温泉科学（J. Hot Spring Sci.），64，42-52（2014）

原著

Geochemical and Isotopic Characteristics of the Mineral
Spring and Natural Spring Waters in the Setogawa
Group of the Western Yamanashi Prefecture, Japan
Muga Yaguchi1）*, Yoichi Muramatsu2）, Hideyuki Nagashima3）,
Fumiaki Okumura4） and Masumi Yamamuro1）(Received Mar. 31, 2014, Accepted May 16, 2014)

山梨県西部の瀬戸川層群における鉱泉および湧水の地球化学的・同位体的特徴
谷口無我1)*，村松容一2），長島秀行3），奥村文章4），山室真澄1)
要旨

山梨県西部の瀬戸川層群北部に位置する雨畑地域における鉱泉・湧水を対象に，主要溶存成分および水素・酸素・硫黄（δD・δ18O・δ34S）安定同位体分析を実施し，湧水地点から採取した岩石片の顕微鏡観察を実施した．得られた結果を基に，当該鉱泉水および湧水の起源と水質形成機構を検討した．
試料水はいずれも降水起源であり，主要成分の溶存割合は湧水で Ca2+＞Na+＞Mg2+ およびHCO3－＞SO42－＞Cl－，鉱泉水で Ca2+＞Mg2+＞Na+ および HCO3－＞SO42－＞Cl－である．これらの成分のうち，最も主要な Ca2+ および HCO3－ は瀬戸川層群に含まれる石灰質の溶解作用，
SO42－は表層での黄鉄鉱の酸化反応に規制されており，SO42－が持つ δ34S 値 は瀬戸層群の硫化物が持つ δ34S 値を反映して特徴的に低い．この SO42－は地下の嫌気環境における硫酸還元菌の作用によって H2S を生じていると思われ，鉱泉地では硫化水素臭を伴っている．黄鉄鉱の酸化によってもたらされた H+ は石灰質の溶解を引き起こして中和される．
キーワード：山梨県西部，瀬戸川層群，鉱泉水，水-鉱物相互作用，水質形成機構，安定同位体比（δD・δ18O・δ34S）
1)

Department of Natural Environmental Studies, Graduate School of Frontier Sciences, The Universityof Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8563, Japan. 1）東京大学大学院新領域創成科学研究
科自然環境学専攻 〒277-8563 千葉県柏市柏の葉 5-1-5．*Corresponding author：yaguchimu@nenv.
k.u-tokyo.ac.jp
2)
Department of Liberal Arts, Faculty of Science and Technology, Tokyo University of Science, 2641
Yamazaki, Noda, Chiba 278-8510, Japan. 2）東京理科大学理工学部教養科 〒278-8510 千葉県野田市山崎 2641.
3)
Faculty of Science, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601,
Japan. 3）東京理科大学理学部 〒162-8601 東京都新宿区神楽坂 1-3.
4)
Research Center, Japan Petroleum Exploration Co., Ltd., 1-2-1 Hamada, Mihama-ku, Chiba 261-0025,
Japan. 4）石油資源開発株式会社技術研究所 〒261-0025 千葉県千葉市美浜区浜田 1-2-1.

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Abstract
Chemical and stable isotopic compositions (δD, δ18O, δ34S) of the water samples from awell drilled for mineral spring source and from a natural spring in the Amehata area locatednorthern Setogawa Group of the western Yamanashi Prefecture, additionally disseminatedminerals of a rock sample from the natural spring site, were investigated to determine theorigin of the water and formation mechanisms of the water quality.
The sampled waters were meteoric in origin and the ionic dominance pattern observed
was Ca2+＞Na+＞Mg2+ and HCO3－＞SO42－＞Cl－ for natural spring, and Ca2+＞Mg2+＞Na+ andHCO3－＞SO42－＞Cl－ for mineral spring water. The primary dominant ionic contents of Ca2+and HCO3－ were derived from dissolution of calcareous material, and SO42－ was derived
from pyrite oxidation at the aerobic surface layer. The SO42－ reflected highly negative δ34S
values of the sulfides in the Setogawa Group, and it may be reduced to H2S by sulfatereducing bacteria in the anaerobic underground. H+ released by pyrite oxidation triggered
this dissolution of calcareous material and was neutralized through the reaction.
Key words : Western Yamanashi Prefecture, Setogawa Group, Mineral spring, Water-rockinteraction, Water formation mechanism, Stable isotope ratio (δD・δ18O・δ34S)

1.

