Natural radioactivity in groundwater--a review.

Abstract

The issue of natural radioactivity in groundwater is reviewed, with emphasis on those radioisotopes which contribute in a significant way to the overall effective dose received by members of the public due to the intake of drinking water originating from groundwater systems. The term 'natural radioactivity' is used in this context to cover all radioactivity present in the environment, including man-made (anthropogenic) radioactivity. Comprehensive discussion of radiological aspects of the presence of natural radionuclides in groundwater, including an overview of current regulations dealing with radioactivity in drinking water, is provided. The presented data indicate that thorough assessments of the committed doses resulting from the presence of natural radioactivity in groundwater are needed, particularly when such water is envisaged for regular intake by infants. They should be based on a precise determination of radioactivity concentration levels of the whole suite of radionuclides, including characterisation of their temporal variability. Equally important is a realistic assessment of water intake values for specific age groups. Only such an evaluation may provide the basis for possible remedial actions.

Full text

Page 1

Thi s art i cl e w as dow nl oaded by: [Akadem i a Gorni czo Hut ni cza], [M r Kazi m i erz Rozanski ]
On: 14 Decem ber 2011, At : 08:00
Publ i sher: Tayl or & Franci s
Inf orm a Lt d Regi st ered i n Engl and and W al es Regi st ered Num ber: 1072954 Regi st ered
of f i ce: M ort i m er House, 37-41 M ort i m er St reet , London W 1T 3JH, UK
I sot opes i n Envi ronm ent al and Heal t h
St udi es
Publ i cat i on det ai l s, i ncl udi ng i nst r uct i ons f or aut hor s and
subscr i pt i on i nf or m at i on:
ht t p: //www. t andf onl i ne. com /l oi /gi eh20
Nat ur al r adi oact i vi t y i n groundwat er – a
revi ew
Nguyen Di nh Chau a , Mar ek Dul i nski a , Pawel Jodl owski a , Jakub
Nowak a , Kazi m i er z Rozanski a , Moni ka Sl ezi ak a & Pr zem ysl aw
Wachni ew a
a Facul t y of Physi cs and Appl i ed Com put er Sci ence, AGH
Uni ver si t y of Sci ence and Technol ogy , Kr akow, Pol and
Avai l abl e onl i ne: 14 Dec 2011
To ci t e t hi s art i cl e: Nguyen Di nh Chau, Mar ek Dul i nski , Pawel Jodl owski , Jakub Nowak, Kazi m i er z
Rozanski , Moni ka Sl ezi ak & Pr zem ysl aw Wachni ew ( 2011) : Nat ur al r adi oact i vi t y i n gr oundwat er – a
r evi ew, I sot opes i n Envi r onm ent al and Heal t h St udi es, 47: 4, 415- 437
To l i nk t o t hi s art i cl e: ht t p: //dx. doi . or g/10. 1080/10256016. 2011. 628123
PLEASE SCROLL DOW N FOR ARTICLE
Ful l t erm s and condi t i ons of use: ht t p://w w w. t andf onl i ne. com /page/t erm s-and-
condi t i ons
Thi s art i cl e m ay be used f or research, t eachi ng, and pri vat e st udy purposes. Any
subst ant i al or syst em at i c reproduct i on, redi st ri but i on, resel l i ng, l oan, sub-l i censi ng,
syst em at i c suppl y, or di st ri but i on i n any f orm t o anyone i s expressl y f orbi dden.
The publ i sher does not gi ve any w arrant y express or i m pl i ed or m ake any represent at i on
t hat t he cont ent s w i l l be com pl et e or accurat e or up t o dat e. The accuracy of any
i nst ruct i ons, f orm ul ae, and drug doses shoul d be i ndependent l y veri f i ed w i t h pri m ary
sources. The publ i sher shal l not be l i abl e f or any l oss, act i ons, cl ai m s, proceedi ngs,
dem and, or cost s or dam ages w hat soever or how soever caused ari si ng di rect l y or
i ndi rect l y i n connect i on w i t h or ari si ng out of t he use of t hi s m at eri al .

Page 2

Isotopes in Environmental and Health Studies
Vol. 47, No. 4, December 2011, 415–437
Natural radioactivity in groundwater – a review
Nguyen Dinh Chau, Marek Dulinski, Pawel Jodlowski, Jakub Nowak, Kazimierz Rozanski*,
Monika Sleziak and Przemyslaw Wachniew
Faculty of Physics and Applied Computer Science, AGH University of Science and Technology,
Krakow, Poland
(Received 29 July 2011; final version received 21 September 2011)
The issue of natural radioactivity in groundwater is reviewed, with emphasis on those radioisotopes which
contribute in a significant way to the overall effective dose received by members of the public due to the
intake of drinking water originating from groundwater systems. The term ‘natural radioactivity’ is used
in this context to cover all radioactivity present in the environment, including man-made (anthropogenic)
radioactivity. Comprehensive discussion of radiological aspects of the presence of natural radionuclides in
groundwater, including an overview of current regulations dealing with radioactivity in drinking water, is
provided.The presented data indicate that thorough assessments of the committed doses resulting from the
presence of natural radioactivity in groundwater are needed, particularly when such water is envisaged for
regular intake by infants. They should be based on a precise determination of radioactivity concentration
levels of the whole suite of radionuclides, including characterisation of their temporal variability. Equally
important is a realistic assessment of water intake values for specific age groups. Only such an evaluation
may provide the basis for possible remedial actions.
Keywords: committed effective dose; groundwater; natural radioactivity; radioactive isotopes; review;
water cycle
1.Introduction
RadioactivesubstancesareubiquitousonEarth.Theycontainradioactiveisotopes(radionuclides),
generating different types of ionising radiation in the course of nuclear transformations. Radionu-
clides present in the environment can be grouped into several distinct classes with respect to their
origin: (i) primordial radionuclides, (ii) cosmogenic radionuclides, (iii) radionuclides produced
in natural decay series and (iv) anthropogenic radionuclides.
PrimordialradionuclidescompriseradioactiveisotopeswhichhavebeenpresentonEarthsince
its formation. Because of their very long half-lives, comparable with the age of the Solar system,
they have not yet decayed beyond the point of their detection. The most prominent member of
this group is the radioactive isotope of potassium (40K), which contributes with approximately
16% to the annual effective dose received by members of the global population due to ionising
radiation originating from natural radionuclides [1,2].
*Corresponding author. Email: rozanski@novell.ftj.agh.edu.pl
ISSN 1025-6016 print/ISSN 1477-2639 online
© 2011 Taylor & Francis
http://dx.doi.org/10.1080/10256016.2011.628123
http://www.tandfonline.com
Downloaded by [Akademia Gorniczo Hutnicza], [Mr Kazimierz Rozanski] at 08:00 14 December 2011

