hydrochemistry of the groundwaters in the izu collision...

309

Geochemical Journal, Vol. 45, pp. 309 to 321, 2011

*Corresponding author (e-mail: [email protected])

Copyright © 2011 by The Geochemical Society of Japan.

tectonic difference between the two areas since theMiocene.

Drilling of thermal wells for hot-spring bathing pur-poses since the 1980’s was performed extensively on adeep thermal aquifer at the depths more than 1000 m inthe non-volcanic area of the Kanagawa Prefecture. As aresult, geological structure and hydrochemistry of the areahave been reported by many investigators (e.g., Imahashiet al., 1996; Seki et al., 2001; Oyama et al., 1995; Ozawaand Eto, 2005), while details of the water-rock reactionsand flow systems of the groundwater in the area have notbeen clearly described previously. In this paper, wepresent the results of chemical and stable isotopic (δD,δ18O, δ34S) compositions of rivers and groundwaters fromthe wells in the non-volcanic area of the Kanagawa andsoutheastern Yamanashi Prefectures, to constrain the flow

Hydrochemistry of the groundwaters in the Izu collision zoneand its adjacent eastern area, central Japan

YOICHI MURAMATSU,1* YUTA NAKAMURA,2 JITSURO SASAKI3 and AMANE WASEDA4

1Department of Liberal Arts, Faculty of Science and Technology, Tokyo University of Science,2641 Yamazaki, Noda, Chiba 278-8510, Japan

2Department of Chemistry, Faculty of Science, Tokyo University of Science,1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan

3Department of Pure and Industrial Chemistry, Faculty of Science and Technology, Tokyo University of Science,2641 Yamazaki, Noda, Chiba 278-8510, Japan

4Japan Petroleum Exploration Co., Ltd., Research Center, 1-2-1 Hamada, Mihama-ku, Chiba 261-0025, Japan

(Received April 8, 2010; Accepted March 6, 2011)

Chemical and stable isotopic (δD, δ18O, δ34S) compositions of rivers and groundwaters, mineral constituents of rocksamples from wells, and δ34S values of anhydrite in the Izu collision zone and its adjacent eastern area, southern KantoPlain, central Japan, were analyzed to constrain the water-rock reactions and flow systems of the groundwaters.

Inside the accreted Izu–Bonin–Mariana (IBM) basin, a two-dimensional map of the geothermal gradient calculatedroughly using the discharge groundwater temperatures and the borehole temperature logging data confirms that the aqui-fer is recharged by the local meteoric water (LMW) and the high density seawater in the area. The oxygen and hydrogenisotopic compositions reveal that the Ca·Na–SO4 groundwaters in the Tanzawa Mts. and the high Na–Cl groundwaters inthe coastal area are of meteoric water and weakly altered fossil seawater origins, respectively. Sulfur in the SO4 richgroundwaters is derived from anhydrite and gypsum based on the sulfur isotopic compositions. The sulfate-typegroundwaters were produced by the following process: the LMW infiltrated downward with dissolution of the sulfateminerals from hydrothermal veins in the Tanzawa Group, produced the Ca–SO4 groundwater as a result of Ca2+ exchangepartly on Na–smectite layer of mixed-layer chlorite–smectite in the sedimentary rocks of the Tanzawa Group. The Ca–Clgroundwaters in the eastern margin of the Tanzawa Mts. were produced by mixing of LMW with fossil seawater rechargedfrom the surface of the coastal area, and Ca2+ exchange of the mixed-layer mineral in pyroclastic rocks of the TanzawaGroup.

Outside the accreted IBM basin, the Na–HCO3 groundwaters in the shallow aquifer were formed by dissolution ofauthigenic calcite with LMW, and Na+ exchange in the Kazusa Group. The moderate Na–Cl groundwaters in the deepaquifer were formed by mixing of the deep seated fossil seawater with the Na–HCO3 waters in permeable sandstone andconglomerate of the Kazusa Group.

Keywords: Izu collision zone, groundwater, hydrochemistry, formation mechanism, recharge, anhydrite, cation exchange

INTRODUCTION

Within non-volcanic area of the Kanagawa Prefecture,central Japan, the northern tip of the oceanic PhilippineSea Plate is generally thought to have been colliding withthe continental Eurasian Plate at the northern margin ofthe Izu Peninsula. Intense Quaternary crustal movements,such as active faulting with high slip-rates and rapid up-lift or subsidence, occur along this inland plate boundary(Yamazaki, 1992). The flow system of groundwaters inthe accretion area of the Philippine Sea Plate may differfrom those in the eastern non-volcanic area based on the

310 Y. Muramatsu et al.

system of the groundwaters around the accretion area ofthe Philippine Sea Plate.