Introduction

Although the Kofu basin in Yamanashi Prefecture has been considered poor in hot springsources in the past (Tanaka, 1989), a number of wells for hot spring bathing purposes have been
drilled extensively in the last several decades. Many researchers have reported on water
quality characteristics and distribution of hot spring waters in Yamanashi Prefecture (e.g.,
Akiyama, 1972 ; Sugihara and Shimaguchi, 1978 ; Aikawa, 1995), however, the origins of thewater and formation mechanisms, such as details of the water-rock interactions that control
water quality, have not been satisfactorily defined. Stable isotope data can be used to estimate
the origin of water and dissolved chemical species of hot spring waters (e.g., Muramatsu et al.,2012, 2013, 2014), but reports on the hydrogen, oxygen and sulfur isotopic compositions of hot
spring waters in Yamanashi Prefecture are very few (Muramatsu et al., 2011 ; Yaguchi et al.,2014).
As Nishimura (2006) discussed the mechanism of well interference between existing andnewly drilled hot springs of the Nishiyama hot spring area at the western part of Yamanashi
Prefecture based on geological evidences, geological interpretation of water formation mechanism
is indispensable for protection and sustainable development of hot springs. In Yamanashi
Prefecture, about 70% of the drinking water comes from groundwater, thus many researchers
have extensively investigated the groundwater of Yamanashi Prefecture (e.g., Yanai andTsugane, 1957 ; Sakamoto et al., 1990 ; Kobayashi and Koshimizu, 1999 ; Nakamura et al., 2008 ;Kobayashi and Horiuchi, 2008 ; Kobayashi et al., 2009 ; Kobayashi et al., 2010). We also reviewed
these reports for reference, however, most previous works were conducted principally in the
Kofu Basin and focused on the distribution of anthropogenic pollutants and on the temporalchange of groundwater quality. These were conducted in order to investigate the anthropogenic
influence of groundwater quality in a densely populated area. Thus, interrelationships between
geology and groundwater quality in Yamanashi Prefecture has not yet been clarified.
Amehata area is located in the northern Setogawa Group of the western part of Yamanashi
43

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温泉科学

Prefecture. This area has one of the highest amounts of rainfall in Japan (mean annual rainfall ismore than 2,100 mm ; Ministry of Land, Infrastructure and Transport Government of Japan(2014), and has been designated as one of several watershed conservation areas by the
Yamanashi prefectural ordinance (Yamanashi Prefectural Government, 2012).

Further,mineralogical and sulfur isotopic properties of Koei vein deposit in the Amehata have beenreported by Shimizu et al. (1995). Thus, this area is considered a suitable site to discuss therelationships between water quality and geological characteristics without anthropogenic
influence. The water supplies of this area, such as well and spring discharge, have been used
for mineral spring bathing and other domestic purposes, but the origin of the water and theirchemical components have not been described previously.
In this study, water samples were collected from a well drilled for mineral spring source
and from a natural spring located in the Amehata area and subjected to chemical and isotopic
analysis of hydrogen, oxygen, and sulfur. A phyllitic rock sample was also taken from a natural
spring site for microscopic observation to check for the presence of any disseminated minerals.
Based on the analytical results, we discussed the origin of the water and water-rock interactions,which determine the quality of the water.

2.

Overview of Geology

A simplified geological map and sample locations are shown in Fig. 1. The Setogawa Belt of
the Shimanto Belt is an Early to Middle Miocene accretionary complex situated at theeasternmost part of the Akaishi Mountains in Central Japan. It is bordered by the ItoigawaShizuoka Tectonic Line (ISTL) at its eastern end and the Sasayama Tectonic Line to the west
(Sugiyama, 1995). The Belt is comprised of three Groups (the Setogawa, Oigawa, and Ryuso
Groups, from west to east), which are separated by the Fujishiro and Jumaiyama Faults
branching from the ISTL (Sugiyama, 1995). The Setogawa Group forms a major part of the
Setogawa Belt, which is mainly composed of slaty to phyllitic shale and sandstone (Sugiyama,
1995 ; Yagi et al., 1996 ; Ozaki et al., 2002). The Oigawa Group is mainly composed of shale andinterbedded sandstone and shale. Furthermore, the Ryuso Group is mainly composed of rhyolite
volcanic rock and lava (Sugiyama, 1995 ; Ozaki et al., 2002). Several ore deposits are recognizedin the Setogawa group (Shimizu et al., 1995 ; Sugiyama 1995 ; Yakushi and Enjoji, 2004).

