three-dimensional mathematical groundwater flow m odel.

DAVIDSEGAR NGU-BULL 432,1997 - PAGE119

Numerical modelling of an unconsolidated aquifer, Birkelandsmoen, Sauda, Southwest Norway

DAVIDSEGAR

Segar,D.1997:Numericalmodelling of an unconsolidated aquifer,Birkelandsmoen,Sauda,SouthwestNorway.

NorgesqeoloqiskeundersekelseBulletin432,119-126.

The aquiferformed bythefluvial depositof theRiver Storelvaat Saudain Rogalandhasbeen investigated by geo- physical measurements,drilling,pumpingand groundwatermodellingtechniques.This aquiferisbeingconsidered as apotential watersupplysource for Sauda municipality.Theprincipalobjectivesof the studywerei)to provide a more accurateassessmentof theaquifer'scapacitythanthatprovidedby previous studies,ii)to determinetheeffect of municipalabstract ion onan abstraction boreholeownedbyamineralwatercompany andiii)to assess the useof artificialrechargeto reduceanysucheffect.

Thegroundwaterflowmodel MODFLOW andtheparticle-trackingmodelMODPATHwere usedto modeltheaqui- fer.Thismodellingindicatedthatthecapacityof theaquiferisapproximately20l/s.The modellingalsoindicated, however,that any significa nt municipalabst ractionwillalter the groundwater flow patternin theaquifer andmay leadto increasedpumpingcosts forthemineral watercompanyasaresult of increaseddrawdownsinthe aquifer. Themodellingresultsalsoindicatethat althoughartificial recharge mayenablemunicipalabstractionto be increa- sedwithout resultingin additionaldrawdown at themineral water borehole.the integrityof themineralwatermay be compromised.

DavidSegar, Norgesgeologiskeundetsekelse,Postboks3006 Lade,7002Trondheim,Norway.

(Presentaddress:5ThePaddocks,Cockermouth,Cumbria,CA 13OZA,England)

Introduction

Due to the requirem entfor an alte rn at ive water so urce to thecurrentsurf ace wate r supply,themunicipal ityof Sauda inthecounty of Rogaland asked theGeolog icalSurvey of Norway(NGU) to assess potential gro u ndwat er sou rcesin themunicipa lity.Thecapa cityof theunco nsol id at ed aq ui fer at Birkelandsmoen hasbeen demonst rat ed in previous stu- dies (Viak AlS 1991 and 1992).However,concerns over the possible effect of developmentof theaquiferon anexist ing abstract ion borehole being operat ed by a min eral wat er company led to the req uest by Sauda municipality for an assessment of the effects of the construction of a new ground w at er pumping station on theground waterflowin the aq ui fer and,in part icu lar,on the mineralwater abstrac- tion currentlytaking place.

Numerical groundwater modelling techniques were chosen to assessthe poten tialeffects of theco nst ruct io n of the pumping station as a method was required whichcould cope with the complexity invo lved in the assessment of the effects of abstraction from an anisotropic heterogeneous unconsolidated aquifer oflimited lateral extent interacting with a river.

The initial field investigationsco nsist ed of aGeorad ar su rv ey and a site visit . Data fro m prev iou sfield investiga- tionson theaq u ife rwerealso used inthe study.Thesedata incl ud edinvestig at ion boreholesand test pumping results (Viak A/S1991 and 1992)anda seismic survey(Geo t eamA/S 198 1).Thesedat a werethen interpretedandentere dintoa

three-dimensional mathematical groundwater flow m odel.

Follo w ing calibration,the mo d el wasused to sim ulat ethe groundw at erflow in the aqu ifer under various abstraction scenario s.The possibilityof usingartifici alrech argewas also assessed using the model.A part icl e-tr ackin g mo del was used to det erm in egroundw ater flow pat hs andflowrates und erthese scenari o s.

Geology and hydrogeology

The aqui fer studied lies in the area of Kleiv f lata, Bir ke land smoen,nextto the RiverStorel va in themunicipal- ity of Sauda , 80 km northeastof Stavanger(Fig.1).This aqui- fer isa fluvio -glacialdepositand consistsofint erlayered gra- vels,sandsandsilts.There are no overlyinglow-permeability layersandthe aq uiferisconsequ ent lyunc onfin ed.

