Groundwater transit times in a small coastal aquifer at Esebotn, Sogn og Fjordane, western Norway

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Groundwater transit times in a small coastal aquifer at Esebotn, Sogn og Fjordane, western Norway

HELGEHENRIKSEN,NORALFRYE&ODDM UNDSOLDAL

Henriksen,H.,Rye,N.& Solda l, 0.1 996:Gro undwat er transittimesina sm all coastal aquifer at Esebotn, Sog nog Fj ordan e,weste rn No rw ay.Nor.geo/.unders.Bull.431,5-17.

Ashallowalluvialaq uiferinEsebotn, Sog nog Fjordane,Nor w ay,hasbeen inv est ig at ed fromahydrological sta nd - pointin order toobt ainestim atesof groundwater transit time s.Waterlevels, oxyge n isotopes,wat er tem perat ures andpreci pi ta tionweremo nit ored overaone-yea r period.Groun dw ate rtransittimes werecalculatedap ply ingatra- ditio nal Darcyan ap p roachand byusingoxygenisotopes and temperature astracers.All methodsgivetransittimes of60-90 days.Theoxygenisotopesindi cat ethat groundwater recharg edueto infiltrat ionfrom theriver Yg leelvi makesup about80%of thetotalground waterrechargein thecentralpartsof the aquiferduringthesu mmerseason.

He/geHenriksen, SognogFjor dan e College, Departmentof Natura/Sciences,P.Box133,N-S801Soqndal,Norway.

Nora/fRye,Universityof Berqen,Geotoqical tnstitu te,Allegt.4l,N-S007Berq en,Norway.

OddmundSoldal,Geofut ur umAIS,P.Box.22,N-S049 Sandsli,Norway.

Introduction Hypothesis

Only 13%of the population in Norway use groundwater for drinking water purposes.However,thereis anincrea- singawarenessof the utilisation of groundwater;main ly because of betterraw-waterqualityand source protec- tio n,and a higher cost-effectiveness of the watersupply compared wit h surfacewater(Elli ngsen & Banks1993).

Moreover,astronger emphasishas been placedonwater qualitydueto the ratificationof the EU direct iveon drinking water quality. Particularly,the food and beverage ind ustr y and allaccommodationestablishments will have to document water qualityaccording to these regul at i- ons. This has strengthened the efforts to locate new groundwater resources. In coastal Norway, many aquifers in smallriver deposits and deltashavebeen considered as marginal or second-class with respect to water quality and capacity.The main argumentsagainst the exploitation of these aquifersfor drinking water have been the supposed short transit timesand the risk for sea waterint rusion.How ever, compared with surfacewater, which isoften bacteriolog ically contaminated and may also carry humi c substa nces,the exploit at ion of these marg inal groun dwa ter resources may be more cost- effectiveas itwillrequir elesswater treatmentto obtain high-qualit y drinking water.

Theaquifer in Esebotn isa typicalexamp leofthis sort of marginal aquifer.The aquifer has earlier beeninvesti- gate d by geophysi cal and hydr og eol ogi cal meth ods (Bergersenetal. 1987,Eneset al. 1992,Soldalet al.1994).

In thefoll owingaccoun twe will showhow theuseof simple hydrologic almet hods can givevaluable inform ati on about the proportionof infilt rated riverwaterin the aquifer, groundwater flow and transit times;which are all important parameters pertaining to aquifer prote ct io n and waterquality.

Our hypothesis is that oxygen isot opes providea useful means of elucidatinggroundwatertransittimesin coastal areas where climaticcondit ionsare variable.Temperatu- re measurements and hydraul iccalculations arealsosim- ple methodswhich canadequately be applied to calcula- te groundwater transit times,and the reliab ilityofthese methods can be controlled by oxygen isotopes provid ing that statisticaltoolsare used.We claim that smallfluvial deposits are adequate aquifers with respect to water supplies and that they can be protected in areasonable manner.

