Study On Hydrochemical Characteristics And Evolution Mechanism Of Groundwater In Songnen Plain

The Songnen Plain has a groundwater system containing multi-layered aquifers,groundwater plays an irreplaceable role in promoting socio-economic development and maintaining ecological balance in the region,however,as the population growth and economic development,problems of water environment have affected the chemical composition and evolution process of groundwater to a certain extent.In order to understand the evolution rules of grondwater quality scientifically and use groundwater sources reasonably,it is necessary to study the evolution mechanism of groundwater in the area.In this thesis,storing conditions,hydrochemical characteristics and evolution rules of groundwater were discussed systematically,and based on the hydrogeochemical theories,the formation processes of the groundwater chemical composition were studied qualitatively and quantitatively,respectively,moreover,the influencing mechanism of natural process and human activities were revealed.The main contents and conclusions can be summarized as follows:1.Study on the visualization model of aquifers in Songnen PlainGMS and GIS technology were applied to build a three-dimensional visualization model of the existential space of groundwater in the study area.According to results show of geologic body,layering and cross-sections,the topography fluctuation in Songnen Plain is very obvious,the northern elevation is significantly higher than the south,and the eastern and western parts are also having slight bulge,so the northern,western and eastern regions are leaning to the south together.The Quaternary Holocene sediments are mainly dispersedly distributed along the Songhua River and the Nen River,the aquifer is thin with weak storage capacity.Late-Pleistocene series with rich water is mainly located in the western part of the area,for the tilted terrain of the western piedmont plain and coarse particle size,the supply conditions are better.Middle-Pleistocene series distribute more extensively,roughly from the northeast to the south,the groundwater burial is gradually getting deepened,and the aquifer thickness is increasing and water content is gradually getting better.The early-Pleistocene and Tertiary aquifers are mainly distributed in the central low plain of the study area,and the thickness of Tertiary sandstone aquifer varies obviously.Due to multiple aquifers of Quaternary and Tertiary period overlying,the Cretaceous aquifer is buried deep in western Songnen Plain,so it is not considered to be a major groundwater exploitation layer,and in eastern place,due to the large-seale lacuna of Tertiary,early and late Pleistocene aquifers,Cretaceous aquifer is buried shallow under the middle-Pleistocene series,or even close to the ground surface,providing certain research value and supply significance of groundwater.The analysis results of the groundwater existential space,which by means of the combination of the model and basic data,showing that the model has good guiding significance for the buried depth,lateral extension,lithology and water-abundance of aquifers and aquitards(aquicludes),which has certain practical value in the study of hydrogeology and can lay the foundation for analysis of hydrochemical characteristics and study on evolution rules.2.Study on hydrochemical spatio-temporal distribtution characteristics of groundwater in Songnen Plain(1)The spatial distribution regularities of groundwater hydrochemical comprehensive parametersThe spatial-zoning maps of water chemical comprehensive parameters were drawn based on hydrochemical sampling test results of groundwater,and the distribution characteristics were discussed.Results show that most of the groundwater in the study area is freshwater,brackish water are mainly distributed in Quaternary aquifers of the central low plain.The total hardness of the Quaternary groundwater rises from all around to the middle region,and the phenomenon of exceeding standard is more serious in Quaternary shallow groundwater.The total hardness of Tertiary groundwater is generally low,while the total hardness of Cretaceous groundwater has a clear regional difference.The hydrochemistry types of Quaternary shallow groundwater are very complex,the main anionic hydrochemical type is HCO3 type,while the spatial distribution of the main cations present zoned characteristics,along the groundwater flow the hydrochemical types are from Ca type to Ca-Mg and Mg-Ca types,and then to Mg-Na and Na-Mg types,and finally translate to Na type in Daqing and Anda city in the central low plain.The hydrochemistry type evolution rules of Quaternary deep groundwater are similar to the shallow groundwater.The Tertiary groundwater types are relatively simple,types of HCO3-Na,HCO3-Ca-Na and HCO3-Na-Ca are common.The hydrochemical types of Cretaceous groundwater are relatively complex,and there are no obvious spatial evolution characteristics in types.