Journal of Nuclear and Radiochemical Sciences, Vol. 8, No. 2, pp. 143-148, 2007

Concentrations of 226Ra and 228Ra in Hot Spring Waters from Tokyo Metropolis and Its
Vicinity, Japan
T. Nakano-Ohta,*, a T. Kubota,a J. Sato,b and Y. MaharaaabResearch Reactor Institute, Kyoto University, Kumatori-cho, Sennan-gun, Osaka 590-0494, Japan
School of Science and Technology, Meiji University, Higashi-mita, Tama-ku, Kawasaki-shi, Kanagawa 2148571, Japan

Received: December 6, 2006; In Final Form: May 2, 2007
The average concentrations of 226Ra and 228Ra in the water in hot springs in and around metropolitan Tokyo were
39 mBq L –1 and 55 mBq L –1 for NaCl-type water and 0.6 mBq L –1 and 0.8 mBq L –1 for simple alkaline-type water,
respectively. From the chemical characteristics, the hot spring waters from the Kazusa and Miocene formations
are classified as NaCl and that from the Shimanto and Chichibu formations is simple alkaline. This indicates thatthe concentrations of Ra isotopes are high for the NaCl type and low for the simple alkaline type and arecorrelated with the relevant sedimentation layer from which the hot spring water is welled up. Good correlation
was observed for the ratios in the concentrations of Ra/Na and Ra/Cl in both types of hot spring water. The
correlation patterns of Ra/Na and Ra/Cl for the simple alkaline type and sedimentary rock were different fromthose for the NaCl type and seawater, suggesting that the origin of the Ra isotopes may be different. The
228
Ra/ 226Ra activity ratio was found to be related with Si concentration, showing that the activity ratio decreased
with an increase of H2SiO3 for both the NaCl and the simple alkaline type. Assuming that the Ra isotopes in hot
spring water were supplied from the relevant rock in the aquifer together with Si, the activity ratio of 228Ra/ 226Ra
in hot spring water might be different between the NaCl and simple alkaline type, suggesting that the(228Ra/ 226Ra) t =0 activity ratio for residence time at t = 0 might be different for the two types.

1. Introduction
As 226Ra and 228Ra are progenies of 238U and 232Th, respectively, the concentrations of 226Ra and 228Ra in the water of hot
springs is basically dependent on the U and Th content in thesource rock of the aquifer. Since 230 Th and 232Th, which areparents of 226Ra and 228Ra, are supposed to be insoluble in neutral alkaline-type hot spring water, 226Ra and 228Ra in the water
are supplied from the relevant rock in the aquifer into the waterby geochemical leaching and by α-recoil ejection from the Th
isotopes. Assuming that the leaching efficiencies of 226Ra and228
Ra from the relevant rock into the hot spring water is equivalent, the variation of the concentrations of Ra isotopes in hot
spring water might depend on the residence time of the hotspring water1 or the mixing ratio of waters of different origins.
Thus, the concentrations of 226Ra and 228Ra and the 228Ra/226Ra
activity ratio are considered useful tracers of the hydrologicalaspects of hot spring water.
However, the measurements of 226Ra and 228Ra in hot springwater were limited because of tiresome procedures of chemical
separation from large amounts of sample water and difficulties
in measuring low level Ra isotopes. Although by some workers
determined wide ranges of 226 Ra and 228Ra concentrations in
hot spring water from Japan,2–4 the correlation of the concentrations of Ra isotopes and chemical components in hot springwater has not yet been well clarified. Nakano-Ohta and Sato5
classified hot spring water by correlations between the concentrations of 226Ra and 228Ra and the chemical constituents; theaverage concentrations of 226Ra of some hot spring water fromJapan reported in the literature were 104 mBq L –1 for the NaCl
type and 9 mBq L –1 for the simple alkaline type. This indicated that the concentration of 226Ra in the NaCl type was muchgreater than that in the simple alkaline type. The concentra*Corresponding author. E-mail: nakano-o@HL.rri.kyoto-u.ac.jp.
Fax: +81-724-51-2639.