The study area

With regard to topography/catchment,climate and geo- logical architecture,the aquifer in Esebotn is typical for many aquifers in coastal Norway.The Esebotn aquifer is centered around the riverYgleelvi,and is an alluvial fan deposit grading into a coastalfluvial delta. It covers an area of about 0.195 krrr',and is located in a small valley bottom surrounded by steep valley sides which reach alt- it udes of a thousand metresor more (Figs.1&2).The valley alluvium consistsmain lyof coarse graveland sands which have been derived from an older ice-marg inal deposit nearthe head of the valley(FigA). As a result of the isost ati c upliftof the land mass which followed the term inati on of the last ice age about 10,000 years ago,this ice-margi nal depositwas eroded and the sands and gravelswere redepositedfurther downstream on the river plainandin Esefjord(FigA).

Theinvest igated part ofthe aquiferonthe easternside of the river Ygleelvi has an area of about 0.105 krrr'.

Hydrogeological and geophy sical field investi gati ons

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6 Helge Henriken,NoralfRye og Oddmund Soldal GU-BULL 431.1996

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Fiq.l.Locationmapofthe study area show ing theaquiferin Esebotnand itscatchmentarea.

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NGU-BULL 431,1996 He/geHenriksen,NoralfRye&OddmundSo/dol 7

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Fig.2. The aquifer in Esebotnwith parts of itscatchmentarea.Esefjordis seenin theforeground,and the surrounding mountainsreach altitudesof about 1100-1200 metresabove sea level.

Fig.3.Graph showing percentageof catchment area situ ated abovea sp ecified altitud e.

Fig.4.Mapshow ing observationpointsand deducedgroundwaterflow directionsbased onpenetra tion depthofgeoradarsignals from Solda letal. (1994).

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8 ^HelgeHenriksen, NoraffRye& Oddmund Solda l GU-BULL431,1996

(Soldal et al.1994) show that the aquifer in this area is built up of gently seaward-dipping layers of sand and gravel with a total thickness of about10 metres,underla- in by at least 40 metres of silt and clay(Bergersen et al.

1987), At the lower boundary of the aquifer, organic material is encountered in the fine-grained sed iments.

Dating of roots and plant remains encountered during installati on of piezometer B(Fig.4)gave an 14C age of

1085±85 years B.P. (TUa-615, Trondheim). In such a young depositwe conside r it likelythatorganicmaterial was alsoorig inallypresent in theoverlying coarse-grain - ed sediments .However,arapid throug hflowcausedby a high permeability and hydraulic gradient must have enhanced the decomposition of the organicmatte rlea- dingto the present -dayoxidising conditionsthat prevent the dissolut ion of ironandmanganeseoxides.In the fine-

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Fig.5.(a) Profile from YgleelvitoKaldebekken in late spring/earlysummer when the discharge inYgl eelviis high.(b) Th e sam e profile when the discharge in Ygleelvi isIow.

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NGU-BULL431,1996 Helge Henriksen, Norolf Rye&Oddm und Soldol 9

17.09.92 26.11.92 4.02.93 15.04.93 24.06.93 2.09.93

Ygleelvi

Fig.6.Depthstowaterlevelfrom arbritary datums at (a) Ygleelvi; (b) observa tio n pointsS,E,H,FI andKalde bekken.

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grained sedimentsorganic material still remainsdue to a slower throughflow,and reducing conditions exist with possibilities for mobilisationof Fe and Mn.

The catchment has a total areaof about 12 km'. The lower part in the valley bottom comprises only about 5%

of this area, and most of the catchment is situated at high-alti tu de elevations(Fig.3). The main drainageof surface water is through Ygleelvi, supplemented by seasonal drainage through some valley side meltwater and floodwater streams. The average annual precipitationfor the nearest precipitation station at Skarestad in Fjoerland, about 25 km further north, is 1905 mm (Nor sk Meteorol ogiskInstitutt 1987).In our observation period, from October 1992 to October 1993,the precipitation in Fjeerlandwas 2190 mm.Therewas a marked precipitation maximum in the late autumn/early wint er months (Novem ber-January). In the low-lying parts of the catchment much of the winter precipitationwas rainfall,while the area above approximately 800 metres made up a con- siderablesnow-packwhereinmuch of thewinter precipi- tationwastemporarilystored.

