Radon content in groundwater from drilled wells in the Stockholm region of Sweden

GERT KNUTSSON & BOOLO FSSO N NGU-BULL 43 9,2002 - PAG E79

Radon content in groundwater from drilled wells in the Stockholm region of Sweden

GERTKNUTSSON&BO OLOFSSON

Knutsso n, G. &Olofsson, B. 2002:Radoncontent ingro undwater from drilled wellsin the Stoc kho lm regionof Sweden.NorgesgeologiskeundersokelseBulletin439, 79-85.

Radonisthe mostimporta nt contributiontotheradioact ivityingroundwater andhas beenstu diedfordifferent reasons during the lasthun dre dyears inSwede n,inlater timefo cu sed onradon as a health problem.Based on data from anat ion al surveyonrado n concentratio n in drinkin g waterin the 1980s,airbo rneradiometric measurements, geologica l map s anduraniumprospecti ng,a gro undwa te r rad on risk map of Sweden was compiledinthe 1990s.

The map showed thatthere is more pro nou nced riskforrad on ingroundwaterinseveral,fairlylarg e areas in central and northern Sweden.Thebed ro ck in these areasusuallyhas agranitic composition.Many detaile dinvestigations ofwellson region al andlocalscalesweremade durin gthe1980 s and 1990s,altogether31,000radon analyses of groundwa ter from229 mun ici palities inSweden.12.2%ofallwellshadarad on value>1000Bq/L,but some counties had muchhig her percent age,up to23%.The high est valueina sing lewellwas 57,000Bq/L.Dat afro mthe county of Stockholmshow edverylarg elocaldifferences.The concl usio ns arethatthere aresome pron o unced riskareas and a positivecorrelat ionbet w eenhig h rad on concent rat io ns in drilledwellsandgranites,pegmatites,acidicvolcanites andsome acidicgneisses.How ever,wellswit hhigh radon concent rat ionsmay be found locally outsi deareaswith suchrock types.Sta t isticalanalyses ofdatafrom Osteraker andUppsalamun icipalities showthattheinstallatio ns for groundw ater withd raw aland theuseof the wellclearly affe cttheradon content,where asnoclear correlation betw een well dept handsoil depth versusrad o nconte ntis seen.

Gert Knutsson, BoOlotsson.Depar tment ofLand andWater ResourcesEngineering.RoyalInstitute of Technology,SE-lOO 44Stockholm,Sweden.

Radon - occurrence and migration

Radon is the most important contribution to the radioactiv- ity of groundwater. There are three different isotopes of radon, but only"'Rn,with a halflifeof 3.8days,formed within the uranium chain by decay of226Ra,is of interestas the other isotopes are very short-lived.The migrationof radon in the ground takes place in several steps including emanation and diffusion from uranium- and radium-bearing mineral grains,diffusion of radon through permeable rocksand soils, and transport of radon by groundwater flow.Groundwater in uranium-richcrystalline rocks often has an elevated radon concentration in relation tothe radon concentration of the surrounding rock.It ismostlikely explained by the factthat uranium isleached out of the rock and precipitatedtoget her with its decay products, forexample radium, on the surfaces of the fracturesin the rock.Radon isthen emanating from theradium-enriched coatingsdirectly to the groundwater in the fractures (Akerblorn&Lindgren 1997)(Fig. 1).

The consequencesof theseprocesses are not only that the radon concentration in groundwater ishighly elevated, but also that,in a firstste p,leacheduranium and radium can be transportedbygroundwater fairly long distances and is precipitatedalong theflow pathsin the fractures.Then, ina secondstep, radon emanates to groundwate r and istrans- ported farther awaybygroundwater flow. Thismeansthat groundwater with a high concentration of radon may be found in bedrock with low uranium concentrations,for

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Fig.1.Format ionofrad on gasinawater-filled fracture(afterAkerbl o m

&Lind g ren1997).

