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Natural radioactive environment of urban soils in Shihezi, China

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Natural radioactive environment of urban soils in Shihezi, Chinaof,in,urban,soils,China,soil

    Natural radioactive environment of urban

    soils in Shihezi, China

    NuclearScienceandTechniques20(2009)335-339

    NaturalradioactiveenvironmentofurbansoilsinShihezi,China

    GEBenweiLIUAnna2t

    'CollegeofTourismandEnvironment,ShaanxiNormalUniversity,Xi'an,Shaanxi710062,China

    DepartmentofGeography,ShiheziUniversity,Shihezi,Xinjiang832000,China AbstractRadionuclides,shehas23U,

    3Thand?K,

    canbefoundinurbansoil.Toevaluatethenatural

    radioactivityintheenvironment,soilsampleswerecollectedformShihezicityandradioisotopeconcentrationswere

    determinedbyX-rayfluorescence.Thedoserateofurbansoil(mGyperyear,mGy/a)wascalculated.Theresults

    indicatethattheU,ThandKconcentrationsoftheurbansoilswere,respectively,1.2-3.2mg/kg,6.l2.3mg&gand

    2.o5%-2.24%,,?

    imthemeanvaluesof2.47mg&g,10.47mg/kgand2.16%.Doseratesofurbansoilswere 1O.O4

    19.55mGy/awiththemeanvalueof16.31mGy/a.Tl1isdoserateistheperfectandmaximumvalueofnatural

    radiationinsoilanddifferentwith

    l1eairabsorbeddoseratefromterrestrialy-rays.Themeanvalueofairabsorbed doseratewasabout57.42nGy/h.Theannualeffectivedoserateinairwasabout0.07mSv/aandtheaveragevalueof

    Racainurbansoilwas120.37Bq/kg.nerelativecontributionofaparticletothedoserateishigh

erthanthatderived

    from8andy-raysintheurbansoils.

    KeywordsNaturalradioactivity,Radioactiveenvironment,Doserate,Urbansoil,Shihezicit

    y

    1Introduction

    Urbansoilisanimportantpartofurbanecological system,intermsoftheresidentsmayreceive significantdosesinexposuretonaturalradioactivity fromradionuclidesofpotassium,thorium,and uraniuminurbansoil[1'2]

    ,eventhoughtheuraniumand

    thoriumaretraceelements.Theradioisotopes,suchas 38U

    ,

    235U,

    23rh

    ,

    and4oandmanyofthedaughter

    products,emitnuclearradiationswhentheydecayjJ_

    The4uI('inisotopicabundanceofabout0.01%.emits pandT-rays,whiletheheavynuclidesemit,pand

    T-raysTheeffectofparticleishighlylocalizedto withinthesurfacelayerofcrystalquartzandfeldspar inurbansoil,comparedwithpandT-rays.However,

    thegaseousdaughterproducts,suchasZZRn,canbe breathed,henceahazardtohumanhealth.

    Itisnecessarytounderstandthenatural

    radioactivityfromradioisotopesintheenvironment andtheradiationhazards.Naturalradioactivityinsoil samplesorbuildingmaterialshasbeensurveyed,and

    air-absorbeddoserate,anntlaleffectivedoseand externalhazardindex(I-h】【)inmanyareashavebeen

    studied5_31.Inthispaper,thedoserateoftopsoilin ShihezicitywascalculatedbytheU,ThandK concentrationsdeterminedbyXrayfluorescence,so

    astoevaluatethenaturalradioactivity.

    2Materialsandmethods

    2.1Studyarea

    Shihezi(Fig.1),anoasis,issituatedinaridareaof Xingjiangprovince,northwestChina,between 84.7.86.6.Eand43.3.}5-3.N.ItsSOUthboarderison thenorthedgeoftheTianshanMountains,andits northboarder,ontheGurbantunggutDesert.The averageannualtemperatureofthisregionisabout8?.

    andtheaverageannualprecipitationisl2020Oinin.

    AmajorityofprecipitationoccursbetweenApriland July.Thephysiognomyissimpleandcomprisesthe alluvial-proluvialplainanddesert,withamean altitudeof450m.Thesoil,ofadesertgreysoiltype, hasbeenchangedbyurbanconstruction,forestations andcultivation.

