Journal of Nuclear and Radiochemical Sciences, Vol. 1, No. 1, pp. 5−10, 2000

5

Radioactive Disequilibrium of Uranium and Thorium Nuclide Series in Hot Spring and River
Water from Peitou Hot Spring Basin in Taipei
Tieh-Chi Chu*,a and Jeng-Jong WangbaDepartment of Nuclear Science, National Tsing Hua University, Hsinchu, Taiwan 300, R.O.C.
bRadiation Monitoring Center, Kaohsiung Hsien, Taiwan 833, R.O.C.

Received: October 13, 1999
The distributions of the naturally occurring radionuclides 238U, 234U, 232Th, 230Th, 228Th, 228Ra, and 226Ra in riverand hot spring water within the Peitou hot spring basin at the northern part of Taiwan were studied by using alphaand gamma-spectrometric methods and liquid scintillation counting method. The respective concentration rangesof 238U, 234U, 232Th, 230Th, 228Th, and 226Ra in river and hot spring water were 0.80−48, 1.2−51, 0.05−44, 0.09−39,
0.19−342, 3.0−22.5 mBq/L and 2.3−85, 2.9−91, 0.67−73, 1.14−66, 1.01−630, and 4.5−36.5 mBq/L, respectively.
In general, the distributions of uranium-series and thorium-series nuclides in river water, hot spring water were all
pH dependent. The respective radioactivity ratios of 234U/238U in river and hot spring water are observed to be
1.0−1.5 and 1.03−1.5, however, the radioactivity ratios of 228Th/ 232Th in river and hot spring water are 1.4−19 and1.5−8.8, respectively. The high 228Th/ 232Th activity ratios showed that besides the recoil effect, enrichment of228
Ra to 232Th in the waters was another possible factor causing an excess of 228Th. Because radium may exist as(Pb, Ba, Ra)(SO4)X pseudo-colloid in hot spring water, which is the same as the more famous mineral of Peitoustone (Hukutolite), therefore, 228Ra and 226Ra are more abundant in the hot spring sediments, which radioactivity
are18−3010 and 16−230 Bq/kg (dry), respectively.
Introduction
The Peitou hot spring basin is located in the northern part of
Taiwan (see Figure 1). There are volcanic vestiges and hot
springs all around this area. Hot springs here spread over thewestern part of Taiwan between two geological faults. There
are three main streams, Waishuang, Nanhuang and Huangkang
Streams, running through this area, emptying into the KeelungRiver, and finally emptying into the Taiwan Strait through theTamsui River. Owing to a violent material exchange between
the subterranean hot waters and the rock stratum, the erosionof the rocks is greatly increased by the high-acidic sulfurous
waters.
After Okamoto, who first found the radioactive mineral,
Hokutolite, in the Peitou hot spring in this area in 1907,1 studies of the naturally occurring radionuclides in hot spring waterof this area were undertaken one after another.2−5 When a hot
spring water abundant with naturally occurring radionuclides
at high temperatures and its acidity empties into the rivers
nearby, the distribution of the radionuclides may differ fromriver basin to river basin, since changes of the environment cancause fractionation between the naturally occurring radionuclides.
The purposes of this paper is to systematically study the distribution and migration of the naturally occurring radionuclides,
238
U, 234U, 232Th, 230Th, 228Th, 228Ra and 226Ra in the main rivers
and hot springs of the Peitou hot spring basin by α-spectrometry, γ-ray spectrometry and liquid scintillation counting method.
Experimental
Sample Description. Figure 1 shows the sampling spots in
the Peitou hot spring basin. All the water samples collected
were placed in 20-L bottles containing 50 mL of HCl to avoid
hydrolysis and radiocolloid formation. In addition, we collected
100mL of water without HCl, and cooled to room temperature
to measure its pH value with a pH meter. All the temperatures*Corresponding author. E-mail: tcchu@mx.nthu.edu.tw. FAX:
+886-3-572-7310

