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pi lakes om Sou he n
Sweden: na u al adioac i i y
and elemen a y cha ac e iza ion
J. Man e o1,2*, R. Thomas1, E. Holm1, C. Rää 3, I. Vioque2, C. Ruiz‑Cano as4,
R. Ga cía‑Teno io2,5, E. Fo ssell‑A onsson1 & M. isaksson1
Na u al adioac i i y in he en i onmen is a ield gaining mo e a en ion in las decades. This wo k
is ocused on he s udy o na u al adioac i i y complemen ed wi h elemen a y cha ac e iza ion a
o me non‑u ani e ous mining a eas in Sweden. This aim is add essed h ough he s udy o mining
lakes, called pi lakes, which a e wa e bodies gene a ed a e opencas mining. En i onmen al
ma ices (wa e , sedimen s and ocks) om 32 Swedish pi lakes, commonly used o ec ea ional
pu poses we e adiome ically cha ac e ized ia alpha (238U, 234U, 232Th, 230Th, 210Po iso opes) and
gamma spec ome y (238U and 232Th se ies adionuclides). Addi ionally, ambien dose a e equi alen
in he immedia e su ounding o each pi lake was quan i ied. Physico‑chemical pa ame e s (pH,
speci ic conduc i i y, dissol ed oxygen, oxida ion– educ ion po en ial) and elemen al composi ion
(majo and ace elemen s by ICP‑MS) we e analysed in wa e samples and elemen a y composi ion o
sedimen s/ ocks was measu ed by XRF and SEM–EDX in some speci ic cases. A non‑negligible numbe
o pi lakes (26%) wi h enhanced U le els in wa e was ound. A some si es, ocks con ained up o
4% o U in a eas wi h high deg ee o in e ac ion wi h local popula ion. Conce ning he elemen a y
pe spec i e, ano he popula si e (due o i s u quoise wa e ) was ound o ha e ele a ed dissol ed
hea y me al le els. Resul s ob ained in his wo k p o e ha measu emen o na u al adioac i i y is
ano he componen ha should be included in ou ine analysis o cha ac e iza ion in mining a eas,
especially i es au a ion o pos ‑mining si es is in ended o human ec ea ional.
Mining ac i i ies in Sweden, he majo me al mining coun y in he Eu opean Union, in ol ing 63% o i on o e
p oduc ion, Zn (22%), Pb (20%) and Ag (17%) in 20141, imply he gene a ion o eno mous quan i ies o mining
was es. His o ically, mo e han 2,700 mines ga he a ound 30,000 si es ha ha e been mino mines and qua ies
acco ding o he Geological Su ey o Sweden (only 15 ac i e mines in 2015)2. Many o hese si es we e opencas
mines o exploi sul ide, limes one, clay, e c. I is no ewo hy ha open-pi mining has inc eased subs an ially
o e he pas wo decades due o imp o emen o me allu gical echniques ha enable me al ex ac ion om
low-g ade o es3. Du ing exploi a ion by open-pi mining, he wa e able is supp essed o a oid he looding o
ac i e mines. Howe e , when mining ac i i y ceases, he wa e able eco e s i s o iginal posi ion, looding he
open pi s, and gi ing ise o mine pi lakes.
The geochemis y o pi lake wa e s can a y eno mously, depending on se e al ac o s such as local geology,
hyd ology o clima e4–7. A signi ican example is he case o sul ide mine pi lakes, whe e high/ e y high concen-
a ion o hea y me als in wa e s can be ound due o he gene a ion o acid mine d ainage (AMD) p ocesses7.
An impo an ea u e o AMD is ha he sou ces o pollu ion can be ac i e o yea s o e en cen u ies a e mine
closu e8. Howe e , o he an h opogenic ac o s may ha e a signi ican impac on lake wa e s. Fo ins ance, he
a mosphe ic deposi ion o acidi ying compounds eleased mainly by indus y led o se e e acidi ica ion o lakes
and s eams9. Liming has been ex ensi ely used in Sweden since he 1970s o o se he nega i e consequences o
acidi ica ion10. In o al, a ound 8,000 lakes ha e been limed a leas once and abou 20M€/yea has been de o ed
open
1Depa men o Radia ion Physics, Ins i u e o Clinical Sciences, Sahlg enska Academy a Uni e si y o
Go henbu g, 413 45 Go henbu g, Sweden. 2Depa men o Applied Physics II, ETSA, Uni e si y o Se ille,
41012 Se ille, Spain. 3Medical Radia ion Physics, Depa men o T ansla ional Medicine ITM, Lund Uni e si y,
Malmö, Sweden. 4Depa men o Ea h Sciences & Resea ch Cen e on Na u al Resou ces, Heal h and he
En i onmen , Uni e si y o Huel a, 21071 Huel a, Spain. 5Spanish Na ional Accele a o Cen e (CNA), Uni e si y
o Se ille, 41092 Se ille, Spain. *email: [email p o ec ed]
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o liming o su ace wa e s in Sweden11. The majo goals o he Swedish liming p og am we e o keep alkalini y
abo e 0.05meq/l and pH abo e 6.0 in o de o p o ec exis ing lo a and auna and o le species ecolonize. Lim-
ing causes he ans e ence o me als (i.e. Al, Cd, Co, Ni and Zn) om he wa e column o he lake sedimen s
due o inc ease o pH alues. Howe e , he luxes o me als can change i eacidi ica ion akes place, leading o
inc easing me al le els in he wa e column. Despi e he dec ease o acidi ying compound emissions and he
signs o su ace wa e eco e y p ima ily obse ed in Scandina ia, c i ical loads o acidi ying compounds a e
s ill being exceeded in sou he n Sweden by a ac o o be ween wo and i e12. Apa om he po en ial impac
on he ecosys em, many o hese lakes a e nea by su ounded by popula ed a eas and mos o hem a e used o
ec ea ional pu poses (swimming, ishing, di ing, e c.), and hence he po en ial isk om a human pe spec i e
becomes a ele an issue o add ess. The e o e, many o he selec ed si es in his wo k come om a di e ’s o um
in Sweden13. Fo hese easons, he quali y o hese wa e s should be s udied in o de o assess he en i onmen al
and human heal h isks associa ed wi h hese ac i i ies.
