ANNUKKA LlPPONEN NGU-BULL 439, 20 02 -PA G E27
Detection of po tential pat hways fo r contam inants into the Paijanne Tunnel in Finland
ANNUKKAL1PPONEN
Lipponen A. 2002: Detection of potentia lpat hwaysforcontam inantsint o thePaij anne Tunnelin Finland.Norges geologiskeundersokelseBulletin439,27-32.
The potablewater for a million people residingin the Helsinkimet rop olit an areais conveyedfrom Lake Paijanne alongthe 120 km-longPaijanneTunnel. Inform ation on thehydrogeologyofthetun nelis vital forland -useplan- ning,particularlyin thesout hernpart of thetunnelline where ind ust rialactivityand pressurestobuild under- groundcon structi ons areincreasing.InaprojectcommissionedbytheHelsinkiMetropo litan Area WaterComp any, the areas vulnerableto contaminationandpossiblyhydraulically connected tothetunnelhave beenidentifie dand spat iallyanalysed in relationtorisk activ it iesand infrastructu re.The projectcompleme ntsan earlierst udyin which pot ent ial risk activ itieswerelocatedandcharacterised.Relevantinfo rmation on the soil, grou ndwater andbedrock- fracturingconditionsalongthelengt hofthetunnelwas extracted from tunnelconstructiondocumentationand arealenvironmentaldataset s, andassembledas overlaysinGISformat.As aresult,adescript ion ofthe environmen- tal geology of the tunn el zonehas beenprodu ced, and the estimatedarea ofinfluencehasbeen delineatedas a support to land-use planning.
AnnukkaLipponen.FinnishEnvironment Institute.P'O.8ox 140,00251Helsinki,Finland.
Introduction
The Paij anneTunn elis a120 km-long, unli ned water con- veyance tunnel from Lake Paij anne in Asikkala to Silvo la reservoiratthe borderof Helsinkiand Vantaamunicipalities.
Thetunnel is located at anaverage depth of 50-70 m in Svecofennian granite and migmatites,overlain by glacial and glaciofluvialoverburd en.Thecross-section al areaofthe tunnelvariesfrom 13.5m' to 18 m'.The tunnel was con- structed during the period 197 3-1982and has since been in almost constant use. The current flow rate under natur al pressureis2.9
mvs,
The hydraulicconnect ion betweenthe tunnel's waterand groundwat er isindicated bya groundwa- ter leveldecline,in most casestemporary,observed at the timeof tunnelconstructionand duringsubsequentmainte- nancework(Pokki1979).For the northe rnmost100km,the pressurelevelofthe tunnel wateris commonly low er than the local groundwater level.Due to the hydraulic connec- tionandhydraulic gradient tow ards the tunnel, the water is subj ect to riskof contaminant transport.Thecave-ins that occurredin1997and1998demonst rat ethe dynamics ofthe tunnel system and emph asize the significance of regular groundwatermonitoringwithinthe areaoftunnelinfluence (Mikkola &Viit ala1999).The purposeof the st udy was to compile the abundant,scattered tunnel data,to identi fy potent ial pathways for cont aminant s,and to present an accessiblehydrogeologi caldescriptionof thetunnelzonein orderto promote soun d land use andland-useplanning.Methods
Infor matio n on the soil cover, bedrock fracturing and grou ndwaterconditions,collected in the planningand con-
struction stagesofthe tunne lproj ect(Niini1968)and dur- ing subsequent studies,was converted into a GIS-compat i- ble format and spatiallyanalysed to det ect potential flow paths.A summary of the datatypesusedinthe st udyispre- sentedinTable 1. Data analysisand map productionwere carried out using ArcView desktop GISsoftware wit h a Spat ialAnalyst extension.
