Study On Groundwater Chemical Characteristics And Pollutant Migration In An Open Pit Mining Area

The open-pit coal mine in the study area is located in the water source of Dunde Nuur,close to the two lakes of Huitennuur and Dunde Nuur,and the open-pit coal mine has been mined for many years,which may have an impact on groundwater environment.Therefore,it is necessary to carry out research on the identification of groundwater chemical characteristics and pollutant source strength in the study area.Firstly,based on the investigation,sampling and testing of the water environment in the study area,the chemical characteristics and causes of groundwater and surface water in the study area were discussed by statistical analysis,correlation analysis,hydrochemical type analysis,Gibbs chart analysis,ion ratio analysis,cation exchange analysis and isotope analysis.Then,the single factor index method was used to evaluate the groundwater and surface water quality in the study area,and the typical pollutants in groundwater were determined,and the sources of pollutants were analyzed.Then,the sodium fluorescein tracer test was carried out in the study area,and the related parameters of groundwater solute transport were calculated by using the test results.Finally,GMS software was used to simulate the migration and movement rule of F-in groundwater under different intensity of F-pollution sources,and compared with the measured concentration of F-,and finally identified the intensity of F-pollutants in groundwater.The research results of this thesis can effectively guide the monitoring and protection of groundwater environment in the study area.The main achievements of this thesis are as follows:（1）The chemical characteristics of groundwater and surface water in the study area are analyzed:the chemical types of groundwater are mainly HCO₃-Ca type and HCO₃-Na type,and the chemical types of surface water are mainly HCO₃-Na type and Cl-Na type.According to the coefficient of ionic variation,the temporal and spatial variations of groundwater chemical properties are small,while those of surface water are large.The anions in groundwater and surface water are significantly correlated with TDS,the cations Na⁺and Ca²⁺in groundwater are significantly correlated with TDS,and the cations Na⁺and Mg²⁺in surface water are significantly correlated with TDS,indicating that the chemical properties of surface water and groundwater are affected by different effects.（2）The chemical origin of groundwater and surface water in the study area is discussed.Gibbs diagram shows that the chemical properties of groundwater are controlled by water-rock interaction,and the chemical characteristics of surface water are mainly affected by evaporation.The analysis of ion ratio shows that Na⁺ions in groundwater mainly come from weathering dissolution of silicate rocks such as plagioclase.The surface water is mainly replenished by groundwater and a little by atmospheric precipitation.The cation exchange analysis concluded that the dissolution of plagioclase was the main reason for the high content of Na⁺in groundwater,and the increase of Na⁺in water could replace Ca²⁺and Mg²⁺in rock and soil.According to the analysis of stable isotope of water samples,the groundwater is mainly recharged by snowfall melting and affected by water-rock interaction.Surface water evaporation is strong and stable isotopes D and ¹⁸O are enriched.（3）Typical groundwater pollutants were screened out and their sources were determined:the single factor index method was used to evaluate water quality against Class III water quality standards,and the maximum F^-in groundwater was0.15 times of the standard.Considering the high pollution and harmfulness of F^-in groundwater of the study area,F^-is taken as a typical pollutant in groundwater.Combined with geological and hydrogeological conditions of the study area,F^-in groundwater is mainly derived from the weathering dissolution of volcanic tuff,crystalline tuff and other fluorine-containing igneous rocks in the southern mountain area.（4）Field groundwater tracer test was carried out and solute transport rel ated parameters were calculated:100g sodium fluorescein was put into the inj ection hole SZK2 in a one-time transient way,and the tracer concentration in the four receiving holes SZK1,SZK3,SZK4 and SZK5 was monitored regul arly.The whole tracer test lasted about 14,000 minutes,and finally sodium fl uorescein was detected in the SZK1 and SZK4 holes.The peak concentration of SZK1 and SZK4 is 340ppb and 70ppb respectively.The flow direction of groundwater in the test area is the northwest direction.The actual flow velo city of groundwater is 1.3m/d,the longitudinal hydrodynamic dispersion param eter D_Lis 1.1m²/d,and the transverse hydrodynamic dispersion coefficient D_Tis 0.11m²/d.These parameters provide effective parameters for the solute tran sport model of groundwater.（5）The groundwater numerical simulation software GMS was used to i dentify the intensity of F^-pollution sources in groundwater.Firstly,based on t he hydrogeological conditions of the study area,a numerical model is constru cted to simulate the groundwater flow movement.Then,based on the migratio n characteristics of pollutants,a numerical model was established to simulate the migration of pollutants.According to the dispersion coefficient obtained fr om the tracer test and combined with the location of the F^-pollution source,t hree strong concentration scenarios of 5mg/L,10mg/L and 15mg/L were set.Finally,when the intensity of the pollution source was 10mg/L,the concentrat ion of F^-pollutants in the study area reached stability close to 2000years,and the simulated concentration of the verification point was 3mg/L.It is close to the measured concentration value of 3.2mg/L at the comparison verification p oint,so it can be inferred that the intensity of F^-pollution source is 10mg/L.The research results of this thesis can provide scientific basis for open-pit mining and water resources environmental protection in the study area,which has important theoretical significance and practical value.There are 29figures,16 tables and 139 references in this thesis.