Source Zone Architecture And Mass Flux Of PCE In Saturated Porous Media

DNAPLs（Dense non-aqueous liquids）are only slightly soluble in water and resistant to biodegradation.Upon released into the subsurface environment,DNAPLs tend to dissolve slowly and persistently into the surrounding groundwater through NAPL-water interface,and act as long-term source of contamination that is difficult to locate and remediate,posing serious and persistent risks to the groundwater environment and human health.Up to date,there has been substantial domestic research focused on the infiltration and remediation of DNAPLs in groundwater,while there is still a lack of knowledge regarding the dissolution process of DNAPLs.Therefore,systematic research on the migration,distribution and dissolution of DNAPLs in groundwater is in urgent need.Findings from this work will provide scientific basis for the risk assessment,pollution prevention and control,and development of remediation strategies for DNAPL-contaminated sites,which have great theoretical and practical significance.In this thesis,tetrachloroethylene（PCE）was selected as a representative of DNAPL,and a series of one-dimensional and two-dimensional experiments were conducted in laboratory to systematically study the source zone architecture and mass flux of PCE in saturated porous media.The partitioning tracer methods and the light transmission method were used to quantitatively depict the source zone architecture with multiple parameters,and meanwhile the PCE dissolution was monitored using headspace gas chromatograph.The effects of important factors such as media type,wettability and hydrodynamic conditions（flow rate）on PCE source zone architecture and mass flux were systematically investigated.Moreover,ethanol flushing was adopted to obtain different PCE distribution in the porous media,and the corresponding changes in source zone architecture and dissolution concentration were comprehensively analyzed,to further explore the relationship between DNAPLs mass transfer and its source zone architecture.The main conclusions were drawn as follows:（1）The results of partitioning tracer tests in one-dimensional column experiment showed that NAPL-water interfacial area A_nw was obviously different due to the type and wettability of the media.The measured A_nw in quartz sand columns was larger than that in glass bead columns under PCE initial residual state.The important reason for this phenomenon is the difference of particle morphology and surface roughness between quartz sand and glass beads and the morphological distribution of NAPL phase.The A_nw formed in the oil-wet media was much larger than that in the water-wet media,which is caused by the different distribution characteristics of PCE in different wetting media.Due to the sufficient contact between NAPL phase and water phase in PCE initial residual state,the PCE dissolution concentration in the three experimental media was close to the solubility of PCE.（2）The PCE distribution in the columns was changed by washing with ethanol,and several sets of partitioning tracer tests were carried out and PCE dissolution concentration was monitored under different PCE distributions.The results showed that PCE dissolution concentration was closely related to NAPL-water interfacial area A_nw,and the change trends of both were obviously consistent in glass beads,water-wet and oil-wet quartz sand media.The A_nw and PCE dissolution concentrations in the water-wet quartz sand column and glass bead column both decrease with the increase of ethanol flushing volume,and the decrease rate of A_nw slows down significantly.It was noticed that when A_nw decreased to a certain extent,the PCE dissolution concentration in both media dropped suddenly.Because the flushing of ethanol may lead to mobilization and redistribution of residual PCE in the oil-wet media,it is observed that the A_nw increases and the corresponding PCE dissolution concentration also increases in the case of lower PCE saturation.（3）The effect of flow rate on PCE dissolution concentration in the column experiment showed that PCE dissolution concentration in glass beads,water-wet and oil-wet quartz sand media all decrease with the increase of flow rate in PCE residual state,and the extent of decrease increased with the decrease of A_nw.When the A_nw is large,PCE concentration remained at a high level and decreased slowly with increasing velocity,which can be attributed to the sufficient contact between the NAPL phase and the water phase,resulting in a higher mass transfer rate.However,at low NAPL-water interfacial areas velocity can be an important factor,the PCE dissolution concentration decreases significantly with the increase of flow rate.（4）Several sets of two-dimensional（2-D）flow cell experiments were conducted,and the source zone architecture and mass flux of PCE were continuously monitored in the infiltration and stable distribution in flow cell.The results showed that the migration of PCE in homogeneous porous media was mainly downward infiltration,accompanied by lateral spreading caused by capillary force,and when the groundwater flow rate is 1m/d the influence of groundwater velocity on PCE lateral spreading is very weak.Because the whole leakage amount was small,the PCE source zone was mainly composed of residual ganglia trapped in pores along the migration path when the PCE distribution reached stable.PCE dissolution concentration was closely related to PCE distribution area,and the obvious consistency of changes of both was observed,which increased rapidly at first and then stabilized gradually.（5）The PCE distribution in the two-dimensional（2-D）flow cell was changed by multiple ethanol washing,and the changes of PCE source zone architecture and dissolution concentration were compared and analyzed under different PCE distributions.The results showed that PCE saturation in source zone became lower,while the spatial spread of PCE was less affected by ethanol flushing.With increasing ethanol flushing volume,the PCE distribution area decreased significantly and thereby led to lower dissolution concentration,and a linear relationship between these two variables was observed with a correlation coefficient（R²）of 0.76.In addition,continuous monitoring results also showed that the PCE distribution area and dissolution could still remain stable for a period of time（at least 16PV）regardless of whether it is washed with ethanol or not.