Ano he pe spec i e o he physico-chemical cha ac e is ics o pi -lake sys ems ackled in his pape is hei
adiological en i onmen , which is de e mined by means o adiome ic assays. These echniques/assays a e
mainly ese ed o U mining si es, whe e we can ind pi lakes wi h 238U ac i i y concen a ions in he ange o
15 o 40Bq/L in Kazakhs an14, Tajikis an15 o B azil16. Howe e , in a o me coppe mine pi lake 10.5Bq/L o
238U was measu ed, which is close o he ypical anges o U mining pi lakes17. In Eu ope, he mean geochemical
backg ound U concen a ion in con inen al su ace wa e is in he o de o 0.889µg/L (11.1mBq/L o 238U)18.
Mining ac i i ies commonly inc ease he mine al su ace a ea o ai and wa e and po en ially expose mo e
mine als o wea he ing. Thus, mining ope a ion can lead o he elease o na u al adionuclides o iginally con-
ained in he hos ock o he en i onmen . P e ious s udies on Spanish pi lakes show ha , apa om high
le els o hea y me als, enhanced le els o na u al adionuclides can occu a hese si es19. Ac i i y concen a-
ions epo ed in Spanish pi lakes anged om 14 o 1,110mBq/kg o 238U in sul ide mine pi lakes ( he highe
alue exceeding 100 imes he Eu opean mean U su ace wa e ac i i y concen a ion). This enhancemen was
di ec ly ela ed o he AMD p ocess, and wa e samples had pH alues om 2.2 o 2.7. Howe e , enhanced le -
els on na u al adionuclides we e also ound in phospha e/ca bona e mining pi lakes in he Moulouya dis ic
mining in No he n Mo occo20. In his ecen wo k, su icial wa e wi h 238U ac i i y concen a ion anging
om 235 o 1,027mBq/kg we e ound in pi lakes wi h pH alues o 9.2–9.6. Alkaline pH alues and ele a ed
bica bona e concen a ions in oxidized su ace wa e s a ou he s abiliza ion and mobiliza ion o u anium as
u anyl-ca bona e complex21,22. In con as , he p edominan species in acid, oxygena ed wa e s a e he u anyl
ion and he u anyl-sulpha e complex21. In his sense, liming o acidi ied lakes causes a deple ion o me als om
he wa e column o he sedimen , bu may inc ease he mobili y o na u al adionuclides. Howe e , o ou
knowledge, his issue has no been p ope ly add essed un il now.
The e o e, he main goal o his wo k is o assess he dis ibu ion o adionuclides and elemen a y cha ac e i-
za ion o non-u ani e ous pi lakes om sou he n Sweden, p oducing a da abase o u he and deepe s udies
on speci ic si es.
Ma e ial and me hods
Se e al mining esou ces and da abases we e checked o selec he sampling si es among hund eds o
possibili ies2. The su ey coho consis ed o a subg oup o 23 si es con aining 32 pi lakes ha we e co e ed
wi hin h ee sampling campaigns, pe o med du ing Ap il, July and Oc obe 2015. In he supplemen a y ile,
mo e de ailed in o ma ion is p o ided abou he sampling si e loca ions including pic u es o e e y sampled pi
lake (Supplemen a y ma e ial: TableS1 and Pic u es 1–34).
Sampling si es. The map wi h he loca ion o he 23 examined mining si es (wi h o ally 34 pi lakes) in
sou he n Sweden is shown in Fig.1. A p elimina y sc eening was pe o med a e o e lapping he sampling
loca ion map p o ided by a websi e o pi lakes used o ec ea ional pu poses13 wi h adiome ic U ai bo ne
maps p o ided by SGU (Fig.1a). Mos o he si es we e andomly dis ibu ed in a eas whe e 238U in ocks had a
concen a ion o ca 4ppm o highe .
Conce ning he geological pe spec i e, he bed ock ma e ials o sou he n Sweden a e mainly composed
o P ecamb ian c ys alline ocks, belonging o he Bal ic shield, which is usually di ided in o i e geological
p o inces23. O hem, only he S eco ennian p o ince and he mo e ecen S econo wegian p o ince a e ound
in he wes and eas , espec i ely, o sou he n Sweden. Be ween bo h p o inces, he T ansscandina ian Igneous
Bel (TIB) is also ound. Bed ocks o S eco ennian p o ince a e mainly composed o me asedimen a y and me a-
olcanic ocks and se e al gene a ions o g ani oids, and may hos impo an o e deposi s (i.e. i on and sul ide
o es). The TIB consis s o la gely unde o med g ani oids and associa ed po phy ies. I s e ches om Småland
in sou he n Sweden h ough Vä mland and wes e n Dala na (whe e i is pa ly co e ed by Jo nian sands one)
and hen con inues unde much o he Caledonian moun ain chain up o no he n Scandina ia. The S econo -
wegian p o ince may be u he subdi ided in o an eas e n and a wes e n segmen , which ep esen di e en
episodes o o ma ion and ha e been subjec o ex ensi e me amo phisa ion24. Among hese ma e ials, younge
ocks wi h comple ely di e en o ma ion his o ies can be ound such as dykes o diabase o simila mine als,
bu mo e impo an ly some sedimen a y ocks, mainly o med by limes one, shale and sla e, wi h la - opped
moun ains co e ed by diabase (Fig.1b).