3.

Sampling and Analytical Procedures

Two water samples were taken from a well drilled for mineral spring source and a natural
spring in the Amehata area, northern Setogawa Group in the western Yamanashi Prefecture(Fig. 1). Additionally, phyllitic rock samples were collected from the natural spring site. Well
depth and chemical analysis data for the Amehata mineral spring source were obtained byinterviewing the facility manager. Temperature, electrical conductivity, and pH were directly
measured using a standard, hand-held, calibrated meter (HORIBA D-24). Total alkalinity was also
directly determined by using titration with sulfuric acid to a final pH of 4.8 (HACH AL-DT), andthe total alkalinity is expressed as HCO3－. The amount of free CO2 was calculated based on the44

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山梨県西部の瀬戸川層群における鉱泉および湧水の地球化学的・同位体的特徴

Fig. 1 Simplified geological map and sample locations of water samples in thenorthern Setogawa Group of the western Yamanashi Prefecture (modified
after Sugiyama, 1995 ; Ozaki et al., 2002).

alkalinity and pH values. Main compositions such as Na+, K+, Mg2+, Ca2+, Cl－, NO3－ and SO42－
were measured using ion chromatography (SHIMADZU LC-VP). ∑Fe2+・3+ was measured byatomic absorption spectrophotometry (SHIMADZU AA-6200). The SiO2 was analyzed byspectrophotometry using molybdenum yellow method (SHIMADZU UV-1650PC). Concentrationof boron was measured by carmine method (HACH DR-2800) and expressed as HBO2. The rocksample was observed using a digital microscope (KEYENCE VHX-2000) after polishing.
Isotopic compositions of hydrogen (D/H) and oxygen (18O/16O) were reported in terms of δD
and δ18O (‰), relative to the Vienna Standard Mean Ocean Water (V-SMOW) standard. These
were also measured by a mass spectrometer connected on-line to a gas chromatograph (GVInstruments Isoprime-EA). After decomposing a sample by heating at 1,050℃ for H2 analysis
and 1,260℃ for CO analysis in an oxygen-free environment, the product gases werechromatographically separated and introduced into an ion source for analysis with a mass
spectrometer. Sulfur isotope (34S/32S) analysis of dissolved SO42－ was also performed using an
Isoprime-EA mass spectrometer. The dissolved SO42－ in the water samples was collected as45

谷口無我，村松容一，長島秀行，奥村文章，山室真澄

温泉科学

BaSO4 and mixed with V2O5 as a combustion
aid. The mixture was then heated in order todecompose it into a SO2 gas, and obtained SO2
Sulfur isotopic compositions were reported as
δ34S (‰) variations relative to the Canyon Diablo
Troilite (CDT) standard. Analytical precisions
are ±1.0‰ for δD, ±0.1‰ for δ18O, and ±0.2‰
for δ34S values, respectively.

4. Results and Discussion
4. 1

Chemical and isotopic compositionsof the water samples
The analytical results of mineral spring andnatural spring waters are listed in Table 1. The
extremely low concentration of NO3－ in water
samples, which is one measure used in thewater pollution index, suggests that there arelittle or no anthropogenic effects. Natural springwater showed slightly alkaline pH and ionicdominance patterns of Ca 2+＞Na +＞Mg 2+ andHCO3－＞SO42－＞Cl－. The mineral spring water
gave off a pervasive hydrogen sulfide odor, andthe chemical contents were 10 times thicker
than those of natural spring water, but it had
the approximately same ionic dominance as thenatural spring, i.e., Ca2+＞Mg2+＞Na+ and HCO3－＞SO42－＞Cl－.
Hot spring analysis certificate (Yamanashi
Prefecture Food Hygiene Association, 2009)
show slightly higher concentration of major
cation and anions such as, Na+, K+, Ca2+, Mg2+,
Fe2+・3+, Cl－, HCO3－, and SO42－ ions, than theseof hot spring source water sampled in thisstudy, but show the same ionic dominance pat­tern. As shown in Table 1, all these analytical
data, including data in this study and the hotspring analysis certificate (Yamanashi Prefec­
ture Food Hygiene Association, 2009), show
slightly higher concentrations of boron in thewater samples (0.1 to 0.7 mg/L as B) than that of46

