| Groundwater is the source of drinking water for most of the world’s population,and its quality is crucial to human health.Currently,the pollution of underground aquifers by organic pollutants is becoming increasingly serious,and density non aqueous phase liquids(DNAPLs)are the key and difficult points in groundwater pollution remediation due to their low solubility,low mobility,high density,and high toxicity.Surfactant enhanced remediation technology enhances the mobility of organic pollutants in aquifer media by reducing interfacial tension and increasing solubility,allowing a large amount of DNAPLs from the adsorbed and residual phases to enter the aqueous phase,and finally extracting it to the surface for treatment.Although surfactant flushing can solubilize and increase the flow of pollutants,it has poor effect on the removal of DNAPLs from heterogeneous aquifers.And there are core issues such as excessive remediate time and difficulty in completely solving the concentration reverse diffusion and tailing in the lens.It is also necessary to couple treatment with other insitu hydrodynamic remediate technologies to further improve the removal efficiency.Groundwater circulating well(GCW)technology can increase the vertical erosion area and flow rate of low permeability lenses,strengthen the entry of surfactants into the lenses and solubilize and increase the solubilize and flow of DNAPLs therein,enabling them to migrate out of the low permeability region,thereby effectively removing them.Therefore,GCW enhanced surfactant remediation for heterogeneous aquifers is an efficient method for removing DNAPLs.In this paper,an experimental tank and numerical simulation were used to establish a remediation scenario for groundwater circulating wells.Nitrobenzene was selected as the characteristic pollutant of DNAPLs.The research focuses on the circulation flow field and solute migration laws of the remediation technology for circulating wells in heterogeneous aquifers,the impact of operating process parameters on the circulation flow field and circulation efficiency under numerical simulation,and the enhancement effect of the circulation wells on the remediation of nitrobenzene contaminated heterogeneous aquifers by surfactants.The main conclusions are as follows:(1)During the visual dye migration process of bright blue solution in heterogeneous aquifers,the range of circulating flow field assumed an elliptical shape,and the solute migration rate in the transverse direction was greater than that in the longitudinal direction.The distribution of dye migration distances was basically symmetrical,with the longest lateral and longitudinal migration distances being approximately 48 cm and 34 cm,respectively.The phenomenon of "circumflow" occurred as erioglaucine disodium salt passed through the lens during migration,and gradually enters and accumulates in the lens area during circulation.The maximum concentration of erioglaucine disodium salt in the lens area was 38.48% of the injection concentration,and the vertical hydraulic scouring effect during circulation was the key factor for erioglaucine disodium salt to enter the lens area.When studying the influence of different heterogeneous media combinations on the spatial water head in the affected area of the circulating well,the change of the water head in the lens area in the heterogeneous aquifer relative to the water head of the main medium was significantly reduced.The closer the lens was to the circulating well,the more obvious the effect of the water head change was.The larger the permeability coefficient of a lens,the greater the amplitude of water head change within its region.(2)The correlation between laboratory water head results and numerical simulation results was high,and the simulation results were considered to be of reference value.The changes in flow field,influence radius,and particle recovery rate were explored by simulating different process parameters.Particle recovery rate is the ratio of the amount of particles released from the injection screen section to the amount that can be recovered by the extraction screen section.This recovery rate can reflect the circulation efficiency in the flow field.The results showed that increasing the circulation time can improve the influence radius and particle recovery rate of the hydrodynamic circulation well.When the time increased to a certain extent,the influence radius will still gradually increase,while the increase in particle recovery rate was no longer obvious.The increase in the distance between the pumping and injection sieve sections lead to a decrease in the circulation intensity between the pumping and injection sieve sections,and the radius of influence increased linearly with the increase in the spacing between the sieve sections.Particle recovery decreased first and then increased.Under different pumping and injection flow cycles,the groundwater flow field was elliptical,and the increase in flow does not change the overall shape of the circulating flow field.The hydraulic disturbance effect on the heterogeneous aquifer increased with the increase in flow.The thickness of the lens region has a certain impact on the shape and radius of influence of the circulating flow field.The larger the thickness,the more difficult it is for particles to flow to the extraction screen section,and the smaller the radius of influence and particle recovery.When the permeability coefficient of the lens differd by 33 times from the main medium,the ratio of the migration rate through the lens area to the inside of the main medium is about 1/9.(3)In the enhanced flushing experiment of circulating wells,the sodium dodecyl sulfate(SDS)n-butanol complex system had a significant removal effect on nitrobenzene contaminated heterogeneous aquifers.The concentration of nitrobenzene in the lens near the circulating well reached a maximum of 6.11 g/L at 20 minutes,and the concentration of nitrobenzene decreased to below 50 mg/L at 120 minutes,with most pollutants migrating out.The concentration of nitrobenzene at the water outlet was 349.20 mg/L at 60 min,and nitrobenzene underwent a certain degree of lateral and vertical diffusion,but the pollutant were still distributed within the influence range of the circulation well flow field,without uncontrolled migration.The maximum outflow concentration of nitrobenzene was 963.08 mg/L in 70 min;after 120 minutes,the concentration of nitrobenzene at the water outlet was 218.9 mg/L,which can cumulatively remove 59.85% of nitrobenzene in the heterogeneous aquifer.Compared with 200 m M SDS and pure water flushing systems,the removal rate of nitrobenzene in the 200 m M SDS+20% n-butanol system can be increased by 21.45% and 38.78%,respectively,after 80 minutes.When the cumulative removal rate of nitrobenzene in each flushing system was 50%,the total flushing time required for the composite system was 40 minutes less than that for a single SDS system,and the flushing goal was achieved faster.The surfactant flushing system was conducive to the dissolution of nitrobenzene in the aqueous phase,and the hydraulic disturbance of the circulating well was conducive to the migration of nitrobenzene in the aquifer,which not only improved the removal efficiency,but also reduces the amount of flushing fluid and flushing time. |
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