Observation s on bed rock fracturing and tunnel rein- forcements as indirect indicators were visualized against a backgroundof a supe rficia ldepo sitreliefmaptolocatefrac- ture zones.Figure1show sa tunnelsecti onasanexampleof the source data used for overburden thickness and rein- forced zones,the latte r of which were also availabl e in numericform.The superficialdeposit relief map(Fig. 2)was constructed bycombin ing adigital elevation modelanda dig ital soil map by a method sim ilar to the one used by Palmu (1999).A bedrock map of the tunn elandaeroma g- neti c data were applied in recognising bedroc k structures onanarealscale,andin estimatingthe continua tionof frac- ture zones detected by drilling or seismic sounding,as observations in the tunnel or inferred from reinforcement data.Groundwa terinfl ow s,measured with Thompson weirs at thetime of tunnel construction,provid e informa t ion on hydraulic conductivit iesof fractu re zones andwere utilised in qualitativelyestimating the riskof contaminanttranspo rt. Suppor tingobservatio ns forlocat ing zonesof leakage were recordedin October2001 during visitstoselected sections of thetunnel.
Digitalsoil data at a scale of 1:20 000 wereapplie dto out line the surface distribution of soil types withhigh per- meabilitie s.To give aroug hestimateof therelative perme-
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Table1.Geolog ical and technicaldatautilised in the study.
Datatype Sou rce Sign if ican ce Application Limitations
Tunnel reinforc ement s CON heavy suppo rt indicati ve of locationof fracturezones not all fracturezones
significantfracturin g hydraulically conductive
Seismicprofiles r-sv thickness, soiltypeandstrat igraphy of estim ationof flowdirection error+/- 10%,non-unifor m coverage overburden; topography of bedrocksurface andoverburdenpermeability
Drill logs TECH thickness, soil type andstratigraphy of est imat ionof flow direction commo nly restrictedto the CON overburden;topography ofbedrock and overburd enpermeability uppermostfew metres
surface,bedrock fracturing
DigitalElevationModel NLS topographyindicatesbedrock fractur ezone interpretation, fracturingand probable direction s estimationof flowdirection of surfaceand groundwater flow
Map ofQuaternarydeposit s, GTK surfacesoil type com binedwith stratigraph y only refersto theuppermostmetre
scale 1:20 000 forpermeability estimat ion
Groundwaterinflow psv ind icates hydraulicpropertiesof relative importanceof fracture inaccuracy ofthe Thompson
measureme nts fracturezones zonesforgroundwaterflow weirmeasurements
and contam inanttransport
Classifiedgroundwater FEI hydrogeological condition sin significant direct ion ofgroundw aterflow, information scarceon many
areas formations of the overburden depthtoground water surface ofthe smaller areas
Aeromag net ic data, GTK abruptchangesinintensityindicative supportsfracturezone inter- interpretation subjective,preferably scale1:100 000 offault s andfracturezones pretation,recognitionof bedrock coupled to independentobservations
str uctu re trends inregional scale by other methods
Riskactivities FEI current hazards ident ificatio nofareasalready atrisk hazard situationevolves constantly Roadsandrailways FEI transport routesof hazardouschemicals
Records of well drawdow n PSV ind icates the areaof tunnelinfluence estimati onof flow connectio n qualit y variousfacto rscontrib ute todrawd own Groundwaterlevel CON flow,groundwaterrelati vetoground surface interpretat ionof flowdirection also individualmeasurements that give
measurements no informationon fluctuations
Map of litholo giesand GTK minor st ructu res(e.g.cleavage) estimation of fractur ezone atsurface fractur esformbroadzone s;
struct uresfromthe tunnel commonly indicate the orientation fractur ing subparallel to surfacediffi cult
orientation of major features to observe;3D geometrydifficult to
estimatedueto curvingof fractureplanes Abbreviat ions: CON=consultan ts,PSV=Helsinki Metropolitan AreaWater Company,TECH=geot echni caldepart ment s of cities,NLS=NationalLand
Surveyof Finland,FEI=Finni shEnviron mentInstitute,GTK=Geological SurveyofFinland
Fig.1. A tunnelsect ionshowi ng the overburdenthicknessand rein- forced zones,extending betweenthekilomet re readings 100and 104.
