Seasonal variations of fluoride content in groundwater from wells drilled in bedrock

NGU-BULL 435,1999 -PAGE53

Seasonal variations of fluoride content in groundwater from wells drilled in bedrock

ASGEIR BARDSEN,KJELL BJORVATN,KARISAND & DAVID BANKS

Bardsen,A.,Bjorvat n,K.,Sand,K.&Banks,D.1999:Seasonalvariations offluorideconte nt ingroundwa ter fromwells drilled in bedrock.NorgesgeologiskeundersekelseBulletin435,53-58.

Theaimofthe presentstudy wastoinvestigateseaso nalvariationsin theconcentration of fluoridein water sam ples from bedrockgroundwaterwells inthecount y ofHordaland,Norway.Watersamples from 42deep wellswere obtained once amont h for a periodofoneyear(1995). Sampleswere analysed for fluorideandpH.Someseasonal variati onin fluoride concentrationwasfou nd in all thesampledwells.As comparedtolow-fluoridewells,high -fluo - ridewellsshowedsignificantl y high erseasonal variat ions in fluoride cont ent.The statisticalassessment revealed an asymptotic significantdifferencebetweenthefluoridecontentin groundwaterfromthe different bedrocktypes.

There was also anasym ptotic sign ificantdifferencein the seaso nalvariation ofthe fluoridecontentofwaterin the different bedrockgroups.A negativecorrelationwas found between variationinthe fluoridecont ent and theage of the well.Thefluoride concent rationinthegroun dw aterstypically declinedafter period sofheavy rain,withanappar- ent delay of up to3month s.

AsgeirBardser;KjellBjorvatn,LaboratoryofDent alResearch,FacultyofDentistry,UniversityofBergen,Berge n,Nor way.

KariSand,Depart me n tofPlan nin g and Development,Hordaland Coun tyCouncil,Berqen,Nor way.

DavidBanks,Currentaddress:86Holymoor Road,Holym oreside,Chesterfield,Derbyshire,S42 7DX, England.

Introduction

Traces of fluoride may be foundin most naturally occurring waters. Rain water, rivers and lakes are normally low in fluo- ride« 0.1 rnq/l ),while the fluoride content of groundwater variesgreatly (Smith &Ekstrand 1996). Depending on the aquifer'slit ho logy, groundwater may contain hig hfluoride concent rations;recent surveys inNorway fou nda numberof groundwater wellswith fluoride concentra tio nsaround 10 mg/I (Bjorvatn et al. 1992a,1992b, Bardsen et al. 1996). espe- ciallyin high-pH,calcium-poor waters, where the f1uoritesat- uration 'ceiling'on fluorideconcentrations is relatively high (Morland et al. 1997).

In most reports,the cited fluor id econtent is basedupon theanalysisof one or onlya fewwate r samples.It has,how- ever,been demonstrated that significant seasonal fluctua- tionsmay occur in the fluorideconcentration (Larsen et al.

1989, Bjorvatn et al. 1992a).As small variations in thedaily int ake offluoride may critically influencethe mineralisat ion ofhumanteeth(Baelum et al.1987, Burt1993). knowledgeof the 'fluoride-profil es' of relevant drinking water sources is needed.In the county of Hordaland,western Norway, water withafluoride content rangingfrom 0.50to 1.49 mg/I has beenshown to increase the odds for developingdental fluor- osis 10 times as compared to children receiving drink ing water with a fluoridecontent less than0.10mg/l(Bardsen et al.1999).

Theaimof thepresentstudywas to monitorthefluoride content of a selectedgroup of bedrock groundwater wellsin westernNorway overa one-year period,in 1995,and relate the findings tofact ors suchaslocal geology, seasonalprecip- itation,well depth and well age.

Study area and bedrock geology

The study was conductedin the county of Hordaland,on the western coast of Norway.With approximately430,000 inhab- itants and an area of 15,500km²,the county rep resents,res- pectively,10%and 5%of Norway in terms of population and area.