Geophysicalinvestigation s(Soldalet al.1994)indicate that groundwater recharge takes place mainly by infiltration of river water through an eastward-facing bend in the Ygleelvi (Fig.4, area I)about 200 metres upstream from its outletinto the Esefjord.Precipitationand surface water whichinfi ltr at es the aquiferfrom the valley sidesin the east is considered to form only a small contribution to the groundwater recharge. Groundwater discharge

takes place in a marshy area on the western side of Kaldebekken(FigA), attested by the water levelsin a piezometer-nest (F1-F2-F3)in thisarea. The dischargetakes place from natural springsand from variousdug ditches and river-canals, in particular by the groundwaterdomi- nated stream Kaldebekken (Fig A). Surface runoff from the streamBat skredi and themountainsides in the eastis estimated to make up 0- 15%of the totaldischarge of Kaldebekken.In dry and cold periods,Batskredi has prac- tically no discharge. In short periods of heavy rainfall, however,the dischargeof Batskredimay make up a signi- ficant componentofthedischargeinKaldeb ekken.

The discharge of Ygleelvi controlsthe water table in the aquifer, which fluctuates with the water level in Ygleelvi (Figs. 5&6).The hydraulicgradientin the directi- on from the recharge area to the discharge area at Kaldebekken is roughly the same as the gradientof the fan-delta, i.e.about0.06.If we take the water levels in Ygleelvi and Kaldebekken to represent the hydraulic headsin the recharge and discharge areas,respectively, the hydraulic gradientis found to varybetween0.03and 0.05.The hydraulic gradient reaches its highest value during the main snow melting in late spring and early summer.

The pressure of the groundwater flowing through the deposit clearly overprints the tidal effect of the sea water;

which infl uences the water levels in the aquifer in a noti- ceable manner onlyin the observationwells D and P clo- setothe shoreline(Enes et al. 1992).Geoelectric measu-

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10 HelgeHeruiksen, oraliRye&OddmundSo/do/

rements (Soldalet al. 1994)clearly show that sea-water int rusionisleast pronounced during the spring andearly summer when the aquifer is flushed wit h largevolumes of freshwater from the snowmelt.

M eth ods

Stud ies of residence timescanbe carried out by theuse ofhyd rauli c calculat ionsorbyenviro nmenta l or artificial tracers.

The hydraulicapproach is dependenton gathered field data such asgrainsizeand piezometricheadsin orderto estimate hydrau lic conduct ivity, porosit y and hydr aulic gradient. From these data, average linear velocit y and hence residence timescanbe estimated.It has been clai- med that the accuracyof thismethod is limi ted, main ly due to the uncertainties in the estimates of the aquifer properties.The use of tracing techniques has therefore been recommended(e.g. Freeze&Cherry 1979, pA 27).

Naturallyoccurring tracers such as oxygenisoto pes or physical parameters suchastemperat ure haveprovento be appli cable in studies of residence times in aquifers where the residence time isshort« 1 year) (e.g. Walton 1970,Haldorsen et al.1993),providingthatthereexistsa seasonal variation for the measured parameterin precipitation or infilt rat ing surface water. By monitoring the time of propagat ion of the seasonal maxima or minima from recharge areato dischargearea,informationabout transit times can be obtained .The advantagesof using oxygen isotopes and tempe rature are many: they are inexpensiveand simp leto measureandmoveasintegral parts of the normal ground waterflow wit hout retar dati- on due to adsorption,etc.In Norway,the use of oxygen isotopes in groundwater studies has yielded fruitfu l results in small aquifers situatedininland areaswit h stable climat ic condit ions and well defined catchments (Haldorsen 1994).

