Interpreting DNAPL saturations in a laboratory-scale injection with GPR data and direct core measurements

Ground penetrating radar (GPR) is used to track a dense non-aqueous phase liquid (DNAPL) injection in a laboratory sand tank. Before data reduction, GPR data provide a qualitative measure of DNAPL saturation and movement. One-dimensional (1D) GPR modeling provides a quantitative interpretation of DNAPL volume within a given thickness during and after the injection. This is confirmed qualitatively by visual inspection of cores and two-dimensional GPR modeling. DNAPL saturation in sub-layers of that thickness could not be quantified because calibration of the 1D GPR model is non-unique when both permittivity and depth of multiple layers are unknown. Accurate quantitative interpretation of DNAPL volumes using 1D GPR modeling requires: 1) identification of a suitable target that produces a strong reflection and is not subject to any multidimensional interference; 2) knowledge of the exact depth of that target; and 3) use of two-way radar-wave travel times through the medium to the target to determine the permittivity of the intervening material, which eliminates reliance upon reflection amplitude. With geologic conditions that are suitable for GPR surveys (i.e., shallow depths and low electrical conductivities), the procedures in this laboratory study can be adapted to a field site to identify DNAPL source zones after a release has occurred.