Rev. Int. Contam. Ambie. 30 (3) 317-329, 2014

ELECTROMAGNETIC METHODS APPLICATION FOR CHARACTERIZING A SITE CONTAMINATED BY LEACHATES

Salvador Isidro BELMONTE-JIMÉNEZ1*, Alberto BORTOLOTTI-VILLALOBOS1, José Óscar CAMPOS-ENRÍQUEZ2, Marco Antonio PÉREZ-FLORES3, Omar DELGADO-RODRÍGUEZ4

and María de los Ángeles LADRÓN DE GUEVARA-TORRES1

1 Centro Interdisciplinario de Investigación para el Desarollo Integral Regional-Oaxaca, Instituto Politécnico Nacional, Hornos 1003, Col. Noche Buena, Santa Cruz Xoxocotlán, Oaxaca, México. C.P. 71230

2InstitutodeGeofisica,UniversidadNacionalAutónomadeMéxico,Coyoacán,04510,MéxicoD.F.,México3DepartamentodeGeofísicadeExploración,CentrodeInvestigaciónCientíficayEducaciónSuperiordeEn-senada,Carr.Ensenada-Tijuana,3918,ZonaPlayitas,Ensenada,BajaCalifornia,México.C.P.22860

4InstitutoMexicanodelPetroleo,EjeCentralLázaroCárdenasNorte152,SanBartoloAtepehuacan,07730,México DF., México

* Autor de correspondencia; salvador.belmonte.j@gmail.com

(Recibido diciembre 2013; aceptado junio 2014)

Keywords:contamination,geophysicalmethodsVLFandLIN,electricconductivity,landfill

ABSTRACT

Twoelectromagneticgeophysicalmethods,verylowfrequency(VLF)andlowinductionnumbercoils(EM-LIN)wereusedtoobtaintheresponsetothepresenceofleachatesfromawastedisposalsiteusedformorethan24years,coveringanareaof0.16km2. This landfillislocatedinfracturedshaleandsandstoneassociatedwiththeOaxacaFault.Thestudywasperformedonsixprofiles,fourofwhichwerecommontobothmethods,withlengthsof325,320,300and645m,intwoothersonlytheVLFmethodwasused.TheinterpretationofVLFdatausingtheHjeltandKarousfilterresultedincurrentdensitysections.Thecurrentdensityvariationwasassumedtoindicatethepresenceoffractures,alongwhichtheinfiltrationofleachatetakesplace.TheinterpretationofEM-LINdataprovidedtwo-dimensionalmodelsshowingthedistributionoftheconductivityofthesubsoil.Theintegrationoftheseresultsshowsamainconductiveanomalouszoneinthesoutheasternpartofthelandfillthatincreasesinthicknesstowardsthemiddleandwithadepthupto30-40m.Correlationwithnaturalsurfacerunoffenablestoinferthattheconductiveanomalousbodyindicatesthepresenceofleachates.Bothelectromagneticmethods provided a good response in fractured zones.

Palabrasclave:contaminación,métodosgeofísicosVLFyLIN,conductividadeléctrica,tiraderodebasura

RESUMEN

Seutilizarondosmétodosgeofísicoselectromagnéticos,frecuenciamuybaja(VLF,porsussiglaseninglés)ybobinasabajonúmerodeinducción(EM-LIN,porsussiglaseninglés), para estudiar la presencia de lixiviados de un sitio de disposición de residuos sólidosmunicipales,elcualoperadesdehacemásde24añosycuyasuperficieesde0.16km2.EltiraderoseencuentraenunmediocuyofracturamientoestáasociadoconlaFallaOaxaca.Elbasamentodeltiraderoestáconstituidoporlutitayarenisca.Elestudio

S.I. Belmonte-Jiménez et al.318

serealizóenseisperfilesdeloscualescuatrofueroncomunesparaambosmétodos,conlongitudesde325,320,300y645m,ydosmás,únicamenteusandoelmétodoVLF.LainterpretacióndelosdatosVLFutilizandoelfiltrodeKarousyHjeltdiocomoresultadoseccionesqueindicanaltadensidaddecorrienteinterpretadascomoasociadasafracturasquefavorecenlainfiltracióndeloslixiviados.LainterpretacióndelosdatosdeEM-LINbrindamodelosbidimensionalesquemuestranladistribucióndelaconductividaddelsub-suelo. La integración de estos resultados indica una zona anómala conductora principal en lapartesurestedeltiraderoque,deacuerdoasuformageométrica,seincrementahacialapartecentraldelmismoyseobservahastaunaprofundidaddeentre30y40m,coincidiendoconlazonapordondeocurreelescurrimientosuperficialnaturalalolargodelcualfluyenloslixiviados,yquehanaprovechadoalgunasfracturasparainfiltrase,infiriéndosequelas zonas anómalas detectadas son debidas a la presencia de estos contaminantes. Ambos métodos electromagnéticos han proporcionado buena respuesta en un medio fracturado.