Abedrockshelfthat extends from Herheim inthe north- west to Seermyrhauqene in the southeast dominates the geometry of the aquifer. Thelo cat io n of this bedrock she lfis shown in Fig.1.Borehole9103 is locatedim m ed iately adja- cent to this shelf and shows 12-13m of fine-grainedmaterial overlying the bedro ck.The thicknessof the aquifer incre ases rapid ly dow nst ream and by boreh ole 9102 the aq uifer is app rox ima tel y 20 m thick.The aq uiferinthe area of this bore- hole is much coarser grain ed and consistspredom inantlyof sand s and gravels.The aquifer shows areduction in grain sizeagain to w ard sborehol e9104whichindicat edunsorted med ium-to fine-g rainedsands and gravelstoadepth ofat least 19m.TheGeorad arprofilestaken in the area support thisinterpretatio n.

NGU-BULL 432,1997 -PAGE 120 OAVIDSEGAR

Fig.1.Map oftheKlelvflataand Birkelandsm oen st udyareashowing the investigation boreholesand Georadar profiles.

No clearvertica ldivision of the aq u ifer can be est abli she d on the basis of geologica l logs from boreh ol es or the Geo rada r profil estaken in the area. Crud e field measure- ments of perm eab ilit y taken by Viak A/S (199 1) sug ges t, howe ve r,thatthe geo logical prof ilecanbedivid ed intofour layers.Thehyd rau licpropert iesof these layersvary conside- rably lateral ly, however,and in areas suc h as the bed ro ck shelf,nogeolo gi cal different iat ion of the vertical profile is possible.A summar y of the gene ralised characteri sti cs of theselayer sisgiven inTabl e1.

Theaq uife r's lateral exte ntand thickn ess were assessed on thebasisof topog raphy,a seis mi c investig at ion carr ied out by Geot eam A/S (1981),a Geo rad ar su rvey and field

9106() lnvesu gauon/Observationborehole o Test pumpingborehole

$ Mineralwaterabstractionborehole

t..

Georadar profile with starti ng

~i tiOn(lOOmintervals mark ed )

mappingundertaken byNGUin 1996 and anint erp retatio n of apu mping test on a borehole adjacent to borehole 9105 by Viak A/S (1992).Areas with exposed bedrock were map- ped in the field.These areaswere considered to represent the physic al limi tof the aq ui fe r.

Dataon the aq u ife r'sperm eabil it y,poros it y andspe cifi c yieldwere obtain edfrom grain -sizedistribu t io ns using the Bayer meth od (Langgut h & Vo ig t 1980).Estim at es ofthe aq u ifer's transm issiv it y andspecificyieldwere also obtained from the testpumping resultsfrom ViakA/S(1992) usingthe Neuman(1975) and Cooper & Jacob(1946) methods.

Interpretation of drawdown data from borehole 9108 duringthetest pumping by ViakA/S(1992) usingStallman's method (Krusem an & de Ridder 1992) ind icat es that low- per m eab ilit y material exists approximate ly 32 m from this boreh ol e(t he method gives no indicatio n of direction).A site visit revealed that bedrock was expose d in an area so utheast of borehole 9108 that was consequen tly inter- prete das anaquifer bo und ary. Interpreta tionof drawdown dat afro m bo reho le 9104 usingStall m an's methodalsoindi - cate d that there islow-perm eabili ty materialapproximately 93 m from thisborehole.The sitevisitshowedthatthe reare springsin thisarea.

Layers1,2 and 3 are assumed to extend over the ent ire area but boreholelo g s,grain-sizedistributionsand Georadar profiles from the area indicate that these layers varyconside- rab lyin co mposit ion both verticallyand horizontally.

The aqui fe ris rechar gedby precipitationand infiltration fro mtheRiver St o relva,wh ich is regulated.Gro un d w at er lev- els inthearea along theRiver Storelva are controlled by the waterlevelin the river(ViakA/S1991).Viak A/S(199 1) also sta te that this infiltration is reversed during periodsoflow flowinthe river.Theflow in the river is usua lly low and flows between 0.7 and1.7m3/sare most common(NVE1996)but it can increase to between 2-85 m3/s for short periods.

Recharg e from precip itation wasesti m at ed as a percentage oftotal precipitation .A prelim inary value of 770mm/yr(35%

of thetotal precip itation)was used.

