Water Chemistry And Sulfur-Oxygen Isotopes Geochemistry Characteristics Of Xijiang River

In recent years,the massive use of fossil fuels such as coals and petroleum has led the sulfur compounds from human sources to enter natural sulfur cycle,causing air pollution and acid deposition.Sulfuric acid participating in surface rock weathering is essentially a process of net release of CO₂,which must be distinguished in the calculation of weathering rate and carbon sink.Therefore,the mechanism of carbonate weathering by sulfuric acid and its relationship with regional carbon cycle are currently hot topics that have attracted great concern of scientists.Rivers are the channels of atmospheric and terrestrial substances to the ocean.The hydrochemical characteristics of rivers are a comprehensive reflection of the natural environment and human activities in the basin.Identifying the sources of solutes in water is an important prerequisite for rational development and utilization of river water resources.The Pearl River Basin is located in the karst area of Southwest China,which is also one of the serious acid rain areas in China and the world.Acid deposition damages the ecological environment and restricts the development of regional economy.Carbonate rocks and coal-bearing rock groups are widely distributed in the upper reach of the Pearl River.The sulfuric acid produced by coal mining and coal-burning activities greatly affects the rock weathering rate and sulfur cycle process in the basin.In this study,water samples of the Pearl River were collected in the wet season?July 2014?and dry season?January 2015?.The concentration of main cation and anion and the isotope value of sulfates in water were determined to know the physical and chemical properties,the temporal and spatial variations of main ions and sulfur and oxygen isotopes.The purposes of this paper are to explore the influence of factors such as lithology,precipitation and human activities on the chemical characteristics of river water and to calculate the rock weathering rate and atmospheric CO₂consumption rate in the basin,so as to evaluate the influence of sulfuric acid from different sources in the basin on the regional carbon cycle.The main conclusions are as follows:1.Chemical characteristics of river water where sulfuric acid participates in weathering of carbonate?1?The solutes in the Pearl River are mainly derived from carbonate weathering by carbonic acid and sulfuric acid,while the dissolution of evaporite and silicate contribute little to the water ion.In the Xijiang River Basin,24%of carbonate rocks are weathered by sulfuric acid in the wet season and 41%of that in the dry season.In the upstream area,the Nanpanjiang and Beipanjiang,sulfuric acid is more involved in the weathering of carbonate.?2?The pH value of the Rearl River water ranges from 7.52 to 8.45 during dry season,with an average value of 7.95.In the wet period,the pH value ranges from6.36 to 8.43,with an average value of 7.70.Both EC and TDS in dry season are higher than those in wet season,besides,the EC and TDS in main stream water decrease gradually from upstream to downstream,indicating that the rock weathering and erosion in the upstream region is stronger.?3?The sequence of main cation concentration in river water is Ca²⁺>Mg²⁺>Na⁺>K⁺and the sequence of anion content is HCO₃^->SO₄^2->NO₃^->Cl^->F^-.The sum of Ca²⁺and Mg²⁺accounts for about 90%of the total cationic content.The mean proportion of HCO₃^--equivalent concentration in the total anionic content during dry season is 70%,and that in the wet season is 74%.The main hydrochemical type of river water is Ca-HCO3.?4?The concentration of ions in the upstream is obviously higher than that in the middle and lower stream,and the concentration in the dry season is generally higher than that in the wet season.SO₄^2-accounts for 17%and 21%of the total anions in the river water during the dry season and wet season,respectively.The SO₄^2-in the the middle and lower stream is largely delivered by the upstream water.?5?The correlation between SO₄^2-,Cl^-and NO₃^-in the water shows that human activities?agriculture,industry and mining,domestic sewage?generally affect the river water in the basin.The contents of SO₄^2-,NO₃^-and Cl^-in the upstream water are about 2 to 3 times that of 15 years ago,indicating that the influence of human activities have intensified in recent years.2.Sulfur isotope geochemistry of river water?1?The?³⁴S_SO4 values of the main river during dry season is between-4.45‰⁺6.00‰with an average value of-0.73‰.In the wet season,the?³⁴S_SO4 values in the Pearl River basin is between-2.86‰⁺5.13‰with an average value of+0.03‰.The range of?³⁴S_SO4O4 in main river is almost in the range of?³⁴S_SO4 of the upper branch rivers,and the?³⁴S_SO4O4 values of the middle and lower main river is close to the?³⁴S_SO4O4 values of Hongshuihe and Liujiang river.The average?³⁴S_SO4 value of the middle and upper tributaries is lower than that in the lower tributaries,and the difference range between the tributaries is larger than that in the lower tributaries.?2?The main sources of SO₄^2-in Beipanjiang are sulfide oxidation of ore deposits,atmospheric precipitation and coal.The main three end-members of sulfate in other regions of the Pearl River basin are atmospheric precipitation,sulfide oxidation of ore deposits,domestic sewage and industrial wastewater.That is to say,the main pollution sources of water body are acid rain,domestic and industrial waste water.3.Rock weathering rate and CO₂ consumption?1?The weathering rate of carbonate rocks in Xijiang River Basin is 76.1t·km^-2·a^-1 and that of silicate rocks is 7.06 t·km^-2·a^-1.CO₂ consumed by weathering of carbonate rocks in xijiang river basin is 198.2×10⁹ mol·a^-1 and CO₂ consumed by weathering of silicate rocks is 29.8×10⁹ mol·a^-1.The amount of CO₂ released by carbonate rocks weathering by sulfuric acid in Xijiang River Basin is 0.19×10⁶mol·km^-2·a^-1(66.5×10⁹ mol·a^-1)which is equivalent to 34%of the CO₂ consumed by carbonate rocks weathering in the basin,twice the amount of CO₂ buried by silicate weathering.