INTRODUCTION

A studywas conducted in a landfill, locatedabout 15 km to the south ofOaxaca city, alongthe road fromOaxacaCity to PuertoAngel, inthe District of Zaachila (Fig. 1),wheremorethan500tonsofwastesperdayaredeposited.Thissiterepresents an important environmental problem because of the presence of leachates that seep into the ground, contaminating the surfacewater andthelocalaquifer.Duetotheneedforfreshwater,peopledig shallowwellsnear thisdump, riskinghealth problems as in other places (i.e., Adepelumi et al.2005,Samsudinet al.2006).ThepossibilitiesofcontaminationoftheaquiferinthelandfillareaofOaxacacityarehigh,sinceaccordingtostudiesconducted to evaluate vulnerability to ground-water contamination (Aragón et al. 2006), thetransitfactorofthelandfillinfiltrationrangesfrom 0.172 × 10–6to34.546×10–6/s i.e., over the maxi-mum value, 3 × 1010/s,establishedbytheMexican

OfficialStandard(NOM-083-SEMARNAT-2003inSEMARNAT 2013). This large transit ensures that leachates infiltrate the aquiferwith a significantcontaminant charge.

Inthissituation,anyactiontakentodiminishthepollutioncausedbyleachateswilllikelysupporttheeconomic development of the region.Fortunately,geophysics plays an important role by helping todeterminethegeometry,spatialextent,anddepthofa contamination plume (i.e., Soupios et al. 2007). Geophysicshasbeenusedtoexploretheundergroundanddetermineiftheleachatesoriginatedinlandfillsreachedthedepthlevelwheretheaquiferisfound(i.e.,BusquetsandCasas1995,Mondelliet al. 2007). Amongothersadvantages,geophysicalmethodsarefast and are non-destructive, less expensive than di-rectmethodsandprovideanoverviewofthestudyarea. Some authors likeCossu et al. (1991) and Soupios et al.(2007)reportthatgeophysicalmethodsare favorable for investigation of dumps because they usually containmaterials characterized by ahighelectricalconductivitythatcanbedetectedbygeoelectricalmethods.Karlik(2001)recommendeddirect current (DC) methods and VLF as tools for mappinggroundwatercontaminationand todeter-mine the extension of the contamination plume.

Inthispaperwereportontheuseoftwoelectro-magneticmethods,specifically,VLFandEM-LIN(workingatlowinductionnumberssimilartoEM-34, EM-31 andEM-38 equipments), to evaluatetheirresponseinafracturedzoneaffectedbyleach-ates.IntheEM-LINcase,measurementsweretakenin both horizontal and vertical loops modalities for thethreeseparationsbetweenthecoilsallowedbytheEM-34.Thisisequivalenttosixmeasurementsforeverysite,increasingtheinformationobtainedfrom the ground.VLFmeasurementswere takenatthreedifferentfrequenciesinordertoselectthe

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Scale 1:25 000

Legend

North

WellRiversLandfillUrban areaContoursRoads

Fig. 1. LocationmapofthelandfilloftheOaxacacity

EM METHODS APPLICATION FOR CHARACTERIZING A CONTAMINATED SITE 319

groundstationwiththebestcouplingwiththeun-derground structure.

Hydrogeology of the studied areaThegarbagelandfillislocatedintheAtoyacRiver

basin.Therunoffscoefficientsrangebetween10to20% and theyare favoredby the regional topographicalslope.Thedrainagenetworkconsistsofabasinsystemthroughwhichthesuperficialrunoffsflowintermit-tently.Inthissystemthelandfillplaysanimportantrole, since the leachatesmovepermanently to thesurfacedrainage.Thenumberofwellsintheareaislowandthemaingroundwaterflowdirectionistowardthesouthwest(Belmonteet al.2005).Theaquiferisfree,withthewatertablelocatedatdepthsbetween6to10minconsolidatedandstronglyfracturedrocks,sincedue theirorigin to the tectonicactivityof theOaxacaFaultthatcontrolstheregionalgroundwaterflow.Thereissecondarypermeabilityinsomeareas.

