温泉科学（J. Hot Spring Sci.），61，116‑122（2011）

原著

Geochemical Study of Water and Gas Samples from
Funagoya Spa in Fukuoka Prefecture, Japan
Kazutoshi OUE1）, Shinji OHSAWA2）, Makoto YAMADA3）,
Taketoshi MISHIMA2） and Takuya SAKAI1）(Received Aug. 9, 2011, Accepted Aug. 31, 2011)福岡県船小屋温泉の温泉水とガス成分の地球化学的研究
大上和敏1)，大沢信二2)，山田 誠3)，三島壮智2)，酒井拓哉1)

要旨

福岡県筑後市の船小屋温泉およびみやま市の長田鉱泉から湧出する鉱泉水，炭酸成分および希ガスの起源を明らかにするために，鉱泉水および付随ガスの化学・同位体分析を行った．
鉱泉水の安定同位体組成（δD およびδ18O）から，船小屋温泉および長田鉱泉の鉱泉水はいず
れも天水起源であることが示された．また，付随ガス中の 3He/4He 比と 4He/20Ne 比の関係より，
船小屋温泉水中には大気起源 He の混入はほとんどなく，マントル起源および地殻起源のHe が混合したものであることが示された（マントル起源 He：60％，地殻起源 He：40％，
大気起源 He：0.1% 以下）．C/3He とδ13C の関係より，付随ガス中の CO2 に占める炭素の起源を推算したところ，マントル起源 CO2（Mantle derived CO2）が 35％，海成炭酸塩起源 CO2（Marine carbonate CO2）が 56％，堆積有機炭素（Sedimentary organic carbon）が 9％という結果が得られた．さらに，付随ガス中の CH4/（C2H6＋C3H8）比と CH4 中のδ13C の解析により，付随ガス中の CH4 は地下深部における有機物の熱分解によって生じたことが示唆された．
キーワード：炭酸泉，ヘリウム，全溶存炭酸，安定同位体，起源，筑後平野

Abstract
We studied mineral water and associated gases of Funagoya spa, and mineral water ofNagata spa, in Fukuoka Prefecture to identify the sources of their respective waters, inorganic carbon components, and noble gases. The stable isotope composition (δD and δ18O)of the mineral waters showed that the waters of the Funagoya and Nagata spas originate
from rainwater. However, the relation between the 3He/4He ratio and 4He/20Ne ratio in the1)

Faculty of Education and Welfare Science, Oita University, Dannoharu, Oita, 870‑1192, Japan. 1）大分大学教育福祉科学部 〒870‑1192 大分県大分市大字旦野原 700 番地．
2）
Beppu Geothermal Research Laboratory, Kyoto University, Noguchibaru, Beppu, Oita 874‑0903,
Japan. 2）京都大学大学院理学研究科附属地球熱学研究施設 〒874‑0903 別府市野口原．
3）
Kyousei Science Center for Life and Nature, Nara Womenʼs University, Kitauoyahigashi-machi, Nara,
630‑8506, Japan. 3）奈良女子大学共生科学研究センター 〒630‑8506 奈良県奈良市北魚屋東町．

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第 61 巻（2011）福岡県船小屋温泉の温泉水とガス成分の地球化学的研究

associated gases indicates that He ﬂowing out from Funagoya spa contains almost no He
derived from the atmosphere : it originates from deep underground (mantle-derived He, 60% ;crustal He, 40%). Furthermore, the relation between C/3He and δ13C in the associated gases
shows that mantle-derived CO2 accounts for 35%, deep subducted marine carbonate for 56%,and sedimentary organic carbon for 9%. Furthermore, analyses of the CH4/(C2H6＋C3H8)
ratio and the δ13C of CH4 in the associated gases suggests that the CH4 from the Funagoya
spa is produced from thermogenic decomposition of deep organic sediments.
Key words : Carbonate spring, helium, dissolved inorganic carbon, stable isotope, origin,
Chikugo plain

