two-dimensional electrical imaging for detection of ......a. godio1* and m. naldi2 1 politecnico di...

Near Surface Geophysics, 2003, 131-137

© 2003 European Association of Geoscientists & Engineers 131

Two-dimensional electrical imaging for detectionof hydrocarbon contaminants

A. Godio1* and M. Naldi2

1 Politecnico di Torino, Italy 2 Techga Servizi S.r.l – Torino, Italy

Received September 2001, revision accepted March 2003

ABSTRACT

The effects of a long-term diesel oil pollution due to leakage from buried tanks have beeninvestigated using electrical resistivity tomography. The reliability of 2D electrical resistivityimaging of the subsoil was assessed using a numerical modelling approach that simulated thedifferent behaviour of the contaminated zone. The effects of inversion parameters, such asthe damping factor and smoothing matrix, have been studied in order to evaluate the opti-mal parameters to process real data.

The results of the field test indicated that highly conductive anomalies can be related tothe biological degradation of hydrocarbons: geochemical analysis performed on severalgroundwater samples confirmed the presence of biodegradation activity. Chemical analysispointed out an anomalous concentration of iron and manganese cations dissolved in thegroundwater. Very low values of resistivity can be associated with a marked modification ofthe cation exchange capacity of the soil mixture due to degradation of these hydrocarbons.Chemical and physical interactions due to hydrocarbon pollution affect the electrical prop-erties of soils and groundwater.

INTRODUCTION

In the site characterization of contaminated land, the costsand environmental impact of drilling numerous boreholesmake it desirable to develop less costly and more environ-mentally friendly techniques. Many non-invasive geophysi-cal methods have recently been applied to reduce theadverse impact of drilling on the environment and to lowerthe costs of site characterization, monitoring and remedia-tion. Electrical Resistivity Tomography (ERT) is a geophys-ical technique that has been successfully applied to monitorcontaminated groundwater (Barker 1996). We describe itsuse in investigating leachate plumes caused by leakage fromunderground storage tanks that contain hydrocarbons.

Geoelectrical and electromagnetic methods have beenwidely employed in environmental geophysics over the last10 years: high-resolution electrical methods have beendeveloped to detect and monitor leachate in landfills, tomap the waste distribution in controlled and abandonedlandfills and to detect organic and inorganic contaminants

in soils and groundwater. There is an increasing interest inthe use of high-resolution resistivity surveys to investigatecontaminants, particularly concerning the plume migration.

The behaviour of the electrical resistivity of contami-nants, with respect to the host environment, depends on sev-eral factors, such as the host lithology, the soil moisture andthe solubility of the contaminants in the groundwater. Fororganic contaminants that are not miscible with water, suchas diesel fuel and gasoline, the complexity of chemico-phys-ical reactions with the host environment makes it unrealis-tic to determine a typical range of resistivity which couldenable differentiation of the plume from its surroundings.These phenomena can significantly affect the electrical andelectromagnetic properties of the soil and groundwater andmake the clear and unambiguous detection of contaminantsvery difficult. Many papers have reported seemingly contra-dictory results on the electrical and electromagneticresponse of different contamination events. Mazac et al.(1990) pointed out the shortcomings of surface electricalmethods in the detection of thin layers of the free hydrocar-bon phase above the water table, although borehole investi-gations and tomographic data processing can overcome this*[email protected]

A. Godio and M. Naldi132

© 2003 European Association of Geoscientists & Engineers, Near Surface Geophysics, 2003, 1, 131-137

problem under some conditions (Morelli and LaBrecque1996; Godio and Morelli 1998). The electromagneticresponse at radio frequency is complicated by the possibleeffects of the vapour phase and by the different behaviourof the contamination in the vadose zone and underlyinggroundwater zone (Benson 1995). Geophysical methods areunable to detect some contaminants at the low concentra-tions of parts per billion that are of regulatory concern,although toluene can sometimes be detected with the com-plex resistivity technique, due to its chemical reactions withclay minerals. Complex electrical tomography has beenreported to be successful in the detection of diesel pollutionin the vadose zone (Kemna et al. 1999) even though greatcare must be taken to avoid electromagnetic couplingeffects on the experimental data. Perhaps, the best way ofidentifying contaminants in small concentrations is bydetecting their movement through repeated time-lapse geo-physical measurements (Barker and Moore 1999).