Abstract ion from the aq uiferat the mineral water bore- hole (seeFig.1)curre ntly takes place at an average rate of app roximately1.7

l i s.

Layer Geologicaldescription Approximate thickness(m)

1 Predominantlyhighly permea ble 11

sandygravels.

2 Permeabletohighlypermeable 3-6 coarse sandsand gravels.

3 Permeablegravelsandsands.Highly 3-6 permeablenear borehole9105.

4 Bedrock -

Table 1. Simplified characteristics of the geological profile in the Kleivfl ataarea.

Simulation of groundwater flow in the aquifer

The United States Geolog ica l Survey (USGS)thr ee-d im en- sio nal groundwater flow model MODFLOW (McDo nald &

Harbaugh 1988)was selected to sim ul at e the ground water flow in the aquifer. The USGSparticle tracking code MOD- PATH(Po llo ck1989)was used to calculate groundwaterflow lines and residence times.By calculatinggroundwaterresi- dence tim es inthe aqui fer under different abstractionscen- arios,MODPATHcan be used to investigatethe effect of ex- plo it at io n of the aquiferfrom a newgroundwaterpumping stationat Kleivflata.

DAVID SEGAR

Thedatafileswhich MODFLOWand MODPATH requ ire were constructed using VisualMODFLOW(Waterloo Hyd rog eol- ogic Inc.)whichwasalso used to presenttheoutput results graph ically.

The mathematical model

The finite-diffe rencegridand the hydraulicboundariesused to simul atethe aquifer areshown inFig.2.Thegridconsistsof fourlayers, which represents layers 1-4show n inTable1.The outer edge of the gridrepresents ano flowboundarywit h the river acti ngin sucha way as to directthe regional ground- waterflowin asout hwesterly direction.Thecellsrepresenting theaquiferoccupy the valley trending northeast-southwe st across thegrid.Outside of the aquifer the grid isformed of cells representing low-permeability bedrock.In areas where bedrockis consideredto crop out, all layerswereassigned the hydraulic properties of the bedrock. In order to check the vali- dity of the grid boundaries,the model was tested wit h alter- nativeboundary conditions.As no discernibleeffect could be seen on the groundwater flow in thevicinityof theaquifer it was consideredthat the boundariesweresatisfactory.

Thegrid consistsof smallerblocksin thevicinity ofthe pumping boreholes in order to increasethemodel'saccur- acy in thisarea.Each block in each layerisassignedapre- liminary value for aquifer thickness,permeability,recharge

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Fig.2. The MODFLOW finite-differencegrid usedtosim ulatethe aquiferat Birkelandsmoen(grid numbersshownin metres).

NGU -BULL 432,199 7 - PAGE 12 1

rate (from precipit ation)and a starting groundwa te r head value.Thepreliminary valuesforaquife rthickness,permeabili- tyandrechargeratewere adj ustedduringmodel calibration.

A topo graphic map ata scaleof 1:5,000ofthe area was digit ised using SURFER(GoldenSoft ware Inc.)and the resul- ting data file was importedinto Visual MODFLOW. A digit i- sedmap of physical features in thearea(roads, rivers,build- ings, etc.) was also con structed using CorelDraw and im- port ed into VisualMODFLOW.

TheRiverStorelva was simul ated using theriver package in the MODFLOWmodel. Waterlevels in the riverwere taken from leveldatameasuredby Saudamunicipalityduringthe pumping testcarriedout byViakA/Sin 1992.Theflowin the river at thetime these measurement sweretaken wasapp- roxim ately 0.7-1.5 m3/s (NVE 1996).The transient model took intoaccount observedchangesin theriverwaterlevel during the pumping test sim ulated.River leakage factors wereadj usted during the modelcalib ration process.

Calibration of the MODFLOW model

Asteadystate versionof the MODFLOW model wascalibrated by simul ating the observed ground water levelspriorto the testpumpin g undertaken by Viak A/S(1992).The esti mated rechargerate wasfixedduring thecalibration procedure and the estimated hydraulic parameters adjusted until the simula- ted groundwaterlevels were sufficientlyclose to the obser- ved levels(basedona minimisingofthe rootmeansquare of the errors in observedversus simu latedgroundw ater head).

Adjustments were onlymadeto parametersforwhichimpre- cise esti mateswereavailableorwhere data were lacking.