Ma ices. Su ace wa e was sampled a each pi lake (n = 34 wa e samples) in 5 L polye hylene je y cans
(FISHER, USA). A i s cleaning wi h dis illed wa e was applied o each bo le in he ield, ollowed by a insing
wi h wa e om he pi lake be o e sampling. Samples we e manually collec ed om he sho eline. A second
45mL aliquo was also sampled o chemical composi ion de e mina ion and i was immedia ely acidi ied wi h
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concen a ed HNO3 (65%) o pH < 2 o p e en p ecipi a ion and adso p ion eac ions wi hin he con aine
walls. A blank sample was also p epa ed by adding 65% HNO3 o 45ml o dis illed wa e . These samples we e
il a ed in he lab wi h a quali a i e il e pape (35–40µm po e size) o pa icle e en ion (WHATMAN, Eng-
land). The main aim a his s age o he p ojec was o measu e he o al elemen a y con en in wa e (no only
he dissol ed ac ion whe e a 0.45µm po e size is used). The 5 L aliquo , in ended o adiome ical analysis,
was also acidula ed o pH < 2 and was no il a ed in o de o enable an assessmen o he o al concen a ion o
adionuclides.
Sedimen s we e collec ed om he sho eline (20–30cm dep h), aking ca e o sample only he op-1-cm laye .
Only 10 si es allowed sampling o sedimen s wi h a o al o 14 samples. When possible, di e en spo samples
(~ 0.5kg we weigh in each spo ) along he sho eline we e collec ed o make a composi e sample om he en i e
lake. Mos o he pi lakes a e small in size wi h wa e su aces below 1 hec a e, and o ha eason he sampled
sedimen s could be conside ed as ep esen a i e o he pi lake. Sedimen s we e packaged in zip plas ic bags.
Once in he labo a o y samples we e homogenized, d ied a 80 ºC, milled and inally sie ed o 1mm size o
chemical composi ion (X- ay luo escence) and adiome ic de e mina ions (alpha and gamma spec ome y).
In addi ion, some ocks we e also sampled on si es whe e ele a ed ambien dose a e equi alen was eco ded.
The ock samples we e milled and sie ed o he same size as o he sedimen s (1mm).
Techniques and measu ing sys ems. Me hodology and sys ems used du ing his wo k is desc ibed wi h
mo e de ail25 oge he wi h he quali y assu ance p og am. As a consequence, only a b ie desc ip ion will be
p o ided abou me hodology in his sec ion.
Physico‑chemical pa ame e s in wa e . Physico-chemical pa ame e s as empe a u e, pH, speci ic conduc i -
i y (SC), oxida ion– educ ion po en ial (ORP), o al dissol ed solids (TDS) and dissol ed oxygen (DO) we e
measu ed immedia ely in he ield wi h a mul ipa ame ic p obe. Du ing he i s sampling a P o essional Plus
mul ipa ame ic me e (YSI, USA) was used, while o consecu i e samplings, a MS5 mul ipa ame ic me e
(HYDROLAB, USA) was used. These ins umen s we e calib a ed wi h ce i ied s anda ds solu ions om he
p obe supplie s (pH, SC, ORP and TDS) be o e each sampling campaign.
ICP‑MS. Elemen a y composi ion o samples wi h majo (Na, Mg, P, S, K, Ca) and ace (Fe, Mn, C , Cu, Zn,
As, S , Ba, Pb, U and Th) elemen s we e de e mined by Induc i ely Coupled Plasma-Mass Spec oscopy, model
Agilen 7500c. Two aliquo s pe sample o he 45ml wa e sample ial we e dilu ed a ac o 50 be o e being meas-
u ed by ICP/MS. Sample in oduc ion was pe o med wi h a PFA (pe luo oalkoxy) mic o low au o-aspi a ing
nebulize combined wi h a double-pass sp ay chambe (AGILENT TECHNOLOGIES, Japan). A Mul i-elemen
Calib a ion S anda d solu ion in 5% ni ic acid media (p o ided by Agilen Technologies) con aining: Al, Sb,
As, Be, Cd, C , Co, Cu, Pb, Mn, Mo, Ni, Se, Tl, Th, U, V and Zn was p ope ly dilu ed and used in e e y analysis
sequence which also included blank samples o con ol he pe o mance h oughou he measu emen sequence.
The ela i e unce ain y (95% con idence le el) o hese elemen s anges be ween 15 and 20%.
Alpha spec ome y (U, Th and Po iso opes). En i onmen al samples (wa e /sedimen / ock) unde wen h ee
main p ocesses: p e-concen a ion, sepa a ion and inally he alpha sou ce p epa a ion. Depending on he
Figu e1. (a) Loca ion o sampling si es shown a a map o U-238 ac i i y concen a ion om ai bo ne gamma
measu emen s o iginally supplied by SGU30 and modi ied using he g aphic edi o Pain included in Mic oso
Windows 10. (b) Bed ock map, o iginally om50 and modi ied by using Co el D aw 10 so wa e.
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ma ix, he p e-concen a ion s age di e s, while sepa a ion and alpha sou ce p epa a ion p ocedu es we e he
same o wa e and sedimen / ocks.