Hydrocarbons usually show very high resistivity values,but the electrical behaviour of hydrocarbons in soil or ingroundwater can be affected by biodegradation. This phe-nomenon usually leads to an increase in the conductivity ofthe aquifer (Atenkawa et al. 1998). This leads to remarkablevariations in the dissolved ions in the fluid phase of the sub-soil. For example, high concentrations of dissolved iron areassociated with benzene biodegradation coupled to ironreduction. On the other hand, biodegradation of the ben-zene consumes sulphate and reduces the total amount offree sulphate ions. Contaminated soils and groundwaterusually show very low resistivity values due to the imbal-ance of the sulphate reduction and the production of dis-solved iron.

In this complex scenario, our experience has shown thatpollution caused by LNAPL (light non-aqueous phase liq-uid) hydrocarbons (diesel oil) can be investigated usingelectrical tomography. In a study of an industrial site wherehydrocarbon contamination was thought to be a problem,several electrical imagings were carried out to verify thepossible leakage of diesel oil from three underground stor-age tanks into the subsoil. The study involved geochemicaland geophysical investigations of the industrial site.

GEOLOGY OF THE SITE AND DATA ACQUISITION

The investigated site is located in the southern part of thealluvial plain of Turin (Italy).The upper part of the sedimen-tary sequence is built up of recent alluvial deposits (silty andsandy gravel with some rare clayey lenses) which overlie anolder fluvio-glacial sequence made up of silt and claydeposits with gravel. The phreatic water table is quite shal-low, located at a depth of about 5 m, and the flow directionis to the south-east.

The location of the ERT profiles were planned in orderto cross the groundwater flow direction in the industrialarea, taking the available space inside the industrial site intoaccount. Measurements were acquired by a Syscal Jr instru-ment connected to lines of 48 electrodes, using the Wennerconfiguration. A small electrode spacing of 1 m was adopt-ed, in order to be able to provide considerable detail of anyplumes related to leakage from the underground tanks. Thelocations of some lines, close to the diesel oil leaking source,are shown in Fig. 1.

It is well known that the Wenner array is relatively sensi-tive to vertical changes of resistivity below the centre of thearray, but is not so sensitive in delineating narrow verticalstructures. On the other hand, good quality data can beacquired in comparison with other conventional arrays innoisy areas. Therefore, the choice of the array was condi-tioned by the compromise between spatial resolution andthe need to acquire good quality data in an area with poten-tially high background noise due to the presence of theasphalt coverage and man-made structures.

CONSIDERATIONS ON THE ELECTRICALRESPONSE OF HYDROCARBON-CONTAMINATEDSOIL

Different physico-chemical behaviour can be determinedfrom the interference of hydrocarbon pollutants and thesubsoil. It is useful to simplify the electrical response of thehydrocarbon pollution of the subsoil using two differentresistivity models. In the first case, the contaminant plumeprovides a very high conductive zone in the saturated andvadose zone, because of the presence of strong naturalbioactivity. The second reference model considers a thincontaminated layer above the water table, simulated by ahighly resistive layer (free hydrocarbon phase).

FIGURE 1

Plan view of the site with location of the ERT lines.