Athorough description ofthe complexlocallit hologies has been given by Morland et al.(1997). Climatein the area is wet (At lanti c)wit h an averageyearly precipitation of 2250 mm and a yearly meantemperatureof7.rC(St ati sti sk Sen- tralby ra1997).

Material and methods

Onethousandwells were located in Hordaland, as described inaprevious study(Bardsen et al. 1996).All wellswere drilled in crystalline Palaeozoic or Precambrian bedrock,and were at least 20 mdeep.Water sampleswere collectedandanalysed for fluoride.Asthe upper limit for acceptable fluoride con- centrations in Norwegian drinking water is 1.5 mg/I (Sosial- og helsedepartementet 1995),the wells were divided into a 'high-f luoride' group(f luoride content~1.5 mg/l)and a'low - fluoride'group(fl uoride content<1.5 mg/I).lnorder to study possibleseasonal variations in the fluorideconcent rat ions in grou ndw ater,21wellswere randomly selectedfrom each of thetwo groups.

Water samples werecollect ed monthly(between the15^th and 20^th of each month)from all the 42 wells, during the period January - December 1995.Thesampleswere taken directlyfrom well-headtaps orfrom nearby domesticcold water tapsafter letting the water run forat least 5 minutes, according to established standards (Laxen & Harrison 1981, Morland et al.1997).The waterwas kept in sealed, clean poly- ethylenevials (30 mi) and immedi at ely dispatched to the

12 NGU-BULL435, 1999-PAG E 54

Laboratory of DentalResearch,University of Bergen,where the samples were stored cool(4° C) foramaximu mof 7 days.

Samp les were then analysed forfluorideand pH at roomtem- perature (20-22°C). The fluoride concent ratio n was deter- mined wit h an ion specificelectrode(model96 0900;Orion Research,Cambridge , Mass.,USA).The lowe r detection limit was10-⁶M(-0.02 rnq/l ) (Orion Research Ine.LaboratoryProd - ucts Group 1991).The pHwas determinedbythe use ofapH elect rode connected to a digit alpH meter(Orion 429A,Orion Research Ine. Laboratory Products Group 1991).

Thewater samples collected in January were all included inamoreextensiveanalysis(radon,fluo rideand62 elements as determined by ICP-MS),as described byMorla nd et al.

(1995,1997),Reimann et al.(1996) and Bjo rvatnet al.(1997).

Technical informationon thewells, such as year of drill- ing, depth of well andwat er yield, were takenfrom Bardsen etal.(1996). Meteorologicaldatafrom the appropriateareas werecollected fromthe Norwegia nMeteorological Inst it ut e, Division Bergen(Meteorologicaldatawere from thefollow- ing statio ns: Bergen-Florid a, Bergen-Stend, Eikanger-Myr, Takle and Eidfjord-Bu).The lit hology of the aquifers was ded ucedfrom available geolog ical maps(Torske 1973,Quale 1981,Andersen etal. 1988,Fossen & Thon 1988).

Variation in fluoridecontent and pH was characterisedas the interquartilerange(diff erence betweenthe75^thpercen- tile and the 25^th percent ile) of values

measured in wat er samples from each well throughouttheyear.

11

ASGEIRBARDSEN,KJEL L BJORVATN,KARISAND

s

methoderrorwasevaluatedby Dalberg'sstatistics(Dalberg 1940),and showedacceptablereproducib ilit y(Se=0.031).

Results

Some seasonalvariation influoride concentration wasfound inallthesampledwells(Fig.1).The fluctuation,however, was more prono unced inthegroup of high -fl uoride wells than in the low-fl uoride wells. Some variation in pH of the well waterswasalso observed throughoutthe year(Fig. 2).