Du ing p e-concen a ion s age, and a e he spiking o he selec ed aliquo s wi h a known amoun o ace s
(232U, 229Th and 209Po) in he case o wa e s, 0.5 L we e submi ed o i on hyd oxide p ecipi a ion while in he
case o sedimen s/ ocks, 1g o sample we e mic owa e diges ed wi h 40% HF plus aqua egia. Once he samples
we e diges ed, he solu ions unde wen also an i on hyd oxide p ecipi a ion p ocess. F om he wa e and sedi-
men / ocks i on p ecipi a es he Th, U and Po ac ions we e isola ed by combining liquid–liquid sepa a ion and
ex ac ion ch oma og aphy echniques. The isola ed ac ions o U and Th we e hen elec opla ed on o s eel discs
while ins an deposi ion on o Cu discs was applied o Po iso opes. Comple e desc ip ion abou adiochemis y
used in hese wo k can be ound in25.
Alpha sou ces we e measu ed wi h Passi a ed Implan ed Plana Silicon de ec o s om Canbe a in an Alpha-
Analys sys em and ULTRA Ion-Implan ed-Silicon Cha ged-Pa icle de ec o s om O ec in an Alpha Ensamble
sys em. Due o he low ac i i y concen a ion le els in en i onmen al samples, he acquisi ion ime was chosen
o 200,000s o ob ain ela i e s ochas ic unce ain ies in ac i i y concen a ion a ound 5% and , in gene al, a
minimum de ec able ac i i y (MDA) below 0.5mBq o he di e en U, Th and Po iso opes.
Gamma spec ome y. Sedimen s/ ocks, we e d ied, g ounded and, a e 1mm sie ing, packed in a plas ic
cylind ical 35mL geome y wi h only 10mm heigh o minimize sel -abso p ion e ec s. As e e ence ma e ial
o pho opeak e iciencies, wo ma ices we e used: IAEA-RGU-1 o 238U- and 235U-se ies adionuclides and
IAEA-RGTh o he 232Th-se ies adionuclides. As ou ine me hodology, sel -abso p ion co ec ion was applied
acco ding o26.
NORM adionuclides om 238U se ies s udied we e 210Pb, 234Th, 226Ra (de e mined by secula equilib ium
using 214Pb and 214Bi) and also 234Pam (measu ed when possible due o i s e y low gamma yield). F om 232Th
se ies: 228Ra was ob ained ia 228Ac and 228Th ia 212Pb, 212Bi and 208Tl. Addi ionally, 40K and he an h opogenic
137Cs we e measu ed. Gamma measu emen s we e pe o med in an ex ended ange ge manium coaxial de ec o
(X Ra) o 37.1% ela i e e iciency. Fo a 200,000s acquisi ion ime, his sys em p o ides MDA alues ang-
ing om 1.5 o 5Bq/kg o adionuclides om he 238U and 232Th se ies, ~ 15Bq/kg o adionuclides in he
235U-se ies, ~ 10Bq/kg o 40K and a ound 1Bq/kg o 137Cs.
X‑ ay luo escence (XRF). The elemen a y composi ion (mainly ace elemen s as Fe, Mn, S, Ba, Pb, Zn, S ,
C , Cu, As, Th, U) in sedimen s and ocks we e pe o med a he X-Ray Labo a o y o he Uni e si y o Se ille
by wa eleng h Dispe si e X-Ray Fluo escence (WDXRF). A ound 0.1g o sedimen / ock is mixed wi h 0.01g
agglome an (LICOWAX) and is p essed o 1min o 200 kN on op o a bo ic acid mould. Finally a 40mm
diame e cylinde o bo ic acid wi h a 10mm inne cen e ed cylinde con aining he sample is p oduced and
measu ed wi h an AXIOS sys em (MALVERN PANALYTICAL, Uni ed Kingdom /Ne he lands). Re e ence
ma e ials we e used: MBL-1 (basal ), GYP-B (o e SO4), JCRM R041 (Mulli e) and NCS DC71305 ( ock) o ali-
da e his me hodology. Rela i e unce ain y (wi h 95% con idence le el) anges om 1.7% o S un il 30% o
P, a e aging 11.5% o all elemen s. De ec ion limi s ange om 1ppm o Th un il 100ppm o Mn a e aging
22ppm.
Ambien dose a e equi alen . Two independen ex e nal gamma dose me e s (Rados SRV 2000 Compensa ed
GM- ube wi h an ene gy ange: 50keV–3meV and a dose a e ange: 0.05 µS /h–10S /h), calib a ed o ambi-
en dose a e, H*(10), we e placed 1m abo e he g ound a di e en places a ound each si e du ing he sampling
ime. Each sys em p o ided an a e age alue o e 15 o 20min measu emen . The esul used o ep esen he
inal es ima e o he ambien dose a e a he si e was he a e age o se e al measu emen s a di e en spo s. The
wo dose a e me e s we e quali y checked be o e use.
Scanning elec on mic oscopy‑ene gy dispe si e X‑ ay spec oscopy (SEM–EDX). In some pa icula cases whe e
ocks we e ound wi h enhanced le els o na u al adionuclides, SEM–EDX was used o a mo phological cha -
ac e iza ion and o local elemen a y composi ion. Fo ha pu pose, a JEOL 6460LV scanning elec on mic o-
scope was used, equipped wi h acquisi ion o digi al images in bo h seconda y (SEI) and backsca e ed (BEI)
elec on imaging modes (maximum esolu ion 3.5nm). This de ice was coupled o an EDX mic op obe and
i ed wi h an ATW2 be yllium window ( esolu ion 137eV a 5.9keV). The semi-quan i a i e analysis was pe -
o med using he Ox o d INCA so wa e.
Da a unce ain ies. All epo ed unce ain ies/e o ba s a e shown wi h k = 1 c i e ia. Rega ding ICP-MS
measu emen s, 20–25% unce ain y is epo ed (semi-quan i a i e analysis), while XRF p o ides 15–20% unce -
ain y. Gamma spec ome y p ecision is in he ange o 10–15%, while i is 7–10% o alpha spec ome y. These
unce ain ies depend on how close he measu ed alues a e o he MDA o he echnique. So wa e used o
s a is ical ea men o da a was O iginP o 8.0 (ORIGINLAB, USA).