NGU -BULL 439, 2002 - PA G E 80

example, an artesian spring insedimentary rocks mayhave acquired its high radon in contactwith precipitated radium, which originates from underly inguranium -richgranite.

National overview

Studiesof radon in groundwater have been carried out for different reasonsduring thelast hundred years in Sweden.

The first investigations were related to mine ral and spa water- radioactivewater was thought to be healthyat that time. The radon concentration of the spring water at Vastersel(announced as'healt h water')was more that 5000 Bq/L.Some springsandwellswit hconsiderably high radon concentrations were found, thehighest in syenit e(Sahlbom 1916).Radon concentra tions became of interest again in

N

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Gene ra l ris k Spo radic ri sk Low risk

200 km

Fig. 2. Preliminary groundwater radon risk map of Sweden (after Akerblom&Lind gren 1997).

GERTKNUTSSON & BO OLOFSSON

connectionwith prospectingof uranium ores in the 1950s;

the highest value was measured in a spring area at Masungsby,Lapland,with a bedrock of granite with peg- mati te(Armands 1967).

During the 1960s and 1970s,radiation-protection spe- cialists studied radon in drinking water, in milk and mine- water (and air).Measurable concentrations of radon were found in milk from 13 dairies of around 300 dairies allover Swe den.Eleven of these dairies were situated in the granitic area of sout heasternSw ed en, where detailed investigations of ground w aterfor water supply to the farms in two areas werecarried out (Snihs 1973,Knutsson 1977).The highest radon conc ent rat ion s were foundin drilledwells in granites (maxi mum2900 Bq/L),aboveall in a red granite susceptible toboth chemical andmechanical weathering.Seasonal fluc- tuat ions were obvious but different in various types of groundwaterand wells.

A national survey ofradon concentrationin household wate rwascarried out bythe Swedish Radiation Protection Institu te (SSI)in the 1980s,andradon analyses of randomly selected wells were made during hydrogeological surveys by the Geological Survey of Sweden (SGU). During the 1990s, agro undw ater radonrisk map of Sweden was com- piledbySGUand SSI(Akerblom & Lindgren 1997) (Fig. 2).It was based on differenttypes of data:a data set of radon in drilledwells, airborne radiometric measurements,geological maps andolddatafrom the areas of uranium prospecting.

The risk map shows areas with an elevated risk for radon in three classes: more general risk,more sporadic risk and rela- tively low risk.More widespread general risk areas were ide ntifie din the Bergslag en province of central Sweden.In nor the rn Sw eden, Norrland,two large areas with general risk are marked,one areainthe southof Norrland and onein the nor t h, thecent ral part ofLap land(Fig.2).The areas are char- acteris edby bedrock with common occurrences of uranium- rich granites,pegmatitesand aplites.

Region al outline

Regional authorities (count y boards)as well as many local autho rit ies (t he municipalities) have carried out a lot of investig ations of rado n in groundwater during the 1980s and1990s. Aregional stu dyin Bohuslan(1982), on the west coast, of selected wells in areas with high radiation concen- trations showed that 31%of thewells had a radon concen- tration above 1000 Bq/L (t he compulsory limit for private wells in Sw eden)with a maximum at 11,166Bq/L (Nylund 1987). All wells withthe highest concentrations(4%>4000 Bq/L)weredrilled in the uranium-rich Bohus granite,a com- paratively'young'Precambrian granite.Even higher values were measured in a nearby'young; Precambrian,uranium- rich granite,the Blo mskog granite in the municipality of Arjang, Varrnland county. An inventory from this area show ed that 54%of the wells had a radon concentration above 1000 Bq/Land 15%above 4000 Bq/Lwit h a maxi- mum at 27,890 Bq/L (Nylund 1987).Later on,the highest

GERTKN UTSSO N

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NGU-BULL 439,200 2 - PAGE81

D <100 D 100-1000 • >1000 Bq / L

100%

90%

80%

70%

60% 50%

40%

30%

20%

10%

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Fig. 3.Radoncontentinwellwater inSwedishcounties(BasedondatafromSOU 2001).

