Spectral induced polarization for the characterization of free‐phase hydrocarbon contamination of sediments with low clay content

Abstract

The identification of organic pollutants in the soil and the subsurface is a goal of primary importance in the management of contaminated sites. However, only a few non-invasive techniques can be useful towards this goal. One such technique is spectral induced polarization. In this study, we investigate the spectral induced polarization effect of changing fluid saturation in a well-characterized porous medium, analysing the difference between air and hydrocarbons, at different degrees of water saturation. The experiments were conducted on fine colic sand samples coming from an experimental site near Turin, Italy. Octanol and benzene were used as non-aqueous phase liquids. Samples were initially saturated with water having controlled electrical conductivity and Subsequently de-saturated stepwise with injection of air at known pressure. The colic sand samples were then re-saturated with the same water contaminated with hydrocarbons and then a non-aqueous phase liquid phase (either octanol or benzene) was injected in volumetric steps, in order to compare the effects of air and non-aqueous phase liquid invasion. At each saturation step, spectral induced polarization measurements were conducted in the 0.01 Hz to 1 kHz range using the ZEL-SIP04 impedance meter developed at the Forschungszentrum Juelich. The measurement setup guaranteed a 1 mrad phase precision for the entire frequency measurement range. Measurements were conducted under temperature controlled conditions at 20 (+/- 0.5)degrees C. All spectral induced polarization curves show a peak in the range 0.01-1 Hz that changes in intensity and frequency with varying saturation and a high-frequency phase shift increase dominated by capacitive coupling effects of the measuring system. A Multiple Cole-Cole model was fitted to the data. The effects of de-saturation on the low-frequency Cole-Cole parameters are that a) resistivity increases with decreasing water saturation but increases less with non-aqueous phase liquid than with the same volume of air; b) chargeability increases with decreasing water saturation but in presence of non-aqueous phase liquids its value is sometimes lower, sometimes higher and sometimes similar to the one observed in presence of air; c) the time constant tau increases with decreasing water saturation and is consistently larger with non-aqueous phase liquid than with air. These differences between air and non-aqueous phase liquid injection can be explained in terms of differences in non-aqueous phase distribution within the porous medium, as observed by X-ray micro-CT: while air is homogeneously distributed, non-aqueous phase liquids segregate under density effects. In summary, all spectral induced polarization effects of air and non-aqueous phase liquid injection in the considered porous medium are volumetric, i.e., are not due to interaction with grain surfaces or other electrical-chemical effects but are caused by pore obstruction by the electrically non-conductive phase

Monitoring and modelling of soil–plant interactions: the joint use of ERT, sap flow and eddy covariance data to characterize the volume of an orange tree root zone

“…At each de-saturation step, the electrical conductivity of the sample was measured under temperature-controlled conditions using a ZEL-SIP04 impedance metre (Zimmermann et al, 2008). A completed description of the set-up is given by Cassiani et al (2009b). Figure 6 shows two example experimental results on samples from two different depths.…”

“…Daily et al, 1992), from catchment (e.g. Weill et al, 2013) and hillslope characterization (Cassiani et al, 2009a) to agriculture and eco-hydrological processes Ursino et al, 2014).…”

Influence of non-aqueous phase liquid configuration on induced polarization parameters: Conceptual models applied to a time-domain field case study

“…This has been observed in a number of lab and field studies on different kinds of NAPLs (e.g. Chambers et al, 2004;Cassiani et al, 2009;Naudet et al, 2014;Power et al, 2015). However, at field sites, the situation is complicated by the fact that the age and degradation status of the contaminants change the groundwater chemistry.…”

Review: Some low-frequency electrical methods for subsurface characterization and monitoring in hydrogeology

“…Vanhala (1997) showed that the introduction of motor oil decreases the magnitude of the phase of glacial sediments (sands and tills). Cassiani et al (2009) performed an investigation showing the effect of the saturation of hydrocarbons upon induced polarization (using a non-wetting oil) but they investigated only a few extreme values in the oil relative saturation (volumetric fraction of the pore space occupied by the oil phase). Figure 20 shows a cross-borehole imaging result from the Strasbourg-Entzheim site showing the in-phase (real) component, the quadrature (imaginary) component, Fig.…”