Table 1 Chemical composition and pH of water samples collected from the northern Setogawa Group in the western
Yamanashi Prefecture.

gas was then analyzed by a mass spectrometer.

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rain and river water of central Honshu (0.009 ppm and 0.042 ppm as B, respectively ; Utsumi and
Isozaki, 1967). This boron content indicates that the quality of water samples is influenced by
the interactions between water and sedimentary rock of the sampling site of the waters because
the boron content is generally high in sedimentary rock (Ishikawa and Nakamura, 1989). Yazaki
et al. (1981) have pointed out that the groundwater in this area tends to have ionic dominance of
Na + ＞Ca 2+ and Cl －＞HCO 3 － with a slightly alkaline pH.

However, these water quality

characteristics were virtually focused on an investigation of waters associated with natural gas.
Therefore, relationship between water chemistry and sedimentary rock of the Amehata area has
not been clearly identified. Regarding the sample waters in the present work, water quality issimilar to general water qualities of shallow groundwater (Ca-HCO 3 type) rather than thefeatures determined by Yazaki et al. (1981).
The δ34S values of the dissolved SO42－ in the samples were characteristically highly negative.
This may be one of the features, which found in the water samples from Setogawa Group of
Amehata area.
4. 2

Origin of the water and formation mechanisms of water quality
The δD and δ18O values of the water samples suggested that the waters were meteoric in

origin because the δ values of the water samples (Table 1), were similar to the annual mean valuesof the spring waters in the Kofu basin, which are meteoric in origin (δD＝－66‰, δ18O＝－9.7‰ ;Nakamura et al., 2007).
In general, the water quality of the groundwater derived from meteoric water is controlled
by several water-rock interaction processes, such as the dissolution and precipitation of calcite,
plagioclase weathering, and ion-exchange reactions of clay minerals. This is especially true atshallow layer levels where a dissolution of calcite as shown in reaction (1) is the most dominantfactor affecting the chemical properties of groundwater because the calcite dissolution rate isexceedingly faster than that of plagioclase (Rimstidt, 1997).
CaCO3＋CO2＋H2O → Ca2+＋2HCO3－

(1)

In the Setogawa Group, calcareous rocks, which contain several species of fossils, such as
foraminiferal, nanno, molluscan and radiolarian fossils, have been occasionally recognized (e.g.,
Honjo and Minoura, 1968 ; Iwasaki and Ono, 1977 ; Ibaraki 1983, 1984 ; Sugiyama, 1995). Thus
the calcareous content of the rocks seems to be the source of the Ca2+ and HCO3－ in the water
samples. However, the average concentrations of the Ca2+ and HCO3－ in the natural waters(including spring water, groundwater, and river water) from every region of Japan are 14.7 mg/
L and 50.7 mg/L, respectively (Yabusaki and Shimano, 2009). Based on this, the Ca2+/HCO3－ meq
ratio is calculated at 0.88. Conversely, the Ca2+/HCO3－ ratio of the water sampled in the present
study is slightly high, reaching 1.17 for mineral spring and 1.01 for natural spring water.
Next, we considered the origin of the SO42－ in the water samples. According to Shimizu etal. (1995), the Koei vein deposit (Fig. 1) in the Amehata area consists of an abundance of sulfide
minerals, such as pyrite, pyrrhotite, chalcopyrite, and sphalerite. Furthermore, those sulfides
have highly negative δ34S values at －14.6‰. Those δ34S values are remarkably similar to thevalues of the SO42－ in the water samples (－14.4‰ for mineral spring ; －13.7‰ for natural
spring), suggesting that the origin of the SO42－ was from an oxidation of the sulfide minerals. As
47