Fig. 4. Radon content in groundwater in the county ofStockholm, n=5666.(Based on datafrom Stoc kho lmCountyBoard 2000).

self and sent itto an approved laboratory,recommendedby themunicipality.Theresult ofthisst udy is summarised in Fig. 4 andshowsthatthe average cond it ionsare fairly close to themeanvaluesfor the wholecountry. 11%ofthewells have aradoncont ent exceeding 1000Bq/L.

The regional differences,however, arevery large(Fig.5).

There isabroad zonein the inte riorofthenorth ern andcen- tralparts of the countywit hnumero uswells wit h a radon content of 500-1000 Bq/Land>1000Bq/L, as wellas some fewclustersofthesehighradon levels on certainislands.The bedrock in the central part is dominated by a young Precambr iangranit e (t he Stockh olmgranit e)wit hnumerous pegmatit es, whereas the northern part and most of the coastal area consistof older granitoids and some smaller st ret ches of metavolc anit es (Iept ite, lept ite-g neiss) and a local massifof basicrocks (gabbro, dior it e)along the coast.

The radon content is,in general,low in the coastal area

« 500 Bq/Lor<100 Bq/L) (exceptoncertainislands)aswell value everfound in Sweden, S7,000 Bq/Lwasmeasuredina

well drilledin the Blomskoggranite (Akerblom& Lindgren 1997).The highest valuein Norway,31,900Bq/Lwasfound in a simi lar type of granite,theIddefj ord granite,on the other side of theborder between Sweden and Norway (Ba nkset al. 1998).

Quite differentresultswere obtained in an investigation of wells in the county of Jarntland in centralSweden.94%of thewells had radon concent ration sbelow200 Bq/L; even in areas of alumshale.Most of the cent ralandweste rn part sof the county consistof Caledon ian metasedimentary rocks, but crystallinegranitic and gneissic rocks dominate in the east ern part and the highest radon concentration,2,368 Bq/L,was found in a drilledwellinaPrecambrian granite in the sout heaste rn cornerof the county.

A comprehensivecompilation of all availabledatain the year 2000 onradon concentrations in drinking water in the municipalities of Sweden has beenpublished in anofficial report on radon (SOU 2001). Altogeth er 31,000 analyses from 229 municipalities are presentedin tabularform.Based on thecompil ation, anoverviewis given for eachcounty in Fig. 3.

The Stockholm region

The Stockholm County Board (2000) has published a progressreport on radonconcentrati onsin public and pri- vatewells in the county of St ockholm.It is based on an inquirytoall municip aliti esinthecountyin 1999.Altog eth er 5666 analyses were collected from the archives in the municipaliti es.Theprocedu retoobtain theanalyses at the local levelhas,ingeneral, beenthatthe representat ives of themunicipalitieshavegiven advice,inform ation andabot- tle to the well-ow ner,who hastakenthe wat er sample him-

>1000 Bq/L 11%

100-500 Bq/L 47%

<100 Bq/L 27%

NGU-BUL L 439,2002 -PAG E82

N

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Wells with radon analysis in the county of Stockholm

GERTKNUTSSON

s

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Radon risk areas calculated using kriging.

(Wliteareas have too fewv.els)

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Fig.5 .Radonmeasurement sin drinkingwater from privatedrilled wellsin the county of Stockholm,and a preliminaryrisk map calculated from exist- ing wells.(Basedon datafrom StockholmCounty Board2000).

Fig.6. Radon content ingroundwater within Osteraker municipality, n=999.(Based on datafromOste rakermunicipality).

radon contentexceeding 1000Bq/L, whereas 16%fall inthe range500-1000Bq/L (Fig. 6).

