温泉科学（J. Hot Spring Sci.），61，88‑96（2011）

原著

Geochemical Study on Hot Spring Waters in the Northern
Satsuma Area, Kagoshima Prefecture
Hiroo ISAGAI1)(Received Dec. 9, 2010, Accepted Aug. 10, 2011)鹿児島県北薩地域における温泉水の地球化学的研究
飯 盛 啓 生1）

要旨

桜島や霧島地域を含む鹿児島県の大部分はいわゆるシラスに覆われているが，鹿児島県北部（北薩地域）は広い範囲にわたって四万十層が分布している．そこでは，四万十層が露出している地域とシラスに覆われている地域が存在する．また，一部では安山岩や花崗岩が貫入している場所もある．
本研究では，北薩地域に湧出する温泉の化学成分について，地質との関連性から検討を行った．その結果，地質によって温泉水の化学成分に違いがみられた．特に，四万十層群から湧出する温泉水の化学成分組成は，ほぼ同じであるのに対し，他の温泉群とは異なっていた．このことから，四万十層群の露出地域から湧出している温泉は他の温泉とは湧出起源が異なることが示唆された． 四万十層群以外の温泉はシラスや安山岩などの貫入の影響が推定される．
キーワード：温泉，化学成分，北薩地域，四万十層群

Abstract
The Shimanto Group is distributed across the southern Kyushu, Japan. Most of theShimanto Group is covered with a pyroclastic ﬂow deposit locally known as Shirasu except
for the northern portion of Kagoshima Prefecture (northern Satsuma area). In addition,
andesite and granite have intruded in this area.
This study examines the relationship between the characteristics of hot spring waterand the geology of the northern Satsuma area. The chemical composition of the hot springwaters in the layers of the Shimanto Group was found to be quite similar each other, butdiffer from that of the hot springs in other parts of the northern Satsuma area. Thissuggests that the hot spring waters in the layers of the Shimanto Group have a diﬀerent
gush origin. It is estimated that the chemical components of the most hot spring waters
except for those in the Shimanto Group area are inﬂuenced by Shirasu and the intrusion of1)

88

Kagoshima University Education Center, 1‑21‑35, Korimoto, Kagoshima 890‑0065, Japan. 1）鹿児島大学
教育センター 〒890‑0065 鹿児島市郡元 1‑21‑35.

第 61 巻（2011）鹿児島県北薩地域における温泉水の地球化学的研究

rocks such as andesite.
Key words : Hot spring, Chemical component, Northern Satsuma, Shimanto Group

1． Introduction
Many hot springs are distributed throughout Kagoshima Prefecture in Japan. Hot springs
are generally classiﬁed as volcanic or non-volcanic. Volcanic hot springs are discharged aroundan active volcanic area. Many volcanic springs exist in Kagoshima Prefecture because of thepresence of active volcanoes (Mt. Kirishima and Mt. Sakurajima). However, some non-volcanic
hot springs are also present in this area (Tsuyuki, 1962). Tsuyuki (1992) classified the hotsprings near Mt. Kirishima and Mt. Sakurajima as either volcanic or non-volcanic. In thenorthern Satsuma area, the non-volcanic hot springs are generally older than the volcanic ones.
The host rock of non-volcanic hot springs consists of sedimentary rock, granite, and igneous rockof the Shimanto Group (Fig.1). Most of Kagoshima Prefecture is covered by a pyroclastic ﬂow
deposit so-called Shirasu, which covers the layers of the Shimanto Group ; however, the surfaceof the Shimanto Group is exposed in air in the northern portion of Kagoshima Prefecture (thenorthern Satsuma area). In addition, andesite and granite intrude the northern Satsuma area
(Editorial Board of Nihon no Chishitsu, 1992). Some hot springs in this area are discharged from

Fig. 1 Sampling points and geology of the northern Satsuma area (A : Shirasu. B : ShimantoGroup. C : Pyroxene andesite. D : Granite. E : Rhyolite. F : Hornblende andesite. G : Basalt).
① : Sendai-taki. ② : Shibi. ③ : Yu-kawauchi. ④ : Shiraki-kawauchi. ⑤ : Takaono. ⑥ :Shibagaki. ⑦ : Suwa. ⑧ : Ichihino. ⑨ : Miyanojyou. ⑩ : Tougou. ⑪ : Sendai. ⑫ : Kushikino.
⑬ : Ichiki. ⑭ : Yunomoto-A. ⑮ : Yunomoto-B