A time-variantversionof themodel wascalibrated by sim- ulating thegroundwater levelsduring thetestpumping by Viak A/S(1992).Thesteadystate version of the model was usedas a starting point for thisprocess.Theestimat ed hy- draulicparametersfrom the steadystate versionof theMOD- FLOW modelwere furtheradjusteduntil the model success- fullysimulated the groundwaterlevelsduring thefirst14 days of the pumping test. Following asuccessful simulation of these data,themodel wasthenrun again as asteady state model in orderto checkwhet her the adj ust ment s thathad beenmadehad altered thesteadystate version ofthe model.

This process was repeated unt ilthemodelcouldsimu lateboth the steadystategroundwater levelsbeforethetestpumping and time-variantdrawdo wnduringthe test. Assessmentofthe successof the modelcalibrationprocess wasundertakenbya visualinspect ionofacomparativeplot ofsimulatedand ob- servedground waterheads.Thepumping test wascarriedout duringaperiodof negligible recharge from precipitat ionand the errors involved in the time-variant calibrati on areconse- quent ly expected to becomparatively small.

Simulation of flow in the aquifer

during different abstraction scenarios

Following calibration,MODFLOWwas usedto simulate the

NGU-BULL 432,1997- PAGE 122

grou ndwaterflow in the aquiferusingdifferent abstra cti on scenario s.The particl e-trackin gmodel MODPATH was then run usingthe data out putfrom MODFLOW.Theuse of MOD- FLOW and MODPATH tog et her produced data on the groundwater flowdirections,flowrates,residencetimesand groundwater head valuesunder a number of abstraction scenarios.In order to investigate the hydraulic conditions existingin the aquiferand the likely conditionsgiven further exploitation,the models were run using five abstraction scenarios:

1. Current conditionswit h pum pingfrom the mineralwater abstractio n borehole.

2. Assessmentofthe capacityoftheproposed groundwa ter pumping sta tio n at Kleivila ta. This scenario assumes pumping from both the mineral wate r borehol e and the propo sed groundwa te r pumping station.MODFLOWwas run a num ber times wit h increasing abst ract ion rates fromthepumping stati on unt ilchanges in groundwater flows and drawdowns reached an unaccepta ble level withrespecttothe mineralwaterabstractionborehole.

3. The effectof constructionof a groundwate rpumpingsta- tion.TheMODFLOWand MODFLOWmodels were used to assess the changesin groundwaterflow direct ions,resi- dence times,etc. whichwould occur following the con- struction of a groundwaterpumping stationat Kleivflata.

4. The effectof a reduced abstraction fromthe proposed qro-

DAVIDSEGAR

undwaterpump ingstation.In thisscenario itis assumed that abstraction from elsewhere intheaquifer(at a dis- tance where no effecton the Klelvflata areawill occur) results in areduced abstraction requireme nt from Kleiv- flata.

5. The effectofartificial recharge.This scenario assesses the use ofarti ficialrecharge toincrease the aquifer's capacity and reducethe drawdow natthemineralwater abstrac- tionboreho le.

The simulation scenarios

Current conditions

A map ofthe groundwater table elevation immediately prior to the test pumping by Viak A/S(1992)isshown in Fig.3.This map showsthat the regional groundwaterflow in the aqui- feris towards the sout hwestand that the RiverStorelvacon- trolsthe elevation ofthewater table.TheRiverStorelvare- chargestheaquifer inthe areaimmediatel y upstrea mof the bore holesatKleivfliita but the groundwat erdischarges into the river dow nstreamof thisarea.

Figu re3 showsflow lines for groundwater traced from the mineral water abstraction borehole back toit srecharge area.This figureshowsthat groundwaterabstractedat this borehol eis derivedpredominant ly from the aquifernorth - east of the borehole.The figurealso showsthat some of the mineral water abstractedisrechargedupstre amofthebed- rock shelf.

- - -

100

m

o

N 1

- 65- Groundwater head(m) $ Observationborehole

++

Groundwaterflow line

(each arrow represents365days traveltime) 0 Mineralwater abstractionborehole

Fig.3.Simu latedgroundwater flow pattern occurringduringpumpingfrom themine- ralwaterabstractionbore hole(1.7l/s),Grid numbersshowscale in metres.