Sedimen s as ma ke s o pollu ion. In o de o use he elemen a y composi ion measu ed in sedimen s o iden-
i y any po en ial pollu ed si e, di e se me hods we e ound in he li e a u e. The e a e se e al app oaches o
s udy he in e ac ion be ween he wa e column and sedimen s by pa ame e s such as en ichmen ac o (EF) o
index o geoaccumula ion (Igeo)27,28. Howe e , we decided o use he me hod p oposed by Håkanson29, de el-
oped o lakes in Cen al Sweden, he same a ea conside ed in his su ey. In acco dance wi h Håkanson’s model,
sedimen composi ion can be used as a diagnos ic ool o wa e pollu ion con ol. A pa ame e de ined as deg ee
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o con amina ion (Cd) is assessed based on ela i e elemen a y composi ion in lake sedimen s o e e ence le el
based on 8 elemen s (Hg, Cd, As, Cu, Pb, C , Zn and PCB, polychlo ina ed biphenyl), and ollowing he equa ion:
whe e
Ci
0−1
is he concen a ion o he i- h elemen in he sedimen (sampled om 0 o 1cm laye ) and
Ci
e is
he s anda d p eindus ial e e ence le el de e mined om a ious Eu opean and Ame ican lakes.
Ci
is de ined
as con amina ion ac o (C ) o he i- h elemen .
Elemen a y composi ion o 14 composi e sedimen samples will be included in Eq.(1) o assess bo h deg ee
o con amina ion and con amina ion ac o s on each si e.
Resul s and discussion
elemen al and adiome ical cha ac e iza ion o su ace wa e . Physico‑chemical pa ame e s o
su ace wa e . The physico-chemical pa ame e s (i.e. pH, SC, ORP and DO) in su icial wa e samples om
he 23 sampling si es a e shown in Fig.2 (Raw da a in TableS2 om supplemen a y ma e ial). Resul s show low
SC alues o 47–597 µS/cm (a e age alue o 260 µS/cm). The maximum SC alues s emmed om Si e 4 (due
p obably o he p esence o sul ide-bea ing schis s in bed ocks ou c opping in he d ainage a ea acco ding o he
SGU local bed ock map30. A e age pH alues o 7.6 we e obse ed in he s udied lakes, wi h an in e qua ile be-
ween 7.1 and 8.2. Such high alues could be due o he Swedish liming p og am ini ia ed in 1977 o coun e ac
he an h opogenic acidi ica ion obse ed in Swedish su ace wa e s31. Howe e , he minimum alue o 4.9 was
obse ed a Si e 14, due o sul ide mining ac i i ies in his si e, a de elic sil e mine ope a ed discon inuously
om 1,483 o 1,900, leading he o ma ion o a pi lake o a ound 240m dep h. The oxida ion o galena and o he
sul ides ound o iginally in his si e may ha e caused such low pH alues. The a e age ORP alue in lake wa e s
was 95mV (in e qua ile ange o 41 o 100mV; Fig.2), al hough a maximum alue o 300mV was obse ed a
Si e 4, whe e sul ide-bea ing schis s appea o ou c op in he d ainage basin. Wa e samples we e well oxygena ed
wi h DO alues om 7.8 o 12mg/L (65–99% o sa u a ion) and a e age 9.9mg/L. Such high concen a ions o
dissol ed oxygen oge he wi h he low alues o o al phospho ous (a e age alues o 2.5mg/L, in e qua ile
ange o 1.3 o 3.8; Fig.3), seem o indica e an oligo ophic na u e o lakes s udied.
Majo elemen s and ace elemen s in su ace wa e . The con en o dissol ed elemen s in lakes is p ima ily
con olled by ock wea he ing, a mosphe ic p ecipi a ion and e apo a ion-p ecipi a ion p ocesses32. Ca and S
a e he main elemen s in disolu ion; in s udied pi lakes a e age alues o 140 and 280mg/L o Ca and S (in e -
qua ile ange o 50–220mg/L and 110–420mg/L), espec i ely, we e eco ded (Fig.3a). Raw da a o p oduce
Fig.3a,b) can be ound as TableS3 in supplemen a y ma e ial. The main sou ces o S may be d y deposi ion and
he acid ain in indus ialized and u ban a eas; i.e. mos lakes s udies a e ound close o la ge ci ies (S ockholm,
Ö eb o, No köpping, e c.) and o a lesse ex en he oxida ion o mino amoun s o sul ides p esen in he d ain-
age ca chmen , while he high concen a ions o Ca ound in hese lakes should be ela ed o liming and o a
lesse ex en , o he dissolu ion o ca bona es in old ma ble qua ies. Compa ed o hese elemen s, he concen a-
ion o o he s such as Na, Mg o K is no iceably lowe ; a ound 20mg/L we e obse ed (Fig.3a). Howe e , maxi-
mum alues exceeding 100mg/L o Na and Mg we e ound in Si e 4 and Si e 11, espec i ely. The la e si e was a
o me qua y whe e ma ble (calcium-magnesium ca bona e) was exploi ed. The in ense wa e – ock in e ac ion
a e looding may ha e eleased signi ican concen a ions o Mg o he wa e column.