Figure 1. Sampling sites (•) in the Peitou hot spring basin in Taiwan.
H: Huangkan stream, N: Nanhuang stream, V: Waishuang stream,
P: Peitou hot spring, H: Hsingi-lu hot spring,
L: Liuhuang-guu hot spring, M: Matsao hot spring.

recorded were made by a Hg-thermometer at the sampling
spots. In the Waishuang Stream, we collected samples at six
spots (V1 − V6). In the Nanhuang Stream, samples were collected at eight spots (N1 − N8). In the Huangkang Stream, atfour spots (H1 − H4) were the samples collected. In addition,at the Peitou hot spring, we collected water samples at four
spots (P1−P4) and sediment samples at two spots (P3 − P4).
At the Hsingi-lu hot spring, we collected water samples at two
spots (H1 − H2) and sediment samples at one spots (H2). At
the Liuhuang-guu hot spring, one water sample (L1) and one
sediment sample (L1) were collected. And, only one hotspring water sample (M1) was collected at the Matsao hot
spring.
Separation of Uranium. 6 232U was added as a tracer toeach sample, followed by addition of CyDTA to mask the thorium in the sample. Uranium was sorbed on a Chelex-100
resin (Na-form; 50−100 mesh), and then desorbed by ammonium carbonate solution which was subjected to successive treatment with nitric, perchloric and sulfuric acids. The resulting© 2000 The Japan Society of Nuclear and Radiochemical Sciences

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J. Nucl. Radiochem. Sci., Vol. 1, No. 1, 2000

solution was electrolyzed for 2 hours with platinum as an
anode and a stainless steel plate as a cathode. The cathode wasdried by an infrared lamp and cooled to room temperature foralpha counting.
Separation of Thorium.7 The thorium was concentrated bycoprecipitation of ferric hydroxide and then purified by ananion exchange resin column. 229Th tracer was added to eachsample, and thorium was collected as a ferric hydroxide precipitate. The precipitate was dissolved by nitric acid and thenpassed down a Dowex-1 × 8 resin column (NO3− form; 50−100mesh) to remove thorium. Thorium was desorbed byhydrochloric acid, and treated by nitric, perchloric and sulfuric
acids as in the case of uranium. The final solution was electrolyzed for 1.5 hours, and the stainless steel cathode was driedby an infrared lamp. After cooling, alpha activity was counted.
Alpha Counting. Alpha counting was done using a
NUMELEC Model NU114 grid ionization chamber, and thecounting efficiency of which was determined by means of an241
Am standard source. From the chemical yield and thecounting efficiencies of the instrument, the disintegration ratesof the uranium and thorium isotopes were determined.
Separation and Determination of 226Ra.8 226Ra was concentrated and separated using the barium sulfate coprecipitation method and then the radioactivity of 226Ra was determinedby liquid scintillation counting.
Radium was coprecipitated with barium carrier as sulfate.
After purifying the radium by recrystallization, the precipitate
was dissolved in an EDTA solution. The purified 226 Ra in
EDTA solution was packed into a 20-mL glass vial and thendiluted to 10 mL with H2O. After the addition of 10 mL of a
POP−POPOP cocktail in toluence, the vial containing the sample was composed of two phases, was kept air-tight andallowed to stand for over 30 days to establish a radioactive
equilibrium between 226Ra and 222Rn. The activity of 222Rn wasmeasured on a Packard Model 2560 TR/XR liquid scintillation
counter within an energy window of 335−530 keV, fromwhich the activity of 226Ra was then determined. A 3.824-day
half-life of 222Rn was determined by an activity ingrowth-curveat 335−530 keV using a spiked 226Ra water sample which was
operated in the process mentioned above. Besides, a 100 %counting efficiency, 96.7 ± 2.3 % chemical yields of radium
were estimated using 226Ra spiked water samples which wereoperated in the same process.
Determination of 228Ra. The hot spring sediment samples
were heated to dry under 110 ˚C for 24 hours and kept air-tightin the acrylic containers for 30 days to establish radioequilibrium between 228Ra and 228Ac. The γ-ray activity of 228Ac wascounted using a pure Ge detector, and from which the radioactivity of 228Ra was solved.