DAVIDSEGAR NGU-BULL 432,1 9 97 - PAGE 123

Assessment of the capacity of the proposed groundwater pumping station

Inorder to assess thecapacit y of the proposedgroundwater pumping stationat Kleivflata, MODFLOWwas run anumber of timeswith a constant abstractionof 1.7

l is

from the mine- ral water abstraction borehole butwit h an increasing ab- straction rate fromthe proposedgroundwaterpumping sta- tion. The pumping station'scapacity was consideredto be the maximum abstraction that does not result inan unac-

Fig.4.la)Simulated relationshipbet weenpumpingstationabstract ion rate anddrawd ownat the mineralwaterabstract ionboreho le.No artifi- cialrecharge. (b)Simulatedrelation shipbet ween thepumping station abstraction rate and the groun dw ater level at the mineral water abstractionborehole.Groundwater levelwithoutabstracti on from the pumping stationor min eral waterborehole isassumedto be 3.44m below groundlevel.Noartificialrecharge.

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1 0 20 30 40

Pumping station abstraction rate (lIs)

10 20 30 40

Pump ing station abstraction rate (lIs)

ceptable drawdown at themineral water abstraction bore- hole.

Agraph ofabstra ct ion from the pumpingstation against steady state drawdown at the mineral wat er borehol eis shown in Fig.4a.The corresponding groundwater levelsas- suming that thegroundwater level with no abstraction from eitherthepumping stati on or the mineralwater borehole liesat 3.44mbelowground level(b.g.l.).This levelrepresents the levelwhichexiste d priorto the pumping testcarri edout by ViakA/S(1992);theresults are shown inFig.4b.There are no concrete criteria that can be used to determine what drawdown would be unacceptable at the mineral water borehole and this complicatesthe determination of anew pumping station'scapacity.Amaximumdrawdownof 1.5m was selectedbasedon the groundwa terlevels andassumed filte rlevelat themineral water borehole andthelikely errors involved in the modelling process.Using this maximu m drawdow n,a capacity of approximate ly20

l is

is obtainedfor the proposedpumpingstatio n.

The effect of the construction of a pumping station at Kleivflata

The flow linesfor groundwaterabstracted at the mineralwater borehole and theproposedpumping station (w ith an abstrac- tion of 20

l is )

areshown in Fig.5.Thisfigureshowsthat,even wit h an abstractionof 20

l i s

fromthe pumpingstation,most of the water abstracted from the mineralwater borehole is now derivedfrom the River Storelva.The groundwaterpumping station has taken over thecatchment area of themineral water borehole.The residence time of the groundwate r abstractedat the mineral water borehole is thereforeconside- rably shortened andthis may have some effect on thechem- istry of the abstracted water, possiblyresulting in a soften- ing ofthe water.The disturbanceof the groundwaterflowin the aq uifermay also have unfavourableconsequences wit h regard to marketi ng of the abstra cted water as 'natural mineral water: The increased drawd ow n at the mineral wate r borehole willalsoresult inincreasepumpingcosts for the companyresponsiblefor theabst ract ion.

The effect of a reduced abstraction rate from the groundwater pumping station

Wit h abstraction from another part of the aquifer the ab- straction required from the proposed groundwater pump- ing station at Kleivflata can be reduced.A scenario with a red ucedabstraction rate of 10

l is

was therefore considered usingthe MODFLOWand MODPATHmodels.

Figures 4a and 4b showthat ared uced abstraction from the pumping station gives a much reduced drawdown at the mineral water borehole.Thegroundwa ter flow pattern simulated by MODPATH is,however, very similar to that show nin Fig.S.Theresultsofthis sce n a r io thereforein d ica te that even wit h thislowerabstraction rat e the groundwater flow patternsin the vicinity of the mineral water boreholes are significantlyaltere d.This is due tothe fact thateven an

NGU-BULL 432 .1997 - PAGE 12 4

N 1

DAVIDSEGAR

7--7-

Groundwater flow line

0

Proposedgroundwaterpumping station (each arrowrepresents 365 daystravel time)

$ Observationborehole

-65- Groundwater head(m)

o

^Mineralwater abstractionborehole ^{Fig.5.Simu lated}ground water flow patt ern occurringduringpumpingfromthe propo - sed groundwaterpumpingstation(abstr ac- tion rate 20 lis) and the mineral water abstractionborehole(1.7lis).

