Study On The Sources And Transformation Of Sulfate And Its Environmental Significance In Northern Mount Huangshan Watershed

Sulfuric acid is one of the most important acid involved in the chemical weathering process of mineral rocks,and has a great impact on the global carbon cycle.But current researches have not reveal the detailed mechanism completely,so that it still need more data to interpret.In recent years,the influence of acid deposition has become more serious in Mount Huangshan.However,few scholars have investigated the role of sulfuric acid in the rock weathering of Huangshan granite and the sources of weathering agents under the background of acid rain.This study selected the Northern Mount Huangshan Watershed（NMHW）as the study area.It is a typical silicate watershed composed of the MaChuan,RangXi,PoXi and SuXi rivers as well as the Taiping Lake.Agents and reaction types of the chemical weathering in the watershed under the background of acid rain were analyzed,based on the methods of major elements,carbon and sulfur isotope geochemistry.An inversion method was used to calculate chemical weathering rate and atmospheric CO₂ consumption rate.Sources of dissolved SO₄^2-in river water were identified and its corresponding contributions were also estimated.This study could provide basic data for the ecological environment protection of the Huangshan granite critical zone and silicate chemical weathering by sulfuric acid.The main conclusions are as following:（1）Ca²⁺and HCO₃^-are dominant ions in river water in NMHW.The proportion of Ca²⁺and HCO₃^-accounting for total cation and anion content in river samples,respectively,in 2016-2017,are higher than those in 2014.Mg²⁺and NO₃^-are second major ions in river water samples in 2014 and that for samples in 2016-2017 are Na⁺and NO₃^-.According to analyses by principal element,Piper diagram and Gibbs chart,chemical compositions of the river water are mainly affected by lithology,followed by atmospheric input,and human activities have little effect on it.（2）The seasonal variance of dissolved SO₄^2-concentration in the whole basin is not obvious among two separate sampling years.Its values in 2016-17 are slightly lower than those in 2014.Theδ³⁴S values in dry season show great more variation than that in the wet season during 2014,but the situation during 2016-17 is opposite.This is mainly attributed to great variations of sources of the dissolved SO₄^2-which affected by human activities.Atmospheric input is major sources of dissolved SO₄^2-in2014 due to serious air pollution occurred during the period.However,the contribution is reduced and sulfide oxidation in soil is increased in 2016-17 which is possiblely resulted from the action of controlling air pollution in China after the occurrence of smog.（3）Based on estimation by model for end-members of dissolved SO₄^2-,it is concluded that the most important source of dissolved SO₄^2-is atmospheric input（51.2%）in dry season,followed by soil input（34.5%）and the anthropogenic input（14.3%）.But in wet season,the soil input is relatively high（39.8%）,the soncond is anthropogenic input（31.4%）and the last is soil input（28.8%）during 2014.The rising activities of microorganisms are the most important reason for the increasing sulfur contribution from soil source in wet season.Different from that in 2014,sulfide oxidation is the dominant source with the proportion of 42.3%and 43.3%,respectively,in the dry and wet season in 2016-17.Correspondingly,soil input account for 35.5%and 33.3%,and that from anthropogenic input are 21.8%and23.4%,respectively,in total contribution.The results verify above analyses on end endmember.（4）According to the combinated analyses of major ions composition,carbon and sulfur stable isotope geochemical data,it can be safely concluded that H₂CO₃,HNO₃ and H₂SO₄ are all take part in chemical weathering processes in NMHW.The role of sulfuric acid in dry season is more obvious than that in wet season.The sulfuric acid which involves the process of chemical weathering is mainly originate from the oxidation of sulfide and atmospheric input in wet season and dry season,respectively.Chemical weathering of these rocks by carbonic acid is the dominant process in the watershed.Sulfuric acid also acts as a weathering agent in the process but its role in it is minor.（5）Based on the calculation for carbon flux using the data of carbon isotope and dissolved inorganic carbon（DIC）,the CO₂ consumption rate is 5.87×105mol·km^-2·y^-11 and the chemical weathering rate is 16.0 t·km^-2·y^-11 in 2014 in NMHW,which are 7.21×105 mol·km^-2·y^-11 and 15.6 t·km^-2·y^-1,respectively in 2016-17.The CO₂ consumption rate of carbonates weathering in wet season is higher than that in dry season in any year.The CO₂ consumption rate of carbonates weathering in 2014 is lower than that in 2016-17.No obvious change of chemical rates are observed between the year of 2014 and 2016-17.Some data show that sulfuric acid has accelerated the weathering of carbonate rocks in NMHW which would affect the supply of mineral nutrients to ecological system in Mount Huangshan and the rate of soil and water loss there.