Two-dimensional electrical imaging for detection of hydrocarbon contaminants 133


The electrical conductivity of the groundwater can beapproximated by summing up the contribution of the differ-ent ions multiplied by their mobility, according to the rela-tionship:

(1)

where Ci is the number of gram equivalent weights of the ithion per cubic metre of water and mi is the mobility of the ithion [m2/s V]. In the experimental test, the chemical analyses,performed on the specimens of groundwater, have indicatedthe presence of anomalous concentrations of iron, man-ganese and also zinc, for the PA borehole (Table 1). Theincrease in ions dispersed in the groundwater, caused bybiological activity, therefore, does not seem to justify the sig-nificant changes in resistivity that are highlighted by theelectrical tomography. This fact was fully analysed, accord-ing to the empirical relationship suggested by Waxman andThomas (1974), where the specific resistivity of shale andsands depends on the cation exchange capacity (CEC), asgiven by the following formula:

(2)

where ρ0 is the bulk resistivity, ρw the fluid resistivity, F is theformation factor, and the parameter B describes the slightinterface conductivity dependence on the ion content, givenby

(3)

The parameter Qv is related to the cation exchange capacity(CEC), according to

(4)

where ds is the grain soil density (g/cm3) and φ is the poros-ity, CEC is given in milliequivalents per 100 g of soil matrix.The effects of CEC on the resistivity of the soil is shown inFig. 2.

In the site experiment, it was possible to neglect the

effect of the double layer on the interface conductivity ofclay because of the absence of clay minerals in the soilskeleton; the soil consists mainly of sandy material. On theother hand, a high CEC can be related to the organic mat-ter that is present and which is enhanced by the biologicalactivity in the contaminated soil.

The hydrocarbon contamination, trapped around the soilgrains, and the biodegradation activity lead to:• a decrease in the effective porosity of the pore water con-

ductivity and an increase in the CEC of the organic mat-ter of the soil; these effects cause an increase in the valueof the Qv parameter;

• a decrease in the fluid conductivity rw as a consequenceof the increasing ion content, which accounts for theincrease of the B value that appears in formula (3);

• and finally, the increase in Qv and B values and thedecrease in the fluid conductivity lead to the decrease inthe resistivity according to formula (2), which is only par-tially balanced by the variations in the formation factorvalue, due to the decrease of the interconnected porosity.

The influence of different factors on the inversion proce-dure was analysed to verify the reliability of the electricalresistivity images. In particular, we wanted to analyse theinfluence of the presence of a thick layer that was character-ized by high resistivity values (the asphalt layer) and theeffect of abrupt changes in resistivity between the unconta-minated and the contaminated soil and the groundwater.The accuracy of our data is better than 3%; bad measure-ments with a noise level higher than 3% were eliminated(about 5% of the data points was excluded). The effects ofinversion parameters on the final results were also analysed.The standard smoothness-constrained least-squares method

TABLE 1 Anomalous concentrations of contaminants in the

groundwater

Particle P1 P2 P3 P4 PAppb ppb ppb ppb ppb

Iron 76 160 260 89 53Manganese 175 541 786 88 37,2Zinc 1,01 3,36 2,49 1,28 878Dissolved hydrocarbons < 3 35,4 < 3 < 3 < 3

FIGURE 2

Resistivity response versus fluid resistivity for different CEC values of the

soil.



is based on the solution of the following data set of equa-tions (Loke 1995):

(5)

where λ is the Lagrangian multiplier or damping factor,used to stabilize the solution of the ill-posed tomographicproblem, J is the partial derivative matrix of the potentialfunction with respect to the model resistivity parameter, C isthe smoothing matrix; vector ∆p is the perturbation of themodel parameters for each iteration and ∆d is the vectorthat contains the differences between the observed andcomputed apparent resistivity values. The smoothing matrixC enables constraints to be introduced on to the vertical orhorizontal elongation of the structures.

The effects of the tomographic inversion reliability wereexplored using a numerical simulation approach (Sasaki1992) that considered the effect of the damping factor andthe array configuration on the quality of the inversionresults. The damping factor leads to a stabilization of thesolution but produces a smoothed resistivity model: asmoothed section provides for a realistic solution where agradual change in contamination or a different effect of thenatural attenuation can be expected. On the other hand, alow damping factor should offer a better interpretable sec-tion in the case of high contrast between the free phase ofhydrocarbon and the uncontaminated groundwater. A near-surface highly resistive layer was considered to approximatethe asphalt coverage; the uncontaminated aquifer was char-acterized by a resistivity of 50 Ωm and the contaminatedzone was considered to be very conductive (2 Ωm), accord-ing to the above theoretical formulation.