Thesectioncoincides wit h the sout hern part of the tunnel linedis- played in Fig.3. By permission from Paakaupun kiseud un Vesi Oy (Helsinki Metropo litanArea WaterCompany),
ability of areas,the soi l type was combined wit h strati- graphic inform ati on fromdrill logs,where available.The cov- erage ofseismic profiles and drilllogsis non-uniform, butin areas of high observation point density, a continuou s bedrocksurface wasrough lyint erpolat ed by alsoaccou nt- ing for the elevatio n data from out crops.An example of
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inte rpola ted bedrocksurfacetopog raphy is shown in Fig,3.
The slope of the bedrock surface givessomeindicati on of the direction of DNAPL transport and groundwater flow in bedro ckfractures,
Field stu diesfor groundwater supply have been carried out by consultants,particularly in theexten siveesker sys- tems of Janiksenlinna in Tuusula and Kukkolanharj u in Harneenkoski,as well as in theFirst and Second Salpausselka ice-marginal formations.In addition to consultant report s, descriptions of groundwater areas resulting from mapping and classification carried out by the Environment Administration,mainlyin the 1980s(Brit schgi &Gustafsson 1996)and protection plans for groundwater areas,provide info rmation on groundwa ter conditions. Data such as groundwater flow directions and groundwater levelsinindi- vidual glacialdrift aquifers,variablein detail, were appliedto delineatethe areaof influence,
The well drawdowns resulting from water pressure decrease in the tunnelindi cat e the qualityof hydrauliccon- nectio n to groundw ater in the overburden,In most areas along the tunnel line,the drawdownobservationsareinade- quate in number to allow outlining of the actual area of influence,Consequently,the extentof hydraulic connection hadto be inferredfrom groundwater levels,soiltypeinfor- mation,topographyandbedrock surface elevation,depend - ing on availability, The delineat ion of the inferred area of
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Digital elevation model,roads,water bodies· National Land Survey, permission 7/MYY/01
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R "I Estimated direction of groundwater flow
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_ Bedrock outcrops
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Peatland_ Fill so il _ Lakes and rivers
Fig.2. Superficial depositreliefmapoftheViit aila area(mo difiedfrom Lipponen 2001).Digital elevationmodel:Nati onal LandSurveyofFinland;soil type map,scale1:20 000:GeologicalSurvey ofFinland.
NGU-BULL439,20 02 - PAG E 30 ANNUKKA UPPONEN
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Thetunnelreinforcem ent s and the interpreteted fract ure zonesare also shown.The area is outlined in Fig 2.
influencewascarried out by visualising the parameters in GISandanalysing the spatial relationships.
Poten tial risk sites identified by Haavisto (1998) and main roadsthat also represent transport routes for haz- ardous chemicalswere viewed in relation to the soiltype andbedrock-fracturinginformation to evaluate the riskaris- ing from currentact ivities.Figure 4displaysthe main infra- structurein relationto the PaijanneTunnel aswell as the dis- tribution ofpot ent ial risksites.
Results
Based on the extent ofcoarse-grained superficial deposits, groundwa ter flow direction and the location of bedrock fractures,theinferred areaof influence was delineated in the vicinity of the tunnel.The areaof inf luence isthe recom- mended zone ofcaut iousness for locating or monitoringrisk activitiesthat could potentially contaminate soil or ground- wate r.From within the area, contaminan ttransportis con- sidered possible, and thereforethe tunnel should betaken into account in land-use planning.The possible hydraulic connection would need to beassessed on a case-by-case basis.Along fracture zonesand eskers,the area ofinfl uence wasexte nded farth er from the tunnel. Thedelineation aims at covering the enlarged area of influence prevailing at times of increasedhydraulic gradient duringa tunnelrepair.
