Using ground penetrating radar (GPR), we monitored a controlled low-density non-aqueous<br>phase liquid (LNAPL) gasoline spill at a test facility at Oregon Graduate Institute, near Portland OR. The<br>results were different from some reported for uncontrolled gasoline spills, in which the gasoline apparently<br>blurs the contrast in dielectric permittivity that usually exists at the top of the saturated zone (SZ), so that<br>GPR reflections from the SZ are subdued. Instead, at OGI we saw a SZ reflection almost everywhere, but<br>this reflection was brighter (higher amplitude) under the spill. The bright spots grew and spread as the spill<br>progressed. We explain this effect by noting that sand grains above the SZ were quite moist, so that values<br>of relative dielectric permittivity (RDP) were relatively high there. As the spilled gasoline displaced this<br>interstitial moisture, RDP values dropped, leading to a GPR high velocity layer and concomitant bright spots.


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