The model and the pseudosection of apparent resistivity,as obtained from the finite-difference forward computation(Dey and Morrison 1979), are shown in Fig. 3. A normallydistributed noise of 5% was introduced into the syntheticdata and the least-squares smoothness-constrained analysiswas performed, according to relationship (5), using differentvalues of damping factor to stabilize the solution. It was ver-ified that a damping factor in the range of 0.01 to 0.25 doesnot affect the resolution of the inverted section to any greatextent and provides very similar final models with an rmsmisfit of about 5%. The conductive anomaly appears to bespatially well resolved, as is the resistivity contrast betweenthe contaminated and clean soil (Fig. 4).

In the second model, a free phase hydrocarbon isassumed to be above the aquifer without any relevant min-eralization effects (biodegradation). In such a context, theshallow aquifer is contaminated by a non-miscible hydrocar-bon phase and by a small amount of the total volume ofhydrocarbon dissolved in groundwater. Only the LNAPL(light non-aqueous phase liquid) contamination was consid-ered as the free phase hydrocarbon; this moves horizontally

above the water table, according to the hydrogeological con-dition of the area. The thickness of the free phase contami-nant above the water table is less than a few centimetres.

The formulation proposed by Worthington (1976), andlater discussed by Mazac et al. (1990), considers the resistiv-ity response of the aquifer, fully saturated by uncontaminat-ed groundwater, oil or a non-miscible mixture of these. Thematrix resistivity value includes the influence of the oil-wet-ting or water-wetting phase in addition to the influence ofthe resistivity of the soil skeleton.

In a more general sense, the resistivity of the fluid phaseincludes the cation exchange capacity effects on the resistiv-ity of the medium due to the interaction between the solidmatrix and the mixture of the fluid phases.The aquifer resis-tivity (ρa) depends on the soil matrix resistivity (ρm) and thefluid resistivity (ρf). It is expressed by the relationship(Mazac et al. 1990),

(6)

FIGURE 3

Model (bottom) and synthetic resistivity (top) for a Wenner array - 48 elec-

trodes, assuming that the hydrocarbon contamination is characterized by a

conductive response (resistivity values in Ωm).

FIGURE 4

Inversion of synthetic resistivity data with 5% of normally distributed noise

λ = 0.25–0.1 and slow horizontal constraints for the model in Fig. 2.



where φ is the soil porosity, while a and n are experimentalparameters that depend on the textural and structural con-dition of the aquifer; the resistivity of the free phase of thehydrocarbon assumes values above 106–108 Ωm.

In the numerical model, the following parameterswere inferred: the hydrocarbon free phase has a resistivity> 5·105 Ω⋅m, the uncontaminated groundwater has a resis-tivity of 100 Ω⋅m and the solid matrix has a resistivitybetween 100 and 10 000 Ωm; the soil porosity is 0.3. As canbe seen in Fig. 5, the difference in the resistivity of the con-taminated layer compared to the uncontaminated soil isnegligible for any matrix resistivity below 200 Ωm. A modelwas determined with a thick overburden layer (asphalt cov-erage at 500 Ωm); the vadose zone was characterized by 300Ωm and the lens of the LNAPL on the water table has aresistivity value of 400 Ωm, as estimated by equation (6)using a=1 and n=2.

The model and the pseudosection of the apparent resis-tivity are shown in Fig. 6, as obtained through finite-differ-ence forward computation. A normally distributed noise of5% was introduced into the synthetic data and the least-squares smoothness-constraints inversion was performed.

The data inversion was carried out with differentsmoothing matrices and with different damping factors inthe range between 0.1 and 0.5. The selected simulation pre-sented here (Fig. 7) considers a damping factor of 0.25 andconstraints on the vertical changes of resistivity. It was notedthat the resistive layer, which represents the hydrocarbonpolluted zone, cannot be resolved in a medium resistiveenvironment; only a small apparent depression of the water-table level is shown. Obviously, the resistivity value of thecontaminated layer above the water table is not wellresolved and the resistive structure is not horizontally elon-gated, as expected.