The areas considered vulnerable tocontamina tion were ident ifi ed and their relevant characteristics indicated.The areas deemed vulnerable displayed a combination of fea- tures including severalof the following crit eria: 1) markedly fractured bedrock (observed or alternatively indicated by heavyreinforcement ),especially if associated with ground- waterinflow; 2)a thin orhigh ly permeable soil cover or a permeablelayer atdept h;3)relat ively thin bedrock roof;or 4)a majo rtransport routeorpot ent ial risk sit e locatedin the immediate vicinity of thetunnel.A description ofthe geol- ogy,hydro geology,essential tunn el structures andpote nt ial risk activities wit h support ing maps wasprod ucedfor each section ofthetunnel.
The greatest natural risk of contaminant transport is linked tofracture zones withgroundwa terleakage. Fract ure zone inter pretationismostreliably doneby iterativelyview- ing topographical lineaments,bedrocksurfacedepressions andvisualisationsof fractu rezones detectedinside thetun- nel.The risk associatedwith hydraulically conduct ive frac- turezones isfurt herincreasedwhen the zones areoverlain by a thinor highly permeabl esoil cover.Insome locations, the contamina nt transpo rt riskmay be greater from the flanks oftopograph icdepressions, where the overburden is commonly thinner than immediately above the deeply weath ered cent ralpart of a fracturezone.Even outside frac-
ANNUKKAL1PPONEN NGU-BU L L439, 2002 - PAG E31
Fig. 4.Generalised map ofthe Paijannetunnel in relationto resident ial and ind ust rial areas, risksites and roads.Thepot ent ial risk activity is concentratedintheHelsinki Metropolitanarea in the south, and in the vicinity of the main roads crossing the tunnel. The residential and indus- trial areas are from the National Land Survey's(NLS)land-use database.
Roadsand water bodies - NLSpermission7/MYY/01.
ture zones, locally significant groundwate r inflow was observed to leak from horizontalfractu ring ,part icu larly in granitic rocks.Itis beneficial to conside rthesurfaceand tun- nel data in combination,to verify whet her those fractu re zoneswithsurface expression are significantfor groundwa- ter flow at the tunnellevel.
Eskers with relativelyhighgroundwater flow rates also present potential flow paths.Coarse-grainedsoil types with high permeability increase therisk of a contami nant being
Southern Finland
200km
20km
N Paijanne tunnel
_ Residential areas Industrial areas
7V. Railvvays
N Main roads
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Risksites
transporteddown with percolating groundwater and reach- ing bedrock fractures. However,despite the groundwater storage available,not all the esker systems displayed high groundwater inflow at the tunnel depth.Apparently,the soil type in the contact zone overlying the bedrock surface and the openness of fractures determine whether the ground - water stored in the overburden leaksint o the tunnel.The significance of the contact zone on the groundwaterinflow was earlier emphasizedby Olofsson(1991).
The present studydemonst rat es the use ofGISas a pow- erful tool for data overlaysof fractured zones in bedrock, overb urden characte ristic sand groundwat er cond it ions for producing aninteg rated description of the geologicaland hydro geological environment.Spati alanalysis of thenatur al environment wit h infrastructu re and hazardous activiti es allow s risk assessmenton ascaledependenton the qualit y of thedata.
Thepressure levelinthetunn el is a major factor affect- ing the groundwater flow in the vicinity of the tunnel. A marked pressure drop at the Kalliornaki pumping station, approximately64 km south of Lake Paijanne, from elevation +78 m to +42 m (asl) steepens the hydraulic gradient towards the tunnel south from the station .Farther to the south,the pressure level gradually approaches the local ground surface and even exeedsit in places,which dimin- ishesthe inflow.The flow towards the tunnel isinferred to have been highest at the time of construction and during subsequent maintenance work when the tunnel had to be emptiedofwater. According to the measurements,ground- water inflow appeared smaller during the repair work in autumn 2001 than at the time of constructionfor most tun- nel sect io ns in thenorthern most 64 km ofthe tunnel.
Conclusions
Potentially hazardous activities sho uld beminim isedin the areasindicatedin thest udyasbeingvulnerabletoconta mi- nationand possiblyin hydraulic connecti onwith thetunnel.