Natural wateriniti ally formedbyevaporatio nof ocean- icwater contains the oxygen isotopes¹⁶0 and ¹⁸0.The isotopic composit ion of oxygen inawatersam pleis normally characterised by the isoto pic ratio ¹⁸0 / ¹⁶0 relative to the same ratio in a known standard water sample. Usually this is'standard mean ocean water'or 'SMOW' (Craig 1961).The relativediffe rence in the isotopic ratios is defined as:

GU-BULL 43 1996

anic water. The

8

¹⁸

0

^%0 valueswill thusnormallybe negative.Fora given geographicarea,the 8¹⁸0 % 0 valuesof precipitatio n will depend on air temperature,the amountandintensity of precip itation,distancefromthe ocean,alt itudeandprevailing winddirection .Onewould consequent lyexpect both regiona land seasonal variations in the

8

¹⁸0 %0values.

Table1. Observationpoint s in the study area and types of parameters measured.ObservationpointFis a piezomet ernestwherethree piezo- metersFl,F2 andF3 areinstalled at diffe rent depths.

Observationpoint Paramete r

Y Ygleelvi Waterlevel.temperature,0"0%0 G Spring Waterlevel,temperature,

o '

0%0 K Kaldebekken Waterlevel.temperature,0"0%0 B Piezometer Groundwaterlevel

F Piezometernest(Fl, F2,F3) Groundwaterlevel H Piezometer Groundwater level

A Precipitationstation Airtemperature,precipitation,O·0%0 E Piezometer Groundwater level

C Piezometer Seawater level

D Piezometer Groundwater level P Piezomete r Grou ndwater level

Data and sampling

In this stu dy 11 observatio n point s were established (FigA, Table 1).Once a week, waterlevels,water tempe - ratures,air temperature and precipitation were measured.Atfour ofthe obervationpoints,watersam pleswere collected for det erminationof8¹⁸0 %0.Thesam ples collected at the precipitation station were accum ulated sam plesfrom theprecedingweek. The observationperi- od lastedoneyear,from October 1992 to October 1993.

In addit ion,precipita t ion andevaporation data from the nearest meteorological observat ion station,Skarestad in Fjcerland,were available( Norsk Meteorolog isk lnsti tu tt 1992a,1992b,1993a,1993b).At observation point sBand F and at severalotherlocations,observation wellshave been drilled and aquifer samples gathered at various depths for estima tio n of hydraulic conductivity from grain-sizegradat io ncurves(Enesetal. 1992).Water-level data for esti mati on of hyd rauli c gradients were also recorded.

Resul ts a nd d iscussion

Darcianapproach

As the relativ e difference between the isotopic ratios is small,itisoftengiven in per mille:

The water vapour inthe atmospherewhich later conden- ses to form precipitationis depleted in¹⁸0 relativeto oce-

Based on the groundwate r flow directionsdeduced by Soldalet al.(1994),the ground wate r residencetime from the recharg e areaIto the discharge areaat Kaldebekk en canbecalculatedfrom theequation:

ne· L(m) t(d)=- - - -

K(m/d )·i

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NGU-BULL431,1996 He/geHenriksen, NoraiiRye&Oddmund So/dol 11

Table 2.Calculatedgroundwaterresidencetimesfromrechargearea(I) to dischargeareaatKaldebekken fordifferenthyd raulicgradientsand effectiveporosities.

dence time is shortest (62- 92 days)in the spring and early summer when the hydraulic gradient is high (about 0.05).

Fig.7. (a) Monthly precipitation data from the nearest metearological observotionsta tio n,SkarestadinFj <erland,during theobservationperiod;

(b)Monthly weighted meansof8'0%0atEsebotnagainsttheprecipitation

datafromSkarestad.

where t is the residence time in days and L is the distance in metres.The hydraulic condu ctivity K is an average valueof a numberof K-values det erm ined by Enes etal.

