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.