Electromagnetic prospection at low induction numbers

TheEM-LINmethod uses two loops, one assource and the other as receiver. The source circu-latesacurrentalongthecoiledwireatafrequencydependingonthedistancebetweensourceandre-ceiver (Table I).Thesourceinducesamagneticfieldwith thesamefrequency into theground. Ifbothloopsareoverawholespace,themeasurementispredictableandisnamedprimaryfield(Hp). When ahalfspaceispresent,asecondaryfieldwillappear(Hs). For a homogeneous half space, Hs isknownandtheratebetweenbothfieldsis(McNeill1980):

4

20 siw

HHP

S μ=

σ (1)

Where:

HS=Secondarymagneticfieldonthereceiver.HP=Primarymagneticfieldonthereceiver.

w = 2πf.f=FrequencyinHz.µ0=Free-spacepermeability.σ=Groundconductivityofthehomogeneoushalfspace (mS/m). s=Distancebetweencoilsinm.i = (–1)1/2

When the Earth is an inhomogeneous half-space, the conductivity in (1) becomes an apparent one.Usingthisequipment(EM-34,EM-31,andEM-38fromGeonicsandGEM-5fromGeoplex),wecanobtain the ground apparent conductivity from thefollowingequation:

20

4wsHHP

S

a μ=σ

(2)

Usingverticalorhorizontalcoplanarloops,wecangetprofilesofapparentconductivityatdifferentseparations (corresponding to several penetration depths).Suchprofilesgiveusaverybroadideaoftheconductivitydistributionintheground.

Inorder toobtain thedepth,geometryand trueconductivityoftheburiedbody,itisnecessarytoinverttheapparentconductivities.Inatwo-dimensionalEarth(2D), this problem can be posed in terms of the integral equationasthescatteringequations.However,weuseanotherintegralequationthatrelatesthemagneticfieldsmeasuredbythereceptorandthegroundconductivity(Gómez-Treviño1987).Applyingtheapproximationforlowconductivitycontrasts,theequationbecomes(Pérez-Flores1995,Pérez-Floreset al.2001):

( ) ( ) ( ) dxdzzxxxzxFxx

xx f f

i

z x

xa ,,,,,0 21

2121 =σ σ (3)

Where σaistheapparentconductivityobtainedfrom (2) that depends on the source (x1) and re-ceiver (x2) positions. Ifwe assume a half-spacediscretizedby a grid of rectangular prismswithcenter coordinates (x, z),andeveryprismhavinga constant conductivity (s), thenF constitutes a weighting function.Thismeans that everymea-surementisakindofavolumetricaverageoftheproductofs’sbythecorrespondingvalueofF. This function depends on the source and receiver posi-tions, the characteristics of the grid (dimensions) aswellasonthemagneticfieldsinducedbytheloops.Whentheloopsarehorizontallycoplanar,thisfunctionis:

TABLE I. ESTIMATED EXPLORATION DEPTH ACCORD-ING THE COIL SEPARATIONS AND FREQUENCY USED

Coilseparation

(m)

Usedfrequencies

(Hz)

Estimated exploration depth (m)

Horizontaldipole

Verticaldipole

10 6400 7.5 1520 2600 15 3040 600 30 60

S.I. Belmonte-Jiménez et al.320

( )( )( )

( )[ ] ( )[ ]dy

zyxxzyxx

xxxxy

xxzxF

++++

+

=

3222

2

32221

212

21,,,

(4)

Theprismshaveinfiniteextensioninthey direc-tion.This integralhasananalyticsolution(Pérez-Flores1995,Pérez-Floreset al. 2001).

Forverticalcoplanarloops,equation(4)ismorecomplex:

( ) ( ) ( )

( ) ( )

[

] dyxzyxExzyxE

xzyxExzyxExxzxF

yy

xx

+

=

21

2121

,,,,,,

,,,,,,,,, (5)

Where Ex and Eyaretheelectricfieldsinthex and y directions for both, source (x1) and receiver (x2). The expressions are developed and explained in Pérez-Flores(1995)andPérez-Floreset al. (2001).

Equation(3)canbeexpressedasalinearequa-tionsystem:

F=aσ σ (6)

Where isavectorwiththeapparentconductivi-ties, Fistheweightingmatrixand is the vector withthetrueconductivitiesoftheground(unknown).Thislinearsystemcanbesolvedinmanyways.Inthisworkweusedquadraticprogramming(Gillet al.1986)andsmoothingfactors for thenumericalstabilization.

VLF prospectingVLFinstrumentsarelightweightandportable,and

theycanbeusedtostudylargeareasquitequickly(Liu et al.2006).TheVLFmethodisbasedontheuseofverylowfrequencyradiowaves(intherangeof15to30kHz)forexplorationoffracturedzones,groundwater contamination andminerals (Jenget al.2004,Drahor2006,Duttaet al.2006,Ganerodet al.2006,Zlotnickiet al.2006,Kayaet al. 2007). It helps to determine the electrical characteristics of theundergroundandshallowrocks(HutchinsonandBarta 2002). There are VLF stations transmitting, for marine communicationprimarypurposes, electro-magneticsignalstravelingbetweentheionosphereand the Earth’s surface.

The signal emittedby the antennas around theworldcanbecapturedinthefieldbymeansofVLFinstruments, and according to the basic electromag-netictheory,atlongdistancesfromthesource,thewaveformapproachesaplanewave(Zlotnickiet al.

