| Resources and environment has become two most important problems across the world in 21th century.More than 50%world’s population lives within 60km from coastline,and seawater intrusion problem in coast area has become main worldwide resources,environment and ecology problem.Massive groundwater exploitation induced seawater intrusion not only strongly threatens local groundwater resources,but also severely impacts local ecological geo-environment,which restricts national economic development.Circum-Bohai Sea Region is the most severe seawater intrusion area in China,and Laizhou Bay coast area in Shandong Province is a representative area of serious seawater intrusion.Seawater intrusion Phenomenon occurred in south Laizhou bay has a increasing trend since the mid 1970’s,which induces water shortage in industrial and agricultural production as well as domestic daily living.The seawater intrusion at the south coast of Laizhou Bay is monitored dynamically over a long period of time.The damage extent of the sweater intrusion is estimated accordingto the reasonable monitoring indexes.Groundwater circulation characteristics are analyzed by hydrochemical and isotope analysis.Meanwhile,the seawater intrusion trend is predicted by a mathematical model.Following conclusions are attained through systematic researches:1.A seawater intrusion monitoring system is established at the south coast of Laizhou Bay.The dynamic change of the seawater intrusion is monitored all-round,including the intrusion on the space,surface,and underground.The dynamic change trend of the seawater is analyzed by comparing the detected data with historical data.Following conclusions are attained:①The remote sensing and ground vegetation of the survey region are studied.For seawater intrusions,ocean tides,soil-forming processes of the sediment,and limits of the microrelief condition,we found that ecological communities are zonally distributed from north to south.The sequences of the ecological community arehalophytes,marsh halophytes and saline water,brackish water,andfresh water plants.Soil and plant are zonally distributed with the change of the TDS.②The groundwater dynamic analysis of the survey region is carried out.We found that theshallow groundwater table at the south coast of Laizhou Bay is gradually declined,since the reduction of the precipitation in recent years and the increase of human activities.With the expanding of the groundwater depression cone in west ShouGuang and HuaiBei from 2002 to 2013,the groundwater divide is formed either between the TianLiu and HouZhen or between the DaJiawa and YangZi.Saline water at north divide transforms toward brineexploitation area at northcoast(groundwater depression cone area).Saline water at south transforms toward groundwater depression cone centerat east and west sides of ShouGuang.Groundwater divides are formed between groundwater depression cone at XiaYing and groundwater depression cone at ChangYi.Saline water at north divide transforms to groundwater depression cone center at XiaYing.Saline water at south divide transforms to groundwater depression cone center at Chang Yi.③Five CTD-Diver groundwater dynamic monitors are installed at monitoring section(ChangYi-LiuTuan section)of groundwater-seawater intrusion.Seawater intrusionmonitoring section and layeredmonitor holes are established.The groundwater rechargesource of various monitor holes is analyzed according to monitoring data in 2013.The groundwater flow field is analyzed.We found thatC01 and C02 decrease with the decreasing of water table,which are fed with brine and saline water at north.The other three monitoring sites are in development of groundwater depression cone at ChangYi,decrease with the decreasing of water table,and are recharged by fresh water in south.④Groundwater table monitoring and water quality analysis of south coast of Laizhou Bay are studied.The groundwater table at seawater intrusionmonitoring sections gradually decreased from 2002 to 2007.The decreasing trend of water table slowed down after 2005,and then the water table kept stable.The interface of saline water and fresh water at seawater intrusionmonitoring sections gradually moved to inland from 2002 to 2007.The move rate was higher between 2002 and 2005,and was obviously lower after 2005.⑤Investigations are carried out at specified seawater intrusion area,for what a Controlled Source Audiofrequency Magnetotelluric method(EH-4)and a high density electric method is adopted.Based on the hydrogeological condition,seawater intrusion boundaries and distribution of fresh water-salt water interfaces are surveyed.These data are basically same with the previous studies.