Groundwater transit times in a small coastal aquifer at Esebotn, Sogn og Fjordane, western Norway HELGEHENRIKSEN, NORALF RYE&ODDMUND SOLDAL Henriksen, H., Rye, N. & Solda l, 0 .1 996: Gro undwater transit times in a small coastal aquifer at Esebotn,Sogn og Fjordane, weste rn Norw ay. Nor. geo/. unders. Bull. 431, 5-17. A shallow alluvial aquifer in Esebotn, Sogn og Fjord ane, Norway, has been investigated from a hydrological sta nd­ poi nt in order to obtain estimates of groundwater transit time s. Water levels, oxyge n isotopes, water tem perat ures and precipi tation were monit ored over a one -year period . Gro undwate r transit times were calculated apply ing a tr a­ di tional Darcyan approach and by using oxygen isotopes and temperature as tracers. All methods give trans it times of 60-90 days. The oxygen isotopes indicate that groundwater recharg e du e to infiltration from th e river Yg leelvi makes up about 80% of the totalgroundwater recharge in th e central part s of the aqu ifer during the summer season. He/ge Henriksen,Sogn og Fjordan e College, Departm ent ofNatura/ Sciences,P.Box 133, N-S80 1Soqndal, Norway. Nora/f Rye, University ofBerqen, Geotoqical tnstitu te, Allegt.4l, N-S007 Berqen, Norway. Oddm und Soldal, Geofutur um AIS, P.Box. 22, N-S049 Sandsli, Nor way. Introduction Hypothesis Only 13% of the population in Norway use groundwater Our hypothesis is that oxygen isotopes provide a useful for drinking water purposes . However, there is an increa­ means of elucidating gro undwater transit times in coastal sing awareness of the utilisation of groundwater; mainly areas where climatic cond itions are variable. Temperatu­ because of better raw-water quality and source protec­ re measurements and hydraulic calculations are also sim­ tion, and a higher cost-effectiveness of the water supply ple methods which can adequately be applied to calcula­ compared with surface water (Ellingsen & Banks 1993). te groundwater transit times, and the reliability of these Moreover, a stronger emphas is has been placed on water methods can be controlled by oxygen isotopes provid ing quality due to the ratification of the EU direct ive on drin­ that statistical tools are used. We claim that small flu vial king water quality. Particularly, the food and beverage depos its are adequate aquifers with respect to water industry and all accommodation establishments will have supplies and tha t they can be protected in a reasonable to document water quality according to these regul ati­ manner. ons. This has strengthened the efforts to locate new groundwater resources. In coastal Norway, many aqui­ fers in small river deposits and deltas have been conside­ The study area red as marginal or second-class with respect to water quality and capacity . The main arguments again st the With regard to topography/catchment, climate and geo­ exploitation of these aquifers for drinking water have logical architecture, the aquifer in Esebotn is typical for been the supposed short transit times and the risk for sea many aquifers in coastal Norway. The Esebotn aquifer is water intrusion. However, compared with surface wate r, centered around the river Ygleelvi, and is an alluvial fan wh ich is often bacteriologically contaminated and may deposit grading into a coastal fluvial delta. It covers an also carry humic substances, the exploitation of these area of about 0.195 krrr', and is located in a small valley marginal groun dwater resources may be more cost­ bottom surrounded by steep valley sides which reach alt­ effective as it wi ll require less water treatment to obta in it udes of a thousand metre s or more (Figs.1&2). The val­ high-quality drinking water. ley alluv ium consists mainly of coarse gravel and sands The aquifer in Esebotn is a typ ical examp le of t his sort wh ich have been derived from an older ice-marginal of marginal aquifer. The aquifer has earlier been investi­ depos it near the head of the valley (FigA). As a result of gated by geophysical and hydrogeological meth ods the isostati c uplift of the land mass wh ich followed the (Bergersen et al. 1987, Enes et al. 1992, Soldal et al. 1994). terminati on of the last ice age about 10,000 years In th e follow ing accoun t we will show how th e use of sim­ ago, t his ice-marginal depos it was eroded and the sands ple hydrologic al methods can give valuable inform ati on and gravels were redeposited further downstream on the about the proportion of infilt rated river water in the aqui­ river plain and in Esefjord (FigA). fer, groundwater flow and tran sit times; whi ch are all The investigated part of the aquifer on t he eastern side important param eters pertaining to aquifer protection of the river Ygleelvi has an area of about 0.105 krrr' . and waterquality. Hydrogeological and geophy sical field investi gati ons 6 Helge Henriken, Noralf Rye og Oddmund Soldal GU-BULL431.1 996 :.