The wells, which for the present study were selected solelyon the basisof previous one-samplewater analyses, wereall drilledin lith ologiessystematisedinto the following subgroup s; (i) gneiss(undifferentiated) (n= 13),(ii)granitic and band edgneiss(n= 16), (iii)granite(n=6),(iv) amphib- olit icgneisswit hpegmatite dykes(n=3),and(v) a groupof 'ot her mino rlit holog ies'suchasquartzite,gabbro and phyl- lit icslates(n = 4).

Median fluoride concent rations in the groundwater s rangedfrom 0.05 to9.04mg/I (Fig. 1).The statisticalproce- durerevealedanasympto ticsignifica ntdifference between the fluoridecontent inthediffe rent bedrock types(p<0.05) (Fig. 3).However,for the multiplecomparison,significantdif- ferences at the group level were found only between the groups'granitic and banded gneiss','granite' and'amphib-

Data analysis

Data were coded,computerised andana- lysed using the Statistical Program for SocialScience(SPSS-PC,Version 7.5).The data distribution s were neit her smooth nor normal;hence,non-paramet ricstatis- tical testswere applied.For example,the null-hypothesisthat the median values of (i)fluoride concentra tion,(ii)pH and(iii) variation in pHand (iv)variation in fluo- ride,werethesamefor subsetsbased on nom inal variables such astypeof bedrock, wastestedusing Kruskal-Wallis(KW)tests.

Ifthe obtaine d value for the KW test was significant,further multip lecomparisons were made to determine which of the groups were significantly different from the others.Correlat ion analyses were car- ried out between inte rval scalevariables usingtheSpearmanrank correlationcoef- ficient(rs)'Inorderto visualisethedist ri- butionwit hin some of thevariables, box- plots, which provide a graph ical data summary, were obtained from theSPSS program.

Erroranalysiswas performed on 150 duplicate measurements.Student'st-test for paired observations revealed no bias between these two series(p

=

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E c 8

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bc 6

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o

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0 5

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wells

Fig.1.Boxplotsshow ingfluoride concentratio n(mgll)through the year from the 42 wells used in the study.The wellsaresortedaccording todecreasingmedianfluorideconcentration.Thelower boun dary ofthe box isthe 25^thpercentileandthe upperboundaryis the 75^thpercentile.The hor- izontal lineinsidethe box represent sthe median.Thelengt h ofthe boxcorrespondsto theinter- quartilerange. Caseswit hvalues that are between 1.5and3 box-lengt hsfromthe upper orlower edge oftheboxare calledoutliers(0),whilecaseswithvaluesmorethan3box-lengt hs from theone of theedges are called ext reme values(*).'Whiskers' show the range tothelargest and smallest non-outlyingdata points.Eachwellisplotted with12 mont hly fluoridedeterminations.

The hor izont alline crossingthe y-axisat the1.5valueshows the action lim itfor fluorideconcentra- tion sindrinkingwaterinNorw ay.

ASGEtR BARDSEN,KJEL LBJORVATN,KARt SAND&DAVtD BANKS

10

6

wells

Fig.2.pHthroug h theyear inallthewells. Thewellsaresorted inthe sameorderasinFig.1,i.e.,indecreasingmedianfluorid e concentrat io n.

olit icgneisswit h pegm atit edykes'as compared to thegrou p 'ot herminorlithol ogie s'(p<0.05).

Thebedroc k groups'granitic and bandedgneiss','g ran- ite'and 'amphiboliticgneisswith pegmatitedykes' all pro- duced water samp les with monthly med ian fluor id e concent rationabovetheupperlimit(1.5 mg/I)of theNor we- giandrinkingwate rstandards(Sosialog helsedepart ement et 1995).

There was an asymptoticsignificant differencebetween the variati on in the differe nt bedrock groupsastoseasonal variati on influori de content (p<0.05)(Fig.4).Statistically sig- nificantdiffe rences at the group levelwerefound when the group s 'granite' and 'amphi bolitic gneisswit h pegmat ite

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typeofbedrock

Fig.3.Fluoride concent ratio n(mg/I)inwat ersamples fromdifferent typesofbedrock.The bedrock lithologies aredivided into5 groups: (1) gneiss;(2) graniticand bandedgneiss;(3) granite;(4)amphibolit icgneiss wit hpegmatitedykes, and(5) other(Guartzite/gabbro/phylliticslates).