OVERVIEW OF GEOLOGY

A simplified geological map of the southern KantoPlain located southwest of Tokyo, including the Kanagawaand southeastern Yamanashi Prefectures, is shown in Fig.1. The western part of the study area is topographicallysituated at the Tanzawa Mountain land (maximum alti-tude of 1673 m above sea level), and the eastern part atthe Sagamigawa low land (10 m above sea level),Sagamihara platform (50 m above sea level), Tama hill(70 to 90 m above sea level), and Shimosueyoshi plat-form (40 to 60 m above sea level) in eastwardly order,while the Miura Peninsula is located in the southeasternpart of the study area at the Miura hill (maximum alti-tude of 241 m above sea level). The geology of the studyarea has been reported by many investigators, based ongeological and logging surveys obtained during drillingof many deep thermal and seismic survey wells and seis-mic reflection surveys (e.g., Ishii, 1962; Omori et al.,1986; Suzuki, 2002; Kanagawa Pref., 2003; Ozawa and

Eto, 2005; Hayashi et al., 2006; Takahashi et al., 2006).The Kobotoke and Sagamiko Groups are correlated

with the Shimanto Group. The Cretaceous KobotokeGroup composed of shale and sandstone is widely dis-tributed through the northwestern Tanzawa Mountains(Mts.) to the Kanto Mts. The Paleaogene Sagamiko Groupcomposed of sandstone, conglomerate, and mudstonecrops out in the southern side of the Kobotoke Group(Sakai, 1987) and occurs deeper than 1005 m below thesurface at Ebina City (location 15 in Fig. 2; Ozawa andEto, 2005). There are normal NE-SW trending faults, andnormal and reverse NW-SE trending faults in the TanzawaMts., and the Tonoki–Aikawa reverse fault runs along theboundary between the Sagamiko and Tanzawa Groups.The Miocene Tanzawa Group, which is equivalent to theGreen Tuff formation, is mainly composed of daciticlapilli tuff, tuff-breccia, andesitic lapilli tuff andtuffaceous mudstone (Oka et al., 1979). It has been meta-morphosed under conditions from zeolite to amphibolitefacies, which were produced by the intrusion of theTanzawa plutonic complex (K–Ar dating of 4.3 to 7.6 Ma;Kawano and Ueda, 1966) composed mainly of tonalitewith minor gabbro (Seki et al., 1969). According to Zhang

3500 m

2500

m2000

m15

00 m

1000

m50

0 m

0 50 km

Subduction in 15 Ma

Tokyo Bay

Doshigawa fault

Kannawa fault

Akiyamakawa fault

Sagami trough

1000 m1500 m2000 mQuaternary system

Volcanic rocksNeogene system

Sedimentary rocks

Volcanic rocks

Granitic rocks

Pre–Tertiary system

Palaeogene system

N

Tonoki–Aikawa fault

Sagami Bay

Makim

e–Susugaya

fault

Kozu–Matsuda

fault

Kanto Mts

Tanzawa Mts.

Accreted IBM basin

Pacific Ocean

Sea of Japan

N

Study area

Fig. 1. Geological map of the southern Kanto Plain (after Hayashi et al., 2006). Thin-broken lines indicate counters of depth ofupper boundaries of the pre-Neogene systems from sea level.

Hydrochemistry of groundwaters in the Izu collision zone 311

and Yoshimura (1997) and Zhang et al. (1997), the lowgrade metamorphic rocks of the southern Tanzawa Mts.can be divided into stilbite, laumontite, prehnite–pumpellyite, epidote, epidote–amphibole and amphibolezones in increasing order of metamorphic grade. TheMiocene Aikawa Group equivalent to the Green Tuff for-mation, which is bordered on the Tanzawa Group by theMakime–Susugaya tectonic line, is mainly composed ofandesitic lapilli tuff, mudstone and tuffaceous sandstone.

The Kannawa reverse fault runs along the boundarybetween the Tanzawa and Ashigara Groups at the south-ern margin of the Tanzawa Mts. The Izu–Bonin–Mariana(IBM) arc of the Philippine Sea Plate is thought to bebordered on the Honshu arc of the continental EurasianPlate by the Kannawa and Kozu–Matsuda faults(Sugimura, 1972). The Pliocene–Pleistocene AshigaraGroup is mainly composed of sandstone, mudstone andconglomerate. The accreted IBM basin (Soh et al., 1998)between the Tonoki–Aikawa fault and the Kannawa,Kozu–Matsuda faults formed by accretion of the IBM arccrust onto the Honshu arc since middle Miocene in thewestern part of the field (Takahashi, 2006).

