Using Ground Penetrating Radar to Estimate the Concentration of Non-aqueous Phase Liquids in Unsaturated Soil
Obtaining proper estimates of NAPL contamination levels in unsaturated soil is critical for developing a successful remedial program. Near-surface GPR is possible tool for remediation experts to consider. GPR is a non-destructive, cost-effective geophysical technique that can produce high-resolution images of the unsaturated soil. However, challenges exist in interpreting the GPR data to estimate the NAPL contamination levels in the soil. Improving the analysis ability of GPR data has been the focus of this study. Initial improvement was implemented by using the complex refractive index model (CRIM) to represent the petrophysical relationship of the soil, instead of Topp's equation. The application of the CRIM model was first used to estimate the water content of landfill waste. The approach was then enhanced to optimize the central antenna frequency and the offset separation distance between the transmitter and the receiver (S) using sensitivity tests, obtaining a frequency of 1 GHz with an offset distance of 0.75m. Using the optimized values reduced the error between modelled and GPR measured water content to 1%. Part of the enhancement process included the development of a novel method to convert MATLAB® output files, representing synthetic GPR scans, to file formats readable by EKKO software, the software used by GPR users worldwide. Using the EKKO software allowed automation of picking the GPR two-way travel times. The application of CRIM and EKKO was then used to interpret synthetic GPR scans to estimate NAPL content as gasoline in three soil types: silty loam, sandy loam and clay. The silty loam soil had a minimum detectible level of the gasoline at 40,100 mgN/kgS. Sandy loam soil had a minimum detectible level of 29,000 mgN/kgS, while the technique failed in the clay soil as the electromagnetic (EM) waves could not penetrate into the subsurface. These threshold values are higher than desired for application to investigate contaminated sites, showing the current limitation of soil contamination scanning with a GPR system.