Further study and quantitative riskanalysisshouldbe con- sidered if there is a potentialcontamination source inthe vulnerabl earea.
Due to the weathe ring of fracture zones,ageing of tun- nel reinfo rcement sandeffects of construction and excava- tionon ground wat erflow patterns, a regular monitoring of groundwa ter cond it ions within the zone of influence around the tunnel isvital.
The presentstudydemonst rat es the applicabilityof ordi- nary geolog ical observati onsand engineeringrecords in an envi ron mental assessment.Municipal and environmental aut hori ti es canutili setheinformat iononthesensit ivity of areasasaguide in land-useplannin g.The hydrogeological descript ioncan also beapplied todirect ing protectivemea- suresand toest imat ing the extentofacti onreq uired in the caseof a chemicalspill.
Risk sit esareconcentratedin the southe rn part of the tunnel,where developm ent is most intense,and inseveral
NGU-BULL439,20 0 2 - PAGE 32
locations where main roads cross the tunnel. Cautionisrec- ommended for the exte nsive underground constructions planned inthe vicinity of Helsinki-VantaaAirport and the Helsin ki metropolitan area's mainring road,since the exca- vations may createflowpaths by disturbing therock matrix and also because fracture zoneswit h associated groundwa- terinf low have been detectedin the area.
References
Brit sch gi, R.& Gust afsson, J.(eds.) 1996: Suo me n luokite ll ut poh- javesialuee t(English abstract:The classifiedgroundwater areasin Fin land, p. 384). Suomen ymp d risto 55. Finnish Environment Inst it ut e,387 pp.
Haavisto,T.1998:l.ikaantumisriskiaaiheuttavatto im innot Paijanne-tun- nelin laheisyydessa(Activit iesposing a contamination riskin the vicinityof thePaijan neTunnel). InFinn ish.Suomenym p dristokeskuk- sen mon istesarj a138.FinnishEnvir onm entInstitu t e,58pp.
Lippon en, A. 2001. Paijann e-tunnelin yrnpa rist oqeo lo q ia ja -riskit (Englishabstract: Enviro nm ent al geology ofthePaij an neTun nel andenvironme ntalrisks,p.139).lnFinnish.Suomen ymparisto525.
FinnishEnvironmentInstitute.
Mikkola ,J. &Viitala,R.1999: Pitkienvesitunneleidenso rt um atjaluj it us.
(English abstract:Cave-ins and reinfo rcem entin longwat er tun- nels).InFinnish.He/sing inkaupunginkiinteistovlrastongeotekn isen osaston tiedote 80, Helsinki City Real Estate Depart me nt,Geo- tech ni calDivi sion,130pp.
Niini,H.1968.Paij anne-Helsinki tunnelitutkimukset(Geolog ical investi- gatio ns fo rthePaij ann eTunnel).In Finnish. Helsingi n alueenveden- hankinna nyleissuu nni telma,liite ERakenn usqeotoq isenyhd istyksen ju/kaisu(Papersof theEngineering -Geo lo gic al Societ yof Finl and)2 (20),28pp.
Olofsson,B.1991:Impact on groundw ater condit ion sbytunneli ngin hard crystallinerocks.TRITA-KUT/91: 1063.RoyalInstituteofTechno- lo gy,Stockholm.165pp.
Palmu,J.-P.1999:Sed ime ntaryenv iro nmentof the SecondSalpausselka ice marg inaldepo sitsin the Karkki la-Lop p i areain so ut hwes te rn Finland.Repo rtofInvestigat ion148.Geol ogical Surveyof Finland, 82 pp.
Pokki,E.1979:Pohjavesikysym yspaij anne-tunnelissa.(Engl ishabstract:
Eff ect of the constructionofthePaijannetun nel on groundwat er, XV-3).ln Finn ish.Kalli omeka n iikanpaiva 1979.Rokenn usqeoloqisen yhd isty ksenju/kaisu(Papersof the Engineering-GeologicalSoci ety ofFinland)13,VII,1-11.
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