EXPERIMENTAL RESULTS

The data processing involved a tomographic inversion usinga high damping factor and constraints on the verticalchanges of resistivity. The results are depicted in four select-ed tomographic sections which are presented in Figs 8–11.The tomographic sections confirm the presence of a surfacelayer with high resistivity that covers the whole area, whichrefers to the asphalt coverage and the coarse gravel andsand level, as confirmed by a subsequent drilling.

The electrical response attributed to the diesel oil release(Figs 8 and 9) is indicated by the high conductivity anomalyin the central zone of the images. This result agrees with thesuggestion that the oil release started several years beforethe investigation. The biodegradation occurred later on andmodified the electrical response of the contaminated zone.

In order to verify the geophysical response, a series of

FIGURE 5

Response of the fully saturated medium versus resistivity of the solid matrix

for different values of fluid resistivity (resistivity values in Ωm).

FIGURE 6

Model (bottom) and synthetic resistivity data (top) for a Wenner array - 48

electrodes, assuming a free phase hydrocarbon contamination as a highly

resistive layer on the water table (resistivity values in Ωm).

FIGURE 7

Inversion of synthetic resistivity data with 5% of normally distributed noise

λ = 0.25–0.1 and low horizontal constraints for the model in Fig. 5.



boreholes was drilled on the site. Soil and water samplestaken from the boreholes were analysed for contaminantdetection. These have confirmed the detection of the plumedefined by electrical tomography (Line 1 and Line 2). Thetwo boreholes (P1 and P2) indicated traces of contaminantswhile no trace of pollution was found away from the mainplume. The results of the chemical analyses have indicated alow concentration of hydrocarbons and an anomalous highcontent of iron (Table 1). The analysis of soil samples indi-cated the presence of TPH in a concentration of 200–250mg/kg in the boreholes close to the tanks at a depth of 3–5m below the surface; a low concentration of dissolvedhydrocarbon was detected in the groundwater samples (35mg/kg). The conductivity of uncontaminated groundwater is350–400 µS/cm; values of 1000–1200 µS/cm were measuredfor groundwater in the boreholes in the leaking zone. Highvalues of dissolved oxygen were measured in the boreholeswhere high conductivity values were pointed out, whereasthe measurements in boreholes outside the plume revealedlow dissolved oxygen values. The anomalous concentrationof iron and manganese values cannot be explained by directcontamination due to the industrial activity and confirmed abacterial process involved in the degradation of hydrocar-bons.

In the authors’ opinion, only the effects of the mineral-ization of the diesel oil can be easily detected by electricalimaging, as this produces an extremely conductive response.On the other hand, the response of a thin layer of diesel oillying above the groundwater table and not involved in thebiodegradation process cannot be detected, as confirmed bythe numerical simulation. When this is the case, electricalimages can lead to a false evaluation of the subsoil volumeaffected by pollution.

CONCLUSIONS

The study pointed out the usefulness and the pitfalls of elec-trical tomography in the characterization of undergroundleakage of hydrocarbons. Experimental evidence, obtainedfrom a joint geochemical and geophysical investigationapproach, indicated that subsoil which has been saturatedwith diesel oil for a long period (> 20 years) exhibits anincreased conductivity. It suggests that electrical tomogra-phy could be useful for monitoring the effects of inducedbiodegradation (bioremediation) through the repetition ofthe survey at different times, in order to observe anychanges in the resistivity due to the increase of free ionsresulting from hydrocarbon degradation. The strong con-ductivity anomaly, attributed to the hydrocarbon pollutionzone, has been explained by increasing the organic activityand modification of the cation exchange capacity of the soilmatrix. However, great care should be taken in the data pro-

FIGURE 8

Electrical resistivity tomography – Line 1, showing positions of the boreholes.

