Hydrogeochemical Characteristics And Evolution Of The Saline/Salty Springs And Subsurface Brines In The Northeastern Sichuan Basin

Subsurface brines exist in the sedimentary basins,and saline and salty springs occur in the shalow buried areas of the salt-bearing strata.The TDS of subsurface brine ranges from tens to hundreds of grams per liter,and elements such as Br,I,Sr,Li,Ba,Cs Rb,B and K are rich in the brines.They are important raw materials for agriculture,industry,and nuclear industry.The hydrochemical and isotopic composition of subsurface brines and saline and salty springs imply important information about salt rocks,evaporation process,and groundwater circulation,which are of great significance for revealing the paleo-evaporite sedimentary environment and the transport of substance in the modern water cycle.The Sichuan Basin is characterized by the formation of salt-bearing carbonate rocks of the Middle and Lower Triassic?T₁j-T₂l?.In this paper,a total of 64 saline and salty springs?drilling wels?in the Sichuan Basin are investigated and analyzed.The geological characteristics of the Sichuan Basin,and the hydrochemical and occurrence characteristics of the saline and salty springs are summarized through analyses of stable hydrogen and oxygen isotopes and ion correlation.The study found that the saline and salty springs mostly emerge at the margin of the basin where the fracture or fault zones exist and within basin where the anticlines occur.The occurrence of the springs is clearly controlled by the regional tectonic lines.The saline and salty springs in the northeastern Sichuan Basin are mainly controlled by the anticlines,occurring in the river valeys.Subsurface brines in the central and western basin are deep buried in the T₁j-T₂l carbonate and evaporite,while saline and salty springs emerge in the core of anticlines in the eastern basin.The origin of recharge,hydrochemical components,water-rock interactions and formation mechanisms of the saline and salty springs in the northeastern Sichuan Basin are discussed in detail.The subsurface brines in the T₁j-T₂l carbonate rocks in the central and western Sichuan Basin are in a deep-buried and stagnant state,and have not been affected by the modern water cycle,with TDS higher than 300 g/L and the hydrochemical types of Cl-Na and Cl-Na·Ca.The T₁j-T₂l carbonate rocks outcrop in the core of the anticlines in the northeastern basin.The TDS of saline and salty springs in the northeastern Sichuan Basin range from 822 to 82933 mg/L,hydrochemical types are of Cl·HCO₃-Na·Ca,Cl·SO₄-Na,Cl-Na type,pH is 7.02-8.4,and temperature is the range of 15-40°C?the subsurface brine is as high as 87°C?.The total contents of rare earth elements of the saline and salty springs decrease with the increasing pH,and their contents in the reducing environment are higher.The total contents of rare earth elements in the northeastern basin range from 0.141 to 0.314?g/L.The light rare earth elements are relatively poor?0.065-0.253?g/L?,the middle rare earth elements are second?0.018-0.091?g/L?,and the heavy rare earth elements are relatively enriched?0.004-0.02?g/L?.Groundwater in the core of anticline is recharged by meteoric water,and percolates to the two limbs of the anticline or deep aquifers forming shalow or deep groundwater?local or regional groundwater flow system?,simultaneously inconcruently dissolves carbonates and evaporites?including anhydrite and halite?in the T₁j and T₂l strata during its subsurface transport.The groundwater of the discharges areas are in the middle of the anticlines or in the low-lying areas of the two limbs of the anticlines where carbonate rocks outcrop,or the low-lying areas of the river cutting the non-carbonate rock where groundwater goes upward through the non-carbonate rocks,in the form of springs.Saline/salty springs with varying TDS and temperature can be encountered.