Simulation Study On The Fluid Migration In The Vadose Zone

Vadose zone is the region of unsaturated stratum between groundwater and surface. Vadose zone is the channel which contaminants flow through into groundwater. Simultaneously, the contaminants may pollute groundwater and be harm to people’s health when the contaminants enter into vadose zone, so the contamination of vadose zone needs control and remediation. To control and remediate contamination of vadose zone, the study of fluid migration in vadose zone is the premise and base.To study miscible and immiscible fluid migration in homogeneous vadose zone and interface between double-deck media in vadose zone, this paper established a serious of simulated sand box experiments.Then the retardation of interfaces of stratum for fluid migration was analyzed, the contaminant’s distribution and migration in different direction in vadose zone were quantitatively analyzed. The results abovecan help for risk evaluation, control and remediation of groundwater contamination.In this paper, desiel and water were used to simulate immiscible and miscible fluid to study their convectional migration in vadose zone. Through the simulated experiments of fluid migration in homogeneous vadose zone, we found that the shape of contaminatedregion was a ellipsoid without the top. The lower the medium permeability of vadose zone was, the stronger horizontal transfer ability of fluid was, the shape of contaminated area was flat ellipsoid. The higher the medium permeability was, the stronger vertical transfer ability of fluid was, the shape of contaminated area was vertical long ellipsoid. Miscible fluid had higher surface tension compared with immscible fluid, so it received higher capillary force from media in vadose zone. In the same vadose zone, miscible fluid had stronger horizontal transfer ability, while immiscible fluid had stronger vertical transfer ability. Based on the experimental results above, a group of predicted formulas for fluid migration distance and contaminated volume were established. When immscible fluid entered into various moisture content vadose zone, as the moisture content increasing, vertical migration rate of fluid increased and then decreased, and the vertical migration rate had crest value when the moisture content was 50% of maximum residual moisture content of media. While horizontal migration rate decreased and then increased as the moisture content of media increased, and had valley value when the moisture content was 50% of maximum residual moisture content of media.Through the simulated experiments of miscible fluid convectional migration in the vadose zone of double-deck media, we found that if fluid entered into low osmosis stratum from high-permeable media, the retardation of interface was not remarkable. Owing to the stronger capillary force fluid received from low osmosis stratum, horizontal transfer ability of fluid was enhanced at the interface between the double-deck media, while vertical transfer ability weakened after the fluid transfered through the interface. When fluid entered into high-permeable media from low osmosis stratum, owing to the stronger capillary force fluid received from low osmosis stratum, the fluid stopped vertical migration, and the retardation of interface was remarkable. The fluid migrated at the horizontal direction in low osmosis stratum along the interface, then concentrated at the interface and even formed a region of water saturated stratum. Fluid broke through the constraint and entered into high-permeable media if the saturated soil was thick enough, but the migration was uneven.Through the study of retardation of interface between double-deck media for fluid migration, this paper proposed that the permeability coefficient of upper and under layer were K1 and K2, we could use K1/K2 to analyse the retardation of interface in the vadose zone of double-deck media for fluid migration. If K1/K2 was bigger than 1, the fluid was entering into low osmosis stratum from high-permeable media. The bigger K1/K2 was, the stronger transfer ability of fluid in under low osmosis stratum was, while vertical transfer ability of fluid weakened. But if K1/K2 was smaller than 1, the fluid was entering into high-permeable medium from low osmosis stratum. The smaller K1/K2 was, the stronger retardation of interface for fluid migration was, and the fluid was hardly entered into high-permeable media from low osmosis stratum.The experimental results were greatly meaningful for grasping cantaminant migration in the vadose zone of different permeability, and understanding retardation of interface between different permeability stratum for fluid migration, and then the foundation of contamination control and remediation can be layed.