(1)
C
d=
8

i=1
Ci
=
8

i=1
C
i
0−
1
Ci
e
Figu e2. Box-and-whiske plo s o physico-chemical pa ame e s (i.e. pH, speci ic conduc i i y (SC),
oxida ion– educ ion po en ial (ORP) and dissol ed oxygen (DO)) in pi lake su ace wa e samples. The heigh
o he box shows he in e qua ile ange, which con ains 50% o he alues, while he ho izon al line inside he
box shows he median alue and he ed c oss deno es he mean alue. The whiske s a e lines ha ex end om
he box o he highes and lowes alues excluding ou lie s (o) and ex emes (*). In his sense, ou lie s ep esen
hose alues being be ween 1.5 and 3 imes la ge han he leng h o he box om i s uppe o lowe bo de
while ex eme a e hose g ea e han 3 imes such alue.

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Conce ning ace elemen s, he mos abundan is Fe wi h a e age alues o 1,200µg/L, ollowed by Zn
(860µg/L), S (120µg/L), Mn (100µg/L), Pb (38µg/L), Cu (23µg/L), Ba (22µg/L), C (4.3µg/L) and As
(1.5µg/L). The maximum alues o Fe and Mn we e obse ed a Si e 13 (13,700 and 1,460mg/L, espec i ely;
Fig.3b), p obably due o he p esence o colloidal ma e ial ich in Fe and Mn passing h ough he po e il e .
This ac would also explain he high concen a ion o o he ace me als such as Zn (8,400mg/L). On he o he
hand, he a e age concen a ion o Th and U a e 85ng/L and 14µg/L, espec i ely, al hough maximum alues
o 750ng/L and 68µg/L we e obse ed. Such di e en alues be ween U and Th may be ela ed o di e ences
in mobili y o U and Th species despi e ha g ani es, he mos abundan ock in he s udied a ea, a e mos ly
en iched in Th o e U33. This en ichmen o Th o e U also applies o alkaline ocks (K o Na > > Ca) acco ding
o Dill34.
Alpha spec ome y o su ace wa e . U anium adioiso opes in su ace wa e . A ange om 0.3 o 1,183mBq/
kg wi h a mean alue o 156 ± 272mBq/kg (mean ± s anda d de ia ion) was ound o 238U iso opes (Fig.4a).
Raw da a o p oduce Fig.4a,b and Table1 can be ound as TableS4 in supplemen a y ma e ial. These alues
could be compa ed wi h he geochemical backg ound alues o 238U o con inen al su ace wa e s anging om
0.02 o 266mBq/kg and a mean alue o 11 ± 21mBq/kg18. The 72% o he si es had le els abo e he mean back-
g ound alue, so he e is in gene al an enhancemen o 238U le el in pi lakes in sou he n Sweden. Fu he mo e,
one na u al lake was sampled a he beginning o each sampling campaign o be used as “ e e ence alue” in
compa ison wi h pi lakes. The 238U in a subse o 3 na u al lakes anged om 0.34 o 11mBq/kg wi h mean
5.0 ± 5.2mBq/kg), which is in he o de o he en i onmen al backg ound alue.
So ing ou 238U ac i i y concen a ion in pi lake wa e s, he highe alues co espond o si es (mBq/kg):
S21 (1183)>S15 (735)>S4 (680)>S3 (609)>S8 (236)>S2 (154)
Figu e3. (a) Concen a ion o elemen s in mg/kg and (b) in µg/kg o su icial wa e samples om he s udied
pi lakes, shown as box-and-whiske plo s ( e e o Fig.2 o explana ion) and so ed ou by mean alues.
Figu e4. (a) 238U ac i i y concen a ion le els in su ace wa e samples om he s udied pi lakes in sou he n
Sweden. The ho izon al line shows he mean geochemical backg ound18. (b) 210Po ac i i y concen a ion in
su ace wa e samples om pi lakes, p esen ed oge he wi h i s no mal dis ibu ion and a box-and-whiske
plo . Raw da a can be ound in TableS4.
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Enhanced le els o 238U we e ound and i is well known he po en ial isk o popula ion due o he chemical
and adiological oxici y o 238U, wi h he wo impo an a ge o gans being he kidneys and he lungs.
In his sense, i is wo h men ioning ha one o he pi lakes is nowadays used as ap wa e ese oi (Si e
2). Acco ding o WHO35 he e is a guidance le el o 10Bq/L in d inking wa e o 238U om he adio oxic
pe spec i e, which is by a one o de o magni ude highe han he maximum 238U le el ound in his sampling.
Howe e , due o he chemo oxici y o U a mo e es ic ed h eshold o 30µg/kg (370mBq/kg) was de ined by
WHO in 2011. In ou case, he 238U ac i i y concen a ion a Si e 2 was 150mBq/kg which is 2.4 imes lowe
han he h eshold, so he chemo oxici y o U is no ele an o local inhabi an s.
On he o he hand, mos o he lakes s udied a e used o ec ea ional pu poses (i.e. ishing, swimming, di -
ing). Al hough de mal con ac is conside ed a ela i ely unimpo an pa h o exposu e due o he limi ed ans e
om skin o he blood, ano he possible ou es o adionuclide inco po a ion o impac should be conside ed
om he dose assessmen pe spec i e.
Rega ding 234U, mos o he samples had highe alues han 238U, anging om 0.3 o 1,700mBq/kg wi h a
mean o 210 ± 375mBq/kg. The 234U/238U ac i i y concen a ion a ios o pi lake wa e samples (Fig.5) we e
all abo e uni y excep o one si e. The 234U o 238U a io should be 1 in case o secula equilib ium bu i is well
known ha he e may exis a disequilib ium in wa e wi h 234U/238U a ios abo e uni y, due o a selec i e leaching,
alpha- ecoil ans e o 234Th di ec ly in o he aqueous phase and he combina ion o he wo p ocesses36. The
a io be ween hese adionuclides in su icial wa e is ypically 1.1 o 1.3, wi h highe alues ela ed o a majo
inpu o unde g ound wa e in o he wa e body. The p esen esul s a e consis en wi h he expec ed ac iona-
ions based on he g ea e mobili y o U and pa icula ly 234U37.
