Dielectric and GPR behaviour of free-phase napl mixtures: implications for the practical assessment of hydrocarbon contamination

Ground contamination by Non-Aqueous Phase Liquids (NAPLs) is one of the World´s most pressing environmental issues. The investigation of NAPL pollution by non-invasive geophysical techniques is becoming increasingly commonplace, with GPR being one of the more popular techniques. Unfortunately, a high percentage of the current NAPL related GPR research focuses on contaminant mapping only, with little emphasis being placed on the nature of the signal and its relationship to material properties. By analysing the spatial variation in signal attenuation, wave velocity, etc, it is possible to obtain additional information on the macroscopic dielectric properties of the sub-surface materials that, in turn, can he used to assess the physical nature of the contaminants. By combining Finite-Difference, Time-Domain (FDTD) modeling methods with the direct dielectric measurement of NAPL contaminated materials, new insights have been gained into the nature of GPR wave propagation and signal attenuation at a coastal, LNAPL contaminated site. The results show that, despite the apparent similarity of some of the material properties, meaniingful variations do exist and may be used to aid data interpretation and analysis.