Removal Of Residual DNAPL In Aquifers By Injecting Ethanol: The Mass Transfer Mechanism And Model Application At Pore Scales

During the process of rapid industrial development,the improper usage,storage and transportation of dense non-aqueous phase liquids(DNAPL)in large quantities result in their leakage into the subsurface environment.Because of physical and chemical properties of DNAPL,such as the high density,high interfacial tension between DNAPL phase and water phase,low biodegradability and low solubility,DNAPL becomes a long-term source of contaminated groundwater that seriously threatens groundwater safety.It is difficult to remove them effectively and quickly by the conventional extraction-treatment technique.Delivering the ethanol co-solvent delivered to the contaminated source area can increase the dissolution and mobilization of DNAPL and accelerate its removal rate from the aquifer.The numerical simulation technology can evaluate the removal efficiency of DNAPL economically and rapidly during ethanol solution flushing,where the mass transfer rate of DNAPL is the key to evaluate the removal efficiency.In most numerical predictions of the DNAPL removal,the dissolution rate of DNAPL is usually calculated by a mass transfer rate coefficient constant,which cannot accurately predict the elution curve and DNAPL removal efficiency.We investigated the DNAPL mass transfer mechanism at pore scale under ethanol co-solvent flushing to reveal these factors affecting the mass transfer rate and developed a mass transfer model based on DNAPL mass transfer mechanism.That can improve the predictive ability of the removal efficiency of residual DNAPL and strengthen the guiding significance in the implementation of residual DNAPL remediation projects in aquifers at the site scale.In this work,we investigated the dissolution process of residual PCE in a transparent micro-porous template under ethanol co-solvent flushing to reveal the mass transfer mechanism of DNAPL and summarize the dissolution law of DNAPL under different mass transfer mechanisms.Meanwhile,based on Raman spectra technique,spatial and temporal distribution of PCE concentration in a hydraulically inaccessible disconnected pore was observed quantitatively.On this basis,a new DNAPL mass transfer model was established.The new mass transfer model was coupled to UTCHEM.Some parametric models related to calculations of fluid properties inside the simulator were corrected.Previous two-dimensional sandbox experimental data were sued to evaluate predictive effects of the new mass transfer model on the elution curve and removal efficiency for DNAPL.Finally,the improved model was used to predict and evaluate the removal efficiency and risk of downward migration for DNAPL under different remediation scenarios at contaminated sites.The main understandings are as following:1.Based on the observation of the dissolution process of residual PCE in a two-dimensional transparent micro-porous model at different ethanol concentrations and flushing rates,the dissolution of PCE can be divided into three stages,which are mass transfer processes dominated by the strong convection,weak convection and diffusion with a limited distance,respectively.In the mass transfer process dominated by convections,the high ethanol concentration,the faster flushing rate,the more uniform PCE distribution and the high PCE saturation in pore media result in the faster dissolution rate of PCE.In hydraulically inaccessible disconnected pores,the PCE concentration at the pore outlet gradually decreases with the dissolution of PCE based on the Raman quantitative observation.However,the PCE concentration at the interface between two phases remains constant,i.e.its solubility.The dissolution of PCE in disconnected pores is dominated by the diffusion with a limited distance.The dissolution rate of PCE is independent of its saturation and distribution characteristics,and increases with increases of ethanol concentration and flushing rate at the exit of the disconnected pore.The new mass transfer model was developed based on the experimental data of PCE mass transfer rate coefficients at different dissolution stages.2.The new mass transfer model was coupled to UTCHEM and some of its internal parametric models were corrected.We evaluated the simulation effectiveness of the improved numerical model based on the image data from a previous experiment.The improved numerical model can predict the heterogeneity of the DNAPL dissolution rate caused by different ethanol concentrations,flushing rates,and distributions of DNAPL saturation,which results in the non-uniform distribution of DNAPL concentrations.It can also better predict the elution curve and remaining PCE volume in the sandbox.3.The improved model was also used to simulate the removal process of DNAPL at actual contaminated sites.The simulation results show that it can effectively increase the ethanol concentration in the contaminated source area and improve the removal efficiency of DNAPL by increasing the ethanol concentration or the injection rate in injection wells,but it would also exponentially increase the downward migration volume of DNAPL.The ethanol solution is injected into the aquifer below the DNAPL contaminated source zone by increasing the depth of the injection well screens.It would migrate upward under buoyancy and contact with DNAPL,dissolving the downward migrating DNAPL and effectively controlling the risk of migration of DNAPL to the deeper aquifer.Under the hydraulic constraint of the pumping wells,the center of the ethanol concentration distribution will migrate downward to the region near DNAPL pool by increasing the ethanol injection depth,which increases the removal rate of the DNAPL pool.