Weathered pyroclastic air-fall deposits called the“Kanto Loam” cover the Sagami Group, in addition tothe Miura, Kazusa and Sagami Groups in ascending or-der at the Yokohama and adjacent district (Mitsunashi andKikuchi, 1982). Geology of the Miura Peninsula is di-vided into the Hayama, Miura, Kazusa and Sagami Groups(Eto et al., 1998). The Early to Middle Miocene Hayama

Group is composed of tuffaceous sandstone and mudstone.The Miocene to early Pliocene Miura Group, composedof tuffaceous sandstone and mudstone, unconformablyoverlies the Hayama Group, and is exposed on the north-ern, central and southern parts of the peninsula. Thereare E-W trending normal faults in the peninsula (Omoriet al., 1986). The Late Pliocene to Early PleistoceneKazusa Group, composed of tuffaceous sandstone andsandy mudstone, unconformably lies on the Miura group.The Sagami Group consists of neritic, lacustrine and flu-vial deposits.

SAMPLING AND ANALYTICAL PROCEDURES

Cutting samples were collected at 5-m intervals fromthe two wells (locations 6 and 16 in Fig. 2) for X-raypowder diffraction analysis. We collected thirty-one sam-ples of river water and groundwater from wells drilledfor hot-spring bathing and domestic purposes in 2006–2007 (Fig. 2). Half of the groundwaters used for hot-springbathing were collected from depths below 1000 m. Twokinds of the groundwaters from the different aquifers werecollected from both shallow and deep thermal wells atthree locations (three pairs of locations 3 and 4, 14 and27, 16 and 24).

Groundwater samples from most wells were collectedafter running for several minutes at the valve located nearthe well head, except for wells at a faucet of a bathtub orthe entrance of a reserve tank. Temperature, conductivity

34

5

16

2

19

20

1221

13

15

16

17

18

14

23

N

Ca Na–SO4

Na–SO4

Na–ClCa–ClNa–HCO3

Ca–HCO3

0 20 km

Sagami Bay

Tokyo Bay

Sagamihara

Otsuki

YamanakaLake

Yokohama

YokosukaOdawara

Ebina

31 7 1122

29

25

26

24

27

FTRRTR

28

YamanashiTokyo

Shizuoka

TAF

30

Hiratsuka

MiuraPeninsula

Fig. 2. Location of the water samples in the study area.


and pH were directly measured using a standard hand-held calibrated field meter. Total alkalinity was measuredby potentiometric titration with sulphuric acid to a finalpH of 4.8. Chloride, SO4, Br, F, PO4, Na, K, Ca and Mgwere measured by ion chromatography, and Al, SiO2 byvisible spectrophotometry.

Hydrogen and oxygen isotopic compositions, reportedin terms of δD and δ18O (‰), relative to V-SMOW stand-ard were measured by an IsoPrime-EA mass spectrometerconnected on-line to a gas chromatograph. After decom-posing a sample by heating at 1050°C for H2 analysis and1260°C for CO analysis in an oxygen free environment,the product gases are chromatographically separated, andafterwards introduced to the ion source of the massspectrometer. Analytical precisions are ±0.1‰ for oxy-gen and ±1.0‰ for hydrogen. Sulfur isotopic composi-tion was analyzed for three groundwaters and oneanhydrite. Powdered anhydrite was dissolved into pure

water at room temperature. The aqueous SO4 was pre-cipitated as BaSO4, and the sulfur isotope analysis wasperformed by an IsoPrime-EA mass spectrometer usingabout 0.1 mg BaSO4 mixed with 10 mg of V2O5. The shiftof raw δ34S values during measurement was correctedusing measured values of the BaSO4 laboratory standard.Sulfur isotopic composition is expressed in terms of δ34S(‰), relative to the Canyon Diablo Troilite (CDT) stand-ard. Analytical precision is ±0.2‰.

Activities of various ions and chemical equilibria forrelevant minerals were calculated using the SOLVEQcomputer programs (Reed, 1982).

RESULTS AND DISCUSSION

Secondary minerals in aquiferThe aquifer is divided into fault and rock facies types

based on the geological structure (Ozawa and Eto, 2005).

0

200

400

600

800

1000

1200

Dep

th (

m)

Lith

olog

y

Qua

rtz

Plag

iocl

ase

Kao

linite

Smec

tite

Primary minerals Secondary minerals

Aqu

ifer

Cal

cite

Mag

netit

e

Hal

loys

ite

1400

Loa

mK

azus

a G

roup

Miu

ra G

.

1500 m

0

200

400

600

800

1000

1200

Dep

th (

m)

Lith

olog

y

Qua

rtz

Plag

iocl

ase

Preh

nite

Chl

orite

(C

h)

Primaryminerals Secondary minerals

Aqu

ifer

Tanz

awa

Gro

up

Green tuff Loam

Anh

ydri

te

Lau

mon

tite

TPC

Gyp

sum

1310 m

Ch/

Smec

tite

(a) Tsukui well (b) Yokohama well

Casing pipeConglomerate

Mudstone Sandstone Sandstone/Mudstone

Quartz diorite

Fig. 3. Stratigraphy, aquifer and distribution of minerals with depth in the Tsukui (a) and Yokohama (b) wells. TPC, Tanzawaplutonic complex. Aquifer below depth of casing pipe is shown in the figure. Solid and dashed lines indicate that minerals areabundant and minor, respectively.