- ::; :::: ::; ;'00 '""'/. ."'1 Fiq.l. Location map ofthestudy area showing the aquifer in Es ebotn and itscatchment area. NGU-BULL431, 1996 He/ge Henriksen, Noralf Rye& Oddmund So/dol 7 1400 . ~ 1200 ui 1000 <ll E 800 ~ 600 ~ 400 « 200 . O -t------,-----,------,-----.---~ o 20 40 60 80 100 % of catchment area Fig.3. Graph showing percentage of catchment area situated abo ve a Fig.2. The aquifer in Esebotn with parts of its catchment area. Esefjord is specified altitud e. seen in the foreground, and the surrounding mountains reach altitudes of about 1100- 1200metresabo ve sea level. Fig.4. Map show ing observation points and deduced grou ndwater flow directions based on penetra tion depth ofgeoradar signals from Soldal et al. (1994). 8 Helge Henriksen, Noraff Rye& Oddmund Solda l GU-BULL 431, 1996 (Soldal et al. 1994) show that the aquifer in this area is 1085±85 years B.P. (TUa-615, Trondheim). In such a built up of gently seaward-dipping layers of sand and young deposit we consider it likely th at organ ic material gravel with a total thickness of about 10 metres, underla­ was also originally present in th e overlying coarse-grain ­ in by at least 40 metres of silt and clay (Bergersen et al. ed sediments. However, a rapid throughflow caused by a 1987), At the lower boundary of the aquifer, organic high permeability and hydraulic grad ient must have material is encountered in the fine-grained sediments. enhanced the decomposition of the organic matte r lea­ Dating of roots and plant remains encountered during ding to the present-day oxidising cond itions that prevent installati on of piezometer B (Fig. 4) gave an 14C age of the dissolution of iron and mangan ese oxides. In the fine- E 1 F Kaldebekken t ~ 2 l ~ V1 g 0 c... <lJ E; -2 3 " , ~ -4 .Cl ';;;- ~ - 6 , .0 , d - . - .. ' - ------- - - ----------- ~ - 8 - - " - ~ ' " .,....- -- -- - 0.. =- _#~_ ..:...-.:.._-:.:-: . ~ Cl -10 - - 12 LEGEND ~ Boulders 'dRiver channel 0 1 . '-~_ . I (0 bbI es and graveI -=--Groundwater level f : ~ ' ~ ' : ;1 Sand and gravel Fine sand, silVclay Q SO metr es o Dry groundwater creeks 00 ~~------ .. - 0 ·0 'c» 0 0 '" ~ 0 0 0 ... <Joc:> ~ ~ -nO O ~t::) ~~ V7o o ' o o # o .o 2 # 0 0 .. 0 0 . 0 ""0 0' ' 0 A 0: .0 °0 "'00 0 0 0 0 "' 0 - 0 . 0 0 "' 0 0 ° 0. 0 0 0' _ 0 0 - - - Cl C> 0 " 0 0 . 0 C> o c» ..- "_ c:... _---:- --.- --~ ---.........- .. ~ --:-:- 0 0 - _ _ . : . -2 "....... 0 0 0 0 ...~ :,. "-: ' - , ", - .- -, :'- __.-... --:----: - -; ., - 4 .- -6 , - - - -- - - - -- - - -- - -- --- ---_...:.......:...- -8 .... v:.: :,...- .....·:--·_- -10 - -- - - - - - - - - - -12 b Fig.5. (a) Profile from Ygleelvi to Kaldebekken in late spring/early summer when the discharge in Ygl eelvi is high. (b) The sam e profile when the discharge in Ygleelvi is Iow. NGU-BULL 431 , 1996 Helge Henriksen,Norolf Rye& Oddmund Soldol 9 -50 0 Ygleelvi 50 100 ~ 0 150 200 250 a) 300 -50 0 50 100 ~ 0 .: : ~ :", . ..... 150 "j . ...... ;.:...... E 200 250 b) Fig.6. Dept hs to water level from arbritary 300 da tums at (a) Ygleelvi; (b) observa tion 17.09.92 26.11.92 4.02.93 15.04.93 24.06.93 2.09 .93 points S, E, H, FI and Kalde bekken. grained sediments organic material still remains due to a takes place in a marshy area on the western side of slower throughflow, and reducing conditions exist with Kaldebekken (FigA), attested by the water levels in a pie­ possibilities for mobilisation of Feand Mn. zometer-nest (F1-F2-F3) in thi s area. The discharge takes The catchment has a total area of about 12 km'. The place from natural spring s and from various dug ditches lower part in the valley bottom comprises only about 5% and river-canals, in particular by the groundwater domi­ of this area, and most of the catchment is situated at nated stream Kaldebekken (FigA). Surface runoff from high-alti tu de elevati ons (Fig.3). The main drainage of sur­ the stream Batskredi and t he mountain sides in the east is face water is through Ygleelvi, supplemented by seaso­ estimated to make up 0 - 15% of the total discharge of nal drainage through some valley side meltwater and Kaldebekken . In dry and cold periods, Batskredi has prac­ floodwater streams. The average annual precip itation fo r tically no discharge. In short periods of heavy rainfall , the nearest precipitation station at Skarestad in Fjoerland, however, the discharge of Batskredi may make up a sign i­ about 25 km further north, is 1905 mm (Norsk ficant component ofthe discharge in Ka ldeb ekken.