Thehorizontallinecrossing the y-axiset the 1.5valueshow stheact ion limit for fluoridecincentration sindrinki ngwat er in Norway.

NGU-BULL435,1999- PAGE SS

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typeof bedrock

Fig.4.Variation (inte rquart ilerange) influo ride concent ration(mg/I)in the diff erent typesof bedrock.Thebed rocklithologies are dividedinto5 groups:(1)gneiss; (2)graniticand banded gneiss;(3)granite; (4)amphi- bolit ic gneisswit h pegm atit e dykes, and(5) ot her(quartzite/gabbro/

phylliticslates).

dykes'werecom paredto thegrou p 'ot her minorlitholog ies' (p<0.05).

Additionally, an asympt ot icsignificant diff erence in pH wasobserved(p<0.05) (Fig.5).At the grou p level,however, a stat ist ically significant differen ce was observed only between the group 'amphibolitic gneiss with pegmatite dykes'andthegroup'other minor lit hologie s' (p<0.05).The variation in pHshowed nosign ificant diff erencebetweenthe differenttyp esof bedrock.

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type of bedrock

Fig.S.pH inwate rsamplesfromthe differenttypes of bedrock.Thebed- rocklithologiesaredividedinto 5groups:(1) gneiss;(2) granit ic and banded gneiss; (3) granite;(4)amp hibo liticgneisswith pegm atitedykes, and(5) ot her(quartzite/gabbro/phylliti cslates).

NGU-BULL435,1999-PAGE56 ^ASGEIRBARDSEN, KJEL L BJORVATN,KARISAND

s

fluorideconcentration(mg/l)inthe January sample

... f'uiIc2wretJ<tkJI

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monthlyprecipitation (mm)

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fluorideconcentration(mg/l)

fluorideconcentration(mg/l) monthlyprecipitation (mm)

~ f I ~

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1 2 3 4 5 6 7 8 9 10 1112 month

Fig.7.(a) Fluorideconcentration(mg/l) inwatersamplesand monthly precipitation(mm)during theperiod January-December1995(month 1- 12),(b)Fluoride concentration(mg/I) in wellwaterJanuary-December 1995(month 1-12)ascomparedto localprecipitation(mm)November 1994-0 ctober 1995 (month 1-1 2) (i.e.,atwo-monthdisplacement shown in thefigure),

Discussion

Geological factors

According to Ko rit nig (1974),felsictypesof igneousrocks, suchas granite,tendto havethe highes t mean fluorine con- tent.Thiscor respo nds with our findingsin so far as the 2nd hig hest fluo rid e con centr ationsoccurinwell s drilled in gran- ite(Fig. 3).Thehighfluorideconcentrationinwater samples from wellsin granit eandgranitic gneissmay be due to disso- lut ion orionexchange processes from fracturemineralssuch as fluoriteor rock-for ming minerals such asfluor it e,apatite, amphibole,muscovite,biotit e,chlorit e,andsome clay miner- als(Gill berg1964,Korit nig1974).

Thehighestmedian concent ration of fluoridein ground- water, how ever,occurs not in granite but in amphibo litic gneiss.The reasons forthis may be three-fold;(i) that the mafic(e.g.am ph ibo le)min eralsinthese rocksare rich in flu- orine,(ii)thatthe rockscontainanab undan ceofpeg matit e dykes, which arelikelyto berich in fluor ide,or(iii)thatthe mafic mine ralsproduce water sof a higher pH,promoting anion exchange.In fact,Fig.6indicates that ionexchange of F' forOH' at highpHis a majorsource of fluoride in ground- wate r.