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−50.

1−8

.57

31

Min

amits

uru

2

07/

26/2

007

15.8

7.

810

.32.

60.

514

.42.

93.

60.

76.

249

.0<

0.1

<0.

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17.9

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6

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1

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88

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4

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039

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410.

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1975

026

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67.5

Tabl

e 1.

C

hem

ical

com

posi

tion

of

wat

er s

ampl

es f

r om

the

sou

ther

n K

anto

Pla

in

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T, D

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empe

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re;

TG

, G

eot h

erm

al g

radi

ent ;

EC

, E

l ect

ric

cond

uct i

vit y

; F

TR

, F

aul t

typ

e aq

uif e

r; R

TR

, R

ock

f aci

es t

ype

aqui

f er;

RW

, R

i ver

wat

er.

*1 Tot a

l de

pth.

*2,*3 P

aren

t hes

i s s

how

wat

er t

empe

rat u

re m

easu

red

at a

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of b

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996)

.


The fault type aquifer (FTR) where the groundwater flowsalong faults is distributed in the accreted IBM basin fromthe Tanzawa Mts. to the southern Miura Peninsula (Fig.2). The Miocene sedimentary and plutonic rocks are dis-rupted by northeast-trending normal faults and northwest-trending normal and reverse faults in the Tanzawa Mts.(Oki et al., 1967; Omori et al., 1986). Figure 3a showsthe distribution of primary and secondary minerals withdepth in the Tsukui well (location 6 in Fig. 2) located thewestern Tanzawa Mts. as a representative well at the FTRarea. The groundwater is reserved near the intersectionof these faults in the Tanzawa plutonic complex at 1290m depth (Oyama et al., 1995). Anhydrite and gypsumoccur widely as minor secondary minerals accompaniedwith laumontite and prehnite in the Tanzawa Group andTanzawa plutonic complex. Clay mineral is a randommixed-layer chlorite/smectite, which is identified by abroad peak, non-integral spacing of 001 reflection and asystematic response of d001 to the air-dried and ethylene–glycol treatments.

In contrast, the rock facies type aquifer (RTR), wherethe groundwater flows horizontally along permeable lay-ers such as sandstone and basal conglomerate in theKazusa and Miura Groups (Ozawa and Eto, 2005), is dis-tributed in the eastern area adjacent to the accreted IBMbasin from the Yokohama district to the northern MiuraPeninsula. Figure 3b shows the distribution of primaryand secondary minerals with depth in the Yokohama well(location 16 in Fig. 2) as a representative well in the RTRarea. The groundwater is reserved in the basal conglom-erate layer of the Kazusa Group from 1215 to 1270 mdepth (Ozawa and Eto, 2005). Kaolinite, smectite andcalcite occur widely as minor secondary minerals in theKazusa and Miura Groups.

Groundwater quality and its geographical distributionThe analytical results and the seawater composition

from the Kamogawa offshore, Boso Peninsula, Chiba(Imahashi et al., 1996) are listed in Table 1. The chemi-cal compositions of the waters described in terms of rela-

Fig. 4. Trilinear diagram of the water samples. Numbers refer to location numbers in Table 1.


tive concentrations of ions allow us to distinguish thefollowing types of waters (Fig. 4):

(1) Sulfate-type groundwaters: Compositions varyfrom Ca·Na–SO4 to Na–SO4. The Ca·Na–SO4 watersfound in the western Tanzawa Mts. show temperaturesfrom 21.2 to 37.8°C, pH values between 8.3 and 9.7 andsulfate concentrations between 117 and 2410 mg/L. TheNa–SO4 waters from shallow depths in the Nakagawadistrict, central Tanzawa Mts., show high pH values be-tween 9.5 and 9.9, and sulfate concentration between 65and 393 mg/L.

(2) Chloride-type groundwaters: Compositions varyfrom Na–Cl to Ca–Cl. The seawater like high Na–Cl wa-ters with Cl concentrations of approximately 20 g/L andtemperatures from 17.2 to 33.4°C are found in the coastalarea bordering Sagami Bay to southwestern Tokyo Bay.The moderate Na–Cl waters with Cl concentrations from1160 to 6579 mg/L and temperatures from 32.0 to 43.8°Care found in the Yokohama district (including Ebina City)and Miura Peninsula. The Ca–Cl waters with moderateCl concentrations from 778 to 8024 mg/L are found inthe eastern margin of the Tanzawa Mts. and Hiratsukadistrict.