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the Shimanto Group, whereas others are discharged from layers other than those in theShimanto Group covered by Shirasu.
In this study, hot springs in the northern Satsuma area were divided into three groups : theShimanto Group area ; the Imuta area, which might be inﬂuenced by magma ; and the coastalarea. In addition, the relationship between the chemical components of the hot spring waters
and the geology of the northern Satsuma area, Japan, was examined to determine thecharacteristics of the hot springs.

2． Geology of the northern Satsuma area
In the Japanese islands, the area north of the Butsuzo Tectonic Line is called the Shimanto
Belt. It consists of sedimentary rock, granite, and igneous rock (Miyaguchi, 1993). The
sedimentary rock from Neogene and Cretaceous in the Shimanto Belt is called the Shimanto
Group, and it consists of alternate layers of tuﬀ, sandstone, and shale (Hashimoto, 1962). Shirasu
extends over southern Kyushu, and thus most of the Shimanto Group is covered with a thicklayer of Shirasu. Hence, the exposed layers of the Shimanto Group are typically not observed.
However, the layers of Shimanto Group are exposed in a large portion of the northern Satsuma
area. Pyroxene andesite and basalt intrusions appear in the exposed Shimanto Group. The
geology of the northern Satsuma area is complicated, and the hot springs are surrounded byvarious geological features (Fig. 1). These sampling points include most of the hot springs in thenorthern Satsuma area (Kurokawa, 1997).

3． Experiment
Hot spring water samples were taken in the northern Satsuma area on March 26, 2009 andMay 31, 2009 (Fig. 1). Water temperature was measured by an alcohol stem thermometer at thesampling sites. Electric conductivity (EC, calibrated at 25℃ ) was determined by EC meter (TOA
CM-40S ; Tokyo, Japan). The pH was determined by pH meter (TOA-DKK HM-20P ; Tokyo,Japan).

The concentrations of Na + and K + were analyzed using an atomic absorptionspectrometer (Shimadzu AA-6200 ; Kyoto, Japan ; C2H2/air ﬂame). The concentrations of Ca2+and Mg2+ were analyzed by chelate titration using 0.01 M ethylenediaminetetraacetic acid(EDTA) solution. The concentration of SO42− was analyzed by turbidimetry, while that of Cl−
was analyzed by colorimetry using a spectrophotometer (HACH DR/4000 ; Loveland, USA). Thealkalinity (pH 4.8) was determined by titrating bromocresol green methyl red (BCG-MR) indicatorwith 0.02 M H2SO4 (Japan Society for Analytical Chemistry, Hokkaido Branch, 1994). Theconcentrations of HCO3− was calculated from data of alkalinity (Pharmaceutical Society of Japan,1990)

4． Results and discussion
The chemical components of the hot spring waters are listed in Table 1. The watertemperature of No. 3 was the lowest (32.5℃), whereas that of No. 10 was the highest (59.9℃).
90

III

50.0
51.0
45.0
43.0

Kushikino
IchikiYunomoto-A
Yunomoto-B

12131415

II

May 31, 2009
May 31, 2009
May 31, 2009
May 31, 2009

43.2
45.7
52.0
38.5
59.9
44.5

March 27, 2009
March 26, 2009
March 26, 2009
May 30, 2009
March 26, 2009
May 31, 2009

