Study On The Mechanism Of Hydrogeochemical Evolution In Chahar Right Wing Qianqi-Jining District,Inner Mongolia

The Right Wing Qianqi-Jining District of Chahar is located in the southern foothills of the middle section of the Yin Mountains,which belongs to the junction of the northern agricultural and pastoral staggered belt and the semi-arid area.The topographic and geomorphological conditions of the study area are more complex,the new and old strata are staggered and exposed,the water resources conditions are poor,and the groundwater is the main supporting water source for the residents’lives and industrial and agricultural production in the study area.With the rapid development of economy and population,the requirements for industrial production and residential water in the area have exceeded the scope of the carrying capacity of groundwater resources in the study area.Water shortage has become increasingly severe,which not only constrains economic advance,but also seriously affects the fragile ecological environment of the local area.Based on hydrogeological survey and sampling analysis,the hydrological geochemical analysis method was applied to reveal the chemical characteristics and formation mechanism of regional groundwater.Three typical groundwater runoff paths were selected,then the hydrochemical evolution characteristics on the trail of updressing and displacement were discussed by using PHREEQC software.The following main conclusions were drawn from the study:（1）Groundwater in the study area is mainly weakly alkaline.From the recharge area to the discharge area,the mineralization concentration in groundwater shows a gradual increase trend.From the perspective of the hydrogeological unit of the whole region,HCO₃^-,Ca²⁺are the main yin and yang dominant ions in groundwater,respectively,and the chemical types of groundwater can be roughly divided into HCO₃-Ca type,HCO₃-Ca·Na Type,Cl-Ca·Mg type,Cl-Ca type,Cl-Na type,of which HCO₃-Ca type water is widely distributed in the study area,accounting for about 65%of the total area.Water-rock interaction and cation exchange are the main mechanism of groundwater evolution.（2）Basalt of the Hannuo Dam Formation was exposed in a large area of the low hilly area（recharge area）in the study area.Using mathematical statistics,correlation analysis,ion spatial distribution analysis and other means according to the water chemical laboratory data to identify the chemical characteristics of groundwater.Groundwater quality is generally good and groundwater environment is weak alkaline,and the water chemistry type is mainly HCO₃-Ca type;the mineralization degree is generally low,and the average value is 539.55mg/L.（3）The content of H₂Si O₃ in the basalt terraces in the north-east and north-west directions of the study area was high,and the highest content was 41.88mg/L.Better groundwater runoff conditions and strong water-rock interactions are one of the main reasons affecting the high content of H₂Si O₃ in groundwater in this area.（4）The results of reverse hydro geochemical simulation show that in the chemical evolution of groundwater in three typical paths,calcite,dolomite,gypsum and CO₂ are the main mineral components that control the change of chemical components of groundwater.Water-rock interaction and alternating cation adsorption are the main hydrogeochemical effects of groundwater on the runoff pathway.