Subsurface contamination by light nonaqueous-phase liquids (LNAPLs) is a pressing environmental issue with many industrial sites having some degree of near-surface pollution. Noninvasive, geophysical investigation methods, such as ground penetrating radar (GPR), have become increasingly popular, but their use has tended to be restricted to plume “mapping” and recent contamination events only. Mature LNAPL contamination, where the hydrocarbons undergo significant alteration and biodegradation, complicates the electrophysical properties of the subsurface, making GPR data interpretation more difficult. This is particularly true in shallow, coastal environments where LNAPLs form a disaggregated, smeared zone associated with the seasonally changing groundwaters. Therefore, any method that can improve data interpretation has a significant practical benefit in these complex environments. By using dielectric material property analysis, empirical mixing model evaluation, and three-dimensional, finite-difference time-domain (FDTD) modeling, it has been possible to investigate the nature and spectral content of GPR signal attenuation and scattering within the vadose or smeared zone of a mature LNAPL-contaminated site. Using FDTD models to simulate real subsurface conditions, a comparison between the GPR response of “clean” and contaminated environments has been made where the subsurface materials contain different LNAPL–porewater saturations and contaminant geometries. The results show that uniform mixtures, disaggregated scattering mixtures, and stratified pools of contaminated material all exhibit different, yet characterizable, temporal and spectral GPR responses and that the recorded data can provide new insights into the mode of contaminant distribution and saturation as long as the incident field of the GPR system can be determined accurately.