Page 3

416
Nguyen Dinh Chau et al.
Cosmogenic radionuclides are produced in the atmosphere and the uppermost layer of the
Earth’s crust, in the interactions of cosmic radiation with constituents of those reservoirs. This
groupcomprisesmorethan20isotopesofelementsrangingfromhydrogentokrypton.Themajor-
ity of cosmogenic radionuclides is produced in three main types of nuclear reactions involving
cosmic particles: spallation, neutron capture and muon capture. Spallation reactions are by far the
most common mode of radionuclide production in the atmosphere. Cosmogenic radionuclides
contribute with less than 1% to the annual mean effective dose from natural radionuclides, with
radiocarbon (14C) being the major contributor accounting for almost 100% of this dose [2].
Radionuclides belonging to the natural decay series represent the most important source of
ionising radiation on Earth, contributing with approximately 83% to the total mean effective
dose received by the global population from natural radionuclides [1,2]. They are generated in
successivedecaysofthreeprimordialradioisotopes:232Th,235Uand238U.Thedecayseriesinvolve
nuclear transformations with emission of alpha or beta particles and end up with stable isotopes
of lead (208Pb,207Pb,206Pb for232Th,235U and238U decay series, respectively). The presence
of gaseous isotopes of radon (219Rn,220Rn,222Rn) and their progenies in natural decay series is
largelyresponsiblefortheirlargeshare(approximately50%)intheoveralleffectivedosereceived
by the global population from natural sources of ionising radiation.
Finally,anthropogenicradionuclides,whichoriginatemostlyorexclusivelyfromtechnological
activities of man, are also present in the environment and contribute to the overall effective dose
received by members of the global population. They can be broadly separated into radionuclides
which were produced and dispersed in the environment during the atmospheric nuclear bomb
tests in the 1950s and 1960s, and those which are released during the routine operation of nuclear
installations (power reactors, reprocessing plants), or found their way to the environment in
accidental releases of various nature. Some of the radioisotopes belonging to this group (e.g.3H
or14C) are also generated by interactions of cosmic rays with constituents of the atmosphere.
Others, such as134Cs,137Cs,90Sr or isotopes of plutonium (239Pu,240Pu,241Pu) have not been
present generally in the environment in detectable amounts before the nuclear era.
Natural radioactive substances are often classified into two groups with respect to their distri-
bution in the environment: (i) naturally occurring radioactive materials (NORM), and (ii) techno-
logically enhanced NORM. The second group comprises materials, natural or man-made, which
containelevatedconcentrationsofradioactiveelementsasaresultoftechnologicaltransformations
of various natures. Examples include fertiliser production and fossil fuel combustion.
Although environmental aspects of both natural and anthropogenic radioactivity have been
discussed in numerous publications, as well as in several monographs and textbooks [3–6], the
presence of natural radioisotopes in groundwater as a hazard factor to the public has not been
addressed there in sufficient detail. Growing use of groundwater resources for drinking water
purposes calls for careful evaluation of this aspect of the presence of radioactive substances in
the environment. Radioactive isotopes such as3H,14C,36Cl or39Ar have often been discussed
in connection with groundwater, but mostly as tools for assessing timescales of groundwater
flow [7–9].
The paper reviews the subject of natural radioactivity in groundwater, with emphasis on those
radioisotopes which contribute in a significant way to the overall effective dose received by the
consumers.Available literature data with respect to the observed activity concentration ranges of
major radionuclides present in groundwater are reported, followed by the discussion of physico-
chemical factors controlling the levels of selected radionuclides in aquifer systems, starting from
the recharge area, through the saturated part, up to the discharge zone.The final section contains a
thorough review of radiological aspects of the presence of natural radionuclides in groundwater,
includingdiscussionofcurrentregulationsdealingwithradioactivityindrinkingwater.Examples
of dose calculations received by different age groups of the public as a consequence of the
consumption of groundwater with specific levels of radioactivity are presented and discussed.
Downloaded by [Akademia Gorniczo Hutnicza], [Mr Kazimierz Rozanski] at 08:00 14 December 2011

Page 4

Isotopes in Environmental and Health Studies
417
Table 1.Major characteristics of selected radionuclides present in groundwater [10].
Radionuclide Half-lifeParticle type and energya(MeV)
3H
14C
40K
210Pb
210Po
222Rn
226Ra
228Ra
232Th
234U
235U
238U
12.32 years
5700 years
1.250 × 109years
22.23 years
138.4 days
3.823 days
1600 years
5.75 years
1.40 × 1010years
2.45 × 105years
7.04 × 108years
4.47 × 109years
β−(0.0186)
β−(0.1565)
β−(1.311), EC
β−(0.0170; 0.0635)
α (5.304)
α (5.489)
α (4.784)
β−(0.0127;0.0391)b
α (3.948;4.011)
α (4.722;4.774)
α (4.366;4.398)b
α (4.151;4.198)
aMaximum energy of beta particles.
bMost probable energies.
PRECIPITATION
- dissolution
- leaching
- recoil
- diffusion
- adsorption
UNSATURATED ZONE
- precipitation
- radioactive decay
- dissolution
- leaching
- recoil
- diffusion
- exchange
SATURATED ZONE
- precipitation
- radioactive decay
- exchange
DISCHARGE ZONE
- precipitation
- radioactive decay
TREATMENT
- co-precipitation
- sand filtration
- lime softening
- ion exchange
- reverse osmosis
Figure 1. Processes controlling the levels of natural radioactivity in groundwater.
2. Factors controlling natural radioactivity levels in groundwater
Basic characteristics of natural radionuclides discussed in this review are presented in Table 1.
Except of tritium, which is an integral part of the water molecule, all other radioisotopes are
associated with dissolved constituents or particulate matter present in groundwater. A sketch
diagram in Figure 1 illustrates major pathways and processes which control the levels of natural
radioisotopesingroundwater,ontheirwayfromtherechargeareathroughthesaturatedpartdown
to the discharge zone, including possible treatment before groundwater is being supplied to the
consumers.
Downloaded by [Akademia Gorniczo Hutnicza], [Mr Kazimierz Rozanski] at 08:00 14 December 2011

