Interpreting TDR signal propagation through soils with distinct layers of nonaqueous-phase liquid and water content

Several studies have demonstrated that time-domain reflectometry (TDR) has an enormous potential to detect and monitor nonaqueous-phase liquids (NAPLs) in uniformly contaminated soils with reference to different-textured soils and under different saturation conditions. Few attempts have been proposed to describe NAPL distribution by using TDR when a contaminated front propagates through a soil. In this study, the TDR response in the presence of NAPL-contaminant transport processes was investigated in a homogeneous soil under confined conditions with a series of laboratory-controlled tests. The laboratory procedure involved measurements of dielectric permittivity (εband electrical conductivity (ECband the acquisition of the reflected TDR waveforms. The experimental framework was designed to simulate sharp NAPL fronts that moved in the soil columns and generated zones of contrasting permittivity. During the experiments, different initial conditions were assumed, as well as different degrees of contamination, by varying the volumetric water (θwand NAPL (θNAPLcontents. The acquired TDR waveforms were systematically analyzed to deduce the influence of NAPLs on TDR signal propagation. A general procedure to determine the location of an NAPL-contaminated front within a soil column was developed. Equipment calibration, measurement accuracy, and error sources were investigated in relation to the experimental procedure setup and the column preparation conditions. The results show that there may be some difficulties when interpreting TDR signals to locate the NAPL front for two main reasons: first, the expected additional reflection at the interface is not always distinguishable; second, there may be problems even locating both the first peak and the reflection point at the end of the TDR probe, especially when the signal shifts from a low to a high impedance layer. The suggested methodology provides a tool to overcome such intrinsic difficulties in NAPL front detection during the propagation of the contaminant.