2006).Thereisarelationofprimarymagneticfield(Hp)andmagneticsecondaryfield(Hs)createdbyaconductivebodythatactsasasecondsource(Kayaet al. 2007). This means that electric currents in the conductingbody(e.g.,afracture)aregeneratedwhenradiowaves(EMfield)passthroughit,creatingan-othermagneticfield(Hs).

The presence of faults and fractures in a hy-drogeological system contributes to increase thehydraulicconductivityandporosityacting,inturn,asstructurescontrollinggroundwaterflow(SharmaandBaranwal2005,Adepelumiet al.2006).Thus,fracturesmay alsobecomepreferential pathwaysfortheflowofleachate,significantlyincreasingtheelectricalconductivityofthesubsurface(Mondelliet al. 2007, Soupios et al. 2007). Depth of penetra-tion depends largely on ground conductivity, butaccording toOskooitandPedersen (2005) this isless than 100 m.

The resulting vector from the sum of Hp and Hs producesatime-varyingellipticallypolarizedfield.Thisellipticalshapehastwocomponentswiththesamefrequency,butdifferentamplitudeandphase.The in-phase amplitude Hp is the real component, whiletheoutofphaseHpistheimaginarycomponentorquadraturecomponent(Ezeet al. 2004).

Theelectromagneticfieldequationforaconduc-tivemediumcanberepresentedby theHelmholtzequationderivedfromtheMaxwellequations:

=HEwi

HE

μ2 σ (7)

Where E and Harerespectivelytheelectricandmagneticfields,σ (mS/m) theconductivity,μper-meability(Henry/m)andωtheangularfrequency.

Incontrast,boththetiltangle(θ)andellipticity(e)arecalculatedusingtheformulaproposedbySmithandWard (1974, see also Sharma andBaranwal2005,andDuttaet al.2006).Oncesimplified,theyareexpressedas:

)cos(tan 1 senHHp

s= (8)

Where Hp istheprimaryfield,Hsisthesecondaryfield,ϕisthechangeofphasebetweenHp and Hs.

Hs is tilting, and α represents the angle above Hp duetothecouplingbetweenthetransmitterandtheundergroundstructure.Then,itisdefinedHs sen α =ΔHy,thusequation(8)becomes.

EM METHODS APPLICATION FOR CHARACTERIZING A CONTAMINATED SITE 321

)cos(tan 1

p

y

HH

= (9)

WhereΔHycosϕ = real component or in-phase of the Hsfield.

The tangent of the tilt angle is proportional to the Hsrealcomponent,whichismeasuredintheverti-cal direction. Therefore, the measurement of the tilt angleisverysimilartomeasuringtherealcomponent(in-phase) of Hs in the vertical direction.

VLFdatacanbeenhancedbyapplyingfilteringprocedures.Thefilterapplicationisessentialtoobtainareasonablecorrelationbetweentheanomalyandthestructure.ThefiltersaredesignedtodecreasenoisefromtheEMsignal.Fraser,aswellasKarousandHjeltfilters,are twomethodswidelyusedinVLFdata processing.

Fraserfilterisalow-passfunctionforestimatingtheaverageoftiltanglemeasurementsproducedbyasubsurfaceconductor.Inalinearsequenceoftiltangle measurements M1, M2, M3, ..., Mn, the Fraser filterF1isexpressedas:

)()( 21431 MMMMF ++= (10)

ThefirstvalueF1 islocatedbetweenM2 and M3 positions,thesecondvaluebetweenM3 and M4, and so on.ThereareseveralstudiesusingFraserfilter(Ade-pelumi et al.2005,Cossuet al. 1991, Liu et al.2006,Monteiro-Santos et al.2006,Zlotnickiet al.2006).

Karous and Hjelt (1983) developed a statisti-cal linear filter, based on Fraser´s one, whichprovidesaprofileofcurrentdensityvs.depth(H0) and is derived from the magnitude of the vertical componentofthemagneticfield(Hz)inaspecificposition.TheseauthorsusedlinearfilteringfortheanalysisofVLFdip-angledatainanextensionoftheFraserfilter.

Theydescribethemagneticfield,arisingfromasubsurface 2D current distribution, assumed in a thin horizontal sheetofvaryingcurrentdensity situatedeverywhereatadepthequaltothedistancebetweenthemeasurementstations.Their technique involvesfilteringthesamedatasetforvariousdepthsandgivesanideaabouttheconductivitywithdepth,sincehighcurrentdensitycorrespondstogoodconductors.Thistechniquehas foundwidepopularityas it providesa simple, readily implemented scheme for semi-quantitative analysis and target visualization.The apparent current density pseudo section shouldprovide a pictorial indication of the depths of various current concentrations and hence the spatial

distributionofsubsurfacegeologicfeatures(Ogilvyand Lee 1991).

