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SOIL PHYSICS

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

^a Graduate School of Environmental Science, Okayama Univ., 3-1-1 Tsushima-naka, Okayama, 700-8530, Japan
^b Currently at: Nikka-Engineering Corp., 3-12-4 Namiyoke, Minatoku, Osaka, 552-0001, Japan

^* Corresponding author (morot{at}cc.okayama-u.ac.jp).

Groundwater contamination by nonaqueous-phase liquids (NAPLs) has become a serious environmental issue. Therefore, it is necessary to estimate the NAPL content (_NAPL) 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 _NAPL in saturated sandy soil, namely, the noncalibration method (NCM), the permittivity regression method (PRM), and the best-fit method (BFM). 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 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 model were 0.038 m³ m^–3 for light NAPL (LNAPL) and 0.015 m³ m^–3 for dense NAPL (DNAPL) when the constant 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 _NAPL estimated by the BFM were 0.007 m³ m^–3 for LNAPL and 0.018 m³ m^–3 for DNAPL. These results demonstrate that _NAPL was estimated easily and with sufficient accuracy with the model and the PRM.

Abbreviations: ADR, amplitude domain reflectometry • BFM, best-fit method • DNAPL, dense nonaqueous-phase liquid • E, coefficient of efficiency • HFE, hydrofluoroether • LNAPL, light nonaqueous-phase liquid • MD, Maxell–de Loor • NCM, noncalibration method • NAPL, nonaqueous-phase liquid • PRM, permittivity regression method • TDR, time domain reflectometry