Tie-Chi Chu
0.09−39, and 0.19−342 mBq/L, respectively.
3. Radium. The river water investigated in this study, shows
the concentration of 226Ra ranges between 3.0 and 22.5 mBq/L.
It is likely the high uranium, thorium and radium contents ofsome river water samples collected in this area result from thecontribution of the hot spring water.
pH Dependence. The chemical properties of uranium andthorium in the waters are mostly affected by the hydroxide.
Therefore, the content of uranium and thorium in the riverwater is related to pH. Figure 2 shows the effects of the pH on
the solubility of UO 2 (OH) 2 and Th(OH) 4 . The solubility
shown in Figure 2 is based on 25 ˚C, µ = 0 and calculated fromthe solubility product (Ksp) of UO2(OH)2 and Th(OH)4.9 It differs from that of real river water, so Figure 2 is included justfor reference. In Figure 2, obviously, Th(OH)4 and UO2(OH)2
are soluble below pH = 5 and 7, respectively.
Figure 3 illustrates the concentration (ppb) of uranium andthorium in river water as a function of pH. In Figure 3, obviously, when pH ~7, the concentration of uranium and thorium
is extremely low; when pH < 5, the concentration gradually
increases. This indicates that, when pH ~7, uranium and thorium cannot be leached much from the fractures or pores of therock in the form of hydroxide. When pH < 5, uranium andthorium are able to enter the river water in the form of UO22+
and Th4+ which are then suitable for leaching.

Figure 2. A plot of concentration of uranium and thorium vs. pH at
25 °C, µ = 0.

Results and Discussion
I. River Water. The analytical results of the radioactivitiesof the naturally occurring radionuclides of the three mainrivers in the Peitou hot spring basin are shown in Table 1.
Depending upon the mixing ratio of the hot spring water, thetemperature and pH of the river water vary greatly; theirranges are 13˚C−47˚C and 1.80−7.64, respectively.
Radioactivity Distribution. The radioactivity distribution
from some of the river water in certain areas varies greatly
because the hot spring water is emptying into rivers in thoseareas.
1. Uranium. From Table 1, the concentration of uranium inthe river water in the Peitou hot spring basin ranges from 0.064
to 3.8 ppb. The radioactivities of 238U and 234U were 0.80−48
and 1.2−51 mBq/L, respectively.
2. Thorium. The concentration of thorium in the river waterof the Peitou hot spring basin ranges from 0.011 to 10.9 ppb,
and the radioactivities of 232Th, 230Th, and 228Th were 0.05−44,

Figure 3. A plot of concentrations of U and Th vs. pH of river water.