Tw oof thesamp led well swerelocated only30 m apart, in amphibolit icgneiss wit h pegmat itedykes. The depthsofthe two wells and the direction sof drilling werealmostide nti cal.

There was, how ever, a difference in the fluo ride con ten t

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9,5

For mostof the wells, watersamples withthe highest flu- orideconcent rat ions werecol lect ed during thespring(April -June)though some wellshad the maximum value in the Decembersample.Thewatersam p leswit h thelow estfluo- rideconcentrationwere,formost ofthe wells,collecte ddur- ing the winter(January-March).Somewells had the low est value in July and August.

Seventeen of the selected wells had fluoride concentra- tions above 1.5 mg/I throughout the year.In another 17 wells,analyses showedfluo ridevaluesbelow1.5mg/Ifor all months.Thefluorid econten tofthe remai ning8 wells fluctu- atedaroundthisvalue.Thevariati on inthe fluori de concen- trat ion (interquartile range) in the wells was positively correlated with the median fluoride concentration of the wells

tr , =

=

0.01).

The depthsof theselected wells ranged between 41 and

120m(25th,

so "

m, respective ly), and a positive correlation was found betw eenthe dept h of the well and thevariation in fluoride cont ent(r5= 0.31,P<0.01).A negativecorrelationwas found betw eenthe fluoridevariati on and the age of theground wa- ter well (rs⁼-0.39, P<0.01).

Fluoride contentand pH showed a positivecorrelat ionfor every mon th's data; the correlation coefficient (rs) varied from 0.71 to0.81throug houtthe year (p <0.01).Figure 6 showsthescatter plot for fluor ide and pH measuredin the January samp le.

A negative relation (t houg h sometimes delayed) was foundbetweenthe fluo ridecontent of thewellsand the sea- sonal,localprecipitation (i.e., low fluorid eandhig hrain fall).

Thisapplies to all wells,but patterns of seasonal variation were different.In somewells there was nodiscern ible delay in time between precipitat ion and thedrop in fluoridecon- centration (Fig.7a) (rs= -0.86,P<0.0 1).Other wells showeda post-precipitationdelayin the decrease ofthe fluoridecon- tent up to three months.Figure7b showsresultsfromawell wit h a 2-monthdelay(rs⁼ -0.72, P<0.05).

Fig.6,Fluorideconcentration(mg/I)versus pH measured intheJanuary sample.

ASGEIR BARDSEN,KJELL BJORVATN,KARI SAND

s

(median fluoride contentsof 6.51and 3.86,respectively),and different patterns of fluoridefluctuation in thesewells (Fig 1, third and sixth box from left).A similar situation has been describ edby Bazarov et al.(1964) who showed a clearreduc- tion in the fluoride content of water from the outer zone of a pegmatite dyke as compared to the sit uat ion wit hin the more central partsof the pegmatite.

Several authorshavenoted positivecorrelation of fluo- ride and pH, both in hard,crystallinerocks(Englund&Myhr- stad 1980,Banks et al. 1993,Bard senet al. 1996, Morlandet al. 1997) and in sedimenta ry rocks (Edmunds 1981,Banks 1997).The most common explanationfor this phenomenon is that fluoride in groundwater is dominantly derived not from dissolution of fluorite (as many havesupposed) but fromanionexchange offluorid efor hydroxid eon aluminosil- icates (amphiboles, micas) or, conceivably ,apatite.At high pH, hydroxid eactivitiesinsolut ion arehighand theseanions displacefluoride from ion exchange sites. The waters seem typicallyto be undersaturat edto saturated with respectto fluorite; thus, it seems thatfluorite sat urat ion represent sa ceilingfor fluoride levels.The interplayof these processeshas beendiscussedbyMorlandet al.(1997).Thehighestfluoride concentrations are mostly found in relatively calcium-poor water sfrom granitesand from amphibolitic gneisses (the chemicalcomposition of our January water samples are dis- cussed in detail by Morland et al. 1997). Mature granitic groundwaterscommonly tend to berat her calcium-poordue to the sodic/potassic nature of their constituent feldspars, placing the fluorite saturation 'ceiling' high.It islessclear why fluorite-richamphiboliticgneiss groundwaters should becal- cium-poor,but this isoftenthe case.Closer examin ation of the data from the January sample (Mor land et al. 1997) revealsthatthese wat ers generally show high pH, high non- marine sodium excesses, high sodium/calcium ratios, low calcium,and high fluorideconcentrations.Ion exchange of calcium for sodium, commonly observed in sedimentary lithologies as well as crystallin e bedrock may cont rol the solubility of fluoride.Insedime nt ary lithologies, thesefea- tures are commonly associated with the onset ofred ucing conditions (Edmunds 1981).Alternativ ely,low calcium and highpH may simply beachievedby exten sivecalcit eprecip- itation in very hydrochemic ally maturewat ers(Banksetal.

