Noninvasive survey technology for LNAPL-contaminated site investigation

Nonaqueous-phase liquids (NAPLs) usually move through the vadose zone not in a homogeneous spread manner but in the form of disconnected blobs, and they remain in the pores of porous media. A major challenge in remediating an NAPL-contaminated site is to detect and delineate the distribution of NAPLs. Geophysical technologies could act as investigating methods for sites with limited resources and time constraints. Based on the principle of magnetometric resistivity (MMR) method, in this study, we applied an electric current flow pattern and its resulting magnetic field to develop a new application for contaminated site investigation. The physical phenomena about the magnetic distribution, the flow pattern of electric current, and the influence of different media in packed sand were observed via sandbox experiments. A field test was performed at a light NAPL (LNAPL)-contaminated site by combining the magnetic field survey and a soil sample analysis. Two potential contaminated zones were determined according to the specific characteristics of the contour image plotted according to the magnetic field intensity collected on the ground. The quantitative results of the total petroleum hydrocarbons (TPHs) of 18 soil samples were used to verify the results of the magnetic field survey. The comparison results revealed that we obtained a high accuracy of 92.9% in the field test, which indicated that this technology could be used to investigate LNAPL-contaminated sites. Moreover, two phenomena, the scale effect led to poor resolution and the behavior of flow pattern disturbance distorted the survey images, were observed in both the sandbox experiment and the field test.