Page 5

418
Nguyen Dinh Chau et al.
Table 2. CompilationofliteraturedataonU,ThandKconcentrationsinselectedrocktypes.
Rock type U (ppm)Th (ppm)K (%)Ref.
Upper crust (average values)
Igneous/metamorphic rocks
Acid igneous
2.79.62.1 [11]
2.2–6.1
4.0
1.1
0.03
<0.015
1.5
4.8
1–13
0.6
0.1–1
2.0
0.8–9.4
8–33
13
3.9
2.0
<0.05
4.4
17
17.1–139
2.2
0.2–5
5–27
12.7–56.6
[12]
[13]
[13]
[14]
[12]
[13]
[11]
[15]
[11]
[12]
[12]
[15]
2.8
1.4
4
Basic igneous
Ultrabasic igneous
Intermediate igneous
Granite
2
3.34
1.4–6.4
0.83Basalts
Gneiss
2.5–5.5
Sedimentary rocks
Unconsolidated sediments
Limestone
1.7
1.3
∼2
3.3
0.5
0–2.4
2.0
0.3
[13]
[14]
[12]
[12]
[14]
[16]
[14]
[17]
[14]
[14]
[12]
[18]
Dolomite
Sandstone
0.03–2
0.5–5.1
0.68–3.1
5–250
1.7–6.6
10–56
up to 120
50–300
77–143
1.1
1.7–11.6
13
9.5–15.1
0.03–2.39
2.8
1.4–3.96
2.5
Black shale
Shales associated with oil
Phosphate deposits1–5
1–5
1.74–7 1.21–12
Major processes which supply radioactive isotopes to groundwater involve dissolution and
leaching.An important role is also played by the recoil process, a purely physical effect operating
on microscale. During the emission of ionising radiation, the nucleus of a radioactive element
is subject to recoil, which may transfer the daughter product directly to the pore space or may
loose its position within the crystalline structure of a mineral, facilitating its further removal
from the rock matrix via leaching or dissolution.Apart from radioactive decay, radionuclides can
be removed from groundwater through a variety of physical or chemical processes such as ion
exchange, co-precipitation or sorption, which are activated in response to the changing chemical
status of groundwater (e.g. redox conditions) or are imposed on the solution during groundwater
treatment.
Table 2 summarises available literature data on U, Th and K contents in major rock types
(igneous,metamorphic,sedimentary)whichconstitutewater-bearingformationsinthevastmajor-
ityofactivegroundwatersystems.Table3providesacompilationofliteraturedataontheobserved
activity concentration levels of238U,226,228Ra,210Pb,210Po and40K in groundwater systems con-
sisting of specific rock types. It is apparent that wide ranges of activity concentrations of the
radioisotopes in question are observed. For a given rock type (e.g. granite, sandstone), they can
vary from the values below recently attainable detection limits (fraction of mBql−1) to several
Bql−1. In general, there is no apparent relationship between the type of rock or sediment consti-
tuting the water-bearing formation and the observed activity concentration levels of the discussed
radionuclides.
2.1.
Natural radionuclides in precipitation
Concentrations of natural radionuclides in precipitation are generally low. Tritium content in
precipitation in most parts of the world is now back to natural levels [33]. Typical activities of3H
Downloaded by [Akademia Gorniczo Hutnicza], [Mr Kazimierz Rozanski] at 08:00 14 December 2011

Page 6

Isotopes in Environmental and Health Studies
419
Table 3.
various types of host rocks.
Compilation of literature data on selected natural radionuclide concentrations in groundwater occurring in
Water-bearing
formation
238U
226Ra
(mBql−1)
228Ra
(mBql−1)
210Pb
(mBql−1)
210Po
(mBql−1)
40K
(mBql−1)
(mBql−1)
Ref.
Intrusive and metamorphic formations
Granite
Granite
Granite/basalt
Granite/black shale
Gneiss
Basalts
Cristaline rocks
Sedimentary formations
Sandstone
Sandstone/limestone/shales
0–5300
0.6–1080
7.4–3700
6.75–3273
0.1–80
0–4900
20–6500
0–16200–656
<0.5–320
[19]
[20]
[21]
[22]
[23]
[24]
[25]
<0.5–430
37–148 11–37000
2–49210–1500
<0.5-160 0.37–6586
0.74–962 1.1–666
1.1–444
30–1490
0.73–444
37–222
1.5–433
12–877
[25]
[26]
[27]
[28]
[29]
[30]
[29]
[31]
[32]
3.7–18500
0.2–500
0.26–55.6
0.44–274
0.04–3.2
7.4–22.2
0.014–0.132
1.44–138
14–260
Sandstone/clays
Sandstone/limestone/dolomite
Sand
Sand/gravel
Limestone
Phosphate rocks
3–758
2.2–601
0.4–1.1
<0.5-28827–10245
0.08–3.2
37–333
≤74-30808
observed nowadays in mid-latitude continental precipitation are in the order of 1 Bql−1. Lower
values are recorded in the tropics (0.1–0.5Bql−1). In groundwater, higher tritium concentrations,
up to several Bql−1, can be found occasionally. They stem from relatively high tritium levels
in precipitation in the 1960s (up to several hundreds of Bql−1), resulting from atmospheric
nuclear bomb tests. Owing to its relatively short half-life (12.32 years), the tritium content in
recharged groundwater before the bomb era is nowadays close to zero. Minute amounts of other
cosmogenic radioisotopes produced in the atmosphere, such as14C,22Na and7Be, can be also
found in precipitation, but their activity concentration levels are far lower than that of tritium.
Radon-222, the gaseous decay product of226Ra is released from the soils and is present in the
lower troposphere, with typical activities ranging from a few to some tens of Bq m−3. Because
radon is highly soluble in water, its content in precipitation may reach relatively high values, up
to hundreds of Bql−1[34]. The long-lived products of222Rn decay in the atmosphere (210Po and
210Pb)canalsogetdissolvedinprecipitationandtransportedtothegroundaswetdeposition.Typ-
ical activity concentrations of210Pb in precipitation are in the order of tens to hundreds mBql−1,
with the210Po/210Pb activity ratio (AR) varying in the range between ≈0.1 and 0.3 [35,36].
Activity concentrations of other natural radionuclides of interest, such as40K,226Ra,228Ra
and228U present in precipitation, are generally in the order of mBql−1or less, and exhibit large
temporalandspatialvariability[37–39].Itisbelievedthatthemajorsourceofthoseradionuclides
in precipitation is re-suspension of soil particles in air due to wind action.
2.2.
Unsaturated zone
Groundwater systems are replenished mostly via infiltration of rainwater through the unsaturated
zone.Theunsaturatedzoneisacomplex,three-phasesystemwheresolidphasescoexistwithliquid
and gaseous phases. Rainwater percolates under gravity force towards the water table, gaining
dissolved constituents through physical and chemical interactions with the solid matrix and the
gaseousphasepresentintheporespace.Thedepthoftheunsaturatedzonemayvarywidely,from
tens of centimetres to hundreds of metres. Consequently, travel times of the infiltrating water
down to the saturated zone of aquifers may vary from weeks to tens and hundreds of years.
Downloaded by [Akademia Gorniczo Hutnicza], [Mr Kazimierz Rozanski] at 08:00 14 December 2011