1998).

Technical factors

According to previou sfindings,groundw ater fluoride con- cent rat ion decreases wit h the age of the well(Bardsen etal.

1996, Morland1997).lnthepresentstudywealso observed a negative correlation bet weenthevariat ion in fluoridecon- tentand the ageof the well. It maybe that drillingand pump- ing of awellinducean inflowof low -fluoride surface waters orshallow,short-residence groundwater into the well's frac- ture-feeding system, thereby diluting existi ng fluorid e-rich ground w ater.Another possible explanation isthat, dueto better drilling techniqu es,newer wellstend to be deeper than older ones,thus draw ing on deeper, more 'mat ure' wate rs. Finally,thepresenceoffreshdrilling cutt ings,wit h high specif ic surfacearea,hasbeen suspected to enhance

NGU-B U LL435,19 99-PAGE57

concentrations of several paramet ersin newly drilledwells (Banks et al.1993).

Climatic factors

The highestfluoride concentrations were generally found in wat er samplescollectedin theperiod Aprilto June(some in December),whilethelowestvalueswere found in January- March (some in Julyand August).Normally, cold periods wit h low precipitation are expect ed in the winter.More rainfall and meltingofsnow isobserved in spring in westernNorway. In 1995, however,the only month with a meantemp erature below O°Cwas December(-0.8°C).Thewinter rainfall would thus have infiltrated the ground immediately, rather than being stored assnow. The driest periods in the county in 1995included May, June,August and December.The highest precipi tationwasregi steredin January-March,and October.

Thus, theAugustfindings (i.e.,a few wells wit h lowfluoride during August) seem to be the only deviation from our hypothesis of a negativecorrelation between precipitation (or, more strictly,recharge)and fluorideconcentration.

Conclusions

Theresults demon strate thatsignificant seasonal variations occur in the fluoride concentrations in groundwater. The Iithological groups'g ranit ic and banded gneiss', 'granit e' and 'amphibolit ic gneisswith pegmatite dykes'yield edmonthly median fluoride concentrations above the drinking wat er maximum concentration for wells (1.5 mg/I) accepta ble according to Norweg iandrinking waterstandards (Sosial- og helsedepartementet1995).

Seasonal changes in fluoride content of groundwater may be of greatimport anceto health,andindividual coun- selling on oralhealth should bebasedona so lid knowledg e of the fluoridecontentin therelevantdrinking wat er. In areas where groundwater isused,morethan onewater sample should be collected and analysed for fluoride before final counsellingconcerning fluoride useand oral healthisunder- taken.This is especiallyimportantinareas with fluoridecon- centrationsaround 1.5 mg/1.

Acknowledgements

Thank s areduetotheownersof the wells forprov idi ng monthlywate r samples throu ghout1995.The st udywas fina nciallysup po rted by grants fro m TheColgateResearch Fund.

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Man uscrip t received August7998;revised manuscriptaccepted January 7999.