FIGURE 9

Electrical resistivity tomography – Line 2, showing positions of the boreholes.

FIGURE 10 Electrical resistivity tomography – Line 6.

FIGURE 11

Electrical resistivity tomography – Line 7 in the uncontaminated zone.



cessing and interpretation in order to avoid misinterpreta-tion of the effects of changes in soil moisture or pitfalls dueto near-surface conductive bodies.

Numerical simulation permitted the effects of the inver-sion procedure on the resolution to be evaluated using asmoothing least-squares approach with a variable dampingfactor. It was verified that, in the case of long-term hydro-carbon pollution in aerobic conditions, when the effects ofbiodegradation activity can be estimated (mineralization ofhydrocarbon, changes in CEC properties of the soils), thecontaminated zone appears to be characterized as a highlyconductive zone. In the present case, a smoothed solutionusing a high damping factor and smoothing matrix would bepreferable.

Furthermore, under anaerobic conditions, where thebiodegradation process is not so pronounced, it should beconsidered that the LNAPL, approximated by a resistivelayer on the water table, can lead to misinterpretation of theERT results.

ACKNOWLEDGEMENTS

We are greatly indebted to Ron Barker for his previous sug-gestions and for having encouraged us to improve the qual-ity of the work. We thank A. Binley and A. Olayinka fortheir suggestions during the revision of the manuscript.

REFERENCES

Atenkawa E.A., Sauck W.A. and Werkema Jr. D.D. 1998.Characterization of a complex refinery groundwater contamina-tion plume using multiple geoelectric methods. Proceedings ofthe Symposium on the Application of Geophysics toEnvironmental and Engineering Problems, EEGS, Chicago, pp.427–436.

Barker R.D. 1996. Electrical imaging and its application in engi-neering investigations. In: Modern Geophysics in EngineeringGeology (ed. D. McCann). Special publication of the GeologicalSociety.

Barker R.D. and Moore J. 1999. The application of time-lapse elec-trical tomography in groundwater studies. The Leading Edge 17,1454–1458.

Benson A.K. 1995. Applications of ground penetrating radar inassessing some geological hazards: examples of groundwater con-tamination, faults, cavities. Journal of Applied Geophysics 33,177–193.

Dey A. and Morrison H.F. 1979. Resistivity modelling for arbitraryshaped two-dimensional structures. Geophysical Prospecting 27,1020–1036.

Godio A. and. Morelli G. 1998. Mapping of complex hydrocarbonscontaminant using geoelectric and electromagnetic methods.Proceedings of the 4th Meeting of Environmental andEngineering Geophysical Society (EEGS, European Section),Barcelona, Spain, pp. 19–22.

Loke M.H. and Barker R.D. 1995 Least-squares deconvolution ofapparent resistivity pseudosections. Geophysics 60, 1682–1690.

Kemna A., Dresen L. and Raekers E. 1999. Field applications ofcomplex resistivity tomography. 69th SEG meeting, Houston,Texas, Expanded Abstracts, 331–334.

Mazac O., Benes L., Landa I. and Maskova A. 1990. Determinationof the extent of oil contamination in groundwater by geoelectri-cal methods. Geotechnical and Environmental Geophysics:Environmental and Groundwater 2, 107–112.

Morelli G. and LaBrecque D.J. 1996. Advances in ERT inversemodeling. European Journal of Environmental and EngineeringGeophysics 1, 171–186.

Sasaki Y. 1992. Resolution of resistivity tomography inferred fromnumerical simulation. Geophysical Prospecting 40, 453–463.

Waxman M.H. and Thomas E.C. 1974. Electrical conductivity inshaly sands. Journal of Petroleum Technology, 213–225.

Worthington P.F. 1976. Hydrogeophysical equivalence of watersalinity, porosity and matrix conduction in arenaceous aquifers.Ground Water 14, 224–232.

two-dimensional electrical imaging for detection of ......a. godio1* and m. naldi2 1 politecnico di...

Documents