Study On Seawater Intrusion In Typical Areas Of Dalian Coastal Zone

Almost 50%population live in the area within 60 km of the coast.The groundwater is the main source for domestic,industrial,and agricultural water utilization.Excessive exploitation of the groundwater will result in the hazard of seawater intrusion.Dalian is one of the earliest cities in China to suffer from the problem of seawater intrusion.Daweijia Town,once one of the largest water supply sources in Dalian,began to exploit underground water in large quantities without limit in the 1960s,which caused the serious intrusion of seawater in the area,and caused Daweijia Water Supply to stop mining in 2001.In recent years,the field investigation found that groundwater is the only irrigation water source for the main cash crop,i.e.,cherry in Daweijia,and some villages still use groundwater as the drinking water.Therefore,it is of great significance for local economic development and people's health to study the characteristics of groundwater quality in this area.The research topic of this thesis is chosen based on the project of"Coastal Zone in Dalian Seawater Intrusion and Groundwater Circulation Research"and"Dalian Seawater Intrusion in Coastal Zones Area 1:50000 Special Hydrogeological Survey".Taking the groundwater in Daweijia Town of Dalian City as the research object,the characteristics of seawater intrusion in the study area were analyzed by means of data collection,theoretical analysis,field investigation,sample collection and test,and numerical simulation,focusing on the mode,scope and degree of seawater intrusion and its impact on groundwater.In addition,the evolution of groundwater chemical characteristics in this area is analyzed,and the time required to eliminate the impact of seawater intrusion in the study area is numerically predicted under different groundwater exploitation schemes,so as to provide a scientific basis for the rational development and utilization of groundwater in this area.The main research results are as follows:1.Based on the analysis of the geological conditions of the study area,there are two intrusion modes:including the invasion through the shallow space plane of the Quaternary system and the invasion through the dominant flow channels of deep bedrock.Through the analysis of the dynamic characteristics of groundwater in the study area,it is clear that the groundwater dynamic field in the study area is in the state of fresh water draining and driving salt water,and the phenomenon of seawater intrusion in the study area is formed in the historical period.2.The framework of"three subsystems"of groundwater flow system in the study area is put forward,including three subsystems:Qidingshan-Daziying-Zhoujiagou north regional groundwater flow subsystem,Daweijia regional groundwater flow subsystem and southern offshore regional groundwater flow subsystem.It lays a theoretical foundation for clarifying the effects of seawater intrusion and the evolution of groundwater chemical characteristics in the study area.3.On the basis of groundwater flow subsystem division,the spatial variation characteristics and influencing factors of groundwater chemistry in the study area are systematically analyzed.The spatial distribution and evolution of groundwater chemical types in the three subsystems of groundwater flow are consistent on the whole,but their characteristics and evolution are not the same due to the different geological and structural control of groundwater chemistry in each subsystem.4.The spatial distribution range and degree of seawater intrusion are defined.At present,only the coastal area is invaded in the study area,and the maximum invasion distance is about 3 km.According to the concentrations of?D,?¹⁸O and Cl^-in the groundwater,the mixed end-element method was used to calculate the seawater mixing proportion in the seawater intrusion area in the study area.The results show that the proportion of seawater mixing in the seawater intrusion area is 0.56%?5.59%.5.Based on recent water quality data,the suitability of groundwater used for drinking and agricultural irrigation in the study area was analyzed,and the main controlling factors of water quality characteristics were identified.Most of the groundwater in the study area is not suitable for living and drinking,and the groundwater in the coastal area is not suitable for agricultural irrigation.The groundwater in coastal areas is mainly affected by seawater intrusion and salinization,while the inland areas with poor water quality are mainly affected by human activities such as agricultural fertilization.Based on the analysis results,suggestions for controlling groundwater exploitation rational fertilization and monitoring the dynamic change of groundwater quality are put forward.6.Based on the hydrogeological conditions of the study area,the numerical simulation technique is used to analyze the time required to eliminate the seawater intrusion phenomenon in the study area under different groundwater production schemes.The results show that when the annual groundwater extraction is the current exploitation(5.0×10⁶ m³),it will take 40 years for the simulated area to recover to its natural state.Under the premise of ensuring the normal production and life of the villagers,the groundwater extraction amount should be controlled as far as possible.