Sulfur And Oxygen Isotope Constraints On The Fate Of Sulfate In A Typical Karst Catchment In Southwest China

Water pollution is one of the most serious eco-environment problems in the karst area of southwestern China,since it has integrated features with geochemical variability and eco-environment vulnerability.The sources and transformation processes of sulfate in water can have impacts on regional eco-geological environment and water quality.Hence,understanding their sources and transformation processes can provide scientific evidences for evaluating the water quality and protecting the environment in the karst area.In this study,water-soluble ions as well as sulfur and oxygen isotopes of sulfate in atmospheric precipitation,surface water and groundwater in the Houzhai catchment,Puding county,southwestern China,were analyzed,in order to identify their chemical variations as well as the sources and transformation processes of sulfate.The research results obtained from this study are shown in the followings:（1）The sulfur and oxygen isotope ratios in sulfate in precipitation show that the weighted average values ofδ³⁴S-SO₄^2-for precipitation in the lower and upper reaches of the Houzhai River are 1.0‰（-8.5～17.9‰）and 2.2‰（-10.2～21.2‰）,respectively;the weighted average values ofδ¹⁸O-SO₄^2-for precipitation in the lower and upper reaches of the river are 2.2‰（3.7～18.1‰）and 2.2‰（7.2～16.0‰）,respectively.The sulfur isotopic composition of sulfate in precipitation in the study area is close to that of coal and biogenic sulfur from the local area,indicating that coal burning and biogenic sulfur are the two main sources of sulfate in precipitation.In addition,compared with previous studies,it shows a reduction in sulfate concentration and an increase in p H andδ³⁴S-SO₄^2-values,indicating that the pollution level of acid rain in southwestern China is gradually decreasing.（2）The sulfur and oxygen isotope ratios in sulfate in surface water show that the mean values ofδ³⁴S-SO₄^2-andδ¹⁸O-SO₄^2-are 7.6‰（-0.3～18.2‰）and 8.7‰（5.6～15.0‰）,respectively,suggesting that the sulfate in river water are mainly derived from pyrite oxidation and gypsum dissolution.The study displays that the sulfate in river water from the lower reaches of the Houzhai River is significantly more affected by pyrite oxidation than gypsum dissolution,while the sulfate from the upper reaches of the river is significantly more affected by gypsum dissolution than pyrite oxidation.（3）The sulfur and oxygen isotopic compositions of sulfate in groundwater show that the mean values ofδ³⁴S-SO₄^2-andδ¹⁸O-SO₄^2-are 7.2‰（-2.9～25.8‰）and 9.4‰（4.0～16.5‰）,respectively.It indicates that the sulfate in groundwater are mainly affected by pyrite oxidation and gypsum dissolution,which is similar to the sulfate in surface water.The distribution of sulfur in the study area has a spatial difference,i.e.the sulfate of groundwater in the middle reaches of Houzhai River is more affected by gypsum dissolution than that in the lower reaches of the river,and the sulfate of groundwater in the lower reaches of the river is more affected by pyrite oxidation than that in the middle reaches of the river.（4）The research on sulfate in different water bodies shows that the precipitation in the study area changes in different seasons,which has different dilution effects on the sulfate concentration in the surface water.The sulfate concentration of surface water decreases with the increase of precipitation in the rainy season.Compared with the lower and upper reaches of the Houzhai River,there are more sinkholes and springs in the middle reaches of this river.Hence,the sulfate concentration in groundwater from the middle reaches of the river is significantly diluted in the rainy season,since the river water flow increases with the increase of precipitation.Theδ³⁴S andδ¹⁸O values of sulfate in groundwater were strongly correlated with those in surface water,indicating that the sulfate in groundwater and surface water is affected by pyrite oxidation and gypsum dissolution.