-7---7-

Groundwaterflow line

0

Proposed groundwater pumping station

(eacharrow represen ts 60daystraveltime)

$

Observationborehole

C

Proposed artificial rechar ge basin

-65- Groundwater head(m)

o

^{Mineral water abstractionboreh ole Fig.7.Simulated}groundwater flow pattern occurring duringpumping from the propo- sed groundwaterpumping station(abstrac- tion rate 40 lis)and the mineral water abstraction borehole(1.7lis)togetherwith artificialrechargeat 40lis.

DAVIDSEGAR

abstraction rat e of 10l/s is sti ll nearly sixtimes that of the mineral water borehole and the pumping station borehole will thereforestilltake overthe catchmentareaof themine - ral water borehole.lnthis case the min eralboreholeis again fo rced to take more water from the River Storelva.

A reduced abstraction rate from the proposed ground- water pumping station will therefore stillhave a sign ificant eff ect on the groundwaterflow patte rnsin the vicinityof the mineral water borehole.

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Fig.6.(alSimulatedrelationshipbetween abstractionfrom the propo- sedpumping stationandthe drawdown atthe mineral waterabstracti- onborehole.Artificialrecharge.(b)Simulatedrelationshipbetweenthe abstractionratefrom the proposedpumping stationand the ground- waterlevel atthe mineral waterabstractionborehole.Groundwaterle- vel withnoabstraction is assumed tobe3.44m belowground level.

Artificialrecharge.

NGU-BULL 432 ,1 9 9 7 - PAGE 12 5

Artificial recha rge at Kleivflata

Art ificial recharg e represents a potential solution to the problem at Kleivflata,The use of artificial recharge would enable the aquifer'scapacity to be increased at the same time as reducingthe drawdown atthe mineralwater bore- hole.In order to simulateartificial recharge, the MODFLOW modelwasrunwit h artificialrecharge rates of 20 and 40 l/s from a simulatedrechargebasin in the area southwest of the gravel quarry at Kleivfliita(see Fig.2).This areawas selected asthere is a thick unsaturated zone there whichwil l increase the groundwater residencetim es and enable better cleans- ingofthe waterbefore reachingthe saturatedzone.

Figu res 6a and6b showthe simulated drawdownsand correspo nding gro undwa te r levelsthat will occur with ab- st ractio n from the gro undwater pumpin g st at io nandart ifi- cial recharge rates of 20and 40l/snearthe gravel quarry. These figures show that the draw do wn in thevicin it y of the min eral water bo rehol e is great ly redu ced.Theuse ofarti fi- cialrecharge therefo re increasesthecapacity of the aquifer to ap proximate ly40 lis wit ho ut resulti ng in greater draw- dow ns.

Figure 7 shows the groundwa ter flow pattern in the vicinity of the boreholeswith an artificial recharge rateof 40 l/s. This figure shows that the minera lwater boreholeand the groundwat erpumping station now draw most of their water from therecharge basin. The groundwaterresidence timesfro m the basinto the pumping station are less than 20 days and up to 80-100 days to the mine ral water borehol e.

This will result in greater cleansing of the water abstracted from this boreholebut mayhave unfavourabl econsequen - ces interm s ofthemarket ingof thiswater as'nat ural mineral wate r:

Wit halower artificialrecharg e rate the pumpingstat ion and themin eral water boreho lewill drawmo re waterfro m theRiverStorelva. Thiswill reduce the effect ofthe artificial rech arg e on themineral water boreh ol ebutwillalsoreduce any posit ive effec ts in term s of red uced draw do wn in the aquifer.

Theuse oftheMODFLOWand MODPATH modelsthere- fo re indicates that arti fic ial recharge representsa possib le solutio n to the lack of aq uifer capacity at Kleivf lata.The mod elalsoindicates that the use of this met hod may have unfavourab leconseq uen cesforthemineral wate rcompany in terms ofaltering the natural groundwater flowpatterns in the aquifer.

Con clusions

Groundwatermodelling techniques combined wit h deta il- edfield invest igat io ns were successfullyused to assess the aquifercapacity and the effect of the construction ofanew groundwater pumping stat ion in the vicinityof amineral water abstraction boreholeat Kleivflata inSauda municipa l- ity.

