Imaging the movement of toxic pollutants with 2D electrical resistivity tomography (ERT) in the geological environment of the Hoa Khanh Industrial Park, Da Nang, Vietnam

Abstract

This study presents the results of a geophysical survey undertaken by 2D electrical resistivity tomography (ERT) along two traverses. The study was conducted between an industrial park (IP) and a residential area bounding the Hoa Khanh Industrial Park, Da Nang, Vietnam. The first traverse, 300 m long, which is at the boundary between the industrial park and Bau Tram Lake, includes 504 measured data points. The second traverse, which includes 189 measured data points, is 200 m in length and runs alongside the boundary between Bau Tram Lake and the residential area. Bau Tram Lake is located between the IP and the residential area and acts as a natural boundary. After discarding noisy data points, the data were processed using the commercial software Res2dinv which creates a 2D visual section of the resistivity distribution below surface using a finite difference algorithm and the method of least squares inversion. The result of an inversion, which is a simulated 2D resistivity image of the subsurface or Electrical Resistivity Tomogram (ERT), showed the formation characteristics and geological structure along the first traverse from surface to 39.4 m in depth, including two overlapping geological layers with indistinctive local boundaries. The resulting image illustrates the amalgamation of anthropometric transported soil and rocks with local lake material, brought into the lake from other areas, to fill up the lake with rock and improve the engineering foundation for the construction of the industrial park buildings. The research results show that the main components of the second geological layer on both sides of the survey area have the same electrical signature associated with black mud and sand and represent the natural lake sediments. The inferred groundwater level identified in the electrical imagery and two boreholes corresponds approximately to the water level in Bau Tram Lake. At a depth of approximately 10 m along both traverses, the groundwater in Bau Tram Lake and the surrounding residential areas is contaminated by electrolytes and heavy metals. Furthermore, the groundwater inside the industrial park contains a significant amount of industrial electrolytes (with a high probability of chemicals such as KCl, NaCl) and heavy metal contamination (with a high probability of elements such as Pb, Hg, Zn) at a greater density than measured in the residential areas. This suggests that Bau Tram Lake acts as a filter to strongly reduce the movement of these toxic substances from seeping into the groundwater below the residential area.