tions of Ra isotopes in hot spring water from metropolitan
Tokyo were determined with respect to the chemical constituents. The results indicated that the concentrations of Ra isotopes in the NaCl type averaged 70 times greater than those in
the simple alkaline type,5 suggesting that the concentration of
Ra isotopes depends highly on the chemical characteristics.
This paper deals with three investigations regarding Ra isotopes in the hot spring water of metropolitan Tokyo: (1) correlation of 226Ra and 228Ra in hot spring water with relevant rockin the aquifer from which the hot spring water is derived, (2)correlation of the concentration of Ra isotopes with major
chemical components, and (3) the correlation of the 228Ra/226Ra
activity ratio with the concentration of H2SiO3.
2. Experiments
2.1. Sampling sites. Figure 1 shows the sampling sites of
the hot springs and the geology of the studied areas.6 The hotsprings in metropolitan Tokyo are grouped into three localities:(1) the core metropolitan area, (2) the Tama area, and (3) the
Okutama area.7 The chemical characteristics of the water fromthe hot springs have been classified into (1) NaCl and NaHCO3
types for the core metropolitan area, (2) NaCl and NaHCO3
types for the Tama area, and (3) the simple alkaline type for
the Okutama area. The evaporated residual materials of the
hot spring water ranged from 98 to 580 mg L –1 for the Okutama
area,8 150 to 2053 mg L –1 for the Tama area,7,8 and 2,000 to
32,050 mg L –1 for the core metropolitan area.8 This means thatthe chemical components in the hot spring water in the core
metropolitan area are more abundant than those in the
Okutama area.
2.2. Determination of Ra isotopes. Radium isotopes in thesamples of hot spring water were quantitatively collected by
Mn-impregnated acrylic fiber columns4 having a flow rate ofless than 2 L min–1. The column consisted of three small columns (3.5 cmφ, 90 cm long) of 31 g of Mn-impregnated acrylic© 2007 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 10/19/2007

144 J. Nucl. Radiochem. Sci., Vol. 8, No. 2, 2007
fiber connected in series. This collection procedure required
2.5 hours for 100 L of water.
The Mn-impregnated acrylic fibers in the first and second
columns were combined and packed into one airtight tin canister (76 mm φ, 24 mm H); the third column was packed intoanother canister. The canisters were stored for 1 month for γ ray spectrometry so that 214Pb and 228Ac reached radioactive
equilibrium with their parents, 226 Ra and 228Ra, respectively.
The radioactivity of 214Pb and 228Ac were determined by 351
keV and 911 keV γ -rays, respectively, using a Ge detector with
10% relative efficiency. Details of γ -ray spectrometry are

Nakano-Ohta
4given by Saito et al.
The concentrations of 226Ra and 228Ra were calculated by theamounts of 226Ra and 228Ra activities collected by the combined
two Mn-impregnated acrylic fibers, and the sample volumes
employed. Details of the procedure for determining 226Ra and228
Ra are given in the paper of Nakano-Ohta and Sato.5
2.3. Determination of major elements. Hot spring waterfrom Oyedo-onnsen-monogatari, Seta-sanga-no-yu, Jindajiyukari, Kosuge-no-yu, and Hisamatsu were filtered with amembrane filter having a pore size of 0.45 µm, and the water
was diluted with deionized water. The concentrations of Na,

Figure 1. Sampling sites (after Omori,6 modified).

Figure 2. Geological map (after Kanroji8 and Kaizuka,9,10 modified) and the depth of bore-hole for some hot-spring waters.