Table 1. Summa y o ac i i y concen a ion o U, Th and Po iso opes om 238U and 232Th se ies in pi lake
su ace wa e samples (n = 34). Da a a e gi en as minimum, maximum and mean alues oge he wi h SD.
Unce ain ies a e gi en in b acke s.
Ac i i y concen a ion (mBq/kg) Concen a ion a ios (mBq/kg)
238U234U230Th210Po 234U/238U230Th/234U232Th
Min 0.32 ± 0.09 0.28 ± 0.08 0.1 < 0.8 ± 0.2 0.8 ± 0.2 0.0008 ± 0.0003 0.1 <
Max 1,183 ± 30 1701 ± 43 26 ± 3 94 ± 4 1.97 ± 0.42 0.61 ± 0.31 8.8 ± 2.7
A e age 155.8 210.1 2.4 10.5 1.32 0.098 0.9
SD 272.2 375.1 5.0 17.9 0.24 0.157 1.6
Figu e5. Iso opic a ios showing disequilib ium in 238U se ies o pi lakes wa e samples. Sampling si es we e
so ed ou a ending o an inc easing 230Th/234U a io. Raw da a can be ound in TableS4.
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Tho ium adioiso opes in su ace wa e . The ac i i y concen a ion a e age o 230Th (belonging o 238U se ies)
was o 3.1mBq/kg anging om MDA (~ 0.1) o 26 ± 3mBq/kg. The ange o alues ound belongs o he ypical
en i onmen al alues o his iso ope. Tho ium concen a ion in wa e is expec ed o be e y low38,39 al hough
i can inc ease due o soluble complexes wi h humic ma e ial o ca bona es40. These low alues compa ed wi h
hose o 238U (and also compa ed wi h 234U, he 230Th di ec ances o ) can be explained by a e y low mobiliza ion
o Th and i s endency o be ixed o he solid phase. As a consequence, 230Th/234U ac i i y a ios we e all ound
lowe han uni y (Table1 and Fig.5) wi h a minimum alue o 8·10–4.
Rega ding 232Th and i s daugh e s, he ac i i y concen a ion le els we e e en lowe han 230Th (Table1 and
Fig.5), in acco dance wi h Jia e al.41. The MDA o 232Th was 0.5mBq/kg, and 46% o he samples had le els
below MDA. 232Th ac i i y concen a ion a ied om MDA o 8.8 ± 2.7mBq/kg wi h an a e age o 0.9 ± 1.6mBq/
kg. In con inen al wa e s 232Th anges om less han 0.008 o 1.50mBq/kg wi h mean 0.10 ± 0.16mBq/kg18. In
a di ec compa ison be ween Th iso opes, he a io 230Th/232Th is mos ly highe han 1 (Fig.5), poin ing ou he
di e en o igin o hese wo adioiso opes.
Due o he low ac i i y concen a ion o Th iso opes, hese adionuclides will ha e a negligible adiological
impac on exposed indi iduals.
210Po in su ace wa e . A e age ac i i y concen a ion o 210Po, also belonging o 238U se ies, in su ace wa e
samples was 10.5 ± 17.9mBq/kg wi h a a ia ion om 0.8 o 95 ± 4mBq/kg. The dis ibu ion o hese da a can
be seen in Fig.4b whe e 94% o he samples had alues below 25mBq/kg and 76% below 10mBq, which is in
ag eemen wi h en i onmen al le els42.
F om a dosime ic pe spec i e, 210Po is he adionuclide wi h he highes inges ion dose coe icien wha
implies he highe adiological oxici y. As an example, in Si e 2 (men ioned be o e) a pi lake used as wa e
ese oi o human consump ion, a s aigh o wa d assessmen o he annual commi ed e ec i e dose ia
inges ion is showed in Table2. F om his able, he mul iplica ion o ac i i y concen a ion, annual in ake o
wa e (assumed o be 2kg/day in adul s) and he e ec i e dose coe icien by inges ion o adul s, p o ides he
annual dose by inges ión due o wa e including 238U, 234U and 210Po adionuclides (Th iso opes a e neglec ed
o being below MDA). Taking in o accoun ha he h eshold o he e ec i e dose in wa e is 0.1mS /y43, he
o al amoun (0.0136mS /y) ep esen s only 14% o his h eshold.
elemen al and adiome ical cha ac e iza ion o sedimen s. Elemen a y cha ac e iza ion. The
abundance o majo and ace elemen s was de e mined in 14 sedimen samples by XRF (Fig.6), and he ob-
se ed composi ion e lec s accu a ely he li hological cha ac e is ics o he s udy a ea. Raw da a o p oduce
Fig.6 can be ound as TableS5 in supplemen a y ma e ial. The mos abundan elemen is Si (62% o a e age),
ollowed by Al (9.8%) p esen in aluminosilica e. The p esence o Ca (a e age o 4.9%) sugges s he in luence o
liming in he s udied lakes, al hough he exis ence o de elic ma ble qua ies among he sampling si es could
also explain such alues. Lowe abundance o K, Mg, and Na was obse ed (3.2, 3.0 and 1.0%, espec i ely)
ela ed o he wea he ing o bed ock. The p esence o oxides and hyd oxides seems o be limi ed o Fe (a e age
alue o 4.8%), conside ing he low concen a ion o Mn in sedimen s (0.1%).