Page 7

420
Nguyen Dinh Chau et al.
Natural radionuclides enter infiltrating water via several mechanisms, such as leaching and
dissolution of mineral phases, diffusion of222Rn out of mineral grains and dissolution in the pore
water, recoil of daughter products during decay of radioactive elements bound in the solid matrix
(e.g.226Ra,234U). Radiocarbon (14C) is entering the groundwater mainly via dissolution of soil
CO2gas originating from root respiration and decay of organic matter in the soil.
With the exception of222Rn and14C, the levels of natural radionuclides which enter the infil-
trating water are in general significantly lower than those which can be found in the saturated
zone. The activity concentration of14C in the infiltrating water is generally in the order of 0.01–
0.02Bql−1, depending on the partial pressure of soil CO2and the availability of carbonate phase.
By far, the largest contributor to natural radioactivity of the infiltrating water is222Rn, which
levels may reach several hundreds of Bql−1. Leaching of radionuclides from soils is strongly
affected by processes such as preferential flow, transport with colloidal particles, sorption and
desorption, ion exchange, complexation and biological uptake [40–43]. Modelling efforts [44]
indicate that release of natural radionuclides from the unsaturated zone is a complex and highly
variable process, both in space and time.
2.3.
Saturated zone
Below the water table, groundwater is interacting with the solid matrix of the aquifer. Chemical
composition and radioactivity content evolve along the general directions of groundwater flow,
responding to changing physico-chemical parameters such as pH, Eh and temperature, as well as
varying lithology of the aquifer matrix. In the following sections, major parameters controlling
activity concentration levels of specific radionuclides in the saturated part of aquifer systems will
be discussed in some detail.
2.3.1.
Potassium-40
Potassium is one of the major rock-forming elements in the Earth’s crust. Its concentration varies
from less than 1% in limestone to several percent in granite. The average concentration in the
upper crust was estimated to be around 2.1% [11]. Potassium can be found in a variety of miner-
als. It occurs most commonly in the form of aluminosilicates and feldspars. During weathering
processes, potassium readily dissolves in water. Its concentrations in groundwater can vary in a
wide range, depending on specific conditions in the aquifer and on the potassium content in the
host rocks. Concentrations ranging from a fraction of mgl−1up to several hundreds of mgl−1
were reported in the literature. For shallow groundwater systems underlying areas with intense
agriculture, elevated concentrations of potassium related to fertiliser usage may occur.
Potassium-40isalong-livedradioactiveisotope(T1/2= 1.25 × 109years)withanaturalabun-
danceof0.0117%.Ithastwodecaymodes:(i)betadecayto40Ca(89.3%)and(ii)electroncapture
(EC) transition to40Ar (10.7%) with emission of gamma radiation (E = 1.46MeV). For crustal
rocks, the activity concentration of40K is around 600Bqkg−1[3]. Some granites exhibit higher
values, up to several thousands of Bqkg−1. The activity concentration of40K in soils is generally
lower, with a mean value of ≈410Bqkg−1[2]. In groundwater, the activity concentration of40K
can be calculated, taking into account its abundance in natural potassium and K content in the
investigated water [45]. A distinct relationship between pH and40K activity concentration has
been reported, with lower40K levels corresponding to higher pH values [46].
2.3.2.
Uranium isotopes
Rocksofdifferenttypesaretheprimarysourceofuraniumanditsdecayproductsingroundwater.
Concentrations of uranium in rocks and sedimentary deposits range from trace levels, typical for
Downloaded by [Akademia Gorniczo Hutnicza], [Mr Kazimierz Rozanski] at 08:00 14 December 2011

