Research On The Mechanism And Prediction Of Contaminant Migration In Red Mud Tailings In Karst Valley In Central Guizhou

Guizhou is located in the central southwestern karst mountainous area.Karst landform is mainly developed as exposed and semi-naked patterns.The mineral resources are abundant in the area.Almost all kinds of tailings dump sites in coal,phosphorus,metallurgy,chemical and electric power industries need to be selected.Karst valley is the best choice as a natural tailings dump site.However,the tailings dump that was built and put into use in the last century was not treated properly due to the comprehensive anti-seepage measures in the dump area at the initial stage of construction.Tiny particles,strong acid and alkali wastewater in the tailings then passed through the sinking hole,funnels,karst joints and fissures and enter the aqueous system,causing karst leakage pollution problems.At present,although some tailings ponds have been deactivated,the tailings of the landfills still have leakage problems under the effect of precipitation and leaching,causing serious threat to the surrounding groundwater environment.Bauxite mines are mainly distributed in the central area of Guizhou,and a large amount of red mud,as a by-product of the alumina industry,is stored in karst valleys.This thesis takes the karst valley type red mud tailings dump which is with Guizhou characteristics as an example and carries out detailed research work.This thesis discusses the groundwater pollution characteristics and mechanism caused by the karst valley tailings dump.It provides scientific basis for effectively controlling leakage pollution of tailings reservoir in Karst valley,perfecting corresponding closure measures and protecting groundwater system from pollution.At the same time,it is of great practical significance to the location of new tailings in Karst mountain area and to prevent and control groundwater pollution caused by leaching leakage from tailings.Based on the previous work,this research carries out extensive work such as investigations of the study area,sampling,testing,numerical simulation and so on.Porous media fluid dynamics,fissure flow dynamics,environmental hydrochemistry,karst hydrogeology and other multidisciplinary theories have been used on the comprehensive and systematic study of transport and pollution mechanism of red mud particles and its leaching solution in groundwater system.The thesis established the sewage groundwater flow and solute transport model of the study area by COMSOL simulation software.The following major findings are finally drawn:?1?The groundwater in the study area is dominated by fractured karst water,and precipitation is the main recharge source.The karst spring is the main form of discharge,forming a heterogeneous dual medium storage with pores and micro-fractures as storage,large fractures and pipelines as the main channel of transport and runoff system.Before the red mud dump was put into use,the groundwater hydro-chemical type was weakly alkaline Ca-HCO₃ water.After use however,the water hydro-chemical type changed to K,Na-SO₄ and Cl type.Controlled by the structure and karst development,the distribution characteristics of the red mud leaching filtrate has three pathways that leak to the northwest and southward directions.?2?From the perspective of rock-soil body nature,Red mud is equivalent to a well-classed cohesive soil backfill material.Its particle size is small,generally between 0.11-130.30?m,and the particle size distribution characteristics perform log-normal distribution.For the chemical composition,the main chemical components of red mud are K,Na,Ca,Mg,Fe,Al,Si,Ti and other elements.The distribution characteristics and migration of red mud particles,K⁺,Na⁺and Ca²⁺in the red mud reservoir are affected by the vertical zoning of groundwater.The main factors affecting the migration of red mud particles are particle size,hydrodynamic conditions and the stress of the particles.The difference of hydrodynamic condition determines the maximum particle size of the red mud particles.The stronger the hydrodynamic conditions are,the larger the particle size of the migrated particles will be.?3?Through the static and dynamic leaching test in the laboratory,the effects of solid-liquid ratio and water dynamics on ion release in red mud were revealed.Based on the chloride concentration,the linearity correlation between each ion and chloride was determined.So that the transport characteristics of a single ion can be used to analyze the various ion transports in the red mud leachate.A mathematical model for the release of major ions under dynamic leaching of red mud was established to quantitatively predict the groundwater pollution intensity of the main ion components in red mud under precipitation.?4?Through the lab-experimental study of fissure solute transport,the performance characteristics of groundwater runoff and solute transport under the centralized point-like recharge-discharge t relationship are revealed;the influences caused from fissure width,fissure width ratio,flow rate to solute concentration process curve and peak are revealed;the red mud dump pollutant transport channel is confirmed as a multi-fissure combination.?5?The groundwater flow and solute transport model in the study area were established,and the hydrogeological parameters of the study area were partitioned by the Analytic Hierarchy Process?AHP?.The groundwater flow and solute transport were simulated by COMSOL software.The comparison between the simulated values and the measured values shows that the established pore-fissure medium groundwater flow and solute transport model could be applied to predict groundwater flow and solute transport in the study area.The result provides theoretical basis for the groundwater pollution management.?6?The red mud dump pollution was predicted by two kinds of working conditions in the study area,including wet and the drought seasons.The prediction results show that,the changes of sulfate ion concentration in the monitoring points of the wet and dry years are consistent.In wet season,the sulfate ion concentration of wet years is lower than the sulfate ion concentration in the drought years;in the drought season,the sulfate ion concentration of wet years is higher than the sulfate ion concentration in the drought years,and all the values are close to or exceed the standard groundwater quality level-III.The results could provide theoretical basis for groundwater pollution control in the study area.