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Residual Hydrocarbons in a Water-Saturated Medium: A Detection Strategy Using Ground Penetrating Radar
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.
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