Shibagaki
Suwa
Ichihino
Miyanojyou
TougouSendai

678910
11I

7.68
7.85
7.46
7.64

6.18
6.24
9.38
7.55
7.56
8.14

8.87
9.34
9.32
9.14
8.21

3320
3770
225
463

3440
5710
236
1132
2060
356

305
372
156
422
761

40.7
36.6
4.25
3.61

116
237
2.01
11.2
13.0
1.20

1.60
0.80
1.68
0.64
3.05

6.90
4.28
1.07
0.34

8.50
34.0
0.24
2.92
6.08
0.24

0.05
0.24
0.44
0.10
1.31

644
805
110
131

667
975
73.6
276
552
136

94.3
106
53.6
94.3
202

52.5
35.08.18
8.11

62.2
95.2
1.08
14.1
23.1
11.2

17.5
1.62
0.65
1.80
3.33

256
207
108
105

464
878
85.4
500
1488
115

138
183
61.0
201
373

938
850
12.3
41.4

768
1146
18.4
132
11.7
62.6

5.94
39.4
5.18
10.7
80.2

44.9
562
88.5
77.8

294
584
17.8
2.40
4.00
4.00

40.2
13.6
24.1
36.8
3.60

Water
pH
Electric
Ca2+
Mg2+Na+K+
HCO3−
Cl−
SO42−
temperature
Conductivity（℃）(μS cm−1) (mg dm−3) (mg dm−3) (mg dm−3) (mg dm−3) (mg dm−3) (mg dm−3) (mg dm−3)
42.0
46.0
32.5
41.9
52.0

Sampling Date

Sendai-taki
May 30, 2009
Shibi
March 27, 2009
Yu-kawauchi
May 30, 2009
Shiraki-kawauchi March 26, 2009
TakaonoMay 30, 2009

Name of hotspring

12345Group Sample

Table 1 Physicochemical properties and chemical components of hot spring waters in the northern Satsuma area (Group I : Nos. 1‑5. Group II :Nos. 6‑11. Group III : Nos. 12‑15)

第 61 巻（2011）鹿児島県北薩地域における温泉水の地球化学的研究

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These hot springs were divided into three groups ; Group I, II and III, according to theirgeology, and each group was examined.
The chemical components of the hot springs in the northern Satsuma area were plotted intrilinear diagrams (Fig. 2). In the diagram, the presence of cations in all the hot springs wasplotted in the Na++K+ region. The anions in the hot springs were plotted in various regions ofthe trilinear diagram. The anions in the hot springs in Group I (Shimanto Group) were plottedin the region rich in HCO3−.
In the diagram, no diﬀerence was observed in the distribution of the cation plots for thethree groups, but the anion plots for Group I were more concentrated than those of the othergroups (Fig. 2). The chemical components of the hot springs were plotted in regions C and D
and the chemical components of the Shimanto Group were plotted in region C. These diagrams
show that the chemical components of the hot springs in Group I diﬀered from those in Groups
II and III.
Gram equivalents of Na++K+, Ca2+, Mg2+, Cl−, and SO42− and HCO3− were plotted on a hexa
diagram (Fig. 3). The figure shows that the shapes of the plots for the hot springs in theexposed Shimanto Group (Nos. 1, 2, 3, 4, and 5) are almost similar, suggesting that the chemicalcomponents of the hot springs discharged from this area are similar, except for No. 5. In Groups
II and III, some hot springs are similar to those of Group I, but others are not. It was suggested
that the hot springs of Groups II and III originate in the Shimanto Group, but those that are not

Fig. 2 Trilinear diagram of chemical components of hot spring waters in the northern
Satsuma area. ( ● : Group I. △ : Group II. □ : Group III)92

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Fig. 3 Hexa diagram of chemical components of hot spring waters of Nos. 1 to 15 in thenorthern Satsuma area. Group I : Nos. 1‑5. Group II : Nos. 6‑11. Group III : Nos. 12‑15.

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similar to Group I were inﬂuenced by the Shirasu and magma covering the Shimanto Group.
The water temperature of most samples was 40‑50℃ (Table 1). Because the water from hotspring Nos. 5, 11, 12, and 13 was collected by pumping into a tank, the actual water temperatures
were probably higher than those shown in Table 1.
4．1