1.6±0.1

238

2.0±0.2

1.5±0.1

7.49

13.0

1.8±0.1

1.4±0.1

7.64

14.0

V3

1.9±0.2

1.4±0.1

7.36

13.0

V4

Downstream

1.8±0.2

1.4±0.1

7.18

14.0

V5

1.2±0.1

0.80±0.09

7.43

13.5

V6

Tributary

2.7±0.2

2.0±0.2

7.37

15.5

N1

2.6±0.2

2.5±0.2
2.1±0.2

2.0±0.2

4.39

17.5

N3

Upstream

4.61

18.0

N2

1.8±0.2

1.9±0.2

7.05

22.0

N4

4.0±0.3

3.5±0.3

4.29

21.0

N5

5.9±0.1

4.1±0.1

U/232Th

Th/232Th

238

228

Th/232Th

Ra/ Th230

230

Th/234U

226

230

Th/238U

U/238U230234

1.3±0.2

1.3±0.1

2.2±0.5

1.4±0.2

2.8±0.5

1.3±0.2

3.1±0.5

0.6±0.1

1.5±0.2

0.9±0.3

1.0±0.1

1.4±0.2

1.1±0.2

1.2±0.2

1.0±0.1

2.8±0.3

0.13±0.02 0.053±0.04

0.15±0.01

5.4±0.2

0.65±0.3

19±2

11±2

1.4±0.3

1.7±0.318±2

8±1

2.2±0.4

1.9±0.426±58±3

1.9±0.7

1.6±0.625±39±2

1.6±0.4

1.5±0.3

37±5

10±2

1.7±0.3

1.5±0.311±2

3.0±0.7

1.7±0.5

1.7±0.3

25±3

17±3

4.4±0.2

2.0±0.4

4.4±0.5

1.4±0.2

1.3±0.2

5.8±0.5

3.70±0.06

2.2±0.3

2.0±0.3

6.5±0.617±1

9±1

3.0±0.3

1.5±0.2

3.0±1

7.7±0.4

5.3±0.3

1.1±0.1

6.1±0.2

1.5±0.1

19.0±0.5

1.02±0.04

0.11±0.01

7.5±0.3

0.22±0.03

0.09±0.02

0.05±0.01

1.7±0.1

1.4±0.1

1.01±0.06

16.8±0.557±2

8.0±0.3

5.4±0.3

14.9±0.912.6±0.8

3.20

28.0

N8

Tributary

7.11

42.0

N7

0.8±0.1

2.3±0.2

1.6±0.2

0.57±0.03

1.7±0.2

2.0±0.3

1.2±0.2

1.2±0.1

1.05±0.08

0.75±0.06 0.87±0.09

0.99±0.08

0.86±0.05

5.5±0.1

12.1±0.7

4.6±0.4

4.0±0.3

11.1±0.6

10.6±0.637±1

3.3±0.2

4.3±0.3

14.8±0.8

14.2±0.7

2.56

23.0

H3

1.1±0.1

1.6±0.2

0.37±0.03

0.60±0.04

10.9±0.3

3.8±0.2

1.04±0.08 1.06±0.06

1.07±0.09 0.35±0.02

1.07±0.07

1.14±0.06

22.5±0.7

342±5

39±1

44±1

51±2

48±2

1.80

47.2

H4

Downstream Tributary

6.00±0.06 11.9±0.3

11.8±0.4

1.5±0.5

1.5±0.1

8.2±0.5

7.5±0.5

3.65

19.5

H2

Upstream

3.00

21.0

H1

Huangkang Stream

28±6

4±1

1.8±0.583±20

2.3±0.2

10.6±0.7

1.5±0.1

2.1±0.1

2.7±0.3

3.0±0.3

1.2±0.1

1.2±0.1

5.0±0.5

7.9±0.6

1.0±0.3

4.0±0.4

0.58±0.02

7.8±0.2
3.44±0.03 1.09±0.05

8.6±0.6

0.77±0.07 0.89±0.03

3.6±0.2

0.05±0.01 0.54±0.04 0.41±0.04 0.18±0.06 0.22±0.02 0.76±0.04

0.06±0.01 0.63±0.05 0.43±0.04 0.20±0.09 0.23±0.02 0.81±0.04

1.2±0.110±3

0.85±0.07 0.011±0.003 1.34±0.07

0.47±0.05 0.25±0.03

0.59±0.02

0.28±0.03 0. 21±0.02

14.8±0.4

45.0±0.5

0.32±0.02 2.42±0.06

0.12±0.02 0.28±0.03 0.55±0.05 0.17±0.02 0.69±0.06

1. 4±0.26±1

0.16±0.02

7.1±0.2

1.2±0.1

1.1±0.1

0.11±0.02 0.18±0.03 0.16±0.03 0.13±0.02 0.12±0.02 0.38±0.07 0.09±0.01 0.27±0.03 0.52±0.07 0.18±0.02 0.61±0.05

0.15±0.02 0.24±0.04 0.21±0.05 0.17±0.02 0.15±0.02

1.31±0.2

2.±0.50.14±0.01

5.11±0.07

0.035±0.006 0.047±0.008 0.05±0.01 0.039±0.006 0.035±0.005 0.07±0.02 0.029±0.005

7.9±0.2

Th (ppb)

U/Th (ppb/ppb) 3.71±0.7

0.11±0.01

6.1±0.2

0.20±0.02

0.11±0.01

7.6±0.2

0.11±0.01 0.064±0.007 0.16±0.01

0.12±0.01

6.4±0.1

0.13±0.1

4.6±0.1

U (ppb)Ra (mBq/L)