(2)Partitioned statistical characteristics of ionic components in groundwaterAccording to the main influence factors of groundwater,the shallow groundwater in the study area was partitioned,and deep groundwater was divided into groups according to the geological era.Based on the statistical theory,statistical analysis was carried out by groups for the main ions' content of groundwater samples,and water quality characteristics and main hydrochemical processes were also discussed by the hydrogeochemical point of view.The results show that HCO3-is the relatively stable main anion in groundwater,while Cl-and SO42-are the main variables for anionic hydrochemical type's evolution.Na+ is the main cation in the groundwater system of Wuyuer River-Shuangyang River existing in the central low plain,apart from this,Ca2+ is the main cation of the shallow groundwater,and Ca2+ and Na+ are the main cations together in the middle and deep groundwater.The content of NO3-is higher in shallow groundwater,especially in the eastern high plain,which is directly related to the developed agricultural activities in the area.The distribution of F-is affected by the factors such as terrain,the mineral composition of lixiviation zones,evaporation levels of groundwater and so on,and in shallow groundwater of the low plain the content of F-increases obviously.Moreover,in the middle and deep groundwater the content of F-is obviously lower,and gradually reduce with the deepening of buried depth.Generally speaking,under the influence of watershed systems,terrain and water cycle conditions,the chemical constituents and hydrochemical types in different partitions of shallow groundwater show obvious regional evolution characteristics from recharge area to discharge area,which reflects the main hydrochemical process changing from the weathering-dissolution to the evaporation-deposition process.(3)The vertical distribution characteristics and seasonal evolution rules of compositions in groundwaterThe representative drilling with different exploited positions were selected,and Schoeller diagrams were used to compare the chemical compositions in groundwater of different aquifers,in addition,under the influence of seasonal variation,evolution regularities of groundwater compositions in the typical area were studied.The results show that the groundwater in different depth has certain hydraulic relations in the vertical plane,especially among the different Quaternary aquifers,and seasonal variation can affect the groundwater evolution processes.The mean values of TDS rise during dry season,which generally characterized by the content increasing of Na+,Cl-and SO42-in low-lying areas,while the influence of seasonal variation on the total hardness is not obvious.NO3-pollution is serious in the phreatic water during the dry season,while in the wet season,due to the increase of precipitation infiltration and river's lateral supplies,the concentration of NO3-decrease obviously.3.Study on the formation processes of groundwater chemical compositionBased on the hydrogeochemical theory,statistical theory,isotopic and many water chemical analysis methods,the formation processes of groundwater chemical composition and the interaction between groundwater and environment in the study area were analyzed.Results show that the groundwater in the study area is recharged by atmospheric precipitation;weathering-dissolution of carbonate and sulfate minerals is the primary process involved in the evolution of groundwater,and accompanied by the dissolution of silicate and halite;the silicate dissolution is also a main process in the deep aquifer,in appropriate conditions the silicate minerals inthe study area can be hydrolyzed into kaolin.The leaching is the main process of shallow groundwater in the western sloping plain,the north part of eastern high plain and the deep groundwater.The groundwater system of Wuyuer River-Shuangyang River in the central low plain is obviously controlled by evaporation process,which also affects the evolution process of groundwater chemical composition in some local low discharge areas.Alternating cation adsorption process is a common hydrogeochemical process in the study area.Affected by terrain conditions and the grain size of rock,the cation-exchange adsorption is weak in the northern part and the western sloping plain of the study area,while it is common in the central and eastern parts of the study area,where is of importance for the evolution of groundwater composition.In addition,as widely distributed rivers,mixing is also important for the evolution of shallow groundwater,and to some extent,human activities also changed the groundwater chemical composition and equilibrium state directly and indirectly in recent years4.Reverse hydrogeochemical simulation of groundwaterPHREEQC software was used to simulate hydrogeochemical process of groundwater in different aquifers,and by combining the simulation results with the hydrodynamic conditions,groundwater hydrochemical evolution rules were analyzed quantitatively.The research results show that different degrees of the weathering-dissolution and ion-exchange reactions occur in different aquifers during groundwater runoff in the Songnen Plain.The dissolution of silicate in middle and deep groundwater is often a significant hydrogeochemical process,and the reaction degree and direction of ion-exchange are related to hydrodynamic conditions and hydrochemical composition of groundwater.In the recharge area,the weathering-dissolution amount of soluble salts is less and ion-exchange reactions are relatively weak,however,in the discharge area,the weathering-dissolution amount of soluble salts is significantly increased and ion-exchange reactions are obvious.This shows that the results of weathering-dissolution is closely related to the hydrodynamic field of groundwater,the groundwater hydrodynamic field is an important factor in determining the direction of groundwater evolution,while the dissolution extent of minerals plays a certain role in signing the groundwater recharge,runoff and discharge conditions.In addition,human activities can affect the formation of groundwater hydrodynamic field and then effect the evolution of groundwater chemical field;when the groundwater hydrodynamic field conditions is simlar,due to the different mineral composition of the aquifers,existential characteristics and initial chemical composition of groundwater and other factors,the groundwater chemical composition also exhibit different evolutionary characteristics.