Research On The Source Of Sulfate In Karst Reservoir Water Based On Hydrochemistry And Sulfur And Oxygen Isotopes

Dissolved sulfate is an important component of river water bodies.The source and transformation of sulfate in water bodies affect the ecological environment and water environment quality of river basins.High sulfate concentrations can lead to problems such as acidification of water bodies,increased mineralization,deterioration of water quality,and destruction of soil structure and fertility.As the source reservoir of Qianzhong Water Conservancy Project,the importance of water quality safety of Pingzhai Reservoir is self-evident.At the same time,the basin of Pingzhai Reservoir is located in the southwest acid rain region,and is also a fragile area of karst ecological environment.Therefore,identifying the source and transformation of sulfate input into water bodies has important social value.This study takes the Pingzhai Reservoir basin as the research area,collects water samples from May 2022（normal water period）,August 2022（high water period）,and December 2022（low water period）,and tests and analyzes the main physical and chemical parameters,anion and cation concentrations,hydrogen and oxygen isotopes of water bodies,and sulfate sulfur and oxygen isotopes of water bodies.Using hydrogen and oxygen isotopes to analyze the source of water supply,using spatial interpolation methods to analyze the spatial distribution of water sulfate,using sulfur and oxygen isotopes to trace the source of water sulfate,and combining stable isotope mixing models to estimate the contribution rate of each end element.The results are as follows:（1）The water body in the study area is weakly alkaline water with an average p H of 8.51 and a total dissolved solids（TDS）average of230.98 mg/L,which is weakly mineralized water.It is generally in an aerobic environment,with an average DO of 9.38 mg/L and an EC average of 366μs/cm.The dominant cations in the study area are Ca²⁺and Na⁺,while the dominant anions are HCO₃^-and SO₄^2-.The hydrochemical type is HCO₃^--Ca²⁺type.The concentration of ions in the water body is the highest in dry season,and is close to that in wet season and normal season.The water body in the study area is mainly recharged by precipitation,and is partially recharged by groundwater.During the dry season,it is most affected by evaporation and fractionation.According to the analysis of ion correlation,the input of water solute mainly includes weathering of carbonate and silicate rocks,atmospheric precipitation,human activities,and other aspects.（2）The SO₄^2-ion concentration in the water body of the study area accounts for about 30%of the total anions.The variation range of SO₄^2-concentration in the water body is 17.36-129.83mg/L,with an average value of 62.61mg/L.From the perspective of seasonal variation,the SO₄^2-concentration in the dry season is the highest,and the wet season and normal season are close.From the perspective of spatial variation,the spatial difference of SO₄^2-in the river water body is large,while the spatial difference of SO₄^2-in the reservoir water body is small.The sulfate sulfur isotope of the water body in the study area is generally negative,with an average value of-4.1‰.From the perspective of seasonal variation,the sulfate sulfur isotope in the dry season is more positive than in the normal and wet seasons.The sulfate oxygen isotope of the water body is generally positive,with an average value of 1.6‰.The seasonal variation is the highest in the normal season and the lowest in the wet season.The spatial differences of sulfate sulfur and oxygen isotopes in river water bodies are large,and the isotope values at each confluence point of the reservoir water body are relatively close.（3）Based on the analysis of sulfate sulfur and oxygen isotopes in the water body,it is concluded that the sulfate in the water body in the study area mainly comes from input sources such as coal,soil,sewage,and atmospheric precipitation,with a contribution rate of 39%from coal,35%from soil,11%from sewage,and 15%from atmospheric precipitation.There are certain seasonal differences in the contribution rates of different input sources,and there are also certain differences in the input sources of water bodies in river and reservoir areas.