J. Nucl. Radiochem. Sci., Vol. 8, No. 2, 2007 145

Concentrations of 226Ra and 228Ra in Hot Spring Waters from TokyoK, Mg, and Ca in the hot spring water were determined by ICPAES, ICPS7500 (Shimadzu Co. Ltd.). The analytical lines
used for the ICP-AES measurements were 588.995 nm for Na,
766.491 nm for K, 279.553 nm for Mg, and 393.366 nm for Ca.
Alternatively, the concentration of Cl was determined by ion
chromatography, IC-7000 (Yokogawa Co. Ltd.) with an
Excelpak ICS-A23 anion-exchange resin column. The analytical conditions for Cl were 3.0 mM of Na2CO3 for the eluent and
15 mM of H 2 SO 4 for the regenerating solution. The elution
speed of the sample water was 1.0 mL min–1.
The concentrations of the major elements of hot spring waterfrom Heiwajima-kurhaus, Kamata, Kannon-no-yu, Nomekoino-yu, Matsu-no-yu, and Tsuru-no-yu and the Si concentration
were referred to the notice board of the chemical constituentsfor each hot spring. The chemical components were assumed
constant from the day of determination of the major chemicalcomponents to our sampling day.
3. Results and Discussion
3.1. Correlation of Ra concentration and sedimentation
layer of the aquifer. Table 1 lists the concentrations of 226Ra
and 228Ra in the hot spring waters classified by chemical constituent and depth of borehole. The concentrations of Ra isotopes in the hot spring waters ranged from 0.3 to 84 mBq L –1
for 226Ra and from 0.2 to 104 mBq L –1 for 228Ra, respectively.
The average concentrations were 39 mBq L –1 for 226Ra and 55
mBq L –1 for 228Ra in the NaCl type (n = 5) and 0.6 mBq L –1 for226
Ra and 0.8 mBq L –1 for 228Ra in the simple alkaline type (n =5). This indicated that the Ra isotope concentration in the
NaCl-type hot spring was 70 times greater than that in the simple alkaline type.
Hot springs are generally classified into two groups: (1) nonvolcanic hot springs (fossil seawater or deep undergroundwater) and (2) volcanic hot springs. The hot springs in the core
metropolitan and Tama areas were reported to be non-volcanic
hot springs; those in the eastern part of Yamanashi prefecturenear the Okutama area were either non-volcanic or volcanic hot
springs.8 Figure 2 shows the geology and the depth of the bore-

Figure 3. (a) Correlations between 226Ra and Cl, Na, and Ca in hot
spring waters. (b) Correlations between 228Ra and Cl, Na, and Ca in
hot spring waters. ○: Ca-Ra, ●: Cl-Ra, and □ : Na-Ra.

TABLE 1: Concentrations of 226Ra and 228Ra, and 228Ra/ 226Ra activity ratio in hot spring waters from Tokyo area
226

Ra / mBq L –1*

Ra / 226Ra

Depth**/ m

104 ± 494 ± 473 ± 123 ± 112 ± 1
21 ± 155

1.2 ± 0.1
1.9 ± 0.1
1.7 ± 0.1
1.1 ± 0.1
1.2 ± 0.1
1.3 ± 0.1
1.4

1300
2000
1500
1700
1500
1600

1.2 ± 0.2
0.49 ± 0.09
0.85

3.0 ± 0.4
0.73 ± 0.20
1.9

2.5 ± 0.5
1.5 ± 0.5
2.0

100
60

0.30 ± 0.08
0.94 ± 0.26
0.66 ± 0.08
0.43 ± 0.10
0.62 ± 0.06
0.6

0.24 ± 0.04
0.63 ± 0.04
1.1 ± 0.11
0.62 ± 0.15
1.2 ± 0.2
0.8

0.8 ± 0.3
0.7 ± 0.2
1.7 ± 0.3
1.4 ± 0.5
1.9 ± 0.2
1.3

1300–1500
300
30
1500
1300–1500

Chemical character

Sampling date

NaCl type
Oyedo-onsen-monogataria
Heiwajima-kurhausa
Kannon-no-yub
Seta-sanga-no-yua
Jindaiji-yukaria
Tennyo-no-yud
AverageDec. 9, 2005
Dec. 26, 2004
Apr. 3, 2005
Dec. 6, 2005
Jul. 4, 2005
Aug. 8, 2003

84 ± 258 ± 243 ± 1
21 ± 0.5
10 ± 0.2
18 ± 139

Dec. 26, 2004
Jul. 2, 2005

May 14, 2003
May 20, 2003
May 14, 2003
Jul. 12, 2005
May 10, 2005

NaHCO3 type
Kamata a
Hisamatsua
Average
Simple alkaline type
Nomekoi-no-yuc
Tsuru-no-yuc
Matsu-no-yuc
Kosuge-no-yuc
Yasuragi-no-satoc
Average*Errors represent the counting statistics.
**The depth of bore-hole of hot-spring.
a
Metropolitan area. b Tama area. c Okutama area. d Yamanashi Pref.