Whenwemeasureoverconductors,thein-phasepartof the equivalent currentdistributionhasonlypositive values. Negative parts on both sides of theconductorcanbecausedeitherbythelengthofthefilterorbyadecreaseofcurrentdensityduetocurrentgatheringwhichisnotpresentin2Dstructures(Nab-ighian1982).Inthesimplestform,theFraserfilteris:

)2/(205.0323.0446.1446.1

323.0205.0)2/(2

3210

12

xIHHHH

HHxIz

a

a

++

+= (11)

WhereΔz is the assumed thicknessof the cur-rent sheet, Ia, is thecurrentdensity,x the distance betweendatapointsandalsothedepthtothecurrentsheet. The H2 through H3 values are the normalized verticalmagneticfieldanomaliesateachsetofsixpoints.Thelocationofthecalculatedcurrentdensityis assumed at the geometrical center of the six data points (Sundararajan et al. 2007).

METHODOLOGY

Measurementswereconductedalongsixprofileswith theEM-LINandVLFmethods; fourprofilesare common for both methods, respective lengths of 325,320,300and645m(Fig. 2).Threeprofilesweremade in the southern side, one on the northern side of the landfillwithE-Wdirection.Twoadditional

Garbage Area

plantProcessing

Tank

Profile 1

Dirt road

AzucenasStreet

Natural drain

Profile 2

Profile 3

Profile 6

Profile 4

Profile 5

Perimeter of thelandfill

Los Pinos street

Main access

Guardhouse

Fig. 2. Location of the studied profiles. Profiles 1 to 4weremeasuredwithEM-LIN,5and6profilesonlywithVLF

S.I. Belmonte-Jiménez et al.322

VLFprofilesweremade:oneinthemiddlepartofthe landfillwithE-Wdirection.Theother oneon thewesternside,withaN-Sdirection:lengthsrespec-tivelyof415and275m.Inallcasesthemeasure-mentsweremadeevery5m.

Structuralfeaturesfromrockoutcrops,fracturedegreeanditspreferreddirectionwerealsomeasured.Forthetopographicsurvey,anAshtechPromarkGPSpostprocessingequipmentwasused.

TheVLF survey was done with a Scintrexequipmentwithtransmittingsignalsfromthreesta-tions:NAUlocatedinAguada,PuertoRico,withafrequencyof28.5kHz;NPMlocatedinLualualei,Hawaii,withafrequencyof23.4kHzandNSSlo-catedinAnnapolis,Maryland,USA,withafrequencyof21.4kHz.Fortherespectivedataprocessing,thesoftwareKHFFILT-2006wasused(preparedbyPirt-tijärvi2004),toapplytheKarousandHjelt(1983)andFraser(1969)filteringtoVLFdata.

The control source electromagnetic dataweremeasuredwith anEM-LIN equipment (Geonics),consisting of transmitter and receiver coils; per-formingmeasurementswiththreedifferentintercoilspacing(10,20and40m)atdifferentfrequencies(6400,2600,and600Hz)respectively,andintwoforms:horizontalcoplanarcoils(verticalmagneticdipole) and vertical coplanar coils (horizontal mag-netic dipole). Data processing for the electromagnetic coilsmethodwere performed using the softwareCICEM35-2006.This programapplies the theorydevelopedbyPérez-Flores(1995)andPérez-Floreset al.(2001)fortheEMdatainversion,whichconsidersthemeasurementsobtainedwithEM-34equipmentas aweighted average of the earth conductivitydistribution.

ANALYSIS AND DISCUSSION OF THE RESULTS

TheelectromagneticLINarraycomprisedseveralsource-receiver separations. For horizontal and verti-calcoplanarloops,weobtainedfourpseudo-sections(images)oftheapparentconductivity(mS/m)alongthe profiles comprising several studied depths. Figure 3 shows,asanexample,forprofile1,foravertical dipole (Fig. 3a)aswellasforahorizontaldipole (Fig. 3b), the behavior of electrical conductiv-ityalongtheprofile,fordifferentdepths.

Thesixdatasetsforprofile1aredifferentbecauseeveryarraylooksat thegroundinadifferentway(Fig. 3).However, they show a general behaviorthatcangiveusaverybroadideaoftheconductivity

variationwithdepthandhorizontally.Theobservedconductorsareassumedtoberelatedwithconduc-tivematerials likecontaminantfluidscontained inthe porous media. In order to merge those six data setstogetoneconductivitymodelorimageofthegroundconductivity,wejointinvertedthemintoamodelwhoseresponsefitsthesixdatasetsverywell(Pérez-Flores et al. 2001). In figure 4areshowntheconductivityimagesormodelsforthefourEM-LINprofileslocatedinfigure 2.

