Hydrochemical Characteristics And Its Evolution Modeling Of Groundwater In Jinta Basin,Gansu

Groundwater is the main source of agricultural irrigation in Jinta Basin.With the development of economy and society and the increase use of irrigation water,the groundwater level in Jinta Basin is decreasing because of the long-term exploitation of groundwater,which causes the evolution of groundwater chemical composition.In this study,the chemical distribution characteristics and spatial variability of groundwater are expounded according to the collection of 52 groundwater samples and 3 surface water samples of Yuanyangchi,Jiefangcun Reservoir and Beihaizi and the indoor analysis of chemical composition of groundwater.At the same time,the groundwater quality is evaluated by using Entropy Weight-Normal Cloud Model.Meanwhile,the main recharge sources of groundwater and the formation mechanism of water chemical composition are analyzed in relation to Gibbs diagram,ratio terminal element diagram and main ions interaction,as well as environmental isotope tracing.And the hydrogeochemical evolution process is inversely simulated based on PHREEQC software.The main conclusions are as follows:?1?Na⁺,K⁺,SO₄^2-are the main ions in groundwater.CO₃^2-and NO₃^-are strong variations,K⁺,Ca²⁺,Na⁺,Mg²⁺,Cl^-and SO₄^2-show moderate variations and HCO₃^-is low variations.TDS and total hardness are greatly affected by evaporation concentration,showing a relative similarity.?2?The main hydrochemical type of groundwater is SO₄^-·Na-Mg type,and from recharge area to discharge area the chemical types changes from HCO₃^-·SO₄^{-_</sup>Mg to SO₄-_</sup>Na·Mg type.The classification of groundwater quality is mainly V,and from recharge area to discharge area the classification is changed from III to V.?3?Ca2+,K+,NO₃-,HCO₃-and the groundwater level depth show strong spatial correlation;TDS,total hardness and ions also have strong spatial correlation on the whole,indicating that the distribution of each component content is uniform and random factors have little influence on spatial variation of ions in irrigation area.?4?According to the analysis of hydrogen and oxygen isotopes,there is a close hydraulic relationship between surface water and groundwater in Yuanyangchi and Jiefangcun Reservoir,while the recharge of groundwater by atmospheric precipitation is weak.The northwest margin of Yuanyangchi Irrigation Area may be recharged by ancient groundwater,and the northeast area is enriched by isotopes.The age of groundwater is 3.16±0.2012.16±0.94ka,and the average age is 6.56±0.39 ka?5?According to the simulation using PHREEQC software,it is concluded that from recharge area to discharge area,the chemical composition of groundwater in basin forms by dissolution of dolomite,gypsum,kaolinite and salt rock;sedimentation of chlorite and calcite.In part of the study area it is the sedimentation of aragonite and anhydrite,and dissolution of potassium feldspar and albite.Leaching,evaporation-concentration and cation exchange adsorption are the main factors affecting the formation and evolution of groundwater chemical composition.}