The model results indicate d that unde rcurrent condi-

NGU-BULL 432,1997 - PAGE 12 6

tions themineral wate r abstractionborehol edraws most of itswaterfrom theRiverStorelvatothenort hof the borehole and the aquifer immediately northeast of the borehole.

Groundwaterresidence times are short,being in the order of 30-60 days.

Wit h pumping from bot h the mineral water borehole andanew groundwa ter pumpi ngstati on the groundwa te r flow patterns in the aquifer are altered significantly.The pumpingstation borehole will take over the catchmentarea of the mineral water borehole forcing the latter to draw more water from the River Storelva nearer the borehole.

Groundwater residence time sareshortened,possibly resul- ting in small chemical changes in groundwater chemistry.

These hydraulic and chemical changes will occur if the abstraction from the pumping station is largerthan that of the mineralwaterborehole.

Themodelresultsalsoindicate that20 l/s represents the aquifer capacity given the need to avoid excessivedraw- dow ns at the mineral waterborehole.Any additionaldraw- down at this borehole will, however,inevitably result in increasedpumpingcost s forthe mineral watercompany.

The modelresults have shown that artificial recharge of water from the River Sto relvaor ot her source should not have any negative effect on groun dwa ter quantity at the mineral water borehole.Some changesin groundwat erchem- istry may occur,however,althou gh these changes should not be in any way negative in terms of the water quality.

Inevitablechangesinground waterflow patt erns within the aquiferoccurringas a resultof arti ficialrecharge may,how- ever,have negative consequences in terms of the marketing of the water as'nat ural mineral water:

The use of numerical groundwater modelling techni- questo st udy the hydrog eology inthe Kleivf lataarea has enabled thepred icti on ofhyd raulic and,to acert ain extent, chemicalconditionsinthe aquifer under anumber ofpossi- ble aquifer developmentscenariosfor the development. In particular,the model results have shown that, although there should be no hydrogeologicalortechnical limitations

DAVIDSEGAR

preventing the exploitation of the aqu ifer, consideration must be given to the effects of this exploitat ion on the pumpingcostsof the mineralwater compa ny.The abilit yof the company to cont in ue to market the water as natural mineral wate r mustalsobe assessed.

Acknowledgem ents

I wouldlike to thank lOystein Jaegerfor obtaining much of the data used in thisproject and for many productivediscussions.Thanks are also due to David Banks who contributed many useful comments and sugges- tions on the originaldraft report.I would also like to thank Jan Frederik Ton nesen who undertook the Georadar survey along with lOystein Jaeger.Thanks arealsodueto Saudamun icipali tyfor their co-operatio n andassistance with this project and to NVE for providing valuable hydrological data.

References

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Geoteam A/S1981:Seismiske malinger for grunnvannsforsyning iorn- radetBirkelandsmoen .Report 6481.01.

Kruseman,G.P.& de Ridder,N.A.1992:Analysisand evaluationof pum- ping test data.International Institute for Land Reclamation and Improvement,Publication 47.

Langguth,H.R.&Voigt,R.H.1980:HydrogeologischeMethoden.Springer Verlag,486pp.

McDonald,M.G.& Harbaugh,A.W.1988:A modularthree-dimensional finite difference groundwaterflowmodel.Technical Water Resource PlanningU.S.G.S.Book6,ChapterA1.

Neuma n,S.P.1975:Analysis of pumping test datafromanisotropicun- confined aquifers considering delayed yield gravity response.Water ResourcesResearch 11,329-342.

NVE1996: Unpublished hydrological data from Saudamunicipality.The Norwegian Water andEnergyAuthority.

Pollock, DW.1989: Documentationof computerprogramsto compute anddisplay pathlines using resultsfrom the U.S.GeologicalSurvey modular threedimensionalfinite-differencegroundwaterflow mo- del.U.S.GeologicalSurvey Open FileReport 89-38 1.

Viak A/S1991:Sauda kommune,Grunnvannsundersokelser,Kleivflata. Asplan ViakSorAlS,Rappor tnr.45-4344,D-var8,AD.

Viak A/S 1992: Sauda kommune,Grunnvann Klelvflata.Asplan Viak Sor AlS,Statusrapport nr.45-4344,D-varlS,AD.

Manuscrip treceived June1997;revisedmanuscriptaccepted September1997.