226

Th (mBq/L) 0.19±0.03 0.41±0.05 0.35±0.08 0.26±0.03 0.24±0.03 0.46±0.08 0.53±0.04 0.82±0.08

228

Th (mBq/L) 0.24±0.03 0.36±0.05 0.29±0.06 0.24±0.03 0.21±0.03 0.46±0.08 0.24±0.03 0.71±0.06

230

3.4±0.3

3.1±0.3

2.6±0.3

4.37

21.0

N6

Downstream

Nanhuang Stream

Th (mBq/L) 0.14±0.02 0.19±0.03 0.18±0.05 0.16±0.03 0.14±0.02 0.27±0.06 0.12±0.02 0.57±0.05 0.54±0.07 0.22±0.02 2.37±0.06

U (mBq/L)

232

234

2.1±0.2

7.52

pH

U (mBq/L)

13.0

Temp. °C

V2

Upstream

V1

Location code

Waishuang Stream

TABLE 1: The Radioactivities of the Uranium and Thorium Family Members in the Stream Water

Radioactive Disequilibrium of U and Th
J. Nucl. Radiochem. Sci., Vol. 1, No. 1, 2000
7

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Tie-Chi Chu

J. Nucl. Radiochem. Sci., Vol. 1, No. 1, 2000

TABLE 2: The Radioactivities of the Uranium and Thorium Family Members in the Hot Spring Waters and Sediments
Location code
Temp. (˚C)
pH

P-1
98.0
1.55

P-2
79.5
1.65

P-3
72.3
1.72

238

U (mBq/L)
234
U (mBq/L)
232
Th (mBq/L)
230
Th (mBq/L)
228
Th (mBq/L)
226
Ra (mBq/L)

85±4
91±4
72±3
66±3
630±2036.5±0.7

67±2
69±2
66±3
63±3
560±1529.0±0.8

68±2
70±2
65±2
56±2
443±9
N.D.

234

U/238U
Th/234U
226
Ra/230Th
228
Th/232Th

1.1±0.1
0.73±0.06
0.55±0.04
8.8±0.6

1.03±0.06
0.91±0.07
0.46±0.03
8.5±0.6

Uranium (ppb)
Thorium (ppb)
U/Th (ppb/ppb)

6.6±0.3
17.8±0.9
0.37±0.04

5.4±0.1
16.4±0.8
0.33±0.02

P-4
70.5
1.68

H-1
79.0
1.65

H-2
51.0
2.44

M-1
26.0
2.74

L-1
71.0
3.40

76±3
82±3
73±3
64±3
500±12N.D.

14.4±0.7
20.7±0.915.3±0.514.2±0.5106±3
8.96±0.05

4.9±0.4
7.5±0.5
7.1±0.3
5.9±0.2
21.1±0.6
7.2±0.1

2.6±0.2
2.9±0.2
1.5±0.1
2.0±0.1
6.3±0.3
10.8±0.1

2.3±0.2
3.3±0.3
0.67±0.04
1.14±0.05
1.01±0.05
4.5±0.1

1.03±0.06
0.80±0.05−
6.8±0.3

1.08±0.08
0.78±0.07−
6.8±0.4

1.4±0.1
0.69±0.05
0.63±0.03
6.9±0.4

1.5±0.2
0.79±0.08
1.22±0.06
3.0±0.2

1.1±0.2
0.69±0.08
5.4±0.3
4.2±0.5

1.4±0.3
0.35±0.05
3.9±0.3
1.5±0.2

5.5±0.2
16.1±0.6
0.34±0.03

6.1±0.2
18.0 ±0.7
0.34±0.02

1.16±0.06
3.8±0.1
0.31±0.02

0.40±0.03
1.75±0.07
0.23±0.03

0.21±0.02
0.36±0.03
0.6±0.1

0.18±0.02
0.17±0.01
1.1±0.2

Hot Spring Water

230

Hot Spring Sediment238

U (Bq/kg (dry))
U (Bq/kg (dry))232
Th (Bq/kg (dry))230
Th (Bq/kg (dry))228
Th (Bq/kg(dry))228
Ra (Bq/kg (dry))226
Ra (Bq/kg (dry))

