Estimating the Nonaqueous‐Phase Liquid Content in Saturated Sandy Soil Using Amplitude Domain Reflectometry

Abstract

Groundwater contamination by nonaqueous‐phase liquids (NAPLs) has become a serious environmental issue. Therefore, it is necessary to estimate the NAPL content (θNAPL) in saturated soil to detect and monitor the NAPL contamination in groundwater. The objective of this study was to investigate three estimation methods for the θNAPL in saturated sandy soil, namely, the noncalibration method (NCM), the permittivity regression method (PRM), and the best‐fit α method (BFαM). These methods utilize the permittivity (K) measured by amplitude domain reflectometry (ADR) and two dielectric mixing models: a theoretical Maxell–de Loor (MD) model and an empirical α model. The NCM applies the measured K to the models directly. The PRM applies the measured K to the models indirectly using the regression equation between the measured and estimated K The RMSEs of the NCM for the α model were 0.038 m3 m−3 for light NAPL (LNAPL) and 0.015 m3 m−3 for dense NAPL (DNAPL) when the constant α value was set at 0.5. The RMSEs of the MD model, however, were 0.090 m3 m−3 for LNAPL and 0.070 m3 m−3 for DNAPL. Using the PRM, the RMSEs of the MD model became much better than those of the NCM. The RMSEs for θNAPL estimated by the BFαM were 0.007 m3 m−3 for LNAPL and 0.018 m3 m−3 for DNAPL. These results demonstrate that θNAPL was estimated easily and with sufficient accuracy with the α model and the PRM.