Study On Groundwater Recharge Sources Based On Environmental Isotope In Jiaolai Plain

Groundwater is an important source of fresh water in the world and an important part of hydrological cycle.It plays an important role in maintaining rivers,lakes,wetlands and aquatic communities.The Jiaolai Plain(or Jiaolai Valley)is located between the Luzhong Mountains and Jiaodong Hills.Stretching 30–80km from east to west,and reaching the Laizhou and Jiaozhou Bays to the north and south,respectively,the altitude of most of the plain is less than 50m.Jiaolai Plain is densely populated,the climate has been dry for years,the surface water resources are scarce,and the underground water exploitation has increased significantly.All these lead to water resources and environmental problems,such as the expansion of groundwater funnel,seawater intrusion and land subsidence.In this study,the source of precipitation,the process of river runoff,the source of groundwater recharge,groundwater seawater interaction and other water cycle processes in Jiaolai plain were studied by using hydrogen and oxygen stable isotope technique and traditional hydrochemical method.The main conclusions are as follows:(1)The ?²H of precipitation on the Jiaolai Plain ranged from-89.76‰to-0.54‰(average:-34.43‰),and the ?¹⁸O was between-13.85‰and-0.05‰(average:-5.51‰).The monthly ?²H and ?¹⁸O of precipitation on the Jiaolai Plain exhibited bimodal(“M”-shaped)variations,with the general pattern being low values in summer and winter,and high values in spring and autumn.The LMWL was ?²H=6.38 ?¹⁸O+0.72.These two isotopes in the precipitation also displayed significant temperature and amount effects,but the altitude effect was not significant.The water vapor source was controlled by the East Asian Monsoon from June to September,and was mainly evaporated from the adjacent Pacific Ocean.The source was controlled by the Westerlies from October to May,when it originated from the mixing of water vapor that had evaporated from large-scale waterbodies on land and atmospheric(upwind)water vapor.The impact of polar water vapor was small.The water vapor carried by Typhoon Lekima caused precipitation over the plain to be significantly depleted in ?¹⁸O.The main reason was that the typhoon system has its own microphysical processes,which cause the“cloud–rain zonal effect”in which the ?¹⁸O of the precipitation becomes significantly negative.(2)The ?²H of river water on the Jiaolai Plain ranged from-51.54‰?-14.05‰and the ?¹⁸O was between-7.19‰?-0.13‰.The river water in Jiaolai Plain was mainly derived from meteoric precipitation and experienced evaporation to a certain extent during the process of runoff and runoff.The evaporation line of the river water was ?²H=4.97 ?¹⁸O-15.49.The slope of evaporation line in spring,summer,autumn and winter was 5.70,5.10,5.39 and 5.40,respectively,indicating that the evaporation of river water was more intense in summer.The total dissolved solid TDS of the rivers in Jiaolai Plain ranged from 239.68mg/L to 3698.79mg/L,and the main hydrochemical type of the rivers is Ca²⁺-Na⁺-HCO₃^-type.In different river basins,the values of ?¹⁸O, ?²H and TDS in the lower reaches of Weihe River and Dagu River were higher than those in the upper reaches,the ?¹⁸O, ?²H and TDS values of Jiaolai river did not change significantly in the basin.The hydrochemical composition of Weihe River and Dagu River is mainly controlled by rock weatheringand dissolution,while that of Jiaolai river is mainly controlled by evaporation and rock weatheringand dissolution.The interaction between river water and seawater in the estuary area results in the enrichment of isotope and higher TDS.(3)The evaporation line of shallow groundwater in Jiaolai plain is ?²H=5.20 ?¹⁸O-14.78.The total dissolved solids TDS of the groundwater in Jiaolai plain was varied between 275-2056mg/L.TDS was higher in winter and lower in autumn.The hydrochemical types of shallow groundwater in the study area were mainly Na⁺-Ca²⁺-HCO₃^-and Ca²⁺-Na⁺-HCO₃^-.The groundwater in the South Bank of Laizhou Bay in the north of Jiaolai plain was Na⁺-Cl^-type.The chemical composition of groundwater was mainly affected by rock weathering,and the chemical composition of groundwater in Beijiaolai River Basin was controlled by evaporation.The ?²H and ?¹⁸O of estuarine seawater ranged from-36.05‰to-2.41‰and from-3.94‰to 0.88‰,respectively,with the mean values of-12.63‰and-0.97‰,respectively.The evaporation line of estuarine seawater was ?²H=6.84 ?¹⁸O-6.01.The ?¹⁸O value of seawater was the highest in spring and the lowest in winter.Evaporation was more intense in Jiaozhou Bay than in Laizhou Bay.The variation of total dissolved solids(TDS)in seawater ranged from 19.90g/L to 48.38g/L,low in spring and high in winter,and the hydrochemical type was Na⁺-Cl^-type.TDS was lower in Laizhou Bay and higher in Jiaozhou Bay.(4)The groundwater level in the Jiaolai Plain was higher in the northeast and southwest,and lower in the middle,which was positively correlated with the elevation.The groundwater level flowed from the mountainous areas in the northeast and southwest to the middle,and was lower in spring and summer and higher in autumn and winter in different seasons.The main recharge source of groundwater and river water in Jiaolai Plain was meteoric precipitation,which had undergone weak evaporation before recharge of groundwater,and the evaporation of groundwater in the same basin was weaker than that of river water.The underground runoff in the upper reaches of the basin was discharged from the discharge port to replenish the surface runoff and became the source of the main and tributary streams.The relationship between surface water and ground water in the middle reaches of the basin was weak,and the water was subjected to strong evaporation,and the surface water and groundwater formed atmospheric water through strong evaporation.