1． Introduction
Fukuoka Prefectureʼs Funagoya and Nagata spas are well-known carbonated springs inJapan. They were discovered by the Arima Clan during the late Edo Period (1810) on the northside of the Yabe River, and by the Yanagawa/Tachibana Clans on the south side of the river(Urakawa, 2011). The respective spas prospered during the Meiji to the Showa Periods, withpeak visitation by as many as 20,000 bathers annually. Dozens of mineral spring wells havebeen drilled on both sides of the river, including those at the Funagoya and Nagata spas
(Urakawa, 2011). The dissolved inorganic carbon (DIC) concentration covers a wide range fromabout 50 ppm to 1,000 ppm or more (Urakawa, 2011). In recent years, the springs have attracted
attention for the medical eﬀects of their high DIC (namely CO2) concentrations. Associated gases
are discharged together with mineral waters from the most deeply drilled wells in the Funagoya
spa area.
The DIC of the mineral waters, a particularly interesting characteristic of the Funagoya andNagata spas, is probably attributable to the decomposition of organic matter by methanogens,under anaerobic conditions, buried at a depth of a few hundred meters underground (Urakawa,2011). However, that inference has not been proven. Many unclear aspects remain. For thisstudy, samples of mineral waters and associated gases discharged from Funagoya and Nagata
spas were examined to ascertain, based on the samplesʼ chemical and isotopic composition, thewater sources and their inorganic carbonate components and noble gases.

2． Sampling and analyses
On 1 November 2010, mineral water and accompanying bubble gases were sampled at
Funagoya spa (point A), and mineral water was collected from Nagata spa (point B, Fig. 1). The
water temperature and pH were measured at the sites. The water samples were subjected tochemical analyses of the major components and dissolved inorganic carbon (DIC＝CO2 (aq)＋HCO3−＋CO32−), stable isotope analysis of water (δD and δ18O), and carbon stable isotope measurementof DIC (δ13C) using the method described by Sakai
18(2011). The respective measurement

13

precisions of isotope analyses for δD, δ O, and δ C were ±1.0‰ , ±0.1‰ , and ±0.2‰ .
Bubble gases were collected using the following three techniques [Gas samples 1-3] inaccordance with the items that were analyzed.

117大上和敏，大沢信二，山田誠，三島壮智，酒井拓哉

温泉科学[Gas sample 1] Gas bubbles were temporarily trapped in a soft plastic bag(equipped with cocks) using a plastic funnelof about 20 cm diameter.

The gas wastransferred immediately to a gas collection
vessel (a large glass syringe) containing 5M
KOH aqueous solution. The carbon dioxidegas (CO2), which is thought to constitutemost of the volume of gases, was thereby
absorbed into the alkali solution. Throughrepeated collection, it was possible to obtainan adequate volume of the gas component
that had not been absorbed into the alkali
solution (residual gas, ʻR gasʼ). The gas wasthen brought back to the laboratory.
Chemical composition data for CO2, N2, Ar,
He, H 2, CH 4, and O 2 were obtained fromthese samples. In accordance with Iwakura
Fig. 1 Mineral springs investigated in this study :A, Funagoya spa ; B, Nagata spa.

(2000), CO2 was measured from thealkali solution using the CO 2 electrodemethod. However, N2, Ar, He, H2 and CH4

were analyzed using a gas chromatograph using O2 as the carrier gas. The O2 in the gas sample
was obtained by calculating, based on the ideal gas state equation using those analysis values,
the R gas volume, the atmospheric temperature, and the pressure inside the laboratory. Thenthe CO2 that had been absorbed into the alkali solution was reacted with Sr (OH)2, thereby
isolating it as SrCO3 for carbon stable isotope measurements (δ13C) using the same method asthat used for DIC.
[Gas sample 2] A funnel was used to collect gas bubbles directly, together with a small
amount of hot spring water, in a 10 mL or 20 mL glass bottle with a rubber septum. Using themethod described in Ohsawa(2010), data of the relative composition of C2H6 and C3H8 withrespect to CH4 were obtained from the sample, as were data related to the carbon stable isotope
ratio (δ13C) of these compounds. During transport and storage, external air was blocked bytipping the glass bottle onto its side and setting it so that the connection point with the septum
was sealed with hot spring water, thereby preventing sample gas leakage and air contamination.
[Gas sample 3] Gas bubbles were collected, using a funnel, into a Pyrex glass gas-samplingbottle equipped with a stop cock on both ends. Data for the 3He/4He ratio and 4He/20Ne ratio
were obtained from this sample using a magnetic sector mass spectrometer by the Environment
Department of Kyuden Sangyo Co., Inc. The respective uncertainty errors in the 3He/4He ratio
and 4He/20Ne were estimated as 1% and 10%.