RegardingtheVLFdata,onlythein-phasecom-ponentwasprocessed.Figure 5showsthebehaviorofthein-phaseandquadraturecurvesforprofile1inordertoillustratethedatacollectedinfield.

TheinterpretationofVLFdatawasbasedonfilter-ingproceduresfollowingFraser(1969)andKarousandHjelt(1983)methodswidelyusedbyotherauthors(i.e., Benson et al. 1997, Sundararajan et al. 2007). Fraserfilterturnsthecrossingpointsintopeaksignalsthat enhance the conductive structures. In figure 6 for profile1,wecanappreciatetheresultsofFraserfilter,showingthepercentage(%)inthey-axis.Inparticularasignificantnegativevaluecanbeobservedatposition125mduetochangesintheundergroundconductivity.KarousandHjelt(1983)filteringwasusedtoobtaincurrentdensitypseudo-sections(mA/cm2) for the six VLFprofiles(Fig. 7). Processed data are presented withiso-contourlinesofcurrentdensity.Lowcurrentdensityvaluescorrespondtohighresistivityvalues.

Profile 1. Vertical dipolea

b Profile 1. Horizontal dipole

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Con

duct

ivity

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/m) DV10

DV20DV40

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0 100 200 300 400Distance (m)

Con

duct

ivity

(mS

/m) DH10

DH20DH40

Fig. 3. Behaviorof electrical conductivity alongprofile1. a)vertical dipole, and b) horizontal dipole

EM METHODS APPLICATION FOR CHARACTERIZING A CONTAMINATED SITE 323

BycorrelatingtheEM-LINconductivityimagewiththecorrespondingVLFcurrentdensitypseudo-section, it is possible to construct a geological-

geophysicalmodelforeveryprofile.Inthefollowinginterpretation,onlyanomalieswelldefinedbysev-eralneighboringassignmentpointsweretakeninto

0 50 100 150 200 250 300

Distance (m)

60100140180220260300340

Profile 1Depth

–15 m–30 m–60 m

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SW NE

Conductivity (mS/m)

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Conductivity (mS/m)

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NE

507090110130150170190210230

Conductivity (mS/m)

Depth

–15 m–30 m–60 m

SW0 50 100 150 200 250 300

NORTH

Profile 3

Profile 4

Fig. 4.2Dsectionsobtainedbydatainversion.SeeFig. 2forlocationofprofiles

S.I. Belmonte-Jiménez et al.324

account(i.e.,anomaliesdefinedbyseveralcontigu-ous points along the horizontal and along the verti-cal). Also anomalies located at the upper end points werenotconsidered(i.e.,onlythemiddleportion).Ingeneralthereisafair-to-goodagreementbetweenconductivityzonesimagedbytheEM-LINinversion,andtheareasofhighcurrentdensityobtainedfromtheVLFdata.Eachofthehighconductivityzonesinterpretedandreportedbelowissupportedbybothdatasetstypes,aswellasbythepresenceatsurfaceof leachates.

Alongprofile1(Fig. 8), located near the southern perimeterofthelandfill,twoconductivezonesareob-served at the central part (at 200 m, and around 300 m), whicharethemostimportantintermsofspatialdis-tribution (see Fig. 4a).Theanomalyat200misalsorepresentedascurrentdensityanomalies(Fig. 7c), therebyitisinterpretedasafracturezone.IntheVLFsectionanarrowanomalyisobservedaround150m,whichisalsoassumedafracturezone.Bothzonesliebelowalocaltopographicdepression(seeprofiles2and3),andbelowthesurficialleachateflow.The

topographyisirregular,thegeologyisrepresentedbyalternations of sandstone-fractured shale.

The geological-geophysicalmodel associatedwithprofile2(Fig. 9),whichislocatedoutsidethelandfill,tothesouthofprofile1,showsthreemajorconductivezones(between0and50m,between100and150m,andbetween200and300m)(Fig. 4b). Ac-cordingtoVLF,thereisafracturezonebetween225and275m(i.e.,correlatingwiththethirdconductive zone) (Fig. 7d). Another fracture zone might be lo-catedaround175m.ThethirdconductiveanomalyisassociatedwiththepresenceoffractureswithN-S

80

60

40

20

00 50 100

Distance (m)

Profile 1 VLF

150 200 250 300 350–20

–40

–60

–80

PhaseQuadrature

Fig. 5. Behaviorofthein-phaseandquadratureofprofile1

50Profile 1

0

0 50 100 150Distance (m)

200 250

Real component

300

–50

Res

pons

e

–100

–150

Fig. 6. Profile1wheretherealcomponentorphaseisprocessedbytheFraserFilter

–10

100 200 300 400 500 600

Depth (m) Profile 4 NORTH

a) –20–30

–10

100 200 300 400 500 600

Depth (m) Profile 6

b) –20–30

–10

100 200 300 400 500 600

Depth (m) Profile 1

c) –20–30

–10

100 200 300 400 500 600

Depth (m) Profile 2

d) –20–30

–10

100 200 300 400 500 600

Depth (m) Profile 3

e) –20–30

–10

100

–60

Equivalent current density (mA/cm2)