    SupportedbyInitialfoundationofShiheziUniversityforyoungcoHegeteachers(No.RCZX

    200691)

    Correspondingauthor.E-mailaddress:gebenwei2008@stu.sn/lu.edu.ca

    Receiveddate:2o()9O6_02

    336GEBenweieta1./NuclearScienceandTechniques20(2009)335339

    Fig.1LocationofShihezicityandthesamplingsites. 2.2Samplecollectionandanalyticalmethod Soilsampleswerecollectedfrom14sitesinShihezi (Fig.1).Mostofthemareinthecity'snorthwestpart,

    whichbelongstotheoldtownandhasadevelopment ofabout60years.Theeastandsouthpartsofthecity areyoungerandhavelessanthropogenicinfluence. Soilsampleswerecollectedfromthesurfacelayer f0一一5cm)offivesamplingspotsateachsite.The subsamplesweremixedandweighedapproximately 150g,soastorepresentthesamplingsite.The14 representativesampleswerelabeledasS1toS14.In thelaboratoryofShaanxiNormalUniversity,each sampleairdriedatroomtemperaturewascrushed, sievedthrougha75gmmeshafterremovingplant roots.Finally,4.0gsoilsampleofeachsitewasadded with2.0gboricacid,andpressedintoq533mmx4mm waferunder30tpressure.U,ThandKconcentrations intheurballsoilsamplesweremeasuredbyX.ray fluorescence(XRPANalyticalPW2403).

    3Resultsanddiscussion

    3.1TheU.ThandKconcentrations

    TheU.ThandKconcentrationsinsoilsamplesfrom Shiheziaregivenin6Z.eUconcentrations rangefrom1.2to3.2mg/kg,withanaverage concentrationof2.47mg/kgandstandarddeviation (SD)of0.55.TheTIlconcentrationsrangefrom6.4to 12.3mg/kgwithanaverageof10.47mg/kgandSDof 1.54.TheKconcentrationsrangefrom2.O5tO2.24% withanaverageof2.16%andSDof0.06.The averagedUandKconcentrationsinShihezitopsoilare 1owerthanthemeanabundanceinuppercontinental crust(ucc),whiletheaveragedThcontentsin ShihezitopsoilishigherthanthemeanUCC

abundance13].

    Table1Radioelementconcentrationanddoserateofsoil samplescollectedfromShihezicity,China

    SamplesU/mg?kgTh/mg?kg-K/%Doserate/mGy.'

    Thecorrelationcoelcientsofthethreemajor naturalradionuclidesareshowninFig.2.Noobvious correlationisfoundbetweenThandK=0.2271) andbetweenUandK(=0.3101),buttheThandU

    concentrationsarecorrelatedf'=O.9113).ThisThU

    correlationhasbeenfound,too,intheloessplateau, China(Fig.2d).Correlationbetween23Tlland26Ra (R=0.92),theproductionofradioactivedecayofU andweakcorrelationbetweenRaand4oK:0.

    45)

    andbetween232Thand4oK:0.57)inChinese

    cOmmercia1marble4-171.

    3.2Doserateofthesoilsamples

    Inusingthedoserateofnaturalradioactivityinsoilto evaluatenaturalradioactiveenvironment,thekvalue, or6ccoefficient,hasbeenadopted,because,foragiven amountofabsorbedenergy,.cparticlesareless effectiveininducingOSL(OpticallyStimulated Luminescence)signalthanpandy-rays.Inother

    words,theabsorbeddosemeasuredbyOSLisonly partofthedosecontributedbytheradionuclides.In GEBenweieta1.,NuclearScienceandTechniques20(2009)335-339 thispaper,however,weusethedoseratecontributed bytheradionuclides,ratherthandoseratebytheOSL crystal.hencenoneedofconsideringthecoefficient. Thetotaldoserate(inmGy/a)duetotheU,ThandK

    contentsinthesoilsamplescanbecalculatedby Eq.(1)..(4)

    D=DQ+Dp++Dc(1)

    D=2.31CU+0.644CTh(2)

    DB=0.145CIU+0.0273CTh+0.782CK(3)

    D0.113CTU+0.0478(+0.243CK(4)

    wheretheD?Dp,andarecomponentsofthe

    doseratefromalpha,beta,gammaandcosmicrays, respectively.The,CThandCKaretheU,ThandK concentrationinurbansoil,respectively. Thedoserateofcosmicrays,Dc,(inmGy/a)can becalculatedbyEq.(5)l8,

    )c=O.21exp[-0.070dp+0.0005(@)(5)

    wheredisdepth(m)belowthesurfaceandPisdensity (g/cm~).TheDcinaregionisrelatedtotheregion's

    0)

    E

    ,

    

    

    1234

    u/mgkg''

    22.05212.1522225

    K/%

    337

    altitude,longitudeandlatitude,andthedepthofsoi1. Themeanvalueofdoserateofcosmicraysinurban soilofShiheziis0.223mGy/a(25.41nGy/h),whichis lowerthanthenationalaverageof37.4nGy/handthe

globalaverageof32nOy/h【制.

    Tablelshowstheestimatedtotaldoseratedueto theradioisotopesofU,ThandKandcosmicraysin Shihezi.being10.0419.55mGy/awithmemeanof

    16.31mOy/aandSDof2.53.Anenvironmentaldose ratedependsprimarilyongeologicalandgeographical conditionsoftheregion,depositionalconditionsand agrotype.ThetopsoilinShiheziisdesertgreysoil formedonaproluvia1fan.Parentsoilmaterialofthe urbansoilishomogeneousinageographicarea.Itwas alsofoundthattheU,ThandKconcentrationsarenot affectedbyhumanactivity(architectureandirrigation) overthepast60years.Therefore,theaveragedoserate ofl6-31mGy/acanbeusedtoestimateenvironmental radiationlevelintopsoilofShihezi.

