Groundwater Chemical Formation And Evolution And Inverse Modeling In The Coastal Plain,Downstream Of Yishu River

Fresh groundwater resources are important water supply sources in coastal areas of China.With rapid development of social economy,demand of water resources has increased dramatically.While a series of environmental geological problems have taken place in many coastal areas due to over-exploitation of groundwater.In the northern part of Jiangsu coastal area,there are fissure water,pore phreatic water,the first(I),second(II),third(III)and the forth(IV)confined groundwater.The II?III ? IV confined groundwater are the main mining object.Over-years of over-exploitation to II,III confined groundwater makes water level falls continuely.Formation of cone of depression causes water salination,land subsidence and other issues.To exploit groundwater reasonably and to control salinization of II,III confined groundwater,it is needed to clearly understand recharge,discharge,runoff conditions,TDS distribution and the hydrogeochemical processes of groundwater in the study area.Based on the analysis of the geology and hydrogeology conditions of the study area,this paper studies the characteristics of the groundwater by using water level dynamic analysis and isotope tracer techniques.The mechanism of hydrochemical formation was studied by means of correlation and ion ratios analysis and hydrogeochemical simulation was carried out.The main achievements were as follows:1,Runoff direction of the fissure water is controlled by fracture network.TDS ranges from 400mg/L to 600mg/L,mostly HCO3-Na·Ca type.Fissure water flows through the bedrock and dissolves aluminosilicate minerals.2,In the northern hilly area,phreatic water and I confined groundwater flows from mountain to the sea.Hydraulic gradient is 1.5‰ and water alternates quickly.TDS of phreatic water is 0.5g/L~2g/L,slightly lower than that of phreatic water.Pore water is mostly HCO3-Na·Ca?Cl·HCO3-Na·Ca type.The main water-rock reactions are:(1)dissolution of aluminosilicate minerals;(2)precipitation of calcite;(3)cation exchange;(4)formation of kaolinite and illite.Water in this area is basically not affected by modern seawater.3,In the coastal plains,phreatic water and I confined groundwater flows from inland to the sea.I confined groundwater in the Guan river outlet area leaks to II confined water.TDS of I confined groundwater increases from 400mg/L to 20g/L from inland to the coastal area.The inland groundwater and the coastal groundwater is HCO3-Ca type and Cl-Na type respectively.Due to desalination by precipitation,quality of phreatic water is better than the I confined groundwater.Main effects controlling the evolution of phreatic water and I confined groundwater are:(1)mixing with seawater;(2)dissolution of aluminosilicate minerals and soil CO2 gas;(3)precipitation of calcite;(4)cation exchange.Phreatic water is also affected by evaporation.4,In the coastal plain,due to the leakage of I confind groundwater in Chenjiagang,Yanweigang area,TDS of II confined groundwater reaches up to 20g/L.In other areas,II confined groundwater is mainly fresh water or brackish water,just as III confined groundwater.Distribution of water type is also same between II and III confined groundwater.Groundwater of inland water and coastal is HCO3-Ca·Na type and Cl·HCO3-Na·Ca type respectively.Water chemistry evolution is mainly determined by:(1)dissolution of aluminosilicate minerals;(2)precipitation of calcite;(3)cation exchange;(4)formation of kaolinite and illite;(5)desulphidation.5,The proportion of contribution controlling the chemical formation of groundwater was identified by quantitative simulation.In no-leachage area,cationic alternating adsorption or plagioclase mineral dissolution are the main effect controlling the chemistry formation of deep-seated water.While in the leachage area,the most important effect is mixing,followed by cationic adsorption or dissolution of plagioclase minerals.