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第 61 巻（2011）福岡県船小屋温泉の温泉水とガス成分の地球化学的研究
Table 1 Chemical and isotopic data of spring waters andbubble gases in Funagoya and Nagata mineral springs.
Fukuoka Prefecture.
Funagoya

Nagata
23.1
5.0
14.8
2.4
4.1
23.2
16.5
17.1
64.8
1200 as CO2
−43.5
−6.55
−7.38

spring waters

Water temp (℃)
pH
Na (mg/L)
K (mg/L)
Mg (mg/L)
Ca (mg/L)
Cl (mg/L)
SO4 (mg/L)
HCO3 (mg/L)
DIC (mg/L)
δD H2O (‰)δ18O H2O (‰)δ13C DIC (‰)

23.5
5.6
63.5
3.9
21.8
22.0
18.4
8.9
217
1000 as CO2
−43.9
−7.54
−6.99

bubble gases

CO2 (%)
N2 (%)
CH4 (%)
Ar (%)
O2 (%)He (ppm)
C2H6 (ppm)
H2 (ppm)
C3H8 (ppm)
δ13C CO2 (‰ )
δ13C CH4 (‰ )
δ13C C2H6 (‰ )3
He/4He4
He/20Ne

91.2
6.95
1.73
0.043
0.031
68
25
3.5
0.9
−4.13
−43.1
−39.2
6.64×10−6273

3． Results and discussion
Table 1 presents results of chemical and isotope analyses of the samples of the spring waters and the associated bubble gas. Just like Nagata spa, the Funagoya spa is a mineral springwith water temperature of less than 25℃. The hydrogen/oxygen stable isotope compositions of
the spring waters (Table 1) roughly resemble those of meteoric water (δD＝8δ18O＋10 : Craig,1961), thereby indicating that these are shallow ground waters derived from rainwater. The
Funagoya spa is classified as a carbonated spring (HCO3 type), as is the Nagata spa. The
Funagoya spring water has Na-HCO3 type water quality, indicating that the ground water has
undergone considerable geochemical evolution. However, the main cation in the Nagata springwater is Ca, indicating that the initial water quality of the shallow ground water. The major component of the associated gas of the Funagoya spring water is CO2, but it does not contain negligible amounts of CH4 (Table 1). Therefore, it appears that methane fermentation (2CH2O→CH4＋119大上和敏，大沢信二，山田誠，三島壮智，酒井拓哉

温泉科学CO2) of sedimentary organic compounds is
involved in the formation of the carbonate
spring. One might infer that the springwater is shallow ground water of meteoric
origin which has undergone some process ofhydrochemical evolution. Nevertheless, thegeochemical characteristics of the He andCO2 in the associated gas indicate that theorigin of the spring water is not so simple.
Figure 2 [A] shows the 3He/4He ratio
and 4He/20Ne ratios of the associated gas of
the Funagoya spring water (Table 1), as
displayed on a 3He/4He 4He/20Ne diagram,as in the case of the Hitoyoshi Basin Hot
Springs (Ohsawa

2011). When themodel calculation method described in
Shimizu(2005) was used to find thecontribution rates of He types from
diﬀerent sources in the associated gas, theresults revealed that mantle-derived He
accounted for 60%, crustal He for 40%, andatmospheric He for 0.1% or less.
The δ13C value for CO2 (−4.13
2.86

‰ :

‰ heavy with respect to DIC

attributable to isotope fractionationaccompanying gas liberation) reveals thatt h e w a t e r c o n t a i n s d e e p - o r i g i n C O 2.
However, the possibility cannot be denied,
based on δ13C only, that CO 2 is derived
from material containing marine carbonate
and sedimentary organic carbon.
Therefore, this point was investigated byplotting the data on a graph of C/ 3 He
versus δ13C (Sano and Marty, 1995) (Fig. 2[B]).

