Nondestructive Analysis of OilContaminated Soil

Authors: Yoshito Nakashima Yuji Mitsuhata Junko Nishiwaki Yoshishige Kawabe Shin Utsuzawa Motoharu Jinguuji

Publish Date: 2010/05/22

Volume: 214, Issue: 1-4, Pages: 681-698

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Abstract

Nondestructive measurements of contaminated soil core samples are desirable prior to destructive measurements because they allow obtaining gross information from the core samples without touching harmful chemical species Medical Xray computed tomography CT and timedomain lowfield nuclear magnetic resonance NMR relaxometry were applied to nondestructive measurements of sandy soil core samples from a real site contaminated with heavy oil The medical CT visualized the spatial distribution of the bulk density averaged over the voxel of 031 × 031 × 2 mm3 The obtained CT images clearly showed an increase in the bulk density with increasing depth Coupled analysis with in situ timedomain reflectometry logging suggests that this increase is derived from an increase in the water volume fraction of soils with depth ie unsaturated to saturated transition This was confirmed by supplementary analysis using highresolution microfocus Xray CT at a resolution of ∼10 μm which directly imaged the increase in pore water with depth NMR transverse relaxation waveforms of protons were acquired nondestructively at 27 MHz by the Carr–Purcell–Meiboom–Gill CPMG pulse sequence The nature of viscous petroleum molecules having short transverse relaxation times T2 compared to water molecules enabled us to distinguish the watersaturated portion from the oilcontaminated portion in the core sample using an M 0–T2 plot where M 0 is the initial amplitude of the CPMG signal The present study demonstrates that nondestructive core measurements by medical Xray CT and lowfield NMR provide information on the groundwater saturation level and oilcontaminated intervals which is useful for constructing an adequate plan for subsequent destructive laboratory measurements of coresContaminated soils confined in core sample tubes are often toxic and harmful It is thus desirable to be able to collect data on hydrological properties or contaminated intervals by nondestructive measurements of sealed cores without touching harmful chemical species The collected data are useful for planning subsequent destructive measurements of the cores such as gas chromatography We propose the use of two nondestructive measurement methods medical Xray computed tomography CT and timedomain lowfield nuclear magnetic resonance NMR relaxometry Xray CT visualizes the threedimensional 3D distribution of the bulk density within the soil core samples Petrovic et al 1982 Because the bulk density is significantly different between saturated and unsaturated porous soils the degree of water saturation can be detected by Xray CT Crestana et al 1985 Anderson et al 1988 Tollner and Verma 1989 Thus CT enables tracing the increase in the water volume fraction in soil core samples with increasing depth which is important hydrological information for the site Although some previous CT studies on soil cores exist Peyton et al 1992 Fohrer et al 1999 Jassogne et al 2007 Taina et al 2008 there are very few studies on the application of CT to meterscalelong soil cores sampled from real vadose zones In the present study we performed extensive 3D medical CT imaging of real cores total length as long as ∼20 m to demonstrate the successful detection of changes in water volume fraction with depthTimedomain lowfield NMR relaxometry not imaging using permanent magnets eg Anferova et al 2004 is another promising nondestructive method for soil analysis Proton transverse relaxation time ie decay constant T2 of viscous oil molecules is significantly smaller than that of less viscous water molecules eg Dunn et al 2002 Thus it is possible to distinguish oilcontaminated soils from clean or watersaturated soils by the timedomain data analysis The basic idea for the use of lowfield NMR to analyze oilcontaminated soils was proposed by Hedberg et al 1993 Daughney et al 2000 van der Zwaag et al 2001 Hertzog et al 2007 and Bryar and Knight 2008 However there are few studies on applications to real oilcontaminated soil cores In the present study we performed extensive NMR measurements of the real cores contaminated with heavy oil the same ones analyzed by Xray CT to demonstrate the successful detection of oilcontaminated portions of the long coresOther extensive systematic measurements in addition to medical CT and NMR were carried out In situ measurements using a direct push system equipped with a fuel fluorescence detector FFD and electrical conductivity EC sensor as well as timedomain reflectometry TDR logging were performed at the contamination site Destructive measurement of the total petroleum hydrocarbon TPH using gas chromatographyflame ionization detector GC–FID was carried out in a laboratory to quantify the oil contamination Highresolution microfocus Xray CT eg Seignez et al 2010 was also performed to supplement the lowresolution medical CT The extensive data thus obtained using these other techniques were used to assist in the analysis and interpretation of data by medical CT and NMRPhotographs of the nondestructive measurement systems used a Gantry unit of the medical Xray CT system Four core sample tubes length ∼1 m outside diameter 44 mm from the A12 point are placed in the long urethane foam on the patient bed An example of a twodimensional CT image of the four core samples diameter 42 mm is shown rightward b Magnetic circuit unit using NeFeB and a radiofrequency coil box of the lowfield NMR system A core sample tube length ∼70 cm outside diameter 44 mm is inserted in the coil box The grey arrow is the direction of the static magnetic fieldThe CT images obtained show a spatial distribution of the linear Xray absorption coefficient having a dimension of the reciprocal of the length The linear Xray absorption coefficient is conventionally converted into a dimensionless CT number in Hounsfield units HU considering air and water to be standards eg Taina et al 2008 For example the linear Xray absorption coefficients of bulk samples of air water and quartz are −1000 0 and 1900 HU respectivelyAnother type of Xray CT microfocus Xray CT is useful to image microscopic pores Macedo et al 1998 that cannot be imaged by the lowresolution medical CT shown in Fig 1a We used a microfocus CT system SMX225S by Shimadzu Kyoto Japan The photo of the CT system is omitted because it has been published elsewhere Nakashima and Nakano 2008 A threedimensional cone beam Xray not a 2D fan beam from an Xray tube penetrates a rotating small sand sample of ∼1 cm3 and 3D CT images were reconstructed by a computer in a few hours The directivity of the Xray beam cone is tuned as fine as possible to achieve micrometerscalehigh resolution so that the photon flux of the Xray beam becomes small Thus it was impossible to penetrate a large soil core outside diameter 44 mm and resampling of the soil by opening the sealed core was needed Although microfocus CT imaging is not nondestructive porescale imaging by the microfocus CT was employed in the present study to confirm the predictions of the medical CT that the soil pore space is occupied by more water with increasing sampling depth of the core Highresolution CT images enable us to perform the quantitative porescale analysis eg Peth 2010 In the present study quantitative electric current computer simulation using the microCT images was carried out to confirm the image accuracy by comparing the simulated formation factor ie normalized resistivity of the watersaturated soils with the formation factor measured by the in situ electrical conductivity direct push method

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