(1992).Theresidenc e times calculated (Table2) are considered to represent the dominant water throughflow, ig noring lesser inhomogenet iesin the aquifer.Theresi-

-4

1992 ..E 1993

Oxygenisotopes b^{' 8}0 %0

The precipitation has an annual meanb^{l 8}0 value of -8.58

%0, with maximum and minimum values of 1.17 and -17.40.The winter 1992/93 had periods with unusually high air temperatures, strong southwesterly winds and large amounts of precipitation. Several high b^l8 0 %0 values indicate that the isot opiccompositionwas influen- ced by airborne sea-water aerosol s during episodes of ext reme weathercon ditions. Neverth eless,the monthly weig htedmeansshow a distinct seasonalvariati on, with rising values from Decem ber to June, and decreasing valuesfrom Julyto Novem ber (Fig.?). Oneimporta nt condit ion for applying oxygenisotopes inst udies oftransit times wasthus present. Ourassump t io n isthat recharge ofgrou ndwater takes place mainly by infiltration of river water from Ygleelvi. If seasonal variations in the river's

bI B0 %0values are present, this isotopiclabelling of the river water should be traceable in the aquifer and thus provide a means of determining the residence time for the water in the aquifer.

In drainage systems controlled by surface runoff one would expect the highest^b^{I B}0 %0values in the summer months and the lowest during the winter.In ourareathis could be different,because the temporarily stored winter precipitationwhich meltsduring the spring and summer months can reverse the isotopic fingerprints in the river water. In Ygleelvi one must expect any seasonal variation inbI B0 %0to be a result not only of seasonal variations in thebIB0 %0of the precipitat io n,but also of the supplies of meltwatercarryingwith it the low^b^{I B}0 %0isotopic sig- natu res.ThebIB0 %0 valuesalsodecreasewith alt it ude.

Thisaltitudeeffect will result in aloweringofthebIB0 %0 valuesof0.15-0.50%0per 100 m(Yurtsever&Gat 1981).

A commonly observed alti tu de gradient appears to be near 0.2%0 per 100 m(Eriksson 1983).As much of the drainage area of the Ygleelvi is situated at elevati ons above 800 - 1000 metres,the altitude effect will give rise to lower^b^I^B0 %0 values than one would expect at this distance from the coast.The water samples from Ygleelvi show an amplitude in their ^b^I^B0 %0values of about 1.5 per.mille about an average value of -11.11 .

Compared with the observation points in the discharge area,Ygleelvi has the lowest bI B0 %0 values (Fig.8).

Kaldebekken has the highest values (average -10.32), while the values for observation point G(average-10.82) fall between the values for Ygleelvi and Kaldebekken.

None of these cited averages are weighted for flows.The differencebetweenthe three average valuesis statistical- lysign ificantat the 95%confidence level. The pattern of increasingbI B0 %0values with distancefrom the recharge areais most prominent in the period from April to September, and supports our view that recharge of groundwater takes place by infiltration of river water 14.6 14.6

225 225

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12 Helge Henriksen.Noraff Rye&Oddmund So/dol GU-BULL 43 1. 996

- 6

01.04 .93 Ygleelvi

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17 ,09.92 26.1 1.92 4.02.93 15,04.93 24 .06.93 2.09 .93

Fig.B. Variations in8"0%0during the observation period at(a)Ygleelvi; (b)Observation pointG;and(c)Kakiebekken.In Fig. Be,thewaterlevelin Kaldebekkenis also depieted.The vertical bars indicatestartsofdecreasingtrends for8"0%0.

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NGU-BULL431,1996 HelgeHenriksen,Noralf Rye & OddmundSoldal 13

Mean Mean Mean Mean

Annual Nov.92- May 93- April93-

March 93 Aug.93 Sept.93

8"0Yg leelvi -11.11 -10.93 -11.38 -11.35

8"0Precipita tion -8.58 -10.56 -5.13 -7.13

8"0Kald bekken -10.32 -10.65 -10.04 -10.14

%river infil tra tedgro un dwater 68.55 24.30 78.50 72.50

Ta ble3.Mean8'0valuesfromYgleelvi, pre- cipita tion andKaldebekken and thecalcula- ted contri butio nsofriver-infi ltrated water leaving theaquifer atKaldebekken inselec- ted period sfromOctob er /992toOctober /993

from areaI.The trend of increasing8'80 %0values is considered to refl ect direct surface infiltration of local precipitat ion on the river plain labelled with higher

8

¹⁸

0

^%0

values. In April,any infi lt rati on of melted snow on the river plainwould,due to the altitude effect,have higher