–50 –40 –30 –20 –10 0 10

200 300 400 500 600

Depth (m) Profile 5

f) –20–30

Fig. 7. a,b,c,d,eandf.VLFprofilesprocessedwithKarous-Hjeltfilter.The scaleofvalues represents thecurrentdensity (mA/cm2), associating positive values to con-ductive areas because of the content of leachate seeping into the fractured medium

EM METHODS APPLICATION FOR CHARACTERIZING A CONTAMINATED SITE 325

direction(andinalesserproportionwithanE-Worientation),throughwhichleachatesareinfiltrat-ing. The anomalies have a lesser vertical extension.

Thetopographyisnotflat.Thereisatopographiclocal depression at the eastern half, and the ground comprisesanalternationofshaleandsandstonewithasurfacelayerofthesamematerialsbutalteredandweathered.

Profile3(Fig. 10),locatedoutsidethelandfill,to the south of profile 2, presents only aminorfeaturethatcanbeassociatedwiththepresenceofleachates at the subsurface (Fig. 4c; around 200 m). This indicates that the contaminant plume does not continuesouthwards,orithasdeepenedanditcan

notbesensedduetothelimitedpenetratingpowerof the twousedmethods.VLF indicates thepos-sibleexistenceofthreefractureszones(between50and 100 m, around 200 m, and another centered in 250m;Fig. 7e).Thelastfracturewouldbelocatedbelowthesurficialleachatesflow,andwouldpointtotheinfiltrationofleachateintothegroundthroughit, and can be interpreted as a leachate plume. Geologically, this profile consists of a thin layer ofsand-shalerockfragmentsandthenalternationof shale and sandstones.

Figure 11 shows the geological-geophysicalmodelforprofile4,whichisthelongestandlocatedinthenorthernportionofthelandfillinanareawhere

Simbology

N

Shale

Sandstone

Leachate plume

Fractured zone

Sandstone and Shalefragmented rocks

Profile 2

Distance (m)

Dep

h (m

)0

20

40

0 200100 300 325

Layer of alterated rocks fragments(Sandstone and Shale)

NESW

VLFVLF

VLF

Fig. 8. Geologicalmodelalongprofile1,withsuperposedleachatepresenceinferredbyLINandfracturesbyVLFEMmethods.

Profile 1

Distance (m)

Dep

th (m

)

0

20

40

0 200100 300 325

SW NE

Compacted backfill with fragmented rocks

(Sandstone and Shale)Simbology

N

ShaleSandstone

Leachate plumeFractured zone

Sandstone and Shalefragmented rocks

Fig. 9. Geologicalmodelalongprofile2,withsuperposed leachatepresence inferredbyLINandfracturesbyVLFEMmethods.

Profile 3

Distance (m)

Dep

th (m

)

0

20

40

0 200100 300 325

SW NE

Layer of altered rocks fragments

(Sandstone and Shale)

Simbology

N

Shale

Sandstone

Leachate plumeFractured zone

Sandstone and Shalefragmented rocks

Fig. 10. Geologicalmodelalongprofile3,withsuperposedleachatepresenceinferredbyLINandfracturesbyVLFEMmethods

S.I. Belmonte-Jiménez et al.326

Profile 4

Distance (m)

0

20

40

2000

SW NE

Layer of clay filled garbage

60400 600 645

Simbology

N

Shale

Sandstone

Leachate plumeFractured zone

Sandstone and Shalefragmented rocks

Dep

th (m

)

Fig. 11. Geologicalmodelalongprofile4,withsuperposedleachatepresenceinferredbyLINandfracturesbyVLFEMmethods

Profile 5

Distance (m)

Dep

th (m

)

0

20

40

0 275

NW SE

Layer of clay filled garbage projected

N

100 200 250

Simbology

Shale

Sandstone

Leachate plume

Fractured zone

Sandstone and Shalefragmented rocks

Fig. 12. Geologicalmodelalongprofile5,withsuperposedleachatepresenceinferredbyLINandfracturesbyVLFEMmethods

Profile 6

Distance (m)

0

20

400 100 400 415

SW NE

Depth (m

)

200 300 350

Simbology

Shale

Sandstone

Leachate plume

Fractured zone

Sandstone and Shalefragmented rocks

NLayer of clay filled garbage

Fig. 13. Geologicalmodelalongprofile6,withsuperposedleachatepresenceinferredbyLINandfracturesbyVLFEMmethods

inthepastwastewasdepositedbutnotanymore.Twoanomaliesaremappedat100,extendingfrom250andtowards300m(Fig. 4d).Tworelativeanomaliesareobservedbetween375and425m.VLFindicatesfractures centered at 200 and at 400 m (Fig. 4a), cor-relatingwiththesecondconductivezone.Atfield,leachateswereobservedpercolatingthroughthesefractures.

