Resistivity Imaging Of DNAPLs Contaminants Migration Process In A Sandbox Model

In this paper,the migration and distribution of DNAPLs in heterogeneous porous media with different dimensions were studied by establishing two-dimensional and three-dimensional sand trough models of clay lens bodies with poor permeability.The resistivity imaging method was used to dynamically monitor the migration process of DNAPLs in two-dimensional and three-dimensional heterogeneous media spaces.The feasibility of resistivity imaging to monitor the migration of dense non-aqueous liquids in heterogeneous porous media was discussed.Based on the comparative analysis of the resistivity images of DNAPLs monitored in the two-dimensional flask model,a three-dimensional visual monitoring method for the migration of dense non-aqueous liquids in heterogeneous porous media with cohesive soil lens is obtained.Firstly,based on the resistivity imaging method,a set of measurement and analysis system obtained the water distribution image of the heterogeneous medium containing the clay lens in the three-dimensional sandbox model.The image clearly reflected the effectiveness of the measuring instrument in monitoring the water migration.As well as the change in the influence of the longitudinal lens clay lens on water migration,the water migration process is also clearly depicted.Concluded as follow:?1?During the saturation process,due to the weak water release of the clay lens body,it contains moisture,and the corresponding resistivity is low resistance.When the water containing the lens profile migrates to the position of the clay lens body,it preferentially communicates with the moisture pores in the lens body,and then slowly fills the entire medium.The clay lens body has"anti-diffusion"characteristics,and the sand medium having the same depth as the lens body is occupied by the diffused moisture,and the moisture migration priority is formed in the section without the lens body;?2?During the unsaturated process,the clay lens body has poor water permeability,and there is flow around the water transport of the lens body;the water transport path without the lens body is constant and evenly distributed;?3?The presence of clay changes the water transport distribution and provides a reliable basis for studying the migration of DNAPLs in this flask model.The results of DNAPLs migration test in two-dimensional sand trough model show that:?4?Resistivity images at low concentrations also clearly depict areas of contamination in smaller areas;?5?During the infiltration process of DNAPLs,the relative resistivity value showed a negative value,indicating that the water was flooded by DNAPLs and the resistivity decreased,indicating that the DNAPLs pollutants constantly displaced the pore water under the influence of capillary force and gravity,both vertical migration and infiltration of the law;?6?The change of resistivity image during the whole infiltration process indicates that DNAPLs form migration channels in the medium and migrate to the bottom of the sand trough,which diffuse to the two sides after accumulating at the bottom;the clay lens has a retardation effect on the migration of DNAPLs,and some DNAPLs crushing clay migration through the slit continues downward.The results of DNAPLs transport experiments on heterogeneous porous media in a three-dimensional sandbox model indicate that:?7?DNAPLs migration in three dimensional space having a relatively symmetry;?8?In the pollutant injection test,the symmetry of pollutant migration in the three-dimensional sandbox medium was obtained by analyzing the relative resistivity images of the two sections containing the lens body.During the process of standing,the pollutants still undergo a series of migration and redistribution through their own gravity,capillary action,viscosity and volatility,and the saturation is also changing.The clay lens body has a significant hindrance to the migration behavior of DNAPLs,and the diffusion width increased to 23.5 cm.A small amount of contaminants remain in the clay through the relative resistivity image.Through the analysis of the static image.The area of the contaminated area was 784.8cm² at the initial stage,and the area after standing for 9 hours was reduced to 712.1 cm^2,,it is shown that the high-density resistivity method can fully monitor the migration and distribution of DNAPLs in the heterogeneous porous medium during the standing process.?9?In the process of maintaining a certain hydraulic gradient,the migration characteristics of DNAPLs are as follows:a small part of the pollutants dissolve into the water and migrate to the downstream as the water flows.Residual contaminants in the lens communicate with some contaminants moving downstream to the downstream,and finally most of the contaminants flow around the clay through the clay migration and downstream of the sand trap and gather downstream.Comparing the DNAPLs transport experiments in the three-dimensional and two-dimensional flask models with clay lens body poor permeability,the uniformity of the resistivity images was obtained,which fully demonstrates the feasibility of resistivity imaging for monitoring the transport distribution of DNAPLs in saturated heterogeneous porous media,and further obtain the saturation change and the related parameters of the pollutant migration rate and regional variation according to the resistivity image.However,in the process of injecting pollutants into the two-dimensional sandbox,the contaminants will stick to the side walls of the glass and cause relatively large errors.Therefore,it is more advantageous that the three-dimensional sandbox model studies the migration test of DNAPLs to eliminate the two-dimensional sand.The boundary effects brought about by the box test,a more complete picture of the range of changes in pollutant migration.