- ©2000. AAPG/DEG
The focus of this article is to demonstrate through physical model experimentation a potential means for identifying contaminated areas where a light non-aqueous phase liquid (LNAPL) hydrocarbon has been redistributed by a rising water table in a previously hydrocarbon residual–free vadose zone using ground-penetrating radar (GPR). Analogies of the experimentation conducted in this study are situations where a rise of the water table follows leakage from a tank or pipe at depth or where an LNAPL hydrocarbon plume has migrated laterally from a surface source along the top of the saturated zone and a subsequent rise of the water table occurs. Research to date has provided insight into mechanisms that may offer the potential for LNAPL detection under certain field conditions; however, no studies have specifically focused on developing a potential detection strategy for a case in which residual hydrocarbon is present in a water-saturated medium.
A tank model filled with gravel and sand was designed to allow GPR measurements to be made on the surface before, during, and after water and gasoline injections and fluctuations within the tank. Background GPR measurements were made initially with only water being raised and lowered in the model, and the water table was then raised and lowered beneath a volume of 219 liters of gasoline that was injected into the bottom of the tank. Measurements from the initial raising and lowering of the water with no gasoline present demonstrate the sensitivity of GPR for monitoring changes in subsurface water content and minor fluctuations of the water table. Measurements made during the raising and lowering of the water table with gasoline in the model show differences from the measurements made when only water was raised and lowered, and a comparison of the data show that reflections in GPR data can be enhanced when residual gasoline is present in a water-saturated system because there is less attenuation of the radar signal. Differences in travel times to subsurface reflections between the two stages of the experiment are also caused by the residual gasoline present in the water-saturated medium. Results of this study provide the basis for a strategy that has the potential for successful detection and delineation of LNAPL hydrocarbon–contaminated areas at field sites where the conditions are similar to those modeled through this experimentation.
Key Words: contaminant detection, ground-penetrating radar, hydrocarbon, light non-aqueous phase liquid, vadose zone.
Changryol Kim received a B.S. (1986) in geology from Yonsei University (Seoul), and an M.S. (1997) in geology from The Ohio State University and is currently working on his Ph.D. at The Ohio State University. His research interests include LNAPL detection, the effects of seasonal variations on GPR data, and GPR data processing and modeling. He is a member of EEGS, SEG, and AGU.
Jeffrey Daniels received his B.S. (1969) and an M.S. (1970) in geology from Michigan State University and a Ph.D. (1974) in geophysical engineering from the Colorado School of Mines. He worked for 11 years for the U. S. Geological Survey in Denver with primary interests in borehole geophysics. In 1985, he joined the faculty of the Department of Geological Sciences at The Ohio State University. His current areas of research interests involve using GPR to image the shallow subsurface for engineering and environmental applications. He is a member of AGU, EEGS, SEG, and SPWLA.
Erich Guy received his B.S. (1996) in geology from Mount Union College and an M.S. (1998) in geology from Bowling Green State University, and is currently working towards his Ph.D. at The Ohio State University. His current research interests focus on various aspects of surface and borehole GPR as well as seismic data processing and interpretation for environmental, geotechnical, and geologic applications. He is a member of AAPG, AEG, AGU, DEG, EEGS, GSA, and SEG.
Stanley Radzevicius received a B.A. (1992) in earth science, a B.A. (1992) in mathematics from Saint Cloud State University, and an M.S. (1995) in geophysics from Indiana University, and he is currently working on his Ph.D. at The Ohio State University. His research interests include LNAPL and DNAPL detection using geophysics, electromagnetic modeling, the use of GPR polarization and antenna pattern information to improve subsurface imaging and interpretation, and various aspects of seismology. He is a member of AAPG, AGU, DEG, EEGS, IWRA, OGS, and SEG.