28.1±0.7
29.6±0.757±4
36±3
750±302030±20
187±5

37±1
38±1
66±4
54±4
1080±503010±30230±620.0±0.5
20.6±0.518.8±0.9
16.0±0.8
22±1
31±1
31±2

11.2±0.5
11.3±0.515.2±0.7
15.0±0.7
15.5±0.7
18±1
16±1

234

U/238U
Th/234U
226
Ra/230Th
228
Ra/232Th
228
Th/228Ra
228
Th/232Th

1.05±0.05
1.2±0.1
5.2±0.636±3
0.37±0.0213±1

1.03±0.05
1.4±0.1
4.3±0.446±3
0.36±0.02
16±2

1.03±0.05
0.78±0.06
1.9±0.2
1.6±0.1
0.71±0.06
1.2±0.1

1.01±0.09
1.3±0.1
1.1±0.1
1.2±0.1
0.86±0.09
1.02±0.09

Uranium (ppm)
Thorium (ppm)
U/Th (ppm/ppm)

2.26±0.05
14.1±0.9
0.16±0.01

2.98±0.09
16 ±1
0.19±0.02

1.61±0.04
4.7±0.2
0.34±0.02

0.90±0.04
3.7±0.2
0.24±0.02

234

230

II. Hot Spring Water. The radioactivity concentrations of
the hot spring water and its sediment samples in this study arelisted in Tables 2 and 3, respectively. The temperatures andpH of the hot spring water vary greatly; their ranges are 26.0−
98.0 ˚C and 1.55−3.40, respectively.
Radioactivity Distribution. As shown in Tables 2 and 3,
the radioactivities of uranium, thorium and radium isotopes inhot spring water and sediments are distributed widely. The
ranges of radioactivity in hot spring water for 238U, 234U, 232Th,
230
Th, 228Th and 226Ra are 2.3−85, 2.9−91, 0.67−73, 1.14−66,
1.01−630, and 4.5−36.5 mBq/L, while those in hot spring sediments for 238U, 234U, 232Th, 230Th, 228Th, 228Ra and 226Ra are
11.2−37, 11.3−38, 15.2−66, 15.0−54, 15.5−1080, 18−3010,
and 16−230 Bq/kg (dry), respectively. Furthermore, the concentration ranges of uranium and thorium in hot spring waterand sediments are 0.18−6.6, 0.17−18.0 ppb, and 0.90−2.98
ppm, respectively.
As shown in Figure 4 there is a reverse relationship of theconcentration ratio of uranium/thorium vs. pH of hot springwater between hot spring water and sediment. Obviously, theconcentration ratios of uranium/thorium in hot spring water are
increased with the decrease of the acidity of hot spring water
when pH > 2.5. Nevertheless, when pH < 2.5, the concentration ratios of uranium/thorium in hot spring water are slightly
increased when the hot spring water becomes more acidic.
And simultaneously, we can find the reverse relationship ofFigure 4. A plot of concentration ratios of U/ Th in hot spring waterand sediments vs. pH of hot spring water.

