Time domain reflectometry-measuring dielectric permittivity to detect soil non-acqeous phase liquids contamination-decontamination processes

Abstract

Contamination of soils with non-aqueous phase liquids (NAPL) constitutes a serious geo-environmental problem, given the toxicity level and high mobility of these organic compounds. To develop effective decontamination methods, characterisation and identification of contaminated soils are needed. The objective of this work is to explore the potential of dielectric permittivity measurements to detect the presence of NAPLs in soils. The dielectric permittivity was measured by Time Domain Reflectometry method (TDR) in soil samples with either different volumetric content of water ( w) and NAPL ( NAPL) or at different stages during immiscible displacement test carried out with two different flushing solutions. A mixing model proposed by Francisca and Montoro, was calibrated to estimate the volume fraction of contaminant present in soil. Obtained results, showed that soil contamination with NAPL and the monitoring of immiscible fluid displacement, during soil remediation processes, can be clearly identified from dielectric measurements. IntroductionSubsurface contamination of soil and groundwater with organic compounds from waste disposal sites, industrial spills, gasoline stations, mine tailings and industrial processes constitutes a serious geoenvironmental problem. The detrimental effects are limited not only to deterioration of chemical, physical and mechanical properties of soils, but also constitute a real risk to human health and the well-being of other living species.Non-aqueous phase liquids (NAPLs), are organic compounds immiscible with water. They have low solubility that may still be several orders of magnitude higher than that of acceptable drinking water standards. NAPLs can be further subdivided into those that are denser than water (DNAPLs) and those that are lighter than water (LNAPLs). Chlorinated solvents such as trichloroethylene (TCE) and tetrachloroethylene (PCE) and polychlorinated biphenyl oils (PCBs) are common examples of DNAPLs. Hydrocarbon fuels such as gasoline, kerosene and jet fuels are common LNAPL contaminants which pollute the environment extensively (Illangasekare, 1998;Jury and Horton, 2004).Following a near-surface release, NAPLs penetrate the subsurface as an immiscible oil phase that migrates in response to gravity and capillary forces. This results in substantial sensitivity to the local distribution of soil and aquifer properties (e.g. permeability and porosity) beneath the source (Gerhard et al., 2007). As a result, the NAPL body (e.g. the source zone) is often expected to exhibit a complex heterogeneous distribution of both mobile pools (i.e. connected-phase accumulations) and immobile residuals (i.e. disconnected blobs and ganglia (Mercer and Cohen, 1990).The remediation of contaminated soil sites requires knowledge of the contaminant distribution in the soil profile and groundwater. Methods commonly used to characterize contaminated sites are coring, soil sampling and the installation of monitoring wells for the collection of groundwater samples (Mercer and Cohen, 1...

A soil non-aqueous phase liquid (NAPL) flushing laboratory experiment based on measuring the dielectric properties of soil–organic mixtures via time domain reflectometry (TDR)

“…Dielectric mixing models, in their classical application, have been proposed to estimate the bulk dielectric permittivity of a multi-phase medium, that is, a combination of three or four dielectric phases, and to couple the dielectric permittivity of the medium to the dielectric permittivity of each single phase (Hilhorst, 1998). Recently, after analyzing the effects of organic contaminants on soil dielectric properties, the above models were further developed to estimate the dielectric properties of NAPL-polluted soils (Redman et al, 1991;Persson and Berndtsson, 2002;Francisca and Montoro, 2012;Comegna et al, 2013aComegna et al, , 2016Comegna et al, , 2017. Based on such models, in the present study, we analyze the possibility of predicting the correlations between the volumetric contents of NAPL (θ NAPL ) and the dielectric response (ε b ) of contaminated soil during the progression of a steady-state remediation process.…”

“…Measurements of both dielectric permittivity and electrical conductivity allowed a method to be developed (the two-step method) which measured the dielectric properties of the system against the amount of NAPL in soils. Comegna et al (2016) developed a general TDR-based methodology for evaluating the correlations between the dielectric response and the NAPL content in variable saturated soils with different textures and pedological characteristics.…”