Assessment Of Status And Vulnerability To Seawater Intrusion In A Coastal Area

The coastal area is an important place for human activities which nourishes nearly one-third of the world's population.With the rapid development of urbanization and the continuous growth of population,the demand for domestic water and production water in coastal areas is increasing.As a result,groundwater has been exploited in large quantities,causing many aquifers to be invaded by seawater.Seawater intrusion has become a global environmental problem,and it threatens the utilization of coastal groundwater resources,which has an adverse impact on the safety of residential water use and the sustainable development of the region.A coastal area was selected as the study area in this paper.Based on GQI_SWI,HFE-D and GALDIT,the status and vulnerability of seawater intrusion in the study area were evaluated.The above models were adjusted and improved,and valuable insights and conclusions were obtained.The results provide a scientific basis for formulating groundwater exploitation strategies and seawater intrusion prevention countermeasures in the study area,and provide a reference for the assessment of seawater intrusion in other regions of China.The main contents and conclusions of this paper are as follows:(1)Assessment of status of seawater intrusion in the study area.The evaluation results of single indicator evaluation method(Cl^-,TDS),characteristic ions ratio(SAR,Cl^-/HCO₃^-)and water quality index(GQI_SWI)were compared and analyzed,and the status of seawater intrusion in the study area was comprehensively evaluated.The results showed that the evaluation results of seawater intrusion based on a single index and characteristic ion ratio methods were distributed in blocks,and the results were not accurate enough.The evaluation results of GQI_SWI were more accurate,the situation of seawater intrusion was serious in the S11 and S12,the concentration of Cl^-in the groundwater exceeded 17000mg/L,and the GQI_SWI indexes were less than 10,it was judged that the groundwater was seawater;the GQI_SWI indexes of S8,S9,and S10 were about 50,and it was judged that the groundwater was brackish water;the GQI_SWIindexes of S1?S7 were above 80,and the groundwater was judged to be fresh water.(2)The spatial interpolation of GQI_SWI was improved.The permeability coefficient was introduced into the inverse distance weighted spatial interpolation method(IDW)to obtain an improved IDW,which indirectly combined the permeability coefficient with GQI_SWI,and the result was more accurate.Using GQI_SWI based on improved IDW to evaluate the status of seawater intrusion in the study area,the results showed that the distribution of GQI_SWI based on the improved IDW was more uniform,and the results were more accurate,which solved the problem of the blocky distribution of the interpolation results mentioned above.(3)Hydrochemical analysis of groundwater in the study area.The Piper diagram was used to analyze the chemical composition of groundwater in the study area.On this basis,the hydrochemical facies evolution diagram(HFE-D)was used to represent the evolution of hyrochemical evolution,and the desalination and salinization trend of groundwater can be judged,then the evolution trend of seawater intrusion can be determined.In addition,the HFE-D index was established on the basis of HFE-D,and the spatial distribution of HFE-D was visualized in combination with Arc GIS.The results showed that the hydrochemical type of groundwater in the study area was mostly Ca-Cl type,which was in the salinization stage;the groundwater in the southern coastal area had evolved into saltwater,and the hydrochemistry type was Na-Cl;in northern areas,the groundwater was Ca-HCO₃ type,and some groundwater was Na-HCO₃ type,which tended to be desalinated.But overall,the situation of seawater intrusion in the study area tended to deteriorate.The results of the spatial distribution of the HFE-D index in the study area showed that there was a salty area in the northeast of the study area,it may be because HFE-D only judged the hydrochemical evolution trend of groundwater based on the concentration of Cl^-and HCO₃-,which was easily affected by anthropogenic pollution.The inland areas were affected by saline wastewater,which leaded to a higher concentration of Cl^-than concentration of HCO₃-,and these areas will be judged by HFE-D as salinization areas.(4)Assessment of the vulnerability to seawater intrusion in the study area.Based on the GALDIT model,three improved models(GALDIT_W,GALDIT_R,GALDIT_GQI)were obtained by using single parameter sensitivity analysis,triangular fuzzy number and GQI_SWI,respectively.The optimal model was selected through the correlation between the outputs and the TDS by using the Pearson correlation coefficient method,and the vulnerability to seawater intrusion in the study area was evaluated.The results showed that sensitivity analysis,triangular fuzzy number and GQI_SWI can make certain improvements to GALDIT.Among them,GALDIT_GQI had the highest correlation coefficient with TDS.Therefore,GALDIT_GQI was determined as the optimal vulnerability evaluation model for the study area.The results of vulnerability assessment showed that in addition to the coastal areas where seawater intrusion had occurred,the northeastern area of S9 where seawater intrusion had not occurred was also a high-vulnerability area,the probable reasons were that the permeability coefficient of this area was large,so the permeability of the aquifer was high,which was conducive to the penetration of seawater into the aquifer;meanwhile,the thickness of the aquifer in this area was large,which was conducive to the diffusion of seawater from the bottom of the aquifer,so the vulnerability of this area was high.