谷口無我，村松容一，長島秀行，奥村文章，山室真澄

温泉科学

Fig. 2 Micrographs of finely-disseminated and oxidized pyrite in the polished rock sample.   Cube
type ; (A) stereo microphotograph, (A′) reflected light microphotograph.  Veinlet type ; (B) stereo
microphotograph, (B′) reflected light microphotograph.   Oxidation of the pyrite progressed tothe inside.

a result of this microscopic observation of the phyllitic rock sample collected from the natural
spring site, a finely-disseminated pyrite was recognized and the oxidation of it was seen to
progress to the inside (Fig. 2). Therefore the SO42－ in the water samples were derived from theoxidation of pyrite at the oxygen-rich conditions close to the surface by the following reaction(Appelo and Postma, 2005) :FeS2＋15/4O2＋7/2H2O → Fe(OH)3＋2SO42－＋4H+(2)

According to hot spring analysis certificate (Yamanashi Prefecture Food Hygiene Association,2009), the mineral spring water contains dissolved S2O32－, HS－ and H2S as specific chemicalcomponents of the Japanese Hot Spring Law. Because the slaty rock around Amehata area isrich in organic matter (Yazaki et al., 1981), we considered that H2S was produced via the sulfate
reduction of sulfate-reducing bacteria in the anaerobic underground environment. This is due tothe fact that the decomposition of organic matter can lead to anaerobic conditions ingroundwater. The S2O32－ is possibly an intermediate product of the re-oxidation process of H2S
to SO42－.
While pyrite oxidation progressed at the oxygen-rich surface layer, water samples were notacidified. Since water samples contained free CO2 (Table 1), H+ derived from pyrite oxidation48

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山梨県西部の瀬戸川層群における鉱泉および湧水の地球化学的・同位体的特徴

was consumed through the dissolution of calcareous material with the following reaction :CaCO3＋2H+ → Ca2+＋CO2＋H2O

(3)
2+

－
3

We also considered that the reason for a high Ca /HCO

ratio in the water samples was due

mostly to some dissolution of calcareous material progressing without a HCO3－ release, as shownin the reaction (3).
In this study, chemical and isotopic characteristics of mineral and spring waters of Amehata,a watershed conservation area of Yamanashi Prefecture, were interpreted based on geologicalattributes. To protect and sustainable use of hot springs in Yamanashi Prefecture, further
geological and isotopic investigations should be extensively conducted.

5.

Conclusion

Water samples from a well drilled for mineral spring source and from a natural spring in
the Amehata area of northern Setogawa Group in the western Yamanashi Prefecture, as well asa rock samples taken from the natural spring site were investigated in order to determine theorigin of the water and chemical reactions, which determine the quality of the water. A coupleof discernible conclusions below can be drawn from this study :(1) The ionic dominance pattern observed in the water samples were Ca2+＞Na+＞Mg2+ and
HCO 3－＞SO 42－＞Cl － for natural spring, and Ca 2+＞Mg 2+＞Na + and HCO 3－＞SO 42－＞Cl － formineral spring water. Based on the isotopic characteristics of δD and δ18O, sample waters weremeteoric in origin.
(2) The Ca2+ and HCO3－ in the water were derived from the dissolution of calcareousmaterial in the Setogawa Group. The SO42－ that has highly negative values of δ34S was derived
from the oxidation of pyrite at the aerobic surface layer. The hydrogen sulfide odor of themineral spring might be caused by the SO42－ reduction to H2S by sulfate-reducing bacteria in
the anaerobic underground. H+ derived from the pyrite oxidation was then neutralized throughdissolution of calcareous materials.
Acknowledgments
Sulfur isotopic analyses were performed using facilities of the Okayama University. We are
deeply grateful to Mr. T. Ohira and Mr. H. Katayama of Tokyo University of Science for theirsulfur isotopic analyses. We also wish to express our gratitude to Dr. G. Inoue, Editor in Chief ofJournal of Hot Spring Science, and three anonymous reviewers for their critical and constructive
comments to improve the manuscript.
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