Thegeological-geophysicalmodeloftheprofile5 ispresentedinfigure 12,where twozoneswithhighcurrentdensityareinterpretedasfractures(at85m,andonecenteredat200m;Fig. 7f). Onto this profileisprojectedthelocationofthemainbodyofgarbagewithaclayfillinginthemiddleofit,andwith leachates observed at its surface andwhose

spatialdistributionwasobtainedfromVLFandEM-LINmeasurementsalongprofiles1,2,3,4,and6.Thegeologycorrespondstoanalternationofshale-sandstone.Thisprofileislocatedinthewesternpartofthelandfill.

Figure 13correspondstothegeological-geophys-icalmodeloftheprofile6.Thisprofilewasconductedat thecenterof the landfill, soalmosthalfof it isoveralayerofgarbagewithathicknessfrom10to15mofdepth.VLFindicatesthepresenceoffourpossiblefracturezones(at65m,at150m,at225m,andbetween300and400m)(Fig. 7b). It sounds logic tosuppose that through the twoeasternmostfracturesleachatespercolates,sincetheyarelocatedbelow the local topographic depressionwhere the

EM METHODS APPLICATION FOR CHARACTERIZING A CONTAMINATED SITE 327

maximumthicknessofwastesislocated.Thegeol-ogyisrepresentedbyanalternationoffracturedshaleand sandstone.

The analysis of all themodels shows that theconductive areas are related to the presence of leach-ates andmay suggest a continuousflowof themthroughthelandfillofOaxacacity,andalsothatthesubsurface layers are being impregnatedwith thiscontaminantflowthroughthefracturesuptodepthsof more than 30 m.

Figure 14showsinterpolatedimagesofapparentconductivitymeasuredwiththeEM-LINconsideringthe respective four profiles, in the configuration-modality of horizontal coplanar coils (vertical di-pole). It can be observed that at different depths, the contaminant plume is larger in the N-S preferential directionoftheleachateflow.Wecanobservethat

the high conductivity anomaly correlateswith thelandfilllimits.FracturesinferredfromVLFdataaresuperposed.TwofracturesystemscanbeobservedtorunwithaNW-SEdirection.

CONCLUSIONS

TheOaxacacitygarbagelandfillislocatedinafracturedzonewheretheVLFandEM-LINelectro-magneticgeophysicalmethodswereappliedtostudythe presence of leachates in the subsoil. Accord-ingly, thepreferentialflowof leachates is throughthesetofNW-SEfractures.Wedefinedtheirspatial distribution. The data obtained from EM-LIN coils showhighconductivityzones,interpretedtobedueto the presence of leachates. From the information

Fig. 14. Perspective at different depths for the EM coils processed data, corresponding to the horizontal (A) and vertical (B, C, and D) dipole mode

A B

C D

3.0 3.7 4.5 5.5 6.7 8.2 10 12 15 18 22

746200 746300 746400 746500 746600 746700746200 746300 746400 746500 746600 746700

1872800

1872700

1872600

1872500

1872800

1872700

1872600

1872500

Vertical dipolesIntercoil spacing 10 m

Exploration depth = 15 m

Vertical dipolesIntercoil spacing 40 m

Exploration depth = 60 m

Vertical dipolesIntercoil spacing 20 m

Exploration depth = 30 m

Horizontal dipolesIntercoil spacing 10 m

Exploration depth = 7.5 m

ρa, Ohm.m

S.I. Belmonte-Jiménez et al.328

obtained from the VLF method (in-phase component) about13fracturedzoneswereidentifiedalongthestudied profiles, favoring the leachate percolationthrough the fractured zone. In general, there is a good correlation betweenhigh conductivity areasobtainedwithEM-LINandthosehighcurrentdensi-tiesobtainedwithVLF.

Consideringthatthewatertableofthesurround-ingareasislocatedatadepthlessthan15m,therearehighpossibilitiesthattheaquifercouldbecon-taminated.Intheoutskirtsofthelandfillthereisasemi-confinedaquifer,however,thefracturesfavorleachatesflowinginsidethearea,allowingtheleach-atetoinfiltratetogreaterdepths.Thereareanomalousconductiveareaswithdepthsvaryingfrom60to35m in the case of EM-LIN and VLF method of coils respectively.

Finally,itisconsideredthattheuseofthesetwomethods (VLF and EM-LIN) for the detection of leachatesinthesubsoilissuitableasatechniqueforassessment of contaminated areas. It is faster than othergeophysicalmethodsandnotinvasive.

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