228

Ra / mBq L –1*

228

146 J. Nucl. Radiochem. Sci., Vol. 8, No. 2, 2007

Nakano-Ohta

Figure 4. Correlation between 226Ra and 228Ra, and Cl, Na, Ca, K, and Mg in hot spring waters. ●: NaCl type, ■ : NaHCO3 type, and ○: Simple
alkaline type.

holes of the hot springs at Kanroji8 and Kaizuka.9,10 The
Kazusa and Miocene formations are relevant sedimentation
layers for Oyedo-onsen-monogatari, Heiwajima-Kurhaus, Setasanga-no-yu, Jindaiji-yukari, and Kannon-no-yu whereas the
Shimanto and Chichibu formations are relevant for Nomekoino-yu, Tsur u-no-yu, Matsu-no-yu, Kosuge-no-yu, andYasuragi-no-yu.7–14 The chemical characteristics of the Kazusa
and Miocene formations are the NaCl type whereas the
Shimanto and Chichibu formations are the simple alkaline
type, indicating that different concentrations of Ra isotopes
exist in the different aquifers from which the hot spring water
is welled up. Seki et al.15 reported that the chemical characteristics of well water from depths of 700–2000 m in the eastern
Kanto plain of Japan were classified according to the surrounding geology as follows. (1) The hot spring waters from the
Kanto plain were of the NaCl type, and consisted of fossil seawater. (2) Those from the marginal mountainous area were of

the alkaline type with a low concentration of NaHCO3, whichmight consist of meteoric water reacting with silicic igneousrocks. (3) Moderate alkaline concentration Na/Ca-SO 4 type
mostly from the Kinugawa graben, which might consist mainlyof meteoric water reacting with pre-Tertiary sedimentary rocks
and Tertiary volcaniclastic rocks. The hot springs studied in
the present work and observed by Seki et al.15 were collectedfrom the coastal area of the Pacific Ocean to the mountainous
area of the central part of the Kanto plain. The area we studied
is located in the western part of the Kanto plain, while the eastern part of the area was observed by Seki et al.15 The chemical
characteristics of the presently observed hot springs were similar to those that appeared in the literature:15 a high concentration of chemical components with the NaCl type from the
Kanto plain area and a low concentration of components withthe alkaline type from the Kanto mountainous area. If theconcentration of Ra isotopes in hot spring water depends on theConcentrations of 226Ra and 228Ra in Hot Spring Waters from Tokyo

J. Nucl. Radiochem. Sci., Vol. 8, No. 2, 2007 147

Figure 5. (a) Relation between 226Ra/Na and 226Ra/Cl in hot spring waters. (b) Relation between 226Ra/Na and 226Ra/Cl in hot spring waters. (c)Relation between 1/Na and 1/Cl in hot spring waters. ●: NaCl type, ■ : NaHCO3 type, ○: Simple alkaline type, ◇: seawater, and ×: sedimentary rock.

