-}$, which have been derived largely from chemical weathering of carbonate rocks (limestone and dolomite). The production of ${\text{SO}}_4^{2-}$ has multiple origins, mainly from dissolution of sulfate evaporites, oxidation of sulfide minerals and organic S in the strata, and anthropogenic sources. Most ground water is exposed to soil CO₂ and, therefore, the H₂CO₃ which attacks minerals contains much soil C. In addition, the H₂SO₄ produced as a result of the oxidation of sulfides in S-rich coal seams and/or organic S, is believed to be associated with the chemical weathering of rocks. The major anthropogenic components in the surface and ground water include K⁺, Na⁺, Cl⁻, ${\text{SO}}_4^{2-}\text{and}{\text{NO}}_3^{-}$, with Cl⁻ and ${\text{NO}}_3^{-}$ being the main contributors to ground water pollution in Guiyang and its adjacent areas. The seasonal variations in concentrations of anthropogenic components demonstrate that the karst ground water system is liable to pollution by human activities. The higher content of ${\text{NO}}_3^{-}$ in ground water compared to surface water during the summer and winter seasons, indicates that the karstic ground water system is not capable of denitrification and therefore does not easily recover once contaminated with nitrates.     

Carbon sequestration and decreased CO2 emission caused by terrestrial aquatic photosynthesis: Insights from diel hydrochemical variations in an epikarst spring and two spring-fed ponds in different seasons

Whether carbonate weathering could produce a stable carbon sink depends primarily on the utilization of dissolved inorganic carbon (DIC) by aquatic phototrophs (the so-called Biological Carbon Pump-BCP effect). On this basis, water temperature (T), pH, electrical conductivity (EC) and dissolved oxygen (DO) were synchronously monitored at 15-min resolution for one and two days respectively in January and October 2013 in Maolan Spring and the spring-fed midstream and downstream ponds in Maolan Nature Reserve, China. A thermodynamic model was used to link the continuous data to allow calculation of CO₂ partial pressures (pCO₂) and calcite saturation indexes (SI_C). A floating static chamber was placed on the water surface successively at all sites to quantify CO₂ exchange flux between atmosphere and water so as to evaluate the BCP effect. Results show that, in both winter and autumn, remarkable diel variations of hydrochemical parameters were present in the midstream pond where DO, pH, and SI_C increased in the day and decreased during the night while EC, $\left[{{\text{HCO}}_3}^{-}\right]$, [Ca²⁺] and pCO₂ showed inverse changes mainly due to the metabolic processes of the flourishing submerged plants, with photosynthesis dominating in the day and respiration dominating at night. However, hydrochemical parameters in the spring and downstream pond show less change since few submerged plants developed there. It was determined that the BCP effect in the midstream pond was 285 ± 193 t C km⁻² a⁻¹ in winter and 892 ± 300 t C km⁻² a⁻¹ in autumn, indicating a potential significant role of terrestrial aquatic photosynthesis in stabilizing the carbonate weathering-related carbon sink.     

Thermodynamic modeling of binary CH4–CO2 fluid inclusions

An equation of state (EOS) explicit in Helmholtz free energy has been improved to calculate the PVTx and vapor–liquid phase equilibrium properties of CH₄–CO₂ fluid mixture. This EOS, where four mixing parameters are used, is based on highly accurate EOSs recommended by NIST for pure components (CH₄ and CO₂) and contains a simple generalized departure function presented by Lemmon and Jacobsen (1999). Comparison with experimental data available indicates that the EOS can calculate both vapor–liquid phase equilibrium and volumetric properties of this binary fluid system with accuracy close to that of experimental data up to high temperature and pressure within full range of composition. The EOS of CH₄–CO₂ fluid, together with the updated Gibbs free energy model of solid CO₂ (dry ice), is applied to calculate the CH₄ content (x_CH4) and molar volume (V_m) of the CH₄–CO₂ fluid inclusion based on the assumption that the volume of an inclusion keeps constant during heating and cooling. $V_{\text{m}}-x_{{\text{CH}}_4}$ diagrams are presented, which describe phase transitions involving vapor, liquid and CO₂ solid phases of CH₄–CO₂ fluid inclusions. Isochores of CH₄–CO₂ inclusions at given $x_{{\text{CH}}_4}$ and V_m can be easily calculated from the improved EOS.     