Page 8

Isotopes in Environmental and Health Studies
421
46810 12
Mole per cent dissolved uranium
0
20
40
60
80
100
UF3
+
UF2
+2
UF4
UO2F+
UO2
+2
UO2
+
UO2CO3
UO2(HPO4)2
-2
UO2F2
UO2(CO3)2
-2
UO2(OH)3
-
UO2(CO3)3
-4
pH
46810 12
Mole per cent dissolved uranium
0
20
40
60
80
100
UF3
+
UF2
+2
UF4
U(OH)+3
U(OH)4
UO2
+
UO2(HPO4)2
-2
UO2(CO3)3
-2
UO2(CO3)3
-4
UO2(OH)3
-
(a) - oxidizing conditions (Eh = +100)
(b) - reducing conditions (Eh = -100)
Figure 2.
(b) conditions, calculated using PHREEQC-2 code. The total uranium concentration was equal 4.2μM. Only selected
species with abundances exceeding 1% of the total uranium content in the solution are shown. Black solid lines: U(IV);
black dashed lines: U(V); solid grey lines: U(VI).
Distribution of uranium complexes in groundwater at 10◦C as a function of pH, for oxidising (a) and reducing
common rock-forming minerals (a fraction of ppm to several ppm) to the values characteristic for
ore-grade deposits (hundreds of ppm). Typical concentrations of uranium in different rock types
are summarised in Table 2. In general, the concentrations of uranium in igneous rocks are higher
than in sedimentary rocks. Acidic igneous rocks contain significantly more uranium than basic
and ultrabasic rocks. In sedimentary rocks, uranium is mostly associated with clay fraction and
its concentration may reach several ppm. Similar concentrations are observed in metamorphic
rocks. Highest uranium contents occur in black shales and phosphates.
Figure 2 shows chemical speciation of uranium in weakly mineralised water for moderately
oxidising and reducing conditions, which is calculated by using the PHREEQC-2 code [47]. The
assumed total uranium concentration was equal to 4.2μM. The plots in Figure 2 represent only
selected species whose abundances exceed 1% of the total uranium present in the solution.
Under reducing conditions (Figure 2(b)), uranium U(IV) is a dominant form for a wide range
of pH values. In aquatic environments, it forms insoluble precipitates, is strongly adsorbed on
suspended particles or mineral surfaces, and is attracted by organic matter. All these processes
strongly limit the mobility of U(IV). Under oxidising conditions, the presence of U(IV) in water
is limited to pH values below 4. Uranium in this form occurs mainly as fluoride compounds.
Downloaded by [Akademia Gorniczo Hutnicza], [Mr Kazimierz Rozanski] at 08:00 14 December 2011

Page 9

422
Nguyen Dinh Chau et al.
U(VI) has in general higher mobility and, in aquatic environments, often occurs as uranyl
(UO2+
can be transported to large distances in groundwater.As seen in Figure 2(a), these complexes are
formed mainly in the pH range between 6 and 8. These are typical pH values for a large majority
of groundwater systems.
The occurrence or absence of the given uranium species and their relative proportions depend
alsoonmineralisationofgroundwater.Forinstance,uraniumcomplexeswithsulphateandchloride
play an important role only in situations when concentrations of these ions are sufficiently high.
In solutions with low ionic strength, the concentration of U(VI) can be controlled by cation
exchangeandadsorptionprocesses.Uraniumcomplexescanbeadsorbedonclayminerals,organic
compounds and oxides. The significance of these processes decreases in highly mineralised solu-
tions due to the substitution of UO2+
K+). Thus, elevated mineralisation of groundwater increases mobility of UO2+
sorption/desorption processes may not be fully reversible. Iron and manganese oxides should be
treated as irreversible sinks for uranium. In the presence of complexing ions, e.g. carbonates,
the effectiveness of uranium adsorption can be significantly reduced. Both field observations and
laboratory tests suggest that the pH of water, which controls the speciation of uranium, and the
dissolved carbonate content are the most important factors influencing the sorption of U(VI) on
different minerals in a groundwater environment [48].
Free UO2+
increasing the solubility and mobility of uranium in the unsaturated zone and in groundwater
[49–52].This effect is larger than for other bivalent metals. Consequently, the presence of organic
matter in groundwater in the form of dissolved organic acids or colloids may have an impact on
uranium mobility comparable to that of the presence of carbonates or phosphates.
In nature, the235U/238UAR is constant and equals 1/21.7, both in minerals and in the hydro-
sphere. In contrast, the234U/238UAR usually departs from its equilibrium value. In surface water
and in groundwater, the observed AR values are generally higher than 1. The recoil effect is the
primary process responsible for the observed uranium isotope disequilibrium in groundwater.
After the emission of the alpha particle by the nucleus of238U bound in the rock, the daughter
product (234Th) is displaced from the original position of238U. Thus,234U produced as a result
of subsequent decay of234Th (via234Pa and234mPa) can be more easily leached from the rock
matrix than238U.
The relationship between234U/238U AR values and uranium content, as observed in different
groundwater systems, is presented in Figure 3(a). Almost all waters contain uranium within
the concentration range between ≈0.003 and 400μgl−1with AR values above 1. In waters
associated with uranium mines, the concentrations of uranium can reach extremely high levels,
in the order of 10,000μgl−1as observed, for instance, in the Osamu Utsumi mine, Brazil [53].
Eveninrecultivateduraniumminingdistricts,thegroundwaterisaffectedbyleachingandmixing
processes. In Saxony, East Germany, uranium concentrations were analysed in groundwater and
mine dump sites from the former uranium mining, reaching up to 80mgl−1in dump leachate
[54,55] and up to 700μgl−1in groundwater [56]. Under reducing conditions, the concentration
ofuraniuminwaterisusuallysmall.TheARvalueshouldbehigherthaninoxidisingenvironment
due to the recoil effect and preferential leaching of234U. In contrast, under oxidising conditions,
the concentrations of uranium are higher and theAR values lower.
In bottled waters (Figure 3(b)), both the concentration of uranium and the AR values are
generally smaller than those observed in groundwater systems. This stems from the fact that
bottled waters have to fulfil appropriate regulations with respect to chemical quality and the
maximumpermissiblelevelofradionuclideconcentrationsindrinkingwaters(seeSection3).The
concentration of uranium in these waters usually does not exceed tens of μgl−1. The234U/238U
AR is usually below 10, although a value as high as 77 was measured in one of therapeutic waters
2)ion.Inthepresenceofphosphatesorcarbonates,itformshighlysolublecomplexeswhich
2ions in the exchange sites by main cations (Ca2+, Mg2+,
2
ions. However,
2ions show a tendency to form organic complexes with fulvic and humic acids, thus
Downloaded by [Akademia Gorniczo Hutnicza], [Mr Kazimierz Rozanski] at 08:00 14 December 2011

