Fusion Electrical Resistivity Technology And Image Method For The High-resolution Characterization Of DNAPL Contaminant Process

At present,the problem of organic pollution in groundwater is a hot research topic for domestic and foreign scholars.A large number of organic contaminants,such as Dense NonAqueous Phase Liquid(DNAPL),have entered the groundwater environment during production,transportation,and use,causing serious groundwater pollution.In order to study the influence of a high permeability lens on the migration process of DNAPL in saturated non-homogeneous media.This paper integrated Electrical Resistivity Tomography(ERT)and image method to monitor the migration process of DNAPL in a two-dimensional sandbox model in real-time.By setting up two saturated two-dimensional sandboxes with different media,the effects of highly permeable lens and their interfaces on the migration mechanism of DNAPL were compared and analyzed.The whole process of DNAPL migration after entering the sandbox was recorded by digital photographic technique,and the changes in resistivity distribution inside the sandbox during the contaminant migration were monitored simultaneously by using a high-density resistivity device.The measured resistivity values were calculated by iterative inversion to obtain the resistivity inversion results,and the results of the two methods were compared and analyzed.The experimental results showed that:(1)In the two-dimensional saturated homogeneous sandbox model experiment,the average vertical migration velocity of DNAPL was 0.31 cm/min and the average lateral diffusion velocity was 0.13 cm/min.The vertical migration distance of DNAPL in the sandbox was much larger than the lateral diffusion distance,indicating that gravity played a major role in the transport of DNAPL in the saturated homogeneous medium.The resistivity value near the DNAPL injection point inside the sandbox was monitored by the ERT method and increased by125 Ω·m after DNAPL infiltration,indicated that the ERT method could effectively monitor the migration process of DNAPL inside the sandbox.(2)In the two-dimensional inhomogeneous sandbox model with coarse sand lens,the image method shows that DNAPL presented "finger-like" infiltration,and the migration rate of DNAPL in the coarse sand lens was 0.46 cm/min,which indicated that there was a "dominant channel" for DNAPL inside the lens,which could accelerate its infiltration rate.The DNAPL inside the coarse sand lens was influenced by the difference in infiltration at the bottom and both sides of the "coarse sand-medium sand" interface and stayed inside the lens in large quantities.The "coarse sand-medium sand" interface at the bottom and both sides of the lens played an obstructive role in DNAPL infiltration,and the low permeability of the interface was the main reason for the retention of contaminants inside the lens.When the DNAPL content in the coarse sand lens body reached the peak,the measured resistivity value inside the lens body increased by 400 Ω·m,indicated that ERT could effectively identify the area with high DNAPL content,but the identification accuracy of ERT for areas with low pollutant content was low.(3)The comparison between the image method and the resistivity inversion results shows that in the homogeneous sandbox experiment,the relative error between the inversion results and the image method gradually decreases to 2.53% with the increase of DNAPL content in the contaminated region,indicating that the inversion results match with the image method results when the DNAPL content is high.However,in the sandbox experiments containing coarse sand lenses,although ERT can obviously characterize the region of coarse sand lenses with high DNAPL content,it is not obvious for the lateral diffusion characterization in the sandbox,and the relative error between the ERT inversion results and the image method results reached 19.47%due to the influence of coarse sand lenses.This paper attempted to provide a new and feasible multi-source data fusion method for fine characterization of DNAPL migration process in porous media by fusing resistivity method and image method,which laid a foundation for fine characterization of field organic contaminated sites.