Model reconstruction of nitrate pollution of riverbank filtration using 15N and 18O data,Karany, Czech Republic

Stable isotopes of O (δ¹⁸O) in water and N (δ¹⁵N) in ${\text{NO}}_3^{-}$ have been used as natural indigenous groundwater tracers for sources of water and of ${\text{NO}}_3^{-}$ at two riverbank filtration (RBF) water supply systems. Both RBF systems (Skorkov and Sojovice) have wells in unconsolidated Quaternary sediments close to the Jizera River (Czech Republic) that have been affected by increasing ${\text{NO}}_3^{-}$ concentrations. The area is underlain by Turonian sandstones and marls that form a deeper bedrock aquifer. Sources of ${\text{NO}}_3^{-}$ are local sewerage systems and landfills (point sources) and seasonal application of manure and inorganic fertilizers (diffuse sources).At RBF Skorkov recharge to wells can be modelled using a two-component model with 60% river water contribution and 40% of very shallow groundwater with an average residence time of one month. During periods of abundant precipitation, groundwater originates entirely from the unsaturated zone of the Quaternary aquifer; extensive pumping for over 40 a has created new, bypassing flow paths that preferentially drain the contaminated unsaturated zone. During dry periods, wells are recharged by longer residence time groundwater from the Quaternary aquifer.At RBF Sojovice there is an additional recharge component of groundwater from the Turonian aquifer, which is sandier at this locality; this contains denitrified ${\text{NO}}_3^{-}$ with highly positive δ¹⁵N values.     

Remediation of coal-mine drainage by a sulfate-reducing bioreactor: A case study from the Illinois coal basin,USA

This study reports changes in coal-mine drainage constituent concentrations through an anaerobic SO₄-reducing bioreactor monitored over a 3-a period. The purpose of the study was to identify and monitor over time the biogeochemical mechanisms that control the attenuation of toxic compounds in the mine drainage. This information is needed to investigate bioreactor performance and longevity. The water treated at the case example site, the Tab-Simco Mine, was highly acidic with an average pH of 2.9, a net acidity of 1674 mg/L CaCO₃ equivalent-CCE, and high levels of dissolved ${\text{SO}}_4^{2-}$, Al, Fe and Mn. The results of this study indicated that the treatment system increased the pH of the acid mine drainage (AMD) to 6.2 and decreased the median acidity to 22.7 mg/L CCE, ${\text{SO}}_4^{2-}$ from 2981 to 1750 mg/L, Fe from 450.6 to 1.76 mg/L, Al from 113 to 0.42 mg/L, and Mn from 36.4 to 23.3 mg/L. Geochemical modeling indicates that the bioreactor discharge is saturated with respect to the minerals alunite, gibbsite, siderite, rhodochrosite, jarosite, and Fe hydroxide precipitates. The observed trends also include seasonal variations in ${\text{SO}}_4^{2-}$ reduction and a general decline in the amount of alkalinity produced. The average δ³⁴S value of the ${\text{SO}}_4^{2-}$ in the untreated AMD was +7.3‰. In the bioreactor, δ³⁴S value of ${\text{SO}}_4^{2-}$ increased from an average of +6.9‰ to +9.2‰, suggesting the presence of bacterial SO₄ reduction processes. Preliminary results of a bacterial community analysis show that DNA sequences corresponding to bacteria capable of SO₄ reduction were present in the bioreactor outflow sample. However, these sequences were outnumbered by sequences similar to bacteria capable of reoxdizing reduced sulfur species. This study illustrates the dynamic nature of metal removal in SO₄-reducing bioreactor-based treatment systems.