Detecting NAPL in Unsaturated Soil

Research was completed on ground penetrating radar (GPR) to determine the threshold level where non aqueous phase liquid (NAPL) could be detected in the unsaturated zone. Method development focussed on ideal antenna frequency and offset distance between the transmitter and the receiver. Synthetic GPR scans were analyzed to determine the two-way travel time (TWT) of the reflected waves, giving the dielectric constant of the soil mixture. The dielectric constant value allows determination of total petroleum hydrocarbon concentrations (TPH) on a mass basis. Current GPR equipment gives a TPH threshold for TPH at 40,000 mg/kg for both silty loam and sandy loam soil. Due to attenuation of the GPR signal, TPH could not be detected in clay. These measurement thresholds are high when considering GPR for site investigations. Improvements in GPR equipment and data analysis are needed if GPR is to provide information on site contamination levels that approach typical regulatory cleanup levels.