(3) Bicarbonate-type waters: Compositions vary fromNa–HCO3 to Ca–HCO3, and location 22 belonging to Na–HCO3·SO4·Cl water is included in this type. The Na–HCO3 waters found in the shallow depths of the Yokohamadistrict and Miura Peninsula show relatively low tempera-tures from 17.9 to 20.7°C and narrow pH values between8.4 and 8.6. They are also found in the shallow to middledepths of the eastern margin of the Tanzawa Mts. havingtemperatures from 18.2 to 27.7°C and high pH valuesbetween 9.7 and 10.3. In contrast, the dilute Ca–HCO3waters have low temperatures from 15.2 to 15.8°C andpH values between 7.8 and 8.2 in the river waters andgroundwater above 10 m depth in the Tanzawa Mts. Theyare the most widespread water-type discharged from shal-low wells and springs in the Kanagawa Prefecture(Ishizaka et al., 1993).

Mixing of fossil seawater with local meteoric waterThe stable isotopic compositions of the groundwaters

are shown in Table 1. The δ18O values range from –10.64to –0.98‰, and δD values from –69.2 to –7.9‰. Becausethe groundwaters except for six chloride-types plot closeto the local meteoric water line (LMWL; δD = 8δ18O +16)in the δ18O versus δD diagram (Fig. 5a), they are of me-teoric origin. High Na–Cl groundwaters (locations 12, 13and 14) with Cl concentrations of approximately 20 g/Lhave δ18O and δD values slightly lower than those ofseawater, suggesting that they are fossil seawaters with aweak water-rock reaction. The δ18O and δD values of thehigh Na–Cl groundwater from the Odawara well (loca-tion 12) are similar to those (δ18O = –2.4 and δD =

–7.0‰) from the Adachi well drilled to the Kazusa Grouparound the northern end of Tokyo Bay where a large quan-tity of fossil seawater is reserved in the non-volcanic cen-tral Kanto Plain (Muramatsu et al., 2008).

Dilution of the chloride-type waters is confirmed bythe plots of δD versus Cl (Fig. 5b) and δ18O versus Cl.The moderate Na–Cl groundwaters (locations 15 to 17)from the Yokohama district (including Ebina City) in theRTR area are plotted on the mixing line (ML 1) of thehigh Na–Cl groundwaters (locations 12 and 13) and theLMW (δ18O = –6.68‰ and δD = –42.3‰), confirmingthat they have been formed by mixing of these waters(Fig. 5b). Based on the same reason, the moderate Ca–Cl

–100

–80

–60

–40

–20

0

20

–16 –14 –12 –10 –8 –6 –4 –2 0 2 4 6

21

15

12

13,14

16

Ca Na–SO4 Na–HCO3

Na–SO4 Ca–HCO3

Na–Cl Seawater

Ca–Cl Fossil seawaterLM

WL

–100

–80

–60

–40

–20

0

20

0 5000 10000 15000 20000 25000

Cl (mg/L)

16 15

18

19

21

20

17

1413 12

ML1

Ca Na–SO4 Na–HCO3

Na–SO4 Ca–HCO3

Na–Cl SeawaterCa–Cl Fossil seawater

ML227

ML3

Fig. 5. Relationships between δ18O and δD values and betweenCl concentrations (mg/L) and δD values of the water samples.The solid line (LMWL) is the local meteoric water line (δD =8δ18O + 16). The ML1 and ML3 show the mixing of fossilseawater and meteoric water for the Na–Cl groundwaters ofthe Yokohama district and Miura Peninsula, respectively, andthe ML2 shows the mixing of these waters for the Ca–Clgroundwaters of the eastern margin of the Tanzawa Mts. andHiratsuka district.


groundwaters (locations 19 and 20) from the eastern mar-gin of the Tanzawa Mts. and Hiratsuka district in the FTRarea seems to have been formed by mixing (ML2) be-tween the high Na–Cl groundwaters (locations 12 and 13)and the LMW (δ18O = –8.64‰ and δD = –53.7‰; Fig.5b).

The water circulation system in the Miura Peninsulais independent from that of its adjacent Yokohama dis-trict. As the moderate Na–Cl groundwater from theYokosuka well (location 18) is perfectly plotted on themixing line (ML3) of the high Na–Cl groundwater fromthe Miura well (location 14) and Na–HCO3 groundwaterfrom the Miura well (location 27), it seems to have beenformed by mixing of the fossil seawater and the LMWinfiltrating to depth at the Miura hill.