Just as in the case of He, thecontribution rates of carbon from diﬀerent
sources in CO2 of the associated gas can be

Fig. 2 [A] Plot of 3He/4He versus 4He/20Ne ratios
for the Funagoya gas sample obtained for thisstudy. The upper thick line represents the mixing
line between air and upper mantle components.
The lower thick line is a mixing line for air andcrust components. The three medium lines
show trajectories for mixing between air anddeep-originated gas for various mantle helium
components of 10, 20, and 50%. F, Funagoya
spa ; B, Nagata spa.
[B] Correlation between C/3He ratio and δ13C
of DIC for the Funagoya sample. The ﬁgure
also shows model end members for mantle(M-a, Sano and Marty, 1995 ; M-b, Deines,
2002 ; M-c, Nishio
1998), marine carbonate (Sano and Marty, 1995), and sedimentaryorganic carbon (mean value of those describedin Sano and Marty (1995), Deines (2002), andNishio(1998)). The tie lines of the modelend members are mixing lines. F, Funagoya spa.

estimated (Sano and Marty, 1995). Results
show that mantle-derived CO2 accounted
for 35%, marine carbonate was 56% and sedimentary organic carbon was 9%. As explained
above, most He and CO2 in associated gases were of deep origin, which attests to the fact thatthese gases, rising from deep underground, ﬂow into a shallow groundwater system and thereby120

第 61 巻（2011）福岡県船小屋温泉の温泉水とガス成分の地球化学的研究

produce a carbonated spring.
Figure 3 portrays an agrochemical
discrimination diagram, called a ʻBarnard
Diagramʼ, which is widely used in the ﬁeldof organic geochemistry (Ohsawa
2010). The diagram shows the (CH4/(C2H6＋
C3H8) ratio) against δ13C of CH4, thereby
enabling one to trace transitions inphysicochemical processes such as the CH4
origin and the maturation process ofhydrocarbon gases. The CH4 in gases from
Funagoya spa is produced by thermogenic
13

Fig. 3 δ C of CH4 and the CH4/(C2H6 ＋ C3H8)
ratio of the gas sample from Funagoya spa
expressed as the Barnard Diagram, which is
generally used for discrimination of origin of
CH4 and interpretation of physical and chemical processes of hydrocarbon gases. F,
Funagoya spa.

decomposition of shallow organic sediments,
as explained by assuming that CH4
produced deep underground has migrated
to the surface. Even if CO2 were producedby the microbial oxidation of methane, itwould retain the same chemical and isotopic
c o m p o s i t i o n s a s C H 4.

demonstrated by Waseda

However, as(2002), a great diﬀerence exists between the δ13C of CO2 andthat of CH 4 (see Table 1). Therefore, they are unlikely to have similar origin. It seems
reasonable to infer that hydrocarbon gases including CH4 are added to the shallow groundwaterfrom some external source, as are He and CO2.
In future studies, the authors would like to conduct a similar investigation of othercarbonated springs around the Chikugo River in the Tsukushi Plain, with characteristics
reportedly resembling those of the Funagoya spa (Urakawa, 2011).

Acknowledgments
Hydrogen/oxygen stable isotope analysis was conducted at the Stable Isotope Laboratory of
GNS Science (New Zealand). Chemical analysis and measurement of isotope ratios of hydrocarbon
gas were done by the Japex Research Center of Japan Petroleum Exploration Co., Ltd. Measurement of 3He/4He and 4He/20Ne was supported by the Environment Department of Kyuden
Sangyo Co., Inc. The authors would like to express their thanks to all the staﬀ involved. This
research was partially supported by a Ministry of Education, Culture, Sports, Science andTechnology Grant-in-Aid for Scientiﬁc Research (A) 22244067. Anonymous referees provided
useful comments that led to considerable improvements in this manuscript.
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