Characterization And Evolutionary Mechanisms Of Groundwater-Surface Water Chemistry In The Zhouzhi Section Of The Northern Qinling Mountains

With the largest water supply source in Xi’an built in the Zhouzhi section,the northern foot of the Qinling Mountains is an important water source conservation functional area in Xi’an.The quality of groundwater and surface water directly affects the production and life of12.87 million Xi’an citizens.Based on the field survey and indoor analysis,this study took the Zhouzhi section of the northern foot of the Qinling Mountains as the study area,by using multivariate statistical methods,graphical methods,and hydrogeochemical methods,analyzed and explored the chemical characteristics and spatial distribution pattern of groundwater and surface water in the study area,the transformation relationship between groundwater and surface water,the hydrochemical composition factors and formation mechanism of regional water bodies.Based on reverse simulation using Phreeqc software,the evolution law of groundwater in the study area was elucidated,the quality of drinking and irrigation for groundwater and surface water was evaluated,and reasonable water resource planning and protection suggestions were proposed.The main conclusions are as follows:（1）The pH of groundwater in the study area is weakly alkaline,showing high in the southwest and low in the northeast,low TDS,large TH differences,and an increase in conventional indicators from the pre-mountain floodplain fan to the plain area.The main water chemistry types are HCO₃·SO₄-Ca,HCO₃·Ca and HCO₃·SO₄-Ca·Mg,with the HCO₃-Ca type dominating in the high terrain in the south and gradually transitioning to the HCO₃·SO₄-Ca and HCO₃·SO₄-Ca·Mg types in the north.The pH of surface water is weakly alkaline and gradually decreases from the upstream to the downstream of the Heihe River.TDS and TH are low and TDS gradually increases from the upstream to the downstream.The major water chemistry types are HCO₃·SO₄-Ca,HCO₃-Ca and HCO₃·SO₄-Ca·Na,The HCO₃-Ca type predominates in the upper reaches of the Heihe River,and transitions to the HCO₃·SO₄-Ca type predominating in the lower reaches,and to the HCO₃·SO₄-Ca·Na type after the confluence with the Weihe River.The alluvial floodplain fan of the Black River in the study area is mainly recharged by surface water to groundwater,with a recharge ratio of70%.Groundwater recharge to surface water exists in the front part of the alluvial floodplain fan,and atmospheric precipitation is the main source of recharge for all types of water bodies.（2）The water chemistry of the water bodies in the study area is mainly controlled by the weathering of rocks,which is jointly controlled by silicate weathering and carbonate rock dissolution;Na⁺mainly comes from rock salt dissolution and silicate weathering,Ca²⁺and Mg²⁺mainly originate from carbonate and calcium-magnesium silicate weathering and dissolution;the ion correlation between surface water and groundwater is basically the same,and the cation adsorption alternation and redox are weak.Under the influence of human activities,industrial and mining activities have a greater impact on water bodies than agricultural activities and domestic sewage.Nitrate in water bodies mainly comes from the combined effect of precipitation input,agricultural input and sewage input.（3）According to the direction of underground water flow,four groups of paths were selected for Phreeqc inverse hydrogeochemical simulation to determine the amount of mineral transfer in each path,groundwater dissolution of gypsum,dolomite and salt rock occurred along the runoff direction in general,gypsum dissolution is basically greater than salt rock dissolution,calcite dissolution is small,and some even precipitation.（4）Drinking water quality and irrigation water quality in the study area are superior to groundwater in surface water,in the single factor evaluation of groundwater categoryⅣand V water accounted for 60%.surface water categoryⅣand V water accounted for 15.79%.In the comprehensive water quality index evaluation,groundwaterⅣandⅤwater accounted for10.91%.All surface water is above Class III.Both groundwater and surface water in the south are suitable for irrigation,while groundwater alkalinity in the north is generally at a high level and needs to control the impact of alkalinity hazards on soil and crops.