8

¹⁸

0

%0 values than the meltwater dominat ing the discharge of Ygleelvi. In the summer months,the

8

¹⁸

0

%0 values in Ygleelvi were rising (Fig.8a),but were still much lowerthan the8'80%0values (-5to-7)of the precipitation on the river plain.In water which is a mixture of water from differentsources,the 8'80 %0 value is given by:

where xjis the proportion of water with the isotopic composit ion 8j180 (Gonfiant ini 1981). By using the average

8

¹⁸0 %0value for the periodAprilto September (-11.35) forthe river water and the averagevalue of -7.13 for the precipitation,from Table3, an estimate of the relat ive contributions of river and surface water infi ltr at ion can be obtained by simple iteration.The result indicates that during this period, the water leaving the aquifer at Kaldebekken was a mixture of 72.5 % river-infiltrated water and 27.5%water derived byrecharge of precipitation on the river plain. In the period May- August the contribution of river-infiltrated water was 80 %.These figures are comparable wit h figures from a much larger riverdeltaat Sunnda lsera(Soldalet al. 1994).

80 70 60 50 E ⁴⁰ E ₃₀ 20 10 0 -10

80 70 60 50 E 40 E ₃₀ 20 10 0 -10

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17.09.92 26.11.92 04.02.93 15.04.93 24.06.93 02.09.93

Fig.9.(a)Dailyprecipita tionatthe metereologi cal station Skarestad,Fji£rlan d,(b) Differencebetweendailyprecipitation andevaporation atSsatestad, Fj i£r/and,(c) Air temperatureat Esebotn,weeklymeasurements.

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14 HelgeHenrisen,Noralf Rye& Oddmund Soldal GU-SULL 43.1996

Fig. 10.Tempera turetrendsat(a)Yqleelvl, (b) ObservationpointGand(c)Kaldebekken depict edby 10th. order regression curves.

Thevertical bar indicates theposit ion ofthe temp eratu re min imaateach oftheobser- va tionsites.

YGLEELVI

KALDEBEKKEN 08.04.93

OBSERVATION POINTG )

•

25.02.93

•

17.09.92 26.11.92 4.02.93 15.04.93 24.06.93 2.09.93

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I-w 5 4

In the period November-March the average 8¹⁸0 %0 valueswere-10.93 (Ygleelvi), - 10.65(Kaldebekken) and -10.56 (precipitati on). These figures indicate that the water in the aquiferduring thisperiod was a mixture of 24.3%river-infiltratedwater and 75.7%water derivedby infil t rationofprecip itationon theriver plain. Theclimatic cond ition sduring this winter were, however, extreme wit h large amounts of rainfall-prec ipitation on the river plain. In the highe r alt itudepartsof the catchment the temperatureswere lowerand theprecip itation wasin the fo rm of snow.Thewinter discharge of Ygleelv i was normal,but the amountof precip itatio n infil t ratin g on the riverplainmust have beengreaterthan norm al.

InYgl eelvi, a minimuminlate Novembe r 1992(Fig.Ba) is considered to represent the cont ribution of large amountsof autumn precipit atio n with low8¹⁸0 %0valu- es.It is,however,diffic ult to trace this isotopic signal to the observat io n pointsin the dischargearea.Thewater samples in Kaldebekken have an autumn minimum which occursbefore the autumn minimum in Ygleelvi.

Althoug h the observat ion point s in the discharg e area are predom inantly groundwater springs, they receive

precipitationas surfacewater from smaller flood streams during short and intensive periods of heavy rainfall. The lateautum n 1992 and the early winter1993was characterised by mild weath erwith large amounts of precipitation(Fig.9a),and several episodeswit h storms and floo- ding were recorded.In Kaldebekken,thisledto frequent variati onsin discharge (Fig.Bc)andthe8¹⁸ 0 %0 values musthave beeninfl uencedby precipitat ionsurfacewater from the flood-stream Batskredi. This isotopic labelling appearsto have been received earlier in Kaldebekken thaninYgl eelvi,probablybecausethe latterhas alarger discharge andalso receivesalarger part of its discharge by runoff in the upper soil layers. The autum n 1992 groundwater levels were generally high, and direct gro undinfi lt rati onofprecipitation couldalsohave influ- enced the isotopic composition at the observat ion point s.Weconcludethatthe extremeclimaticcondit ions thatexistedat this timeofthe year were not favo urable forstudyingresidencetimes inthe aquifer.

