Investigation On The Migration And Remediation Of Typical DNAPLs In Saturated Porous Media

Groundwater contamination caused by dense nonaqueous liquids(DNAPLs)has been a serious and widespread problem,since DNAPLs will spill into subsurface during transport and usage.The migration behavior and distribution of DNAPLs in subsurface may be influenced by many factors,including physicochemical properties of DNAPLs,leakage rate,groundwater flow velocity,heterogeneity and so on.Thus,it is important to investigate the combined effect of flow velocity and heterogeneity on DNAPLs migration in complex subsurface system,which will facilitate to identify exact position of the source zones,and design remediation schemes after spill events.Because of low solubility,viscosity and difficult biodegradation,groundwater remediation and source-zone removal are very difficult and challenging.DNAPLs remediation has become a hot problem around worldwide.Among all remediation technologies,surfactant enhanced aquifer remediation is an effective technology for source zone remove.After long-time flushing,contaminant tailing occurs and removal efficiency significantly decreases.In situ chemical oxidation can quickly degrade soluable contaminant,while it shows slow oxidation on NAPL-type contaminant.Based on previous studies,PCE was selected as a representative of DNAPLs.Nine sandbox experiments were performed to explore the individual effect of flow velocity,the combined effect of flow velocity and layered lenses,and flow velocity and complex heterogeneity on the PCE migration.The light transmission(LT)method was used to monitor the migration and redistribution processes of PCE within the sandboxes,and quantitatively measure PCE saturation.To compare with experimental results,some migration processes are simulated.Furthermore,migration of DNAPLs with different viscosity for multiple heterogeneous realizations were simulated toexplore the combined effects of flow velocity and geological heterogeneity.Tween 80,persulfate(PS),citrate and ferrous ion were selected as remediation agents.After PCE immobilization in porous media,the Tween 80 solution,and surfactant(Tween 80)coupled with an effective oxidant(Fe2+-citrate activated persulfate)were flushed through the sandboxes to remove PCE.The LT method was used to monitor the remediation processes of PCE within the sandboxes,and quantitatively measure PCE saturation.The GTP(ganglia-to-pool ratio)was used to quantify source-zone architecture,and the definition was the ratio of volume of PCE ganglia to the volume of pools.The effect of source-zone architecture on the remediation efficiency were evaluated.Findings from this work can provide significant implication for source zone characterization,and the remediation of DNAPLs in soil and groundwater.The main conclusions were drawn as follows:(1)Large flow velocity promoted lateral and vertical migration of low-viscosity DNAPLs(i.e.,PCE)when there were no lenses embedded in porous media.The combined effect of flow velocity and heterogeneity significantly influenced migration paths of PCE.Numerical simulation showed agreement with experimental results.Increasing viscosity of DNAPLs resulted in a decreased effect of flow velocity on DNAPLs migration.The migration of medium or high-viscosity DNAPLs was predominantly controlled by geological heterogeneity and viscosity.This suggested that high geological heterogeneity combined with flow velocity could significantly alter migration paths and position of source zone.(2)Tween 80 flushing can remove 80%of PCE,and the PCE removal influenced by initial source-zone architecture.Increasing discontinuous ganglia increased PCE removal through solubilization by micellar,attributing to a larger contact area between Tween 80 and ganglia than pool.However,the pool was difficult to remove,and contaminant tailing occurred,resulting in a decreasing remediation efficiency.(3)In the system that surfactant(Tween 80)coupled with an effective oxidant(Fe2+-citrate activated persulfate(PS)),batch experiment results showed that citrate concentration played a key role in controlling the activation of PS and needed to be optimized for the oxidation system to achieve the best effect.Thus,the appropriate molar ratio of the PS activation system is very important for the successful application of the technology in in-situ chemical oxidation.Sandboxes experiment results showed that even there some difference existed in the initial source-zone architecture,the cumulative NAPLs(PCE and intermediate products)removal was similar when the same volume of mixture solution was flushed.This indicated that the remediation efficiency was significantly influenced by flushing agents,rather than initial source-zone architecture.The flushing agents decided the remediation efficiency.(4)The combination of surfactant and oxidant was much more effective in NAPLs removal than the only surfactant flushing.The PCE concentrations in the effluent significantly increased during the flushing,and the average NAPLs removal from the sandboxes increased by about 22%.The increasing NAPLs removal was due to the combined effects of the surfactant solubilization and PS oxidation:1)Tween 80-enhanced solubilization resulted in an enlarged effective contact and reaction between PCE and PS;and 2)PS degraded the PCE from the soluble phase and thus facilitated its dissolution from the source zone,resulting in more efficient use of Tween 80.This synergistic effect between solubilization and oxidation promoted the NAPLs removal.Findings also suggest combination of surfactant solubilization with PS oxidation can be a relatively reliable and efficient technology for the in-situ remediation of DNAPLs in soil and groundwater systems.