    3.5

    3O

    '

    .,25

    ,

    2.0

    3

    1.5

    1.0

    0'

    E

    ,

    .Iz

    i---

    22052.12.152.22.25

U/mg?kg''

    Fig.2Correlationsbetweenuraniumandthorium(a),potassiumanduranium(b),potassiuma

    ndthorium(c)inurbansoilShihezi

    andbetweenuraniumandthorium(d)inloessofloessplateau,China.

    3.3TerrestrialT-radiation

    ToestimateexternalterrestrialYradiation,thetotalair

    absorbeddoserate(nGy/h)onemeterabovethe groundduetotheactivityconcentrationsof226Ra ,

    .r}land?K(Bq/kg)wascalculatedusingEq.(6). DB=O.0417CK+0.462CR+0.604Ca~(6)

    whereCR.d'CThandckareeactivityconcentrations ofRa,21'lland?KinBq/kg,respectively.Eq.(6) hasbeenusedallovertheworldtoestimatedoserate insoil,sand,marble.andwater[5q21.Theactivity concentrationsof226Ra.

    32ThandKinthesamples

    wasusuallydeterminedusingT-rayspectrometric system.UsingthedatainRef.[15],wheretheactivity 竹俚?们9876

    ..

    .羊口?J-

    32109876

    338GEBenweieta1.,NuclearScienceandTechniques20(20o9)335-339

    concentrationsof.26Ra.

    .handwere

    respectively26.4638.15Bq/kg(average

    concentrationatabout27.26Bq/kg),32.77-35.63 Bq/kg(averagedconcentrationat32.88Bq/kg),and

    593.37661.54Bq/kg(averagedconcenffationat 598.64Bq/kg),theairabsorbeddoserateinShihezi canbecalculatedas53.362.2nGy/hwithamean

    valueof57.42nGy/h(Table2.Thisislowerthanthe averagevalueofthenaturaly-raydoserateofXinjiang (62.21nGy/h)andtheChineseaverageterrestrial7-ray doserate(81.5nGy/h)measuredbyscintillation detectorandhighpressure.ionizingchamber[16.

    butis

    higherthantheestimatedaverageglobalprimordial doserateof55nGy/ht.Averagevalueofthe

    calculateddoserateswasalsolowerthanaveragedose ratefromterrestrial1,exposure(69.9nGy/h)calculated fromradiOnucHdecontentinsoilinmainland China1.

    Table2ActivityconcentrationsofRa,.ThandKinsoilandliteraturedataforcomparison.

    Estimatedannualaverageeffectivedose

    equivalentreceivedbyanindividualwascalculated usingaconversioncoefficientof0.7Sv/Gy,which wasusedtoconve~thedoserateintheairtohuman effectivedosewithanoutdooroccupancyfactorof O.2p.Theeffectivedoserate,DeinmSv/a),was calculatedby

    Deff=8760Dx0.2x0.7x10~(7)

    Themeanannualeffectivedoserateintheair, calculatedbyEq.(7),wasabout0.07mSv/a.Thisis lowerthanthemeanvalueofChineseeffectivedose ratefromnatural1,rays(0.1mSv/a)andiswithinthe averageannualexternaleffectivedoseratefrom terrestrialradionuclide(460uSv/a)inareaswith

normalbackgroundradiationdefinedby

    UNSCEAR[19.Obviously'theenvironmentofShihezi canberegardedasanareawithnormalnatural backgroundradiation.

    Itisimpoganttoassessthesoil7radiation hazardstohumanhealth.Themostwidelyused radiationhazardindex.radiumequivalentactivity (Raea)canbecalculatedby

    Raea=ClR.+1.43C+0.077CK(8)

    whereCaa,Candaretheactivityconcentrations of6Ra

    ,

    .and?KinBq/kg

    ,respectively.

    AccordingtoEq.(8),1Bq/kgofRa,0.7Bq/kgof .21'hor13Bq/kgof?Kyieldsthesamey-raydose.

    TheaveragevalueofRaeainurbansoilofShiheziis 120.37Bq/kg.ThisislowerthanthemeanRaeqvalue ofXingjiangsoil(131.75Bq/kg)andthesuggested maximaladmissiblevalueof370Bq/kgrecommended bytheOrganizationforEconomicCooperationand Development19.

    3.4Relariveeontribution

    Relativecontributionsofa,pand丫一raystothe

    maximumdoserateofurbansoilareshowninFig.3a. Theaveragerelativecontributionbyindividual componentsofnaturalradioactivityis76.0%from a-ray,14.5%from13-ray,and8.1%from7-ray.1'he radiationfromaand6particlesislocalizedinsoiland cannotaffecttheairdoserateatabout1mabovethe

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