From March to September the weather conditions were more stable .In thevalley bottomthere wasan early snow melt. The amo unt of precipitation was small,and represent ed only about 30% of the total precipitation

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NGU-BULL431,1996

during our monitoring programme. Over long periods, the evapotranspiraton exceeded precipitation (Fig. 9b ).

The recharge of groundwater at this time of the year must have taken place mainly by infiltration ofriver water from Ygleelvi,and the observation points in the discharge areawere hardly affected by precipitation derived surfacewater.Thisperiodseemed more promisingforstudi- es of transit times on the basis of our environmentaltra- cers.

The measurements from Ygleelvi (Fig.8a) depict the start of a decreasing trend for

8

¹⁸

0

^%0from about 1 April.

We consider this trend to represent the input of meltwater from the high-altitudeparts of the catchment, label- led with the winter's low isotopic signatures. At observa- tion point G the start of a decreasing trend is recorded around 15 April (Fig.8b).Provided this is the arrival of the same isotopic signal, a transittime of about 15 days from the recharge area to G is indicated.

The interpretation of the

8

^{18 0}^%0values for Kaldebek- ken is not as straightforward. A decreasing trend during the first weeksof May is succeeded by a rising trend which is followed by a second decreasing trend from about 1 June (Fig.8c). The first period with decreasing

8

^{18 0}^%0values coincides in time with a periodwhere the water levels in Kaldebekken are high, but without any precipitation.The lower parts of the drainage area were free of snow at that time,so we consider this trend to represent the input oflocal high-altitudemeltwater from the flood-and meltwater stream Batskredi (FigA ).The snow pack drainedby Batskredi,however,is small,as the area available for snow accumulation is very limited because of the steep topography. The isotopic finger- print from this input of meltwater has probably only resulted in a small lowering of the

8

¹⁸

0

^%⁰ values in

Hydraulic calculations

Oxygen isotopes

Temperature

.1

HelgeHenriksen,NorolfRye & Oddmund Soldol 15

Kaldebekken. The second decreasing trend, which is more pronounced,can be related to the propagation of the isot opic signal from the meltwater in Ygleelvi. This interpretat ion gives a residence time of about 60 days from the recharge area at Ygleelvi to Kaldebekken. In Ygleelvi there are also two subsidiary maxima, M1 and M2, on the 27 May and 22 July.The arrival of these two isotopic signals is recorded at observation point G on 17 June and 19 August,and at Kaldebekken on 29 July and 23 September.Both these signals givea transittimeof 64 days from the recharge area I to Kaldebekken.

Temperature

The temperature of the groundwater in shallow aquifers is affected by daily and seasonal variations in the air temperature.The temperature in groundwater formed by infiltration of river water will display roughly the same trend as the river water. There will,however,be a time- lag,and the temperature of the groundwater could also be slightly higher (Walton 1970, Kihlstr0m 1993). By monitoring the propagation in the aquifer of a well- defined temperature maximum or minimum in the river water,it should be possible to obtainan estimateof the residence time in the aquifer. The time -temperature regressioncurves(Fig.10) depict similarseasonal variation patterns for all three observation points.In Ygleelvi,a well-defined temperature minimum is ident ified on about 4 February(Fig.10a). Similar temperature lows at observation point G and Kaldebekken are identified around 25 February and 8 April,respectively(Fig.10 b,c).

The temperature of the groundwater leaving the aquifer at this time was about 1

se

higher than the temperature of the cold water entering the aquifer in early February.

92 days 62 days

60 days

64 days

87 days 64 days

24.09.92 26.11.92 04.02.93 15.04.93 24.06.93 02.09.93

Fig. I/.Transit timesobtained bythevarious methods.Seetextfor discussion.