⑥The main reason and influence factor of the saline intrusion are analyzed.They are excessively exploration of groundwater,reduced precipitation in the late 1980s,geological factors,topography,and salt field.2.the ChangYi-LiuTong section of Laizhou Bay,which is an important part of our research zone,is hydrochemically studied.The macro ions that collected from different levels of groundwater,such as Ca2+、Mg2+、Na+、SO42-、HCO3-、Cl-,had been tested.And then it combined with mutiple isotope for the analysis to understand the hydrochemical characteristics and distribution law of this geological section.Based on the above,we can further study the cycle of fresh water,brine water and sea water,by this sea water invasion assessment,our conclusion is below:①According to the result,hydrochemical type of superficial aquifer from southwest to northeast is in the sequence of HCO-—HCO3-·Cl-—CI-·HCO3-—Cl-,the salinity degree(mineralization)increase from 0.03g/L to 116.16g/L,average is 6.32g/L.underground Brine water zone staying in the middle of saline water zone,band-like distributing on the east and west,and nearly moving along the coastline,its salinity degree is 85.97g/L to 116.16g/L.②The regulation of the macro ions in ground water that evoluting from fresh to salty,presenting that the linear correlation of fresh water to brine water is closer than fresh water to sea water.lt introduces that micro salty water and salty water is much more like the complex of fresh water and brine water.③The sea water invasion assessment’s indicators and measures for south beach of laizhou bay is confirmed,and the result is drawed,too.The superficial aquifer groundwater of the entire research zone have invaded by sea water,and there is a trendacy that this invasion is getting serious from south to north.④ Through out the analysis of the isotop 2H,18O,37Cl-in the Changyi-Liutuan geological section of our research zone,saline compositions of salty water and micro salty water come from the migration of salty and brine water from north to south,without obvious relationship to sea water.And the derivative of saline compositions of groundwater in deep layers might relate to sea water invasion.37Cl-in brine water has a small distribution extent,applying a speculation that brine water derived from the vaporization of the ancient ocean,but not from the vaporization of the atmospheric rainfall.⑤The CFCs apparent age and 14C age determination show that run from zhuli to lijiabu to downstream,the groundwater age increases along with the flow direction.The result also represent that the velocity of groundwater is getting slower,and the residence time in acquifer is getting longer,too.⑥Brine water underground of the research zone contains high concentration of sulfate ions,which is low in fresh water.And micro brine water has a complicated situation because it is the combination of brine water and fresh water.The sulfate salinity might derived from sea water invasion and irrigation water complementary in the acquifer.3.In this research,numerical method was adopted to evaluate and predict groundwater quality and quantity.Four specific seawater intrusion scenarios were modeled,and results were concluded as follows:① If current groundwater exploitation scheme maintain unchanged,the groundwater exploitation funnel will migrate northwards and the average groundwater table will decline by 1 to 3 m.Furthermore,the TDS isoline in Haiting area has no much change while the TDS isoline in Changyi area would move 0.5km in north direction.②The influence to groundwater/seawater boundary line due to sea-level rise can be ignored.Two reasons can be concluded:Firstly,seawater boundary is more than 20km far away from the coastline.Secondly,brine mining wells were installed in north area of our research area,which induce the formation of brine mining funnel and resist the seawater intrusion towards south area.Thus,sea-level rise in shallow aquifer has no impact on seawater intrusion in southern area.③Groundwater exploitation amount reduced by 30%and seawater exploitation amount increased by 30%as well as installation of fresh water injection well can have certain inhibition effect to seawater intrusion.After exploitation plan was adjusted,the seawater boundary line of west area has slight change that predicted boundary line would move northwards 60-80 m compare to previous exploitation plan while the middle part of seawater boundary line has no obvious change;the seawater boundary line of east area has obvious change that the predicted boundary line would move northwards 220-300m compare to previous exploitation plan.Fresh water injection plan:groundwater along main canal would desalination,and the boundary would retreat northward induce the formation of long narrow fresh water layer after 10 years.The boundary line near Shouguanghou town can retreat northward by 8km while the boundary line near Changyi can retreat northward by 12km. |
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