An experiment was conducted at the Ohio State University using a sand and gravel-filled tank<br>model, to investigate the influence on the GPR response of vadose zone gasoline vapor phase effects<br>and residual gasoline distributed by a fluctuating water table. After background GPR measurements<br>were made with only water in the tank, gasoline was injected into the bottom of the model to simulate<br>a subsurface discharge from a leaking pipe or tank. GPR measurements were made on a 3-D grid on<br>the surface of the tank for an extended period of time after the injection to monitor changes in a<br>vadose zone with no residual gasoline. Water was then introduced beneath this gasoline and the<br>water table was raised and lowered in stages with GPR measurements made at each liquid level.<br>Results from this experiment show the sensitivity of GPR to moisture content and it’s<br>effectiveness for monitoring changes in the water table. After the injection of gasoline into the tank<br>and prior to water table fluctuations, a decrease in reflection amplitudes within the vadose zone<br>occurred as a function of time. One interpretation of these results is that increasing hydrocarbon<br>vapor pressure displaced redistributed interstitial moisture in the vadose zone, which led to this<br>observed change in the GPR response. Additionally, data show that the GPR response can be<br>enhanced when residual gasoline is present in a water saturated system due to less attenuation through<br>the medium.


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