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16 HelgeHenriksen,NoralfRye&Oddmund Soldal NGU-BULL 431,1996

Weconsi derthe lows on the time-temperature regression curvestorepresent the arrival of the temperature sig- nalthrough the aquifer,thusgiving areside ncetime of about64daysfromthe rechar gearea bythe riverbed in Ygleelvi to the discharge area in Kaldebekken.If we consider only the data points,a subsidiarymin imaaround 1 May may be associated with the sameinp ut of a compo- nentof cold meltwater from Batskredi which caused the lowering of the 8¹⁸0 values in Kaldebekken at that time, Alt ern at ively itcouldbeint erp ret edas a transit tim eof 87 days.

Comparison of results

Calculat ed transit times obtained by the different methods are summarised in Fig,11, The three isot opic8¹⁸0 sig- nalsfrom water entering the aquifer in the period 1 April to 22 July give transit times of 60, 64 and 64 days, Hydr aulic calculations based on the prevailing hydraulic gradient during that period (Le.0.05)givetransittimesof 62 days(ne

=

^0.20)and 92 days(ne

=

^0.30).The temperature signal that entered the aquife raround 4 February gives transit times of 64 days or alternatively 87 days.

Normally we would expect longer transit times in the winter monthsdue to a smallerhydraulic gradientat that time of the year.How ever,thewinter of 1993was extre- mely mild and wet. From early February the temperatures in Esebotn were above 0°C,and therewas a considerable amount of precipitation in February and March (Fig.9).

Hence,the wint er discharge in Yg leelvi in 1993 did not differ much from the snowmelt discharge in the late spring and early summer, and hydraulic gradientsand transittim esforthesetwo periods would not differ signi- ficantly. We thus considerthe results obtained from these three methods to be broadly comparable.The consisten- cy in transit times obtained from the 8¹⁸0 signals sug- geststo us that the oxygenisot o pes are themostreliable and precisemeansfor calculat ing transittimes.

Conclusions

Simple hydrogeological calculations ap pl ying a Darcian approach givetransittimes of about 60-90 daysforthe river wat er from Ygl eelvi ent ering the aquifer in lat e spring and early sum mer.The use of environmentaltra- cers, oxygen isot o pes and temperature givecomparable results. We conclude that the environmenta l tracer ap p roach also hasap plications in areas where climat ic condit ionsare more ext reme and variabl e than ininland areas.A pre-requisite for such an approach isathorough monitoring of thetotalhydr ol ogicaland climaticsyst em.

The monitoring ofthe propagat ion of theisotopi csign al delivered bythe snow-melt in high-alt itudeareas appears to be the mostfruitfulapproach inst udi es of residence times. The oxygen isotopes also provide valuable informationabout the relative contribution sof infi lt rat ed

river wat erand morelocalwat erderived by recharge of precipitationon the river plain.The proportio nof rechar- gedue to river infiltration and precipitation variesin both space and time.Such information is particularly important in the context of groundwater development,asit could be used to select optim um abstrac tion sitesfor groundwaterwithregardto protectionandwat er qualit y.

Acknowledgements

This workwascarriedoutasajointproj ectbetweenSog n ogFjordane CollegeandtheGeolog icalIn stitut e,University of Berg en. The project received financial sup po rt fro m TheNorweg ianResearch Council for Scienceandthe Humanities(NAVF). Oxygenisotopeswereanalysed at the MassSpectro me tric Laboratory atthe Universityof Berge n.Dating oforganicmaterial was carried outatThe LaboratoryforRadiological Dating,Trondheim.Datacollection and observations were conscienti- ouslycarriedout by Stei narGronenq.Wethank SylviHaldor senfor rea- dinganearlydraftofthemanuscript,and thetw o refereesDavid Banks andArve Misundforco nstr uctivecrit icismandvaluabl ecomments.

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Helge Henriksen,Norot!Rye&Oddmund Soldat 17

Manuscript received September1995,revised version accepted March1996.

Groundwater transit times in a small coastal aquifer at Esebotn, Sogn og Fjordane, western Norway