High Frequency Ground Penetrating Radar Monitoring of Ethanol Released over Existing Gasoline Residuals

Abstract

High‐frequency (450 and 900 MHz) ground penetrating radar (GPR) was used to monitor the migration of an ethanol release and the redistribution of existing gasoline residuals within an unconfined sand aquifer. In September 2009, 185L of denatured ethanol mixture (E95) was released directly over a year old gasoline (E10) spill zone. GPR profiling prior to the ethanol release indicated that the gasoline impacted zone was largely confined to the vicinity of its initial release point. GPR profiles collected one month after the ethanol release showed the formation of strong reflection events which propagate laterally away from the trench. After this initial ethanol expansion, decreases in event amplitude were seen until the onset of winter conditions. The effects of the ethanol were also monitored using the apparent velocity “pull‐up” of an underlying stratigraphic reflector; variations in the velocity pull‐up correlated well with the behaviour of the ethanol‐induced reflection. The presence of ethanol inhibited the freezing process of the pore water in the contaminated zone, resulting in a difference in the dielectric properties between the unfrozen and frozen zones. It was found that the unfrozen zone area was significantly greater than the spatial extent delineated by GPR during pre‐winter conditions. The spatial extent and depth of the unfrozen zone imaged by GPR was been confirmed by soil probing with a steel rod. After seasonal thaw, evidence of the ethanol or gasoline was absent until mid summer when the water table dropped to approximately 80cm below ground surface. At this point, strong reflection events were again observed throughout the contaminated zone until the end of the experiment in September 2010 when the site was excavated.