Recharge system estimated from geothermal gradient andseawater fraction

As the deep fluid temperature gradually decreases withupward flow in a well due to heat loss, the aquifer tem-perature corresponding to the deep fluid temperature at afeed point is higher than the discharge water tempera-ture. Because of this phenomenon, several chemicalgeothermometers are usually used to estimate the aquifertemperature of geothermal fields (Fournier, 1991). How-ever, the aquifer temperature at a feed point is reflectedin discharge temperature better than the sil icageothermometer in the non-volcanic central Kanto Plain(Muramatsu et al., 2008). Based on the borehole tempera-ture logging of the Yokohama well (location 16), the tem-perature with a conductive profile is 52°C at 1500 m

bottomhole depth (Muramatsu et al., 2008). The tempera-ture gradient below 900 m depth calculated from the equi-librium temperature estimated from the borehole tempera-ture logging data using the conventional Horner method(Fertl and Wichmann, 1977) is approximately 2.5°C/100m, whereas the gradient estimated from the discharge tem-perature is 1.9°C/100 m (Table 1). The small temperaturedifference (0.6°C/100 m) reveals that the geothermal gra-dient calculated by using water temperature measurementsis possible to use roughly for characterization ofgeothermal activity. Assuming that surface andbottomhole temperatures are equal to the mean tempera-ture (15.6°C) of the river water (locations 28, 30, 31) andthe discharge temperature in a well, respectively,geothermal gradients of the study wells are roughly shownin Table 1.

Compared with the geothermal gradients (1.6 to 2.1°C/100 m) of the RTR area, the FTR area has low valuesfrom 0.2 to 1.3°C/100 m except for some wells includingthe Nakagawa (Fig. 6) as also reported by Kikugawa etal. (2007), indicating that recharge water has regionallyreached to the depths in this area. The lowest gradient(0.2°C/100 m) for the Odawara well (location 12) can beattributed to the down flow of seawater from the surface.The Holocene sediments composed of the permeable al-luvial gravels are distributed over the full depth, and prob-ably continue to 1400 to 1800 m depth in the well(Kanagawa Pref., 2002; Itadera et al., 2004). Consider-ing the stratigraphic data, recharge must have been per-formed by downward plunging of cold high density fos-sil seawater along the Kozu–Matsuda fault (Fig. 1) in the

(1.0)

(0.4)(1.0)

(1.3) 1.1

1.3

0.21.1

1.2

(1.8)

1.9

(1.7)

2.1

0.9

(2.0)

N

0 20 km

Sagami Bay

Tokyo Bay

Otsuki

YamanakaLake

Yokohama

Yokosuka

Odawara

1.3

(1.3)

1.6

1.7

0.8

1.5

1.5

FTR

RTR

Ca Na–SO4

Na–SO4


Sagamihara

Yamanashi Tokyo

Shizuoka

TAF

Ebina

1.5

1.5

4.1

1.7

6.8

6.215.9

4.9

0.000.00

0.00

0.00

0.00

0.01

0.04

0.25

1.02

0.41

0.99

0.33 0.27

0.11

0.06

1.03

0.00

N

0 20 km

Sagami Bay

0.01

0.5

Tokyo Bay

10.70

0.53

0.711

0.5

2

1500

m

1000

m50

0 m

Ca Na–SO4


FTR

RTR

Yamanashi Tokyo

Shizuoka

TAF

Fig. 6. Two-dimensional geothermal gradient distribution. FTRand RTR show fault type and rock facies type aquifers, respec-tively. Parenthesis shows geothermal gradient calculated us-ing water temperature measured at a faucet of bathtub or en-trance of reserve tank.

Fig. 7. Two-dimensional seawater fraction distribution of thedeep groundwater samples. *1Data from Muramatsu et al.(2008). *2Data from Awaya et al. (2002). Dashed line showsthe counter line of basal boundaries of the Kazusa Group(Suzuki, 2002). Depths are shown from sea level. FTR and RTRshow fault type and rock facies type aquifers, respectively.


Odawara district.The fraction of seawater in mixing water of seawater

and LMW is given by:

fm m

m mseaCl sample Cl fresh

Cl sea Cl fresh

=−

−( ), ,

, ,

. 1

Where, fsea is the fraction of seawater in the mixed water,mCl is the Cl concentration, and the subscripts sample,sea and fresh represent the groundwater, the seawater andLMW as end-members, respectively. The mCl,sea andmCl,fresh are derived from the data of the Kamogawa off-shore, Boso Peninsula (Imahashi et al., 1996) andMatsudo City, Chiba (Muramatsu et al., 2008), respec-tively. A contour map of the seawater fraction in thegroundwaters from the wells deeper than 500 m is pre-sented in Fig. 7. Within the FTR area, the waters withhighest seawater fractions of above 0.9 are located in thecoastal area bordering on the Sagami bay to the southernMiura Peninsula, while its fractions are nearly zero inthe Tanzawa Mts. In conclusion, the aquifer may be re-charged by high density seawater and local rainwater inthe accreted IBM basin.