sedimentation layer, the hot spring water from the Kazusa
group and the Narita group in the Kanto plain may contain ahigh level of Ra isotopes, as observed in the present study. The
concentrations of Ra isotopes in three areas of undergroundwater in Mobara, Chiba prefecture, in the eastern part of the
Kanto plain along the coast of the Pacific Ocean were measured to be as high as those in the core metropolitan area.
Details concerning the Mobara observation will be given elsewhere.
3.2. Correlation of Ra concentration and chemical constituents. The concentrations of 226Ra and 228Ra in hot springwaters increased with an increase in the concentrations of Cl,
Na, and Ca, as shown in Figure 3. Figure 4 shows the correlations between the observed concentrations of 226Ra and 228Ra,
and among Na, K, Ca, Mg, and Cl. The concentrations of the
Ra isotopes for the NaCl type increased with an increase in theconcentrations of K, Mg, Na, Ca, and Cl, while those for thesimple alkaline type increased with an increase in the Na concentration, but decreased with an increase in the concentrationsof K, Mg, Ca, and Cl.
Figures 5(a) and 5(b) show the correlations of the ratios of[226Ra/Na] and [226Ra/Cl], and [228Ra/Na] and [228Ra/Cl] for the
NaCl type, simple alkaline type, NaHCO3 type, seawater, andsedimentary rock, respectively. Figure 5(c) shows the correlation of the ratios of [1/Na] with [1/Cl]. A good correlation
between [1/Na] and [1/Cl] was observed for the NaCl-type hotspring. The concentrations of 226Ra and 228Ra in surface seawater from Tokyo Bay in the Pacific Ocean ranged from 1.1 to 1.9
mBq L –1 for 226 Ra and from 0.9 to 1.4 mBq L –1 for 228 Ra,
respectively.16 Although the concentrations of 226Ra and 228Ra
in the surface seawater in the Pacific Ocean are higher thanthose just offshore,17 those in the surface seawater of Tokyo Bay
are on the order of 1 mBq L –1.16,17 The ratios of concentrationof surface seawater were 1 × 10 –4 and 0.7 × 10 –4 for [226Ra/Na]
and [226Ra/Cl], and 1 × 10 –4 and 0.7 × 10 –4 for [228Ra/Na] and[228Ra/Cl], respectively. The ratios of concentration of [Ra/Na]
and [Ra/Cl] for seawater are also plotted in Figures 5(a) and5(b) with an open square. The average concentrations of 226Ra
and 228Ra in sedimentary rock,18 which may be the relevantrock in the aquifer for Nomekoi-no-yu, Matsu-no-yu, Tsuru-noyu, and Kosuge-no-yu, were 36 mBq g –1 and 50 mBq g –1 ,respectively. The ratios of concentration of sedimentary rockwere 1 × 10 –3 for [226Ra /Na], 1 × 10 –4 for [226Ra /Cl], 2 × 10 –3
for [228Ra/Na], and 2 × 10 –4 for [228Ra/Cl] as shown in Figure5(a).
The ratios of concentration of [226Ra/Na] and [226Ra/Cl] and148 J. Nucl. Radiochem. Sci., Vol. 8, No. 2, 2007

Nakano-Ohta(2) Although the concentration ratios of [Ra/Na] and [Ra/Cl]
in hot spring waters showed good correlation, those for thesimple alkaline type and sedimentary rock were different fromthose for the NaCl type and seawater, suggesting that the originof the Ra isotopes for the simple alkaline type is different fromthat for the NaCl type.
(3) The 228Ra/ 226 Ra activity ratio decreased with increasing
H 2 SiO3 concentration for both the NaCl and simple alkaline
types. Assuming that the Ra isotopes were leached from therelevant sediment rock together with Si, (228Ra/226Ra) t=0 for thesimple alkaline-type hot spring water might be different fromthat of the NaCl-type hot spring water.

Figure 6. Correlation between 228Ra/ 226Ra and H2SiO3 in hot-springwaters. ●: NaCl type, ■ : NaHCO3 type, and ○: Simple alkaline type.