Page 10

Isotopes in Environmental and Health Studies
423
Figure 3.
[53,115–133], (b) bottled waters [57,66,134–141]. Broken lines mark uranium concentrations and234U/238U AR for
which the annual committed dose arising from average daily intake of 2l of water by adults (age group >17 years) results
in the reference dose level of 0.1mSv.
Literature data on234U/238U activity ratio as a function of uranium content: (a) groundwater systems
exploited in the Polish part of Carpathian Mountains [57]. Dashed grey lines in Figure 3(a) and
(b)indicatetheuraniumcontentandARvaluesforwhichtheannualcommitteddosearisingfrom
average daily intake of 2l of water by adult persons (age group >17 years, see Section 3) results
inthereferencedoselevelof0.1mSv.ItisapparentthatallbottledwaterspresentedinFigure3(b)
fulfil the regulations as far as uranium is concerned.
2.3.3.
Thorium isotopes
In aquatic environments, thorium occurs only on +4 oxidation state. Typical concentrations of
this element in crustal rocks are usually lower than ≈100ppm, with higher values occurring in
Downloaded by [Akademia Gorniczo Hutnicza], [Mr Kazimierz Rozanski] at 08:00 14 December 2011

Page 11

424
Nguyen Dinh Chau et al.
igneousandmetamorphicrocks(seeTable2).Inacidicwaters,thoriumformsfluoridecomplexes,
whereasatpH > 8itoccursintheformofTh(OH)4.AtpH < 3.5,thoriumoccursmainlyasTh+4
ion.At high pH values, it forms hydroxides, which have a tendency to precipitate.
Thorium is strongly adsorbed on iron hydroxides and humic substances [58–60]. Therefore,
thorium is practically absent in groundwater; its average concentration is below 1μgl−1and
is independent on redox conditions [61]. In the presence of inorganic anions such as carbon-
ates, phosphates, chlorides and nitrates, thorium can form complexes. Relative abundance and
importance of individual complexes depend on the concentrations of these ions and on the pH
of water.
2.3.4.
Radium isotopes
Radiumistheheaviestinthegroupofalkalineearthmetals.Outofthefourradioactiveisotopesof
radium(223Ra,224Ra,226Ra,228Ra),two(226Ra,228Ra)areofimportanceingroundwatersystems.
They belong to two different decay series:226Ra (T1/2= 1600 years) belongs to238U decay series
while228Ra (T1/2= 5.75 years) is a daughter product of232Th. Radium in rocks and sedimentary
deposits appears mainly as oxide or hydroxide compounds. It is assumed that the Ra2+ion is the
prevailing form of radium in an aquatic environment.
Thepresenceofradiumisotopesingroundwaterisgovernedbythreemajorprocesses:(i)leach-
ingofhostrocks,(ii)sorption/desorptionphenomenacontrolledbyphysico-chemicalparameters
of water and (iii) the alpha recoil effect. Consequently, the Ra activity concentration in ground-
water should reflect to some extent the type of rocks which host the given aquifer system.Already
in the early 1980s, attempts were made to establish the link between the observed concentrations
of radium isotopes in groundwater and uranium and thorium contents in the host rocks [25]. Rel-
atively low activity concentrations of226Ra and228Ra (below 1Bql−1) were observed in aquifers
hostedinintrusiverockssuchasgneiss[62].Insandstoneandweatheredgranite,veryhighactivity
concentrations of both radium isotopes were observed, reaching in some places several hundreds
Bql−1[63]. Incontrast,inaquifershostedinsedimentaryformationsofmarineorigin(limestone,
dolomite), very low radium contents in groundwater were measured, in the order of few mBql−1
[64]. Review of literature data on radium content in aquatic environments conducted in the 1980s
[65]revealedaverybroadrangeofactivityconcentrationsofthiselement.For226Ra,thereported
range was from ≈0.5mBql−1to ≈55Bql−1.
Figure4showstherelationshipbetween226Raand228Racontentinvarioustypesofgroundwater
fromPoland,Austria,SpainandSweden[19,28,66,67].Theycoverawiderangeofconcentrations
of both isotopes. There is an apparent linear relationship between the measured concentrations of
226Ra and228Ra, with the data points clustering around the 1:1 line. Similar relationships were
observed also in groundwater systems from other regions [25,68,69]. However, the data points
representingwatersfromsouthernSwedenrevealadistinctlyhigher226Racontentcomparedwith
228Ra.Thismightberelatedtothefactthatwater-bearingformationsinthisareaarecharacterised
by an averageTh/U concentration ratio equal to only ≈0.76 [70] while typical ratios of these two
elements in rocks and sediments are generally larger than 1 (Table 2).
AttemptstomodeltheobservedconcentrationsandARvaluesofradiumisotopesingroundwa-
ter[71–74]confirmtheimportantroleofadsorption/desorptionphenomena,aswellastimescales
of groundwater flow. Shallow waters with a mean residence time significantly lower than the
half-live of226Ra should generally exhibit a226Ra/228Ra AR smaller than 1. In contrast, in old
groundwaters, the226Ra/228RaAR may exceed 1 because226Ra can be more easily leached from
the host rock compared with228Ra. This preference stems from the fact that226Ra is formed after
threeconsecutivealphadecays,comparedwithonealphadecayfor228Ra.Ingroundwatersystems
characterised by relatively low desorption and high adsorption coefficient, such as waters with
Downloaded by [Akademia Gorniczo Hutnicza], [Mr Kazimierz Rozanski] at 08:00 14 December 2011

