Study On The Formation And Evolution Of Hydrochemical Characteristics Of Pore-groundwater In Basin Area Of Datong City

Groundwater,mainly pore-water contained in aquifer constituted by particle sedimentary rocks is the main water source for industrial and domestic use in Datong city.The basin area which being considered as the core part of the city has huge number of factories and companies gathered during last several decades.Serious overall ecological deterioration,water over-extraction and water pollution problems are all constantly escalated following the development of local economy.To carry out research works related to evolution and development of pore-groundwater hydrogeochemistry in this area has significant effects on both environment protection and sustainable economic development.With careful analysis of geological and hydrogeological conditions in the basin area,hydrochemical analysis,ion analysis theory and hydrogeochemical reverse simulation were employed in this research work to find out the formation and evolution of groundwater hydrogeochemical properties.Groundwater quality was evaluated as well through random forest model with an improved rectangular extended grid.The research results are as follows:(1)By compare ion concentrations in different parts of the research area,clear spatial distribution differences can be find and distinguished for each ion.for Na⁺,Mg²⁺,Ca²⁺,Cl^-,SO₄^2-,TH and NO₃^-,the values varied in comparatively dramatic ways throughout the research area.On the opposite,data of p H,K⁺,HCO₃^-,F^-,and TDS has less dispersion in different parts.High values of p H,TH and TDS concentration were found in middle and southwest part of the research area,with lower numbers in north and east part.Ca²⁺,SO₄^2-and F^-,Na⁺,and Cl^-have similar distribution rules.The concentration of NO₃^-is abnormally high in the excretory area.Those variations lead to the changes of groundwater hydro-chemical types in different parts of the research area,which from HCO₃-Ca·Mg at the side mountain area to HCO₃·SO₄-Na·Mg·Ca and HCO₃·Cl-Na·Mg·Ca at the runoff area,and finally evolved into HCO₃·SO₄·Cl-Na in excretion area.(2)The improved rectangular extended grid was used to optimize the random forest model to evaluate the groundwater quality in the basin.The groundwater quality research showed that the amount of“good”and“fair”water account for more than 80%of total volume.Water being classified into“poor”and“very poor”groups is mainly in central and southern part of the research area,which are basically for agriculture uses.The overall water quality is general,with only some specific places have poor quality water.It is much better in the northern part than in the southern part.Following the groundwater flow system(from the recharge area to runoff are,then to the discharge area),water quality shows a trend of gradual deterioration.(3)By employing ion analysis theory,the research work find that water-rock interaction is the dominate factor in the formation of groundwater chemical components,with controlling minerals such as dolomite,gypsum and calcite,the impact of silicate mineral dissolution is very limited.By calculating the saturation index of related mineral,calcite was found to be basically saturated.Gypsum minerals,rock salt and fluorite,on the other hand,were unsaturated.Qualitative research on evaporation shows that it has limited impact on groundwater geochemical components in the research region.Scatter plot of related proportional coefficients shows that the intensity of cation exchange in each groundwater region as:drainage area>runoff area>recharge area.The forward exchange effect is stronger than the reverse exchange effect.After involved human activities into the research work,it is found that human activities could change part of underground water flow field,leading to chemical reactions and concentration changes of groundwater components.Human activities could also lead to pollution components increases and have great impacts on the evolution of hydrochemistry.(4)The reverse simulation and mass balance calculation results of 6 typical paths in the research area show that from the northeastern border mountains to the middle of the basin,The dolomite and gypsum are dissolving,accompanied by various positive and negative cation exchange.The hydrochemical processes from the groundwater supply area to the runoff area and then to the discharge area of the western border mountains are more complex and show a great difference in hydrogeochemistry.Hydrodynamic field and geological structure play an important role in controlling the hydrogeochemical evolution of groundwater in the basin area.