Export Of Total Organic Carbon(TOC) And Its Influence Factors From A Typical Karst Watershed

Organic carbon is the main participant in the carbon cycle,and provides material and energy for heterotrophic animals,plants and microorganisms to survive and grow in terrestrial and aquatic ecosystems.The response and feedback of organic carbon to global change will affect the structure and function of ecosystems to a large extent,even will effect the size of global carbon sink.In addition, the content and variation of organic carbon in water is one of the important factors in effecting water quality and the aquatic ecosystem.So far,there have been many studies that have focused on the rising trend of organic carbon concentration.There were long-term increases in surface water organic carbon on the American and the European long-term monitoring sites.The increase of carbon concentration in water have important effects on water quality, especially on climate change,but research about its influence mechanism is still in the debate.Selecting Xueyu Cave karst watershed in Fengdu,Chongqing as the object of this study,high time-resolution,auto-monitoring techniques were used to obtain the data for TOC and hydrogeochemistry of groundwater,air temperature and precipitation to analyze seasonal variation of TOC and the response to different rainfall intensity in Xueyu Cave groundwater,and then the principal component regression model was built to reveal the variation of TOC in groundwater and its influence factors.After a whole hydrological year,field monitoring,laboratory test and analysis,the contents and conclusions of this paper contain the following three aspects:（1）During the whole monitoring,pH value in Xueyu Cave groundwater is between 7.12⁸.31,the average value is 7.72,which makes Xueyu Cave goundwater neutral and low alkali.Ca²⁺ and HCO3-are the main ions of groundwater,so groundwater in Xueyu Cave is HCO3-Ca type,a kind of karstic water with low degree of mineralization.Hydrochemistrical indexes vary with seasons,which is presents as a high pH value in winter and a low value in summer,and pH negatively correlated with precipitation.SpC 、 Ca²⁺、HCO3- and other ions has the similar trend,which showes higher values in summer and lower values in winter.TOC and turbidity of groundwater showes higher values in winter and summer seasons and lower values in spring and autumn seasons,but organic carbon flux is mainly influenced by flow in groundwater, showing the trend of high in summer and low in winter.（2）Due to the unique surface,underground "double space structure system",thin soil layer and distribution of discontinuity,all kinds of pollutants will be easily carried into the underground karst hydrological system through water hole,shaft,etc.Therefore, after rainfall,especially the intense fall,karst groundwater hydrogeochemical index will have sensitive response to the external environment.So the author selects TOC concentration and various hydrogeochemical index under the condition of different rainfall intensity（light rain,moderate rain,heavy rain,intense rain,rainstorm）in a hydrological year.The results indicate that TOC concentration and hydrogeochemical index of groundwater showed obvious response to rainfall.But there were significant differences among the response time and the process under different rainfall intensity.Generally,TOC showed a rising trend in the process of rainfall,but when the rainfall increase,namely after intense rain and rainstorm,TOC concentration has a downward trend mainly by the influence of dilution effect,but organic carbon flux under different rainfall intensity has similar even same trends with groundwater flow.（3）Taking TOC concentration as the dependent variable,main meteorological index and main hydrochemical index in groundwater as the independent variable,principal component regression model was established to analyze the main factors which influence the variation of TOC.As can be seen from the final regression equation,temperature, rainfall and underground river flow and soil temperature has obvious positive effect on the output of the groundwater TOC, and effect of pH and SO42- on the output of the groundwater TOC is inversely.But compared with the reverse effect of pH on the TOC output, the reverse effect of SO42- on the TOC output is weak.