Page 12

Isotopes in Environmental and Health Studies
425
Figure 4.
Source of data: [19,28,66,67].
Relationship between226Ra and228Ra activity concentration in selected groundwater systems in Europe.
Figure 5.
Source of data: [19,28,66].
Relationship between226Ra and238U activity concentration in selected groundwater systems in Europe.
highsulphatecontent,theconcentrationsofradiumisotopesarerelativelylow,witha226Ra/228Ra
AR lower than 1. In contrast, low adsorption and high desorption coefficients lead to relatively
high radium content and a226Ra/228RaAR higher than 1 [72–74].
Figure 5 shows the relationship between the activity concentrations of226Ra and238U, as
observedinseveralgroundwatersystemsinEurope[19,28,66].Thereisaclearinverserelationship
between the concentrations of both elements in groundwater: low226Ra activity concentrations
(below≈100mBql−1)areobservedforwaterswithrelativelyhighuraniumcontent,whilereduced
concentrations of uranium are associated with high levels of226Ra. The observed relationship
Downloaded by [Akademia Gorniczo Hutnicza], [Mr Kazimierz Rozanski] at 08:00 14 December 2011

Page 13

426
Nguyen Dinh Chau et al.
results from a different behaviour of radium and uranium in groundwater. In relatively young
watersrichinoxygen,dissolveduraniumremainsinthesolution,while226Racontentisgenerally
lowbecausesecularequilibriumforthisisotopeintherock-watersystemhasnotyetbeenreached.
Reducing conditions, characteristic for old groundwater, impose removal of uranium from the
solution, while a larger residence time of water favours higher concentrations of226Ra. This
time factor also explains the apparent increase of the radium concentration with the depth of
water-bearing formations observed in several field studies [28,75,76].
Geochemical properties of radium are similar to those of barium. Consequently, in situa-
tions where both elements are present in groundwater, radium, in presence of sulphate, will
co-precipitate with barium leading to enhanced radioactivity levels of the precipitate. Such situa-
tions occur often in the oil industry where radium-bearing barite (Ba(Ra)SO4) is deposited on the
inner walls of pipes [77], or in mines where inflowing groundwater contains both elements [78].
FactorswhichcanenhancethemigrationofradiumingroundwaterarelowpH,highconcentration
of bivalent metals and the reducing environment [79].
2.3.5.
Lead-210
Out of the four radioactive isotopes of lead (210Pb,211Pb,212Pb and214Pb), only210Pb with a
half-life of 22.23 years is of interest for groundwater studies.210Pb is a decay product of222Rn
gas, which belongs to the238U decay series and originates from the decay of226Ra.
Under oxidising conditions, lead exists as tetravalent or divalent oxide which is insoluble in
water. Thus, it will be easily adsorbed on rock minerals and suspended particles. Under reducing
conditions, divalent Pb(II) is most stable thermodynamically and reacts easily with sulphide to
formhighlyinsolublePbS[80].InCO2-richwaters,leadmayoccurinhydro-carbonatecomplexes
which are soluble. In sulphate or chloride waters, lead may form sulphate or chloride complexes
which are also soluble.
In groundwater, the activity concentrations of210Pb are generally below 1Bql−1. For instance,
in several aquifers in Brazil,
980mBql−1were observed [81,82]. Aleissa et al. [83] report the range from 1.3 to 7.8mBql−1
for groundwater systems in SaudiArabia. In mineral waters occurring in Polish Carpathians, the
observedactivityconcentrationof210Pbwasbelow0.5mBql−1,whileinseveralsprings,itvaried
from few mBql−1to ≈36mBql−1[28].The highest210Pb content (≈350mBql−1) was observed
in highly mineralised, CO2-rich therapeutic water originating from large depths (≈1000 m). This
water also revealed a relatively high226Ra content (≈800mBql−1) supporting the production
of222Rn at depth. A survey of groundwater wells performed in California [84] revealed a broad
range of210Pb activity concentrations, from 3.7mBql−1to 1.48Bql−1.
210Pb activity concentrations in the range between 0.5 and
2.3.6.
Polonium-210
Polonium-210 is a radioactive product of the decay of210Pb. It decays with the half-life of
138.4 days to stable lead206Pb. Polonium is easily soluble in water at low pH values where it
appears as PoO(OH)+, Po(OH)2or PoO2. PoO2−
3
solutions.Inaquaticenvironments,210Pocanbeadsorbedonrockmineralsorsuspendedparticles
[80,85]. Publications dealing with polonium in groundwater are scarce.Aleissa et al. [83] report
poloniumcontentsforseveralaquifersinSaudiArabia.Themeasured210Poactivityconcentrations
variedfrom0to≈13mBql−1(averagevalueof3.5mBql−1).Recently,Seiler[24]reported210Po
concentrations measured in 63 domestic wells and public supply wells in northern Nevada, USA.
The measured activity concentrations of210Po ranged from 0.37mBql−1to 6.6Bql−1.
is a dominating form of polonium in alkaline
Downloaded by [Akademia Gorniczo Hutnicza], [Mr Kazimierz Rozanski] at 08:00 14 December 2011

