Characteristics And Mechanism Of Groundwater Pollution In Two In-situ Leaching Uranium Mining Areas In Yili Basin,Xinjiang

Uranium mining and metallurgy is the basis of nuclear fuel supply and sustainable development of nuclear power.In situ leaching uranium mining technology has been rapidly developed and applied in uranium mining because of its low production cost,less damage to the surface ecological environment and being suitable for mining low-grade mineral resources.Although the pollution caused by in-situ leaching of uranium is much less than that caused by conventional uranium mining,due to the injection of a large number of chemical reagents into the ore-contained aquifer during mining,the content of various components in the groundwater of the ore-bearing strata increases significantly,which leads to the increase of the total mineralization of the groundwater and the excessive content of harmful elements.If not treated in time,the scope of pollution will be further expanded.In this study,acid leaching and CO₂+O₂ leaching processes in Yili basin of Xinjiang were selected to analyze the pollution components of groundwater.The single factor pollution index method and Nemerow comprehensive pollution index method were used to evaluate the quality of groundwater.According to the evolution mechanism of sulfur isotope in groundwater and the thermodynamic principle of pollutant formation,the generation mechanism of groundwater pollutants was explored.The main results are as follows:(1)During acid mining,the content of most components in groundwater is significantly higher than the background value of groundwater and the national groundwater class III quality standard.The main components exceeding the standard are SO₄^2-,heavy metal elements such as Mn,Ni,Fe,Pb,Zn,Cd,As,and radioactive elements such as Th.The effect of CO₂ leaching on the concentration of each component of groundwater is much smaller,and the p H is basically unchanged,the main pollution factors are SO₄^2-,NO₃^-,Cl^-,Mn and Ni,while other components such as Cu,Zn,Cd,Pb,Fe and As do not change significantly.(2)In the acid mining area,a large amount of SO₄^2-and various ions produced by surrounding rock dissolution lead to high salinity of groundwater.In CO₂ mining area,SO₄^2-is the main reason of high salinity of groundwater.(3)In the single pollution assessment,the groundwater in acid method and CO₂ mining area belong to the category of serious pollution.In Nemero comprehensive pollution assessment,groundwater in acid mining area is seriously polluted.The groundwater quality in CO₂ mining area belongs to moderate and severe pollution on the whole,and some of them are seriously polluted.(4)There are significant differences in sulfur isotope composition between acid mining area and CO₂ mining area.In the acid mining area,the?³⁴S of groundwater is almost equal to the standard,and its isotopic effect is weak,while in the CO₂ mining area,the?³⁴S of groundwater is obviously enriched in ³²S,indicating that the SO₄^2-of groundwater in the mining area is not from the acid mining area,but mainly from the oxidation of pyrite in the ore bed.(5)In acid leaching,the groundwater is in strong acid and oxidation environment,and minerals are easy to react with leaching agents,which lead to the increase of chemical components in the water.The chemical reaction of minerals in CO₂ in-situ leaching groundwater is easily affected by external factors.Because the groundwater is in a neutral environment,the content of each component is far lower than that in the acid mining area.The geochemical environment and mineral reaction mechanism of groundwater are obviously different between the two different in-situ leaching processes because of the different leaching agents,which lead to the significant differences in the pollution characteristics of groundwater.