In ordertoanalysethe diverse variable sandtheirinfl u- ence on theradon conten t, well type, well depth,age ofthe well andwellusewere studied versus the radon conten tin theground wa ter(Fig.7).l twas found that the specific use of the wellwassignific antl y aff ecting the radoncontentand a sporadicuse,co mmon atsummerhouses,led to generally highervalues.Someofthe wells were dugin soiland such as in thewholesout hern partof the county,whichis domi-

nated byPrecam brian metasedimenta ryrocks(gneiss,mica schist, metag reyw acke). Consequently,there is a fairly good correlation between the levelof radon in groundwater and the type of bedro ck,indicati ng a high content in 'yo ung' granites and a low content in metasedimen tary rocks.

How ever,it isdifficultto detectany correlation in some local areas.

Local conditions

So m e municipalities have been very active in providing info rmationand sponsorship, and thereforea lot ofsamples have been taken and analysed,especially in the northern part of the county, where the radon risk is greatest. In Norrtalje municipalit y,there are almost 2000 analyses,in Varrndo andOsteraker around 1000 each.Haninge munici- pality in the south has carried out local investigations on three islandsinwells at permanentsettlements(m ost ofthe wells on islands in the archipelago belong to summer- ho uses).All samples collected at Haninge archipelago had radon conten ts<1000 Bq/L.

The databaseat Osteraker municipality has been used for furtherstatist ical analyses.About 20%of the wells have a

>1000 Bq/L 19%

<100 Bq/L 20%

GERT KNUTSSON& BO OLOFSSON NGU-BUL L 439,2002- PAGE 83

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Fig. 7.Analysisofvariance (ANOVA)for variousclasses ofvariab lesversusradonconte nt(Bq/L)in gro un dwater wit hinOste raker munici palit y.Mean andstan da rderrors(SE)arepresented.(n=300-900).(Basedondatafrom Osteraker municip alit y).

Fig.8.Rad oncontentversusgeology,wit hd rawalsyste ms and land use, n= 300.(AfterLewinPihlbl ad 1998).

large water tanks have much lower radon content sthan wellswit hpressurised wit hdrawalsystems(Fig.8). No differ- encewasnoticed with regard to theland use arou nd the wells.

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in crystallinerocks.No clearcorrelationto well depth is seen, since thesmallest class« 10 m)mainlycomprisesdugwells.

Youngwellsshowslight lylow erradon valuescomparedto olderwells (Fig.7).

Comparableresultswereobtained from Uppsala munic- ipality north of Stockholm.The bedrockat Uppsala,mainly gneiss-granite and granite,israthe r simi lar to that in the northern part of Stockholmcounty but therearenoyoung granitesin Uppsala municipality.A fairlydetailedandscien- tificallybasedinvestigati on hasbeencarriedout in Uppsala municipality,comprising 300 wells, which were randomly selected and carefully sampl ed by personn el from the municipalit yinco-operat ion with SGU(Lewin&Simeonidis 1998,LewinPihlblad1998).Fluorideand heavymetals were also analysed.Only 12wells(4%)had radon contents>1000 Bq/Lwit h2363Bg/Las maximumvalueand a mean of 338 Bq/L. Several st at ist ical analyses were carried out but no relat ionshipswerefound betw een radon content and well capacity,well depth orsoil thickness(Lewin Pihlblad 1998).

There is,however,a distinct difference in radon content between the oldest Precamb rian granites (=granitoid in Stockholm county) and acidic and inter mediate volcanic rocks(Fig.8).Anoth erimport ant difference inradon cont ent wasfound in relati on to the ext racti on method. Wellswith