Hot springs in exposed Shimanto GroupThe hot springs in the exposed Shimanto Group (Group I) comprised Nos. 1‑5. The watertemperatures of Group I were 32.5‑52.0℃ (Table 1). The water temperature of No. 3 was thelowest ; this hot spring was discharged from the bedrock of the Shimanto Group. The thermal
origin of hot springs in Group I has been reported to be the subsurface granitic magma of Mt.
Shibi (Tuyuki, 1992). The hot springs of Group I are discharged from the host rock in theShimanto Group. The origin of the waters is considered to be groundwater, which consists ofrain water that percolates through the Shimanto Group, and the water is discharged throughprocesses of dissolution, precipitation, and ion exchange. The water temperature of Group I wasgenerally low. This might indicate that the source of these hot springs is shallow and that thegeothermal gradient is low. The pH of Group I was somewhat high (8.21‑9.34, Table 1). The
concentration of HCO3− of Group I was lower than that in Group II. The concentration of Mg2+of Group I was less than 1.31 mg/L, which is lower than that of Ca2+ in this group (Table 1). The
concentrations of HCO3− , Ca2+, and Mg2+ in No. 5 were the highest in Group I. This suggeststhat the upper layer around No. 5 was a Holocene sediment layer.
4．2Hot springs around Imuta volcano
The hot springs around Imuta volcano (Group II) include Nos. 6‑11. Imuta volcano has long

been inactive. Andesite from the volcano is found in its vicinity (Kagoshima Prefecture, 1990). Itis thought that andesite magma has intruded this area and has not yet cooled. Thus, andesite isthought to be the source of heat for the hot springs of Group II.
EC data suggest that the concentrations of the chemical components of Nos. 6, 7, 9 and 10
were high (Table 1). These observations were similar to those of hot springs in Mt. Kirishimaregion (Kurokawa1993).

The concentration of SO42− in Nos. 6 and 7 was high (Table 1), suggesting that the weak acidity at these points was inﬂuenced by the presence of sulfur oxides. The concentrations of Na+,
HCO3− , Cl− and SO42− in Nos. 6 and 7 were high. The fact suggests that these hot spring waters
were similar to NaCl and Na/HCO3/SO4 type of hot springs. The concentration of HCO3− wasalso high in these four hot springs. It has been suggested that this is a result of CO2 supplied bymagma (Iwasaki, 1970). Our results suggest that the water of these hot springs permeates andmoves through a geothermal area. The concentrations of chemical components of Nos. 8 and 11were low. Thus, these hot springs may not be strongly inﬂuenced by the volcano.
4．3

Hot springs around the west coast of Satsuma Peninsula
The hot springs around the west coast of Satsuma Peninsula (Group III) include Nos. 12‑15.

They are discharged around the coastal area, the geology of which is similar to that of theShimanto Group. Water from these hot springs was pumped up from holes with a depth of94

第 61 巻（2011）鹿児島県北薩地域における温泉水の地球化学的研究

about 1,500 m. The water temperatures of Nos. 12 and 13 hot springs were higher than those in
the other hot springs (Table 1). Their heat sources are believed to be the geothermal ﬂuid. EC
data indicate that the concentrations of the chemical components of Nos. 12 and 13 were high.
These hot springs had higher concentrations of Na+ and Cl− than the other hot springs of Group
III. The concentration of Ca2+ was higher than that of Mg2+ (this result was the same as thatobtained for Groups I and II). This suggests that Group III was inﬂuenced by volcanoes rather
than by seawater. The water temperatures and EC of Nos. 14 and 15 were lower than those of
the other hot springs of Group III. Hence, they are considered to be inﬂuenced by the upper
layer of the Shimanto Group rather than by volcanoes.

5． Conclusion
The water temperatures and pH values of the hot springs in the Shimanto Group (Group I,except for No. 5) in the northern Satsuma area were lower and higher, respectively, than thoseof other hot springs in this area. The Cl− concentration in these spring waters was low ;therefore, these hot springs were considered to be similar to the NaHCO3 type and not the NaCl
type of hot springs. The concentrations of the chemical components of most hot springs inGroup I were low. The pH of most of the hot spring waters in Groups II and III was greaterthan 7, and the concentrations of Na+, HCO3−, and Cl− were high. Hence, our results suggestthat these hot springs are inﬂuenced by the reservoir.

Acknowledgments
The author is thankful to reviewers and Dr. G. Inoue for their helpful discussions andsuggestions.
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(A part of this study was presented at the 62nd Annual Meeting of the Hot Spring Society ofJapan on September 8, 2009)96