Acknowledgment. The authors are deeply indebted to Dr.
Y. Kanroji, Hot spring Research Center, for his guidance in theselection of hot spr ings for t he present obser vation.
Cooperation of Misses C. Shinagawa and N. Tomioka, andMessrs. M. Yoshikawa and K. Fujita, Meiji University, in sampling of hot spring water is gratefully acknowledged.
Referencesof [228Ra/Na] and [228Ra/Cl] showed good correlation both for
the NaCl type and simple alkaline type, as shown in Figure5(a). The extrapolated values of the ratios of [Ra/Na] to [Ra/Cl] for the NaCl type agreed with those for seawater, while thesimple alkaline type agreed with the ratio of [Ra/Na] and [Ra/Cl] of the sedimentary rock. The Ra isotopes in the NaCl-type
hot spring water might be supplied from ‘fossil seawater’ in thesedimentation layer, while those in the simple alkaline-type hotspring water might be supplied from the sedimentary rock bymeteoric water.
3.3. 228Ra/ 226Ra activity ratio in the water of hot springs.
The 228Ra/ 226Ra activity ratio ranged from 0.7 to 2.5, as shownin Table 1. Although the concentrations of 228Ra and 226Ra had
a wide range, the hot springs could be classified by chemical
characteristics, however, the 228Ra/ 226Ra activity ratio but notby the concentrations of the major elements. However, the228
Ra/ 226Ra activity ratio was found to be closely related withthe concentration of H2SiO3, as shown in Figure 6. Namely, the228
Ra/ 226 Ra activity ratio decreases with an increase of the
H2SiO3 concentration for both the NaCl type and simple alkaline type of hot spring. As H2SiO3 in hot spring water is considered to be leached from the rock in the aquifer, the activity
ratios may be classified by H 2 SiO3 concentration in both the
NaCl and simple alkaline types, as shown in Figure 6.
Assuming that the Ra isotopes in hot spring water were supplied from the rock in the aquifer together with Si, the228
Ra/ 226Ra activity ratio may decrease with an increase of theresidence time of the hot spring water in the rock in the aquifer.
An 228Ra/ 226 Ra activity ratio for a 0 ppm concentration of Si
indicates the initial 228 Ra/ 226 Ra activity ratio at time zero.
Based on the assumption stated above, the initial 228Ra/ 226Ra
activity ratio of hot spring water for the NaCl type may be different from the simple alkaline type, that is, the activity ratio,(228Ra/226Ra) t=0, of the NaCl-type hot spring is higher than thatof the simple alkaline type.
4. Conclusion
The concentrations of 226Ra and 228Ra in the hot spring waterfrom metropolitan Tokyo suggested that:(1) The chemical characteristics of the hot spring waters fromthe Kazusa and Miocene formations is the NaCl type and thatfrom the Shimanto and Chichibu formations is the simple alkaline type, from the fact that the concentrations of Ra isotopes
are high for the NaCl type and low for the simple alkaline type,and correlate well with the relevant sedimentation layer fromwhich the hot spring water is derived.

(1) T. Nakano-Ohta and J. Sato, Radioisotopes 54, 531 (2005).
(2) J. Sato, O. Yokozawa, and N. Saito, J. Soc. Eng. Mineral
Springs Jpn. 10, 47 (1975) (in Japanese with English
abstract).
(3) M. Yamamoto, A. Toguchi, K. Sashimono, K. Komura, K.
Ueno, and M. Sakanoue, J. Balneol. Soc. Jpn. 44, 18(1994) (in Japanese with English abstract).
(4) T. Saito, H. Ueda, T. Ohta, and J. Sato, J. Balneol. Soc.
Jpn. 52, 3 (2002).
(5) T. Nakano-Ohta and J. Sato, J. Balneol. Soc. Jpn. 56, 37(2006).
(6) Y. Kanroji, S. Tanaka, and H. Ichiyanagi, J. Balneol. Soc.
Jpn. 27, 32 (2000) (in Japanese with English abstract).
(7) Y. Kanroji (private communication, 2005).
(8) Nichiyo-no-chigaku, Ed. M. Omori, Tsukiji shokan, Tokyo(1989) (in Japanese).
(9) S. Kaizuka, J. Geography 96, 223 (1987) (in Japanese withEnglish abstract).
(10) Tokyo-no-shizenshi, Ed. S. Kaizuka, Kinokuniya shoten,
Tokyo (1979) (in Japanese).
(11) Hagiwara Boring Co. Ltd. (private communication, 2005).
(12) A. Sakai, Geology of the Itsukaichi district. With geological sheet map at 1:50,000, Geol. Surv. Jap. (1987) (inJapanese with English abstract).
(13) T. Mitsuchi, Explanatory text of the geological map ofJapan at 1:75,000, Imperial geology survey of Japan(1932) (in Japanese with English abstract).
(14) F. Kuramochi, J. Geography 65, 46 (1956) (in Japanesewith English abstract).
(15) Y. Seki, T. Nakajima, H. Kamioka, Y. Kanai, M. Manaka,
and K. Tstjunyra, J. Balneol. Soc. Jpn. 54, 1 (2004) (inJapanese with English abstract).
(16) T. Nakano-Ohta and J. Sato, Radioisotopes 55, 443 (2006).
(17) M. Yamada and Y. Nozaki, Marine Chem. 19, 379 (1986).
(18) T. Nakano-Ohta and J. Sato, Bunsekikagaku 54, 219 (2005)(in Japanese with English abstract).