Research On Groundwater Hydrochemical Evolution And Recharge Mechanism In The Chinese Loess Plateau On Catchment Scale

It’s of great importance to study the shallow groundwater along the Yellow River in the middle of Chinese Loess Plateau for the ecological civilization construction of the Loess Plateau and ecological protection and high-quality development of the Yellow River catchment.In this study,200 groundwater samples were collected from 14 major catchments of the Chinese Loess Plateau and their hydrochemistry and stable isotopes were measured.Piper diagrams,Gibbs,correlation analyzed and forward derivation modeling were used to analyze the distribution characteritics and its controlling factors of groundwater chemistry and stable isotopes in the Chinese Loess Plateau on catchment scale,which revealed groundwater quality evolution process and recharge mechanism.The results showed that:（1）The pH and TDS values of shallow groundwater in the Loess Plateau have no obvious distribution characteristics at the catchment scale.The groundwater is weakly alkaline and mainly fresh water.The dominant cation and anion of groundwater were HCO₃^-and Na⁺,respectively.and the main hydrochemical type was Ca-Mg-HCO₃.In addition,91%of shallow groundwater belongs toⅠ-Ⅲclasses,9%belong toⅣ-Ⅴclasses,indicating that most groundwater can be directly drinkable.The water quality of most irrigation water meets the requirement at present,but better drainage facilities should be set up to avoid sodium damage.（2）The chemical composition of groundwater was mainly controlled by rock weathering,and evaporation also played a vital role,atmospheric input has little influence on groundwater solutes.According to the forward derivation modeling,the average contribution of atmospheric input to the groundwater solutes was 6±4%.The average contribution of groundwater solute from human activities was 5±8%.Significantly,rock weathering is the dominant source of groundwater solutes.The average contribution from carbonate rock weathering was 22±21%,groundwater in the northern Loess Plateau and Fenhe River catchment was most affected by carbonate rocks weathering.The average contribution from silicate rocks 41±24%,groundwater in the southern part of the Loess Plateau was most affected by silicate rocks weathering.The average contribution from evaporative rocks weathering was 25±22%,and there was no obvious spatial distribution.In addition,climate change,underlying surface conditions change and human activities are the important factors that cause the change of groundwater volume in the Loess Plateau.Therefore,it is necessary to carry out unified planning and evaluation of surface water and groundwater in the arid and semi-arid areas,which is the key to ensure the sustainable development of rivers.（3）Based on the stable isotopes of hydrogen and oxygen in the precipitation and shallow groundwater in the Fenhe River catchment,the isotopic values of groundwater in this area are similar to those of precipitation from July to September,indicating that the groundwater may mainly origin from this period,and found that the groundwater recharge mode（dominant flow and piston flow）has a spatial variation.In addition,most groundwater in the Fenhe River catchment was characterized by isotope fractionation,and the lc-excess evaporation fractionation varied greatly in the whole basin,suggesting that the groundwater was mainly recharged by dominant flow of precipitation,meanwhile,piston flow was also one of the recharge modes.