Page 14

Isotopes in Environmental and Health Studies
427
2.3.7.
Radon-222
Out of three natural isotopes of radon (219Rn,220Rn,222Rn), only222Rn is of importance in
groundwater systems. It is a decay product of226Ra which decays to218Po with a half-life of
3.823 days.As an inert gas, radon shows high mobility, also in underground environment. Radon-
222 exhibits the largest range of concentration in groundwater among all natural radioisotopes:
activity concentrations from a fraction of Bql−1to hundreds of kBql−1were reported [86,87].
For instance, in a large-scale study of222Rn content in 1604 crystalline bedrock groundwaters
in Norway, the range from <10Bql−1to ≈32kBql−1was observed [88]. In the Ramsar region,
Iran, with one of the highest background radiation of the world caused by the geothermal water
system, radon measurements from drinking water sources showed up to 83.5Bql−1, which is
related to an annual effective dose of 223μSv [89].The very highest222Rn activity concentration
in groundwater (182kBql−1) was measured in Hindenburgquelle, Oberschlema, Germany [87].
The levels of222Rn in groundwater are controlled mostly by the source term, i.e. the activity
concentration of226Ra in the host rock and its distribution within the rock matrix. Equally impor-
tant are mineralogical and petrological characteristics of the host rock, in particular the extent
of rock–water contact and fracture morphology [90]. Although the relatively short half-life of
radon limits its spatial transport in groundwater, radon can still travel relatively large distances
underground through faults and fissures.
2.4.
Impact of groundwater treatment
Before being used as a supply of drinking water, groundwater in most cases has to undergo
adequatetreatment,bringingitschemicalqualitytotherangeenvisagedbyappropriateregulations.
Nowadays, a number of technologies are in routine use for this purpose. They include, among
others, aeration, coagulation, filtration, co-precipitation, ion exchange and reverse osmosis. In
general, those processes lead to a certain reduction of natural radioactivity levels in water. The
degreeofreductiondependsonthetypeofthetreatmentprocessusedandmayreach90%[91–93].
3.Radiological aspects of natural radioactivity present in groundwater
Nowadays, groundwater covers approximately 50% of the global drinking water needs [94].
Radionuclide contents in groundwater are of interest when internal exposures of humans to ionis-
ing radiation are considered. Under typical conditions, exposures resulting from the consumption
of water constitute a small fraction of the average annual committed dose from natural sources.
As stated in the UNSCEAR 2008 report [2], the annual global mean dose from food and drinking
wateringestionwasestimatedat0.29mSv,whichconstitutes≈12%ofthetotaldosefromallnat-
uralsourcesestimatedat2.4mSv.Theradioactiveisotopeofpotassium,40K,whoseconcentration
in the body is maintained at a constant level [2], has the largest share in the dose from ingestion
(0.17mSvyear−1). Apart from40K, the UNSCEAR reports consider radioisotopes belonging to
the natural decay series (238U,230Th,226Ra,210Pb,210Po,232Th,228Ra,228Th,235U) as significant
contributors to the ingested dose originating from food and water consumption.
Neither theWHO guidelines on drinking water quality [95] nor the UNSCEAR reports provide
explicit information on the share that consumed water has in the total ingested dose.This fraction
canbeestimatedonthebasisofthedatapresentedintheUNSCEAR2000report[96].Calculations
based on the reference values for concentrations of natural radionuclides in water (Table 15 in
[96] and [97]) and annual reference intake of water for infants, children and adults [96] show that
Downloaded by [Akademia Gorniczo Hutnicza], [Mr Kazimierz Rozanski] at 08:00 14 December 2011

Page 15

428
Nguyen Dinh Chau et al.
the consumption of drinking water should lead to an age-weighted mean annual dose of 13μSv,
which constitutes ≈5% of the total annual ingested dose of 0.29mSv.
The WHO guidelines for drinking water quality [95] incorporate recommendations of the
International Commission on Radiological Protection and set the reference dose level for the
committedeffectivedoseassociatedwithdrinkingwaterconsumptionat0.1mSvyear−1excluding
tritium,40Kandradonanditsdecayproducts.TheWHOguidelinesrecommendthedetermination
of the total dose only when gross alpha and gross beta activity concentration levels in drinking
water exceed 0.5 and 1Bql−1, respectively.
The European Union legislation [98,99] sets quality standards for drinking water adopting the
WHO-recommended dose of 0.1mSvyear−1as a parametric value (total indicative dose, TID)
for radioactivity present in water for public consumption excluding tritium,40K and radon and its
decay products. A separate parametric value of 100Bql−1is set for tritium activity in drinking
water. The European Union also provides recommendations with respect to indicative reference
levels of222Rn,210Pb and210Po in drinking water to be used for consideration by member states
whether remedial action is needed to protect human health [100].
The US Environmental Protection Agency (USEPA) regulation [101] sets the maximum con-
taminant levels for radionuclides in drinking water at 0.74Bql−1for combined226Ra and228Ra
content, 0.04mSvyear−1for the dose equivalent from beta and gamma emitters excluding tritium
and90Sr,0.555Bql−1foralphaemittersexcludinguraniumandradon,and20μgl−1foruranium.
The EU and US quality standards outlined above are not fully consistent, but they allow for the
presence of comparable levels of radionuclides in drinking water.
Theaccuracyofthecommitteddosecalculationsdependmainlyontwofactors:(i)theassumed
annual consumption of water and (ii) the availability of data on the activity concentration in
water of radionuclides which have the largest share in the dose. The WHO guidelines [95] and
USEPA regulation [101] assume an intake of 2lday−1(730lyear−1) for all age categories, while
UNSCEAR reports [2,96] provide different reference values for infants (150lyear−1), children
(350lyear−1) and adults (500lyear−1). The German Radiation Protection Ordinance [102] also
providesdifferentreferenceintakevaluesfordifferentagecategories,rangingfrom55lyear−1for
breast-fed babies younger than 1 year to 350lyear−1for adults. The issue is further complicated
by the widely varying consumption of bottled water of different type.According to the European
Federation of Bottled Waters [103], the consumption per capita of bottled water in European
countries varies from 16lyear−1in Finland to 189lyear−1in Italy.
Lack of data on radioactivity levels in drinking water for potentially important radionuclides
may lead to a significant underestimation of the committed doses from water ingestion. Many
studies of radioactivity in groundwater are limited to the determination of uranium isotopes and
226Ra, while228Ra,210Pb and210Po levels are rarely determined.222Rn is not considered in
UNSCEAR reports and in water-quality standards as important contributor to the effective dose
from ingestion because it is assumed to be removed to a large extent from water due to agitation
duringtreatmentandduringboiling.Furthermore,thevalueoftheingestiondoseconversionfactor
(DCF) for222Rn is still under discussion [26,82,104–108]. The WHO guidelines [95] estimate
the average activity concentration of222Rn in drinking water supplies derived from groundwater
at 20Bql−1and less than 0.4Bql−1for supplies derived from surface water. The average annual
dose from222Rn in drinking water was estimated at 0.002mSv from ingestion and 0.025mSv
from inhalation [96,109].
The committed effective dose E (hereafter called the committed dose) for a given age group,
causedbyingestionofradionuclidesoccurringindrinkingwater,canbeestimatedbythefollowing
formula:
E = V ×
N
?
i=1
(Ai× DCFi), (1)
Downloaded by [Akademia Gorniczo Hutnicza], [Mr Kazimierz Rozanski] at 08:00 14 December 2011