NGU-BULL439,20 0 2 - PAGE 84

Discussion

The investigations carried out in central eastern Sweden indicate that a high content ofradon in groundwater(>1000 Bq/L) is a common and widespread problem in areasof Precambrian crystalline bedrock. However, many of the investigations are not scientifically reliable,due to the well selection met hod andsampling techniques.The number of wells with a high radon content isprobably overestimated within the local archivesat the municipalities,since the sam- pling has often been carriedout in areaswhere anincreased radon riskisexpected.Thereare very fewscientificinvestiga- tions carried out in Sweden,inwhich various radon-affect- ing variables and their influence on the radon content of groundwater have been analysed.Rock type isclearly oneof the mostimportant variables,giving a high radon contentin acidic granites and pegmatites compared to int ermediate and basic rocktypes. Thesame lithologicaldependence has alsobeen found in Norway(Banks et al. 1998)The reason for this is probably the abundanceof uranium and thorium in acidic magmatic rocks,in combination wit h a more pro- nounced regular fracture pattern which increases the sur- face area of water-rock contacts.However, local geological conditions,suchas the presenceof moreprofuse minor peg- matite veins,which arenot shownon the more generalised regional maps, are probably very important.The island of Lj ustero,in Osterakermunicipality,has,for exampl e,several small villages withveryhigh contentsof radon, which can- not simplybe explained bythe data presented on the gen- eralised geological maps(Table 1).

The actualuse of thewellis anot her variableofinte rest.

Continous extractionof groundwater,such as in permanent housing areas,increasesthegroundwatercirculat ion which decreases the radon content. The technical installations must also be taken into consideration during water sam- pling since installationswit h thepossibil ityof aeration(e.g., usingwater tanks)give lower values than pressurised sys- tems as well asbadwell construction swith inflowof surface water(Nilssen 2001).

Conclusions

The conclusionsfrom thisoverviewof theradoncontent in groundwaterfrom drilled wellsin Sweden,mainly from the Stockholm region, can be summarised as follows:

• There are several pronounced riskareasin Sweden ,as well as large regional differ ences in the county of

GERTKNUTSSON&80OLOFSSON

Stockholm,withhigher and much higher radon contents thanthe compulsory limitfor private wells>1000 Bq/L.

• Wells wit ha highradoncont ent areto be found in small areas and scatt ered sites even outside the main risk region sfor radon,whichwas shown,for exampleon the island of Lj usterointhe countyofStockholm.

• There is a fairly posit ivecorrelation betweenhigh radon content s and Precamb rian granites (especially compara- tively young granites),pegmatit es,acid icvolcanite sand someacidic gneisses.

• Detailedbedrock informa tionfrom the area surrounding the well and even in thedrillhole isneededin orderto investigatemorefullythe precise relationsh ip between radon concent ration androck type.

• The use of the wells and the technical inst allati ons clearlyaffectthe rado n cont ent.

• Datafrom well archivesin the municipal itieshave to be handl ed with great cautiondue to incompleteinfor ma- tion about the wells,different procedures during sam- plin g, different method s for measuring and seasonal changes inradon content.

• Further stu diesareneededofthe relationsh ipbetween rado nconten tand

- Hydrauliccondu ct ivity of the bedrock - Hydraulic connection soil/rock

- Hydraulic gradien tsandtopography around thewell - Type anddepthof soilcover

- Chemical compositioninthewat er of the wells

Acknowledgements

The work was partly granted by internal fundsat KTH.Stockholm CountyBoard andthemunicipalitiesof Norrt aljeand Osterakerhave provideddigital dataofradon cont ent s in drilled wells.We alsothank the reviewers,Erik Rohr -Torp,Gaute Storro andTerje Stra ndfor valuab le comments on the manuscript.

Table1.Radoncontentin drilledwells in some smallvillages onthe islandof l.ju stero,Osterakermunici pality.

Village Tota lnumber Well s wit h radon Interval(Bq/ L) Geology of wells content>, 000 Bq/L

Karnas 46 11(24%) 1,200-15,000 Predomin ant greygneiss-graniteandsome clay

Marum 59 42(71%) 1,200-8,600 Mostlyclaywith some outcropsof red gneiss-granite Sundvik 11 10(91%) 1,800-6,100 Outcropsofred gneiss-granitemixed wit h tillandclay Asattra 17 13(76%) 1,500-8,610 Outcro psof redgneiss-granite mixedwit h tilland clay

Angsvik 9 3(33%) 1,300-15,200 Out crops ofred gneiss-granitemixedwithtilland clay

GERTKN U TS SO N& BO OLOFSSON

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