A four-parameter-based thermo-TDR approach to estimate water and NAPL contents of soil liquid

Abstract

In-situ determination of soil non-aqueous phase liquid content (θNAPL) is necessary for early detection of soil NAPL contamination and preventing the spread of the contamination. Thermo-time domain reflectometry (thermo-TDR), which can simultaneously measure dielectric constant, electrical conductivity, volumetric heat capacity, and thermal conductivity, has the potential to estimate θNAPL. The objectives of the study are i) to establish a relationship between the four thermo-TDR measured soil properties and soil water content (θw) and θNAPL values and ii) to evaluate the sensitivities of the thermo-TDR measured properties to θw and θNAPL, and iii) to develop a four-parameter based approach to simultaneously determine θw and θNAPL. Thermo-TDR measurements were performed on sand and glass beads containing various amounts of water and Canola oil as a NAPL. In all cases θw rather than θNAPL dominated all four thermo-TDR measured properties. A sensitivity analysis also indicated that all four properties were more sensitive to θw than to θNAPL. Among the four properties, the dielectric constant, electrical conductivity, and volumetric heat capacity were somewhat sensitive to θNAPL, while thermal conductivity was not sensitive. A new approach using all four thermo-TDR measured properties (four-parameter-based approach) to estimate θNAPL was found to be more accurate than the existing two property-based approaches, i.e., dielectric constant and volumetric heat capacity-based. The root mean square error (RMSE) values for θNAPL estimation with the four-parameter-based approach were 0.066 and 0.042 m3 m−3 for sand and glass beads, while the two property-based approach had RMSE values of 0.180 and 0.220 m3 m−3. The four-parameter-based approach enabled suppression of the effects of measurement errors by the optimization processes and allowed the high sensitivity parameters to cover for shortcomings in the low sensitivity parameters. Use of thermo-TDR sensors with the four-parameter-based approach to determine θNAPL can contribute to various NAPL soil contamination studies as a NAPL content quantifying approach.