Study On The Monitoring And Numerical Simulation For The Salt-freshwater Interface Based On The Electrical Resistivity Tomography Method

Understanding the migration and morphology of the salt-freshwater interface(SFI)is of great significance for the study of seawater intrusion(SI)in the coastal aquifers.To reveal the degree of SI in the typical profile of western in Longkou city,Shandong Province,electrical resistivity tomography(ERT)in combination with the hydrochemistry characteristics of groundwater samples were used to map and capture the SFI change in the dry and wet seasons.Continuous ERT surveys were conducted in24-hour tidal cycle and 15-day spring-neap tidal cycle in wet season to study the SFI response to tidal action.Finally,the SEAWAT module of the groundwater numerical simulation software GMS was used to establish a two-dimensional groundwater flow and solute transport model for the typical profile,which simulated the groundwater head and Cl~-concentration distribution in the aquifer at the typical profile during the dry and wet seasons,and the result were verified with the ERT inversion results.Four monitoring locations W1,W2,W3 and W4,perpendicular to the west coastline,were located at 3200m,2600m,2100m and 1200m from the west coastline.Based on the research results,the main conclusions were followed:(1)The experimental results of groundwater samples analysis showed that the SFI was approximately located between the W2 and W3 according to the SI criterion with Cl~-concentration of 250mg/L,and the concentrations of Cl~-,TDS and EC showed a decreasing trend from coast to inland.(2)The ERT results indicated that the SFI was located about 2572m from the coastline in the dry season,while in wet season it was about 2560m.Combined with the analysis of groundwater samples and groundwater dynamics,the increased rainfall in the wet season raised the groundwater level,restraining the SFI from moving inland to a certain extent.(3)Due to the delay effects of the 24-hour high and low tidal cycle and different tidal amplitudes in 15-day spring-neap tidal cycle,the SFI retreated to seaward in high tide and spring tide,while it intruded to inland in the low tide and neap tide.(4)The established two-dimensional groundwater flow and solute transport model results showed that the simulated SFI was located about 2130-2729m from the coastline in the dry season,while in the wet season it was about 2120-2524m.The increased groundwater extraction and decreased rainfall in the dry season resulted in lower groundwater levels,making the SFI closer to the inland while it moved seaward in the wet season.Comparing the ERT inversion results with simulated results,the simulated SFI and the interface mapped by ERT were basically the same in the wet season,while there was a certain deviation in the dry season.However,the most SFI zone was still located between W2 and W3,which verified the validity of the ERT to determine the change of SFI during dry and wet seasons.