The Formation And Evolution Mechanism Of Saline Groundwater Since Holocene In Luanhe River Delta,China

Since the Quanternary Period,there have been several transgressions and regressions in coastal zones due to global sea level change.Under this transient marine and terrestrial environmrnt,a large amount of salt in groundwater has been formed far inland in muddy coasts,leading to higher salinity than seawater in some areas.This has limited the sustainable development of economic and ecological environment in these environments.At present,studies on coastal groundwater systems and salinization have focused mainly on the identification of salt-water origin and seawater intrusion processes.However,on the sedimentary environment and evolution patterns of saline groundwater,the literature is very scanty.It is of great theoretical and practical significance to study the fromation mechanism in order to clarify the distribution and migration of saline groundwater,and then prevent groundwater salinization and protect freshwater resources in coastal zones.In this work,the Luanhe River Delta（northwest coast of Bohai Sea,China）was chosen as the typical study area,with fresh,brackish,saline,and brine groundwater as the main water components.Through long term groundwater monitoring,the dynamic groundwater level and quality characteristics were collected and summarized.Based on the hydrochemical and isotopic compositions of water samples（including groundwater,surface water and seawater）,the recharge conditions and salt sources of groundwater,as well as a range of hydrogeochemical processes were discussed.Along with the palaeoenvironmental information contained in the published cores in the study area,the saline groundwater sedimentary environments were identified,and the evolutionary processes of groundwater were constructed.The major findings of our work and achievements are as follows:（1）Seasonal dynamics and characteristics of groundwater level and quality.Based on two groundwater monitoring sections in the study area from Octorber 2016 to May 2019,we found that the groundwater level shows a regular rise during the wet seasons（June to September）,and fall during the dry seasons（October to December）.Between January to May,it flluctuates,and the dynamic type of the shallow groundwater belongs to the climatic type.In addition,there is no groundwater despression cone in the study area,and the range of the depth of groundwater level is related to the topography and geomorphology,and the overall trend decreases from land to sea.The groundwater quality was observed to have been in a stable state for many years,with no obvious freshening or salinization,and only fluctuated at the brackish-fresh groundwater interface influenced by human irrigation.Therefore,the development tendency of modern seawater or saline groundwater intrusion has not occurred in the study area,and the hydrochemical field of the shallow aquifer is in a natural state as a whole.The hydrochemical water types（Ca-HCO₃,Ca?Mg-HCO₃,Ca?Na-HCO₃,Na-HCO₃ and Na-Cl）from fresh to brine water was mainly controlled by mineral weathing,seawater mixing and evaporation.（2）Identification of recharge condition and salt origin.The isotopic results(²H,¹⁸O,¹⁴C)show that deep confined groundwater was recharged during the Late Pleistocene cold period,shallow saline and brine groundwater were recharged during the warm Holocene period,and shallow brackish and fresh groundwater was mainly recharged by surface water.The hydrogeochemical modeling（PHREEQC）results reveal that seawater or evaporated seawater is the primary salty source in salinized groundwater.According to the mineral saturation index（SI）,there is gypsum and calcite precipitation in the saline and brine groundwater,while fresh and brackish groundwater could have gone through mineral dissolution along with stream water recharging（such as gypsum dissolution）.The cation exchange between Ca and Na is a common water-rock interaction in the aquifer and is more noticeable for groundwater with low salinity.The SO₄ depletion but HCO₃enrichment can be explained by sulphate reduction reaction in the aquifers.Moreover,weathering of silicate minerals may provide an additional contribution of dissolved Ca and Na ions.（3）The ¹⁸O-Cl relationship diagram yields three end-member groundwater mixing with two mixing scenarios suggested to explain the freshening and salinization processes in the study area.During the transgression period,the fresh groundwater could have been impacted by overlying seawater or evaporated seawater,indicating the salinization processes in which fresh groundwater mixed with either seawater,evaporated seawater,or a mixture of both.Due to surface water recharge,there is a river water-groundwater mixing trend during the regression period,implying continental runoff dilution.（4）Reconstruction of the formation and evolution processes of saline（brine） groundwater and its link to palaeoenvironmrnt.According to transgression and regression events and the saline groundwater distribution in Bohai Sea coast,the sedimentary environment of salinization is analyzed.With the paleoenvironmental information contained in the published boreholes in the study area,the formation and evolution model of saline groundwater is discussed.The salt migration during the geological history is identified by building a reasonable solute transport model.Our study shows that multiple water types are particularly associated with complex geographic evolution in coastal areas.The variation in sea levels（when it rises）causes lowland coastal areas to be inundated by seawater,which induces palaeo-seawater intrusion.The coastal deltas developed after a significant drop in sea levels.The concentration of saline water in the lagoon environment at the delta-front continuously provided salinity to the groundwater.Thus,brine groundwater was formed under the effects of evaporation,mixing,and dissolution.In contrast,the lateral recharge of surface water and irrigation return would cause a slow wash-out of salinized groundwater in the delta plain.