Conce ning ace me als, a c us al elemen like Ba was among he mos abundan in sedimen s (309ppm),
ollowed by Pb (285ppm), Zn (163ppm), S (82ppm), C (45ppm), Cu (41ppm), Th (18ppm), As and Ni
(14ppm) o U (12ppm). Liming o lake wa e s may ha e caused a ne ans e ence o me als om he wa e
column o he lake sedimen s due o inc ease o pH alues. In o de o es ima e he me al luxes om he wa e
column o he sedimen and ice e sa, assessmen o dis ibu ion coe icien s (Kd) ha e been pe o med.
Dis ibu ion coe icien (Kd) was es ima ed as he a io be ween sedimen and wa e concen a ion o an ele-
men . In some wa e and sedimen samples, alues below he de ec ion limi we e obse ed o some elemen s,
and in such cases hal o he de ec ion limi was assumed in o de o be conside ed in his Kd analysis. Some clea
ends can be obse ed (Fig.7): S (associa ed wi h sul ides) wi h low Kd can easily mo e o he aqueous phase,
while he opposi e beha io was ound o Th wi h he highes Kd showing a clea endency o emain in he solid
phase. In e media e alues we e ob ained o he o he ace elemen s ha ing U wi h anges o Kd alues simila
o C , Cu, Zn, As, S , Ba o Pb. Based on Kd a e age alues and in e qua ile anges, we can so ou he end o
be mobilized in o he aqueous solu ion o he elemen s in pi lakes as ollows:

Highes mobili y, i.e. lowes K
d
S>Cu ∼Zn ∼P≥U≥As ∼C ∼Ba >Fe >Th

Lowes mobili y

Table 2. Annual commi ed e ec i e doses (mS /y) by inges ion in adul s due o wa e consump ion om pi
lake si e 2. a F om ICRP, 201251.
Radionuclide Ac i i y concen a ion in wa e (Bq/
kg) Wa e consump ion in adul s pe yea
(kg/yea ) Commi ed e ec i e dose coe icien
by inges ion (S /Bq)aAnnual dose by inges ion (mS /
yea )
238U 0.154 730 4.5·10–8 0.0051
234U 0.164 730 4.9·10–8 0.0059
210Po 0.003 730 1.2·10–6 0.0026
To al 0.0136
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Nguyen e al.44 epo ed Kd alues o a ious na u al aqua ic sys ems (Aus alian coas al and es ua ies, six
es ua ies in Texas and in se e al o he na u al lakes), wi h log Kd anges o 3.0–5.9, 3.8–6.7 and 3.8–7.2 o Cu,
Zn and Pb, espec i ely. In ou su ey o pi lakes, we ound log Kd anges o 0.78–3.9, 1.0–3.4 and 2.1–3.5 o
Cu, Zn and Pb, espec i ely. A e compa ison o hese da a se s, i is clea ha he mobiliza ion o hese me als
in o he aqueous phase in he su eyed pi lakes is highe han in na u al wa e en i onmen s (lowe Kd alues),
once again demons a ing he need o s udy hese special wa e bodies o he po en ial isk as a sou ce o hea y
me als in he su ounding en i onmen .
As a ool o iden i y whe he he e exis o no a chemical pollu ion isk o he en i onmen in a pi lake,
sedimen elemen a y composi ion can be used acco ding o Håkanson29 p oposal. In his model, a e aplying
Eq.(1), bo h C and Cd a e classi ied in 4 le els: low, mode a e, conside able and e y high (Table3). In he p esen
wo k, Hg and PCB concen a ions we e no de e mined and Cd was ound below de ec ion limi (0.1ppm) in
all sedimen s. Thus 6 me als we e included in he assessmen wha implies a conse a i e Cd alue. A ending o
calcula ed alues (Table3), only one si e (Si e 14) was ound wi h a e y high Cd alue, mainly due o a e y high
Figu e6. Concen a ion o majo and ace me als in sedimen s (n = 14) om pi lakes, p esen ed as box-and-
whiske plo s and so ed ou by mean alues. Fo explana ion o he boxes and whiske s, e e o Fig.2.
Figu e7. Dis ibu ion coe icien (Kd) o elemen s in pi lakes (n = 14) and so ed ou by mean alues. Fo
explana ion o he boxes and whiske s e e o Fig.2.
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Acknowledgemen s
Wo k suppo ed by he Swedish Radia ion Sa e y Au ho i y (SSM2014-3485). The au ho s hank D . Ana Calleja
a Radioiso opes Labo a o y (ICP-MS measu emen s) and he X-Ray Labo a o y s a (XRF measu emen s),
bo h om CITIUS acili ies a he Uni e si y o Se ille. Fu he mo e, he Applied Nuclea Physics G oup a he
Uni e si y o Se ille, is also acknowledged o i s con inuous echnical suppo du ing he di e en s ages o his
p ojec .Open access unding p o ided by Uni e si y o Go henbu g.
Au ho con ibu ions
J.M.: concep ualiza ion, me hodology, alida ion, o mal analysis, in es iga ion, da a cu a ion, w i ing—o iginal
d a , w i ing— e iew and edi ing and unding acquisi ion. R.T.: alida ion, o mal analysis, in es iga ion, da a
cu a ion and w i ing— e iew and edi ing. E.H.: supe ision C.R.: me hodology and w i ing— e iew and edi -
ing. I.V.: in es iga ion, o mal analysis and w i ing— e iew and edi ing. C.R.-C: o mal analysis, da a cu a ion,
w i ing—o iginal d a and w i ing— e iew and edi ing. R.G.-T., E.F.-A., M.I.: supe ision and w i ing— e iew
and edi ing.
Compe ing in e es s:
The au ho s decla e no compe ing in e es s.
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Supplemen a y in o ma ion is a ailable o his pape a h ps ://doi.o g/10.1038/s4159 8-020-70521 -0.
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