Characterization And Numerical Simulation Of Groundwater Salinization Under The Impact Of Surface Mariculture Reserviors In A Multi-Layer Coastal Aquifer System In Daguansha Area Of Beihai City

Salinization of coastal aquifers is not only a worldwide environmental problem,but also is one of the current hot issues in coastal aquifers study.Considering the main inducements by human activities and natural process,the identification of the pollution source and the evolution process has been a very important proposition.With the longitudinal land-based mariculture in coastal aquifers and lateral seawater intrusion caused by groundwater exploitation,the hydrogeological conditions of the multilayer aquifer system become more complex and the research on coastal water environment is more challenge in China.The land-based mariculture area of Da Guan Sha in Bei Hai was studied to support the research of paper.Long time series of water level and quality monitoring from 2014 to present was established in the research.The hydrogeochemical analysis and numerical simulation were used to identify the salinization process on the impact of the land-based mariculture.The main conclusions are as the following:1.Hydrogeochemical analysis.The continuous sampling monitoring of three horizontal wells along a section was accomplished in Da Guan Sha area.Based on the normal groundwater chemical composition and the data including hydrogen and oxygen isotopes,sulfur isotope and strontium isotope,the temporal and spatial variation rules about groundwater salinization,hydrochemistry and isotope composition were summarized.With the researches about recharge sources of the land-based mariculture,salinization path and evolution process,the scientific basis for groundwater utilization and reasonable mariculture project can be provided in Da Guan Sha.?1?The temporal and spatial change of TDS,deuterium isotope,sulfur isotope and hydrogen and oxygen isotopes was accordant and it showed that the groundwater salinization were affected by seawater intrusion and mariculture in the area.Moreover,the local anomalies in the unconfined aquifer can show that the salinization is mainly affected by land-based mariculture and the isotopes spatial variance demonstrated that the seawater intrusion may have an important impact on the salinization of aquifer in the Pleistocene and Pliocene.Also the conclusions can be confirmed by the groundwater salinization and isotope anomalies in summer.?2?Nitrate anomalies were detected in all aquifers and the anomalies were mainly distributed around the monitoring point B.It can indicate that the water from mariculture had an impact on all the aquifers by the obviously interlayer flow in the area.?3?The results of anions and cations in the groundwater samplings showed that the hydrochemical composition gradually approached to seawater during the salinization process.The slight negative deviation of Na⁺and Mg²⁺and positive deviation of Ca²⁺occurred.Therefore,it was speculated that there was a cationic exchange process.The single-process ion exchange information was extracted by?Ca+Mg?/Na and?Ca+Na?/Mg milligram equivalent concentration diagrams,the results showed that the process of groundwater salinization in this region was accompanied by the exchange of Na⁺and Ca²⁺in sediments.In the later stage of salinization,the ion exchange between Mg²⁺and Ca²⁺in sediments also occurred.?4?There was great difference of the hydrogen and oxygen isotopic composition between the Miocene and others.Based on the isotope evolution process,it indicated that a long time of isotopic exchange reaction between groundwater and sediments most likely happened,or the intense evaporation process has been experienced.Therefore,the idea was provided that the groundwater in the Miocene had no close connection with seawater and other aquifers and renewed slowly.It was the reason that the salinization had not occurred in the aquifer.2.Groundwater numerical simulation method.The hydrogeological model was established by the measured and monitoring data and the site selection scheme of land-based mariculture was also discussed.?1?Based on the existing experimental data,the three-dimensional geological model and hydrogeological model of Beibu gulf has been built.Numerical simulation results showed that the confined aquifer was less likely to be polluted by high-place mariculture with the protection of aquifuge in a short period.But if seawater intrusion has occurred in the confined aquifer,it will be polluted mainly caused by mariculture.In addition,the salt water from mariculture will seep into the unconfined aquifer and produce many facets of pollution plume without suitable seepage control measures.?2?With the existing monitoring data,the site selection scheme for land-based mariculture was studied using numerical simulation.The results showed that the pollution sources in the confined aquifer included northwest farm,Gucheng Ling farm and Xibei Ling farm.If the farms were abandoned or anti-seepage treatment were adopted,the salinization can be effectively reduced and little pollution occurred in the study area.And also the water source threat caused by salt water northward migration will greatly ease,which was induced by over exploitation in Long Tan village.3.The main innovation and advantage of the paper is as following:?1?the multi-layered groundwater system was established in the land-based mariculture area to monitor the water level and water quality in a high precision and long period.It provided the possibility and guarantee for the further study of the temporal and spatial variation rule of groundwater salinization and its mechanism;?2?based on the stratified high precision monitoring of water level and water quality with long period?over 4 years?,the occurrence area and mode of seawater intrusion were accurately determined combining the identification of water chemistry process and the numerical simulation;?3?the numerical simulation method for groundwater salinization in multilayer aquifer was studied considering the nature and human activities.And the description of moving seawater boundary on inclined aquifer has made progress.