Hydrogeochemical Evolution And Pollution Tracing In Liulin Karst Water System

Karst water resource is the most important water supply sources in karstic regions.It has a unique environmental and ecological significance on the sustainable development of social economy in northern China.Liulin karst spring is one of the main karst springs in Shanxi Province,which provides guarantee for the water-use of industry,agriculture and human life in Lvliang City.However,the sustained decline of karst spring flow and water level with serious problems on water environment,has become a challenge to the safety of karst water supply.This paper has conducted researches on the pollution sources and paths of karst water in karstic regions of northern China by taking Liulin karst water system as the representative.The research results are of great theoretical and practical significance on scientifically protecting karst water resources and preventing karst water pollution.On the basis of systematically mastering the karst medium field,water flow field and hydrogeochemical field of Liulin karst water system,this paper adopted multiple research methods and techniques,containing field survey on hydrogeology,geological multivariate statistical analysis,trace organic pollution detection,hydrogeochemistry evolution analysis,environmental isotope identification,geochemical simulation calculation,etc.,to figure out the key hydrogeochemistry mechanisms and water pollution processes in study area.By analyzing the dynamic variation characteristics of Liulin spring flow and groundwater table,it discussed the evolution mechanism of surface water and groundwater quality as well as the influence of human activities in study area.Besides,it identified the karst water pollution paths in typical leakage sections.The sources and pollution paths of organic contamination were also analyzed in detail.The major achievements of this paper are as follows:1.The surface water chemical type in Liulin was mainly HCO₃-Ca·Mg.Uunder the effect of human activities,it partly evolved to HCO₃·SO₄-Ca·Na or HCO₃·SO₄-Na·Ca·Mg.Water pollution phenomenon existed in both dry and wet seasons.The concentration of Na⁺,Cl^-and NO3^-in dry season could reach 3 to 5 times of that in wet season.On the basis of chemometrics and theoretical analysis of mineral dissolution of carbonate rocks,the karst erosion by sulfuric acid rain was one of the important factors on the increase of ion contents in Liulin surface water.The corrosion by nitric acid also played a positive role on the evolution of water quality,yet it was weaker than sulfuric acid on the whole.Both the sulfuric acid and nitric acid erosion were distinct in wet season.Mine drainage and human activities are the major causes of sulfate pollution in surface water.2.Karst groundwater in study area was neutral to alkaline on the whole.From recharge area to discharge area,the hydrochemical types of Liulin karst water evolved from HCO₃-Ca/Ca·Mg to HCO₃-Na·Ca,HCO₃-Na·Ca·Mg and HCO₃·SO₄-Na·Ca·Mg,and eventually to HCO₃·Cl-Na·Ca and SO₄·Cl-Na.The ion components in karst water mainly come from carbonate or silicate rock weathering process,and partly affected by the dissolution of evaporite rocks.Based on the pollution tracing by hydrochemistry and isotopic(?¹⁸O,?³⁴S)analysis,it indicated that the major factor of deterioration of karst water quality in Liulin was human activities,including the discharge of wastewater from domestic,industrial and mining enterprises,the mine drainage,and agricultural irrigation.The main source of sulfate pollution was mine drainage,while nitrate pollution mainly come from agricultural irrigation.3.A distinct phenomenon of surface water leakage polluting karst groundwater was found in study area on the basis of comprehensive analysis of hydrochemistry,spatial distribution of ion concentration and environmental isotopes(D?¹⁸O?³⁴S?¹³C)tracing.In major river seepage sections,the leakage influence on the chemical characteristics of karst water is significant with spatial differences.Analysis based on environmental isotopes shows that,the pollution effect on karst waters in Caijiazhuang,Zhaidong and Yagou are strongly affected by surface leakage(content of SO₄^2-greater than 134 mg/L).In Baoan and Shengdi,although the surface leakage is strong,but the pollution effect is weaker(content of SO₄^2-about 45 mg/L).Karst water in Jinluo is to some extent affected by surface leakage pollution.The influence of surface leakage and its pollution effect are both weak in Zhongyang County and Shanglouqiao.4.In study area,the total concentration of PAHs in groundwater ranged from 39.25 to16,830 ng/L,with Nap as the predominant component.In karst waters,the contents of PAHs located in coal mining areas and discharge areas were relatively high.The characteristic ratios show that,PAHs in water samples in study area mainly come from local incomplete combustion of fossil fuels like coal.Based on the further analysis by hydrogeochemistry and stable isotopes,it indicated that PAHs in karst water mainly come from the infiltration of runoff water in bared karst areas during rainfall period.River water leakage might lead to PAHs pollution in karst water and pore water somewhere,but it was not the major path in Liulin karst water system.Based on this research,it is suggested that the phenomenon of karst water pollution in Liulin karst water system is common.Coal mining and human activities are the main sources of contaminants in karst water,and surface leakage is the main path of karst water pollution.In order to prevent karst groundwater pollution,it is necessary to strengthen the joint management of surface water and groundwater in Liulin karst water system.It is extremely urgent to take steps to control sewage discharge to surface water and intensify pollution treatment.