Research On The Dynamic Variation Rule Of Seawater Intrusion And Controlling Measures In A Coastal Region

Seawater intrusion refers to the balance between freshwater and seawater being broken by human activities or natural changes,which leads to the phenomenon of bound moving from sea to land.This will cause serious problems such as undrinkable groundwater,motor-pumped well damage,land salinization,and agricultural issues.Seawater intrusion can highly affect the economic development of coastal cities and the daily life of residents;thus,it is a significant research topic in both academic communities and urban development.This dissertation focuses on a coastal area in northern China and presents three major research contents which are current situation of seawater intrusion,the dynamic change law of seawater intrusion,and groundwater recharge measures for seawater intrusion prevention and control.The primary contribution of this dissertation is to carry out fundamental research on seawater intrusion theory,control measures,and groundwater utilization in both the research area and other coastal cities in China.The primary research contents of this dissertation are concluded as follows:1.Based on the uncertainty and fuzziness of seawater intrusion degree,the improved AHP-fuzzy comprehensive evaluation method is used to evaluate the current situation of seawater intrusion in the study area.The evaluation results are visualized by Arc GIS.In order to improve the accuracy of the evaluation results,the improved AHP method is used to calculate the weight of each index,and the evaluation results of the improved AHP-fuzzy comprehensive evaluation method and single index evaluation method are compared and analyzed.The results show that:(1)The improved AHP-fuzzy comprehensive evaluation method is used to evaluate the present situation of seawater intrusion.The results show that among the 9 monitoring points within 0-10 km from the coastline,5 points were classified as level IV,and 4 points were classified as level III,indicating that the groundwater in this range is seriously invaded by seawater.The seawater intrusion levels of all five monitoring points within the range of 10-12.5 km from the coastline are all II,indicating that groundwater in this area is moderately invaded by seawater;All the monitoring points(32 in total)outside the coastline are 12.5 km are of grade I,indicating that they are not affected by seawater intrusion.(2)The single index evaluation method is used to evaluate the present situation of seawater intrusion.The results show that among the 46 monitoring sites in the study area,the seawater intrusion level of 9 sites is III,indicating that the seawater intrusion was serious.The remaining points are all grade I,indicating that they are not affected by seawater intrusion.(3)The evaluation results of the above two evaluation methods are basically the same,but the improved AHP-fuzzy comprehensive evaluation method has a more detailed classification of the water quality evaluation grade between salt water and fresh water.Therefore,the improved AHP-fuzzy comprehensive evaluation method can make the evaluation results of the seawater intrusion status in the study area more accurate.2.According to the evaluation results of the seawater intrusion,the area with a relatively serious degree of seawater intrusion in the study area,the hydrogeological conceptual model,and groundwater migration model are established.The numerical model of groundwater in the simulation area is established by solving the mathematical model by Visual MODFLOW,and the accuracy of the model is verified.The established model is used to simulate the horizontal and vertical migration of chloride ion concentration in the aquifer,and the simulation results are visualized by Arc GIS,then analyze the dynamic changes of chloride ion concentration in the aquifer.The results show that:(1)The simulation value of groundwater level obtained by the seawater intrusion numerical model fits well with the measured value,with the fitting coefficient R~2 is 0.991,and the error coefficient RMSE is 0.125,indicating that the established model has high reliability.2 In the horizontal direction,the farther away from the coastline,the lower the chloride ion concentration of each aquifer;over time,the width of the saltwater zone of each aquifer in the simulated area shows a decreasing trend.The width of the saltwater zone decreases rapidly in the first 10 years of the simulated period and changes slowly in the next 10 years.Compared with other aquifers,the width of the second aquifer shrinks most rapidly.(3)In the vertical direction,the variation trend of chloride ion concentration between different aquifers is that the chloride ion concentration in the lower aquifers is higher than that in the upper aquifers,indicating that seawater intrudes into the groundwater in coastal areas in a wedge-like manner.The concentration of chloride ions near the coastline remained unchanged after 20 years of simulated migration,indicating that the process of seawater invasion is from point to plane.The concentration of chloride ions is the highest at the point where the invasion occurs,and its concentration does not decrease with the passage of time.3.Based on the orthogonal test method,explore the optimization scheme of seawater intrusion prevention measures for groundwater recharge in the study area.The depth of recharge Wells,the distance between recharge Wells,and the distance between recharge Wells and the shoreline were selected as the three main recharge parameters.The orthogonal experimental design method is used to combine the three levels,and nine different recharge schemes are obtained.The established groundwater numerical model is used to simulate the change of chloride ion concentration in observation Wells under different recharge schemes,and the optimal groundwater recharge scheme for seawater intrusion prevention in the study area is analyzed.The results show that:(1)The groundwater recharge layout scheme in the study area has a better groundwater recharge effect when the injection well is 20m deep,2500m away from the coastline,and 400m apart.(2)Considering the increase of groundwater resources with urban scale development,based on the optimal injection layout scheme,the recharge volume is increased by 5%year by year and the constant amount of water is present.The recharge effects under the above two conditions are compared and analyzed.the results show that the recharge time is about three to five years,5%increase year by year injection volume of injection effect is better.