The geothermal gradient has exceptionally high val-ues from 4.9 to 15.9°C/100 m for the Nakagawa wells(locations 7 to 11) in the accreted IBM basin, probablysuggesting the residual heat source from the Tanzawa plu-tonic complex (Oki et al., 1967). Slightly high value forthe Tsukui well (location 6; 1.7°C/100 m) must be alsointerpreted by the same reason because of the occurrenceof the plutonic rock in the deeper part of the well (Fig.3a).

As illustrated in Fig. 6, the geothermal gradients rang-ing from 1.6 to 2.1°C/100 m in the RTR area are similarto those (2.0 to 2.2°C/100 m) in the non-volcanic area,Kanto Plain (Uchida et al . , 2002). According toMuramatsu et al. (2008), a large quantity of fossilseawater is reserved in the Kazusa Group around thenorthern end of Tokyo Bay. Adding chemical composi-tions of the high Na–Cl waters from three thermal wellsaround the Tokyo–Kanagawa prefectural border (Awayaet al., 2002; Muramatsu et al., 2008), the seawater frac-tion in the waters is inclined to increase from southwestto northeast within the RTR area in the Kazusa Group(Fig. 7). Considering increase of the depth of a basalboundary of the Kazusa Group in the direction of thenorthern end of Tokyo Bay (Suzuki, 2002; Odawara,2008), the LMW recharged probably from the directionof the Tanzawa Mts. seems to flow along permeable rocksin the Kazusa Group northeastward, then gradually mix,in variable proportions, with the deep seated fossilseawater in the Kazusa Group (Omori et al., 1986;Muramatsu et al., 2008).

Formation mechanisms of three groundwatersBecause many groundwaters are derived from mixing

of seawater with LMW as mentioned above, an evalua-tion of the extent of M element relative to a theoreticalseawater encroachment can be obtained through calcula-tion of ∆M indices:

∆[M] = [M] – [M/Cl]sea × [Cl] (2)

where ∆[M] is the difference between the M concentra-tion measured in groundwater (in meq/L) and that ex-pected from calculation based on the seawater M/Cl ra-tio.Sulfate-type groundwaters The δ34S values of three SO4rich waters in the Tanzawa Mts. ranges from +17.0 to+21.7‰ (Table 1), confirming that sulfur in SO4

2– de-rives from dissolutions of anhydrite and gypsum fromhydrothermal veins in the Tanzawa Group because of itssimilar value to anhydrite cuttings from the Tsukui well(+20.0‰). The water temperatures versus activity prod-ucts for anhydrite are plotted in Fig. 8a, and a similar

–20

–15

–10

–5

0

5

10

Log

Q

Supersaturated

Undersaturated

(a) AnhydriteCa Na–SO4

Na–SO4

Na–ClCa–Cl

Na–HCO3

Ca–HCO3

–10

–5

0

5

10

15

20

10 20 30 40 50 60

Log

Q

Supersaturated

Ca Na–SO4

Na–SO4

Na–ClCa–Cl

Na–HCO3

Ca–HCO3

Undersaturated

(b) Ca–smectite

Fig. 8. Water temperature versus log activity product foranhydrite (a) and Ca–smectite (b).


result is also obtained for gypsum. The sulfate-type anddilute Ca–HCO3 waters are saturated and slightlyundersaturated with the minerals, respectively. The re-sult reveals that the chemical composition of the shallowdilute Ca–HCO3 waters recharged during infiltration alongfaults into the deeper level of the Tanzawa Group in theTanzawa Mts. must be modified by dissolution ofanhydrite and gypsum in the Tanzawa Group and Tanzawaplutonic complex (Fig. 3a) to produce the sulfate-typegroundwaters saturated with these minerals.

In spite of dissolution of anhydrite and gypsum, theCa/SO4 mole ratios (0.2 to 0.6) in the sulfate-typegroundwaters are significantly lower than that of anhydrite(1.0). Assuming that Ca and SO4 in the groundwaters werederived from dissolution of sulfate minerals, the depletedCa concentration is calculated by the following equation:

∆A[Ca] =∆[Ca] – [Ca/SO4]anhydrite × ∆[SO4] (3)

where ∆A[Ca] is the difference between the enriched Caconcentration with respect to seawater in groundwater (inmeq/L) and that expected from calculation based onanhydrite Ca/SO4 ratio (1.0) using the enriched SO4 con-centration with respect to seawater. As illustrated in Fig.9a, the values of ∆Na versus ∆ACa in the sulfate-typegroundwaters exhibit a significant negative correlationwith similar magnitude (stoichiometrically balanced) ofNa enrichment and Ca depletion. Based on the wide dis-tribution of mixed-layer chlorite–smectite in the Tsukuiwell (Fig. 3a) and at the surface of southern Tanzawa Mts.(Zhang and Yoshimura, 1997; Zhang et al., 1997), themost probable explanation for the Ca depletion of thesulfate-type groundwaters is Ca2+ exchange with Na+ fromNa–smectite layer of the mineral in the sedimentary rocksof the Tanzawa Group by the following reaction:

6Na0.33Al2.33Si3.67O10(OH)2 + Ca2+

→ 6Ca0.167Al2.33Si3.67O10(OH)2 + 2Na+. (4)

This hypothesis is supported by the mineral equilibriumcalculation results; the sulfate-type groundwaters are su-persaturated with respect to Ca–smectite (Fig. 8b), andplot in the Ca–smectite region of the (Na+/H+)–(Ca2+/H+2)activity diagram (Fig. 10).Chloride-type groundwaters Within the high Na–Clgroundwaters from three wells, the chemical composi-tion of groundwater from the Miura well (location 14) issimilar to that of the fossil seawater from the Funabashi

Fig. 10. Stability fields of Ca–smectite and Na–smectite as afunction of log (aNa

+/aH+) and log (aCa

2+/aH+2) at 25 and 60°C,

and 1 atmosphere. Thermodynamic data are from Helgeson(1969).

0

20

40

60

80

100

120

140

–140 –120 –100 –80 –60 –40 –20 0

Na (meq/L)

AC

a (m

eq/L

)

Ca–Cl

(b)

–25

–20

–15

–10

–5

0

0 5 10 15 20 25

Na (meq/L)

AC

a an

d CC

a (

meq

/L)

Ca Na–SO4

Na–SO4

Na–HCO3

(a)

10

12

14

16

18

20

0 2 4 6 8 10 12 14 16

Log (aNa+/aH+)

Log

(a C

a2+/a

H+

2 )

Na–smectite

Ca–smectite

Ca Na–SO4

Na–SO4

Na–ClCa–Cl

Na–HCO3

Ca–HCO3

Fig. 9. ∆Na versus ∆CCa concentrations in the Na–HCO3

groundwaters (a), and ∆ACa in the sulfate groundwaters (a)and Ca–Cl groundwaters (b). See text for ∆Na, ∆CCa and ∆ACacalculations.


well (Muramatsu et al., 2008) which is characterized bylarge depletions in SO4 and Mg as compared with thechemical composition of seawater (Table 1). These de-pletions may be attributed to bacterial sulfate reduction(Kashiwagi et al., 2006; Muramatsu et al., 2010) and Mg-chloritization of initial neritic deposits in the KazusaGroup (Sudo, 1967), respectively. To the contrary, Na+

exchange with K+ and Ca2+ from Na–smectite may haveled to increase of K and Ca concentrations in the ground-water from the Odawara and Fujisawa wells (locations12 and 13). The water especially from the Odawara wellmust be the youngest weakly altered fossil seawater inthe high Na–Cl groundwaters because it has the nearestδ18O and δD values to seawater (Fig. 6a), along with adown flow profile of the borehole temperature and lowwater temperature (17.2°C).

The ∆ACa values of three Ca–Cl groundwaters indi-cate an inverse linear correlation with ∆Na values, hav-ing a similar magnitude of Ca enrichment and Na deple-tion (Fig. 9b). Because they have been formed by mixingof the high Na–Cl groundwaters (locations 12 and 13)and the LMW as previously mentioned, it confirms thatthe LMW was mixed with seawater intruded downwardalong N-S trending faults (Fig. 1) from the coastal areabordering on the Hiratsuka district. Afterwards, Na+ inthe produced moderate Na–Cl water exchanged with Ca–smectite layer of the mixed-layer chlorite–smectite in thepyroclastic rocks of the Tanzawa Group (Ishizaka et al.,1986) by an inverse of the reaction (4). This is supportedby the results of a column experiment with dilutedseawater displacing fresh water (Beekman and Appelo,1990). That is, Ca concentration in the groundwater hasbeen increased by Ca2+ exchange with injected Na+ andthe salinity effect, where cations must balance the in-creased Cl– during displacement of the fresh water by thebrackish water. This process has been observed in sev-eral clay bearing coastal aquifers affected by seawaterintrusion (Hafi, 1998; Gimenez et al., 2001; Vengosh etal., 2002; Bianchini et al., 2005).Bicarbonate-type waters The dilute Ca–HCO3 waters thatoccur in rivers and a domestic groundwater are plottedclose to the mHCO3 = 2 mCa in the plot of Ca versus HCO3concentrations, interpreted by dissolution of authigeniccalcite in the marine deposits of the Kazusa Group (Fig.3b), according to the reaction:

CaCO3 + CO2 + H2O → Ca2+ + 2HCO3–. (5)

Assuming Ca and HCO3 constituents are derived fromdissolution of calcite, the depleted Ca concentration inthe Na–HCO3 groundwaