Methodology for Optical Imaging of NAPL 3D Distribution in Transparent Porous Media

Three-dimensional mapping of non-aqueous phase liquid (NAPL) distribution within saturated porous media is an important issue in bench-scale geo-environmental studies. In this paper, a methodology for optical imaging of 3D multiphase liquid distribution is presented. The natural aquifer is simulated using a transparent soil surrogate that represents the macroscopic behavior of natural sand. To achieve transparency transparent fused quartz grains were saturated with a matched refractive index mineral oil solution that represents the natural aquifer and injected with a green-dyed sucrose solution to simulate dense NAPL contamination. The spatial volume was first estimated and then reconstructed using orthogonal images acquired at the model boundaries at various stages of contamination and remediation. Color space analysis was employed to segment the NAPL zone and transform pixel information into integrated concentration values using a previously published calibration model. The chromatic components C-R and a of YCBCR and Lab, respectively, were combined to render the spatial concentration profile. A novel iterative reconstruction algorithm named 3D carving was used to resolve three 2D projections into a 3D model. The results show that the proposed methodology provides better efficacy for NAPL zone reconstruction in comparison with conventional image analysis routines. The technology presented in this paper is a sustainable, fast, accurate, non-intrusive, and inexpensive method for spatial mapping of contamination zones using bench-scale transparent soil models.