Limestone water mixing process and hydrogen and oxygen stable isotope fractionation response under mining action.

North China type coalfield are gradually mining deep, and the mixing of groundwater is intensified. Hydrogen and oxygen isotopes are important elements for tracing groundwater movement. The fractionation response mechanism under mining conditions is not clear. In this paper, combined with numerical simulation, MixSIAR isotope mixing model and other methods, according to the δD, δ18O and hydrochemical information of various water bodies, the impact of coal mining on hydrogen and oxygen isotope fractionation is analyzed from multiple perspectives. The results show that summer soil water is the main source of recharge for limestone water, accounting for 30.7%-41.5%, and the Zhan River is the main source of recharge for limestone water. Before groundwater recharge, evaporation leads to the increase of δ18O in surface water by 0.31‰-5.58‰, water loss by 1.81%-28.00%, the increase of δ18O in soil water by 0.47‰-6.33‰, and water loss by 2.74%-35.80%. Compared with the coal mining layer, the degree of hydrogen and oxygen isotope drift and water-rock interaction in the coal mine stopping layer are significantly improved. The results of numerical simulation show that the pumping activity reduces the 18O concentration in the mining layer. The ion ratio is used as a new variable to eliminate the influence of water-rock interaction when calculating the mixing ratio. The results show that the limestone water is in a state of receiving external recharge, and mixing effect increases the δ18O in limestone water by 0.86‰ on average, and the δD increases by 0.72‰ on average. The research results explain the controlled process of hydrogen and oxygen isotope fractionation under mining conditions, which is of great significance to coal mine safety production.

A multiple isotope (S, H, O and C) approach to estimate sulfate increasing mechanism of groundwater in coal mine area.

Groundwater in North China type coal mine area is an important source of domestic, industrial and agricultural water. To explore the sulfate increasing mechanism of groundwater in mining area and identify key influencing factors. In this paper, hydrochemistry and multi-isotope tracer techniques such as δ34SSO4, δ18OSO4, δ2HH2O, δ18OH2O and δ13CDIC were used to study the groundwater circulation law and the migration and transformation mechanism of sulfate and carbonate in coal mine area. The results show that: the hydrochemical types of groundwater in the coal mine area are mainly HCO3- and SO42- anions, while the cations are mainly Ca2+ and Mg2+. The sulfate content is significantly increased, and the pH shows weak alkalinity; the relationship between δ18OH2O and δ18HH2O shows that the dynamic field of groundwater changes significantly after coal mining or closure, and limestone water mainly comes from surface water recharge through 'skylight' infiltration. The relationships between δ18OSO4 and δ18OH2O, δ34SSO4 and δ18OSO4 show that the sulfate in groundwater of coal mine area is mainly derived from sulfide oxidation. The ∆δ18OSO4-H2O value of groundwater in coal mine area is greater than 8 ‰, and the oxygen content in sulfate is 25 %-75 % from oxygen in water, indicating that coal mining has disturbed the groundwater in the study area from reducing environment to oxidizing environment, promoted sulfide oxidation, and accelerated the dissolution of carbonate minerals. The δ13CDIC value and δ34SSO4 value in the coal mine area are inversely proportional. The δ13CDIC of groundwater in the coal mine area is affected by the δ34SSO4 value to a certain extent. Sulfuric acid participates in the dissolution of carbonate minerals, making the pH value weak and alkaline as a whole. This paper expounds the migration and transformation law of sulfate in groundwater in coal mine area, which has practical significance for groundwater quality management. The research results can provide theoretical support for the rational development and utilization of groundwater resources in coal mine areas.

Identification of mixing water source and response mechanism of radium and radon under mining in limestone of coal seam floor.

With the gradual increase of the coal mining depth, the mixing of multiple water sources intensifies and the activity of radium and radon in groundwater increases. Identifying the source of mine water inrush by using radium and radon isotopes is a new choice. In this paper, the mathematical statistics method, radioactive isotope decay theory, the mass conservation principle, and the numerical simulation method are used to analyze the influence of total dissolved solids (TDS), pH, and the hydrochemical ion content in groundwater on the isotope activity of radium, radon, uranium, thorium, and lead. The activity of thorium and lead is lower than the detection limit of the instrument, and the influence of coal mining activities on it is small. The simulation of the radium-radon mass balance in groundwater shows that the greater the adsorption coefficient (k) of solid particles in groundwater is, the more obvious the adsorption effect and the greater the influence on the radium-radon activity balance are. The radium-radon dating method is used to calculate the groundwater age. Results show that the groundwater age in the closed pit coal mining area is generally older than that in the mining coal mining area. Combined with the 222Rn, 226Ra, and 234U radioactive isotopes and temperature, a mixing water source identification model of limestone in the coal seam floor is constructed. The model shows that the radium activity and temperature of the groundwater are inversely proportional to the mixing ratio of the Permian sandstone water. From the closed pit coal mining area to the mining coal mining area, the radium radon activity of the groundwater increases gradually, the groundwater age decreases significantly, the water cycle is accelerated, the mixing ratio of the Permian sandstone water decreases gradually, the mixing ratio of the Ordovician limestone water increases gradually, and the risk of coal mine water inrush increases. The research results prove the feasibility of the new method for accurately discriminating the mixing water sources in coal mine areas, which is of great significance to the improvement of the theory of coal mine water disaster prevention and control.