Radioactive Disequilibrium of U and Th

J. Nucl. Radiochem. Sci., Vol. 1, No. 1, 2000

9concentration ratios of uranium/thorium vs. pH of hot springwater for hot spring sediments. As we have well known, uranium is mainly in the soluble ionic chemical forms of UO22+ inground water at pH < 4.10,11 However, thorium is more likely
to be precipitated in the form of insoluble Th(SO4)20 when pH< 2.5, and mainly to become insoluble complexes with organic
species when pH >2.5.12 Therefore, it causes the higher concentration in thorium than in uranium in the sediments becauseof the formation of insoluble Th(SO4)20 or organic complexesof thorium at pH < 2.5 or pH > 2.5, respectively.
Radioactive Disequilibrium. The uranium and thorium
series nuclides in hot spring water should come mainly fromthe leaching of the volcanic rocks. Leaching is a sorption/desorption process of the elements between the hot spring waterand the rock. According to the age of the rock,13 both uranium
and thorium series nuclides in the rock should reach radioactive
equilibrium, which means that the radioactivities of a radionuclide and its daughter nuclide are equal. If the radionuclide andits daughter nuclide are of the same isotopic elements, it meansthat they have the same chemical properties, their radioactivities
should remain equal even after leaching. However, according
to the results of Table 2, the radioactivity ratios of 234U/ 238U
and 228 Th/ 232 Th were 1.03−1.5 and 1.5−8.8, respectively.
Obviously, they are in radioactivity disequilibria.
1. 234U/ 238U. As shown in Tables 2 and 3 the radioactivity
radios of 234U/238U in hot spring water and sediments are 1.03−
1.5 and 1.01−1.05, respectively. The radioactivity disequilibria
for 234U/ 238U are not very serious in both hot spring water andsediments. It has been proven to be the result of α-recoil proceeding of 238U decay, and the recoiled range of its daughter
nuclide 234Th (T1/2 = 24.1 d) is about ~102 Å.14 234Th recoiled
atom can go through many lattice points and enter latticedefect pores or fractures of the rock and is easier to beleached.15,16
2. 228Th/ 232Th. As it is known, just as 238U and 234U, 232Th
and 228Th should be in radioactivity equilibrium in hot springwater and sediments because of the similar chemical properties. Figure 5, shows a diagram of radioactivity distributionsof thorium series nuclides, i.e., 232Th, 228Ra, and 228Th in hot

spring water and sediments. It is obviously to be divided into
two groups by pH < 2 and pH > 2. As shown in Figure 5, theradioactivities of 228Th in both hot spring water and sediments
and the radioactivities of 228Ra in sediments are more enrichedthan 232 Th for the group of pH < 2. On the contrary, theradioactivities of 228Th in both hot spring water and sediments
and the radioactivities of 228 Ra in sediments are obviously
equal to the same level of 232Th for the group of pH > 2.
Furthermore, as listed in Table 2, the radioactivity ratios of228
Th/ 2 3 2 Th in both hot spring water and sediments are
increased from 1.5 to 8.8 and from 1.02 to 16, with the acidityof hot spring water from pH 3.40 to 1.55. There is obviously
radioactivity disequilibrium between 232Th and 228Th. In the
decay process from 232Th to 228Th, there is a middle nuclide228
Ra with a long half-life (T1/2 = 5.75 y). The chemical properties of radium and thorium are quite different in the aqueous
solution. Therefore, we can assume that the enrichment of228
Th may come from 228Ra. To prove the above assumption, adiagram of the radioactivity ratios of 2 2 8 Ra/ 2 3 2 Th and228
Th/228Ra in sediments vs. pH of hot spring water is shown in
Figure 6. As shown in Figure 6 we can find that 228Ra is moreenriched in sediments than 232Th and 228Th, and the radioactivity of 228Ra in sediments is about 46 times and 2.8 times as highas that of 232Th and 228Th at pH = 1.68, respectively. It is thereason why the radioactivity of 228Th is about 16 times as highas that of 232Th in sediment at pH = 1.68, and leads the radioactivity of 228Th to be about 9 times as high as that of 232Th in hot
spring water at pH = 1.55.

Figure 5. A plot of radioactivity of thorium series nuclides in hot
spring water and sediments.

Figure 6. A plot of radioactivity ratios of 228Ra/ 232Th and 228Th/ 228Ra
in hot spring sediments vs. pH of hot spring water.

Conclusion
The hot spring and river waters at the Peitou hot springbasin in Northern Taiwan abounded with naturally occurring
radionuclides. Generally, the radioactive concentrations ofuranium-series and thorium-series nuclides in both river andhot spring waters were all pH dependent. Moreover, radioactive disequilibria were found both in 234U/238U and 228Th/232Th.
The enrichment of 228Th to 232Th is the most prominent. The
excess of 228Th comes from the contribution of 228Ra.

10 J. Nucl. Radiochem. Sci., Vol. 1, No. 1, 2000
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