Remediation Oriented Modeling Of Solute Transport In Fractured Karst Aquifer

Fracture matrix diffusion and secondary diffusion in the fissure stagnant water zone are two important physical mechanisms that hinder the effective restoration of the pollution of fractured karst aquifers.Based on the realistic remediation issues,transient storage(TS)model was used to simulate the solute transport in fracture stagnant water zone and a semi-analytical solute transport model coupling TS model with fracture-matrix diffusion model was developed in this study.Through the simulation,the efficiencies of multiple remediation schemes were quantitatively evaluated,and effective strategies for restoring contaminated fractured karst aquifers were suggested.In this study,analytical solutions are also obtained for the storage zone concentration of TS with instantaneous source,main flow zone and storage zone concentration of TS model with continuous source,critical time and critical coordinates for mass exchange between the two zones,mass exchange flux,and matrix-diffusion model under complex realistic conditions by theoretical analysis.The main conclusions are obtained as follows:(1)Interpretation of the two sets of tracer breakthrough curves using the linear graphic method,river mass tracing method and TS model shows that there are multiple preferential flow paths in the migration of the tracer in the fractured karst aquifer.Relatively independent flow paths also have their own hydraulic parameters.(2)The simulation results of ten-years’ pump-and-treat show that due to the matrix diffusion in the fractured karst aquifer,the pump-and-treat alone is not efficient,and even lead to the spread of pollutants in the aquifer,which makes the subsequent remediation more difficult.(3)The simulation results of the fissure-matrix diffusion model coupled with TS model show that the combination of pump-and-treat and in-situ near-source chemical remediation can reduce the concentration of pollutants in the fissures,and effectively inhibit the back diffusion,and thus improve the remediation efficiency.(4)It is necessary to consider the corresponding measures to prevent the rebounce of the contaminant level when designing the remediation scheme for fractured karst aquifers.(5)In the remediation of fractured karst aquifers,the scheme of remediation should be centered on source treatment and remediation.