This paper deals with the investigation of the performance and limitations of the GPR method when used to map contamination plumes, and in performing 4D (time and space) monitoring of the diffusion of the oil in the subsurface. In particular, a controlled experiment carried out in the laboratory at the University of São Paulo (USP), Brazil, is presented. The experiment used GPR to monitor in 4D a controlled Light Non-Aqueous Phase Liquids (LNAPL) spill in a masonry tank. In particular, the study has simulated a highly polluting LNAPL spill, usually occurring in electric power stations and distribution substations. The experimental results were supported by FDTD numerical simulations, which helped in characterizing the GPR signal propagation in the investigated scenario. The interesting results showed that the mapping of the contaminant plume by LNAPL spill in a substation is possible when the subsurface is saturated with water, and this opens new scenarios in environmental monitoring by GPR.