GPR Attenuation and Scattering in a Mature Hydrocarbon Spill: A Modeling Study

Abstract

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 signifi cant alteration and biodegradation, complicates the electrophysical properties of the subsurface, making GPR data interpretation more diffi cult. 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 signifi cant practical benefi t in these complex environments. By using dielectric material property analysis, empirical mixing model evaluation, and threedimensional, fi nite-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 stratifi ed 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 fi eld of the GPR system can be determined accurately.

Geophysical Signatures of Microbial Activity at Hydrocarbon Contaminated Sites: A Review

“…The complexity of the subsurface spatial distribution of different LNAPL phases is also illustrated in a GPR-based study by Cassidy (2008). In this study, Cassidy (2008) shows that LNAPL geometries within the contaminated smear zone greatly affect the frequency component of the GPR response and exhibit distinct spectral responses.…”

“…In this study, Cassidy (2008) shows that LNAPL geometries within the contaminated smear zone greatly affect the frequency component of the GPR response and exhibit distinct spectral responses. Models in the Cassidy (2008) study present likely scenarios of LNAPL distribution encountered in a smear zone and include: (1) a uniform mixture of pore fluids (LNAPL/pore water mixtures of different percentages), (2) small-scale disaggregated LNAPL units (consisting of randomly distributed disaggregated blobs of fully saturated LNAPLs within a background of contaminated water), (3) large pools of spatially distinct free phase LNAPLs (thick LNAPL lenses), and (4) a large number of thin LNAPL lenses and a small number of thin LNAPL lenses (Fig.…”

“…Models in the Cassidy (2008) study present likely scenarios of LNAPL distribution encountered in a smear zone and include: (1) a uniform mixture of pore fluids (LNAPL/pore water mixtures of different percentages), (2) small-scale disaggregated LNAPL units (consisting of randomly distributed disaggregated blobs of fully saturated LNAPLs within a background of contaminated water), (3) large pools of spatially distinct free phase LNAPLs (thick LNAPL lenses), and (4) a large number of thin LNAPL lenses and a small number of thin LNAPL lenses (Fig. 2;Cassidy 2008). The Cassidy (2008) study illustrates that the GPR response varies both as a result of the distribution of the various phases of LNAPL as well as the macro and pore scale distribution of the LNAPL.…”

“…Early geophysical studies that reported higher bulk electrical conductivity and attenuated ground penetrating radar (GPR) signals at LNAPL-contaminated sites were linked to microbial activity associated with biodegradation (e.g., Benson et al 1997;Sauck et al 1998). A large volume of literature now exists supporting the early findings of anomalous geophysical signatures at LNAPL contaminated sites undergoing bioremediation (e.g., Atekwana et al 2000Atekwana et al , 2002Atekwana et al 2004a, b, c, d;Abdel Aal et al 2006;Bradford 2007;Cassidy 2007Cassidy , 2008Allen et al 2007). Attempts to understand the role of microorganisms in altering the physical properties of geologic systems has resulted in the development of a new sub-discipline in geophysics called ''biogeophysics'' ).…”

3D Electrical Resistivity Tomography to Locate DNAPL Contamination in an Urban Environment

“…Non-invasive geophysical methods can be very useful as they survey large areas in a short time and at a lower cost than drilling techniques, they avoid infiltration of the contaminant to non-contaminated layers, and they enable more precise information between existing boreholes or piezometers, often separated by large distances. Over the last decades, a variety of geophysical methods have been proposed for the detection of DNAPLs in the shallow subsurface including ground-penetrating radar (Daniels et al, 1995;Bermejo et al, 1997;Cassidy, 2008), resistivity methods (Newmark et al 1998, Benson et al, 1997Atekwana et al, 2005, Chambers et al 2010, induced polarization (Chambers et al, 2005) and self-potential measurements (e.g., Minsley et al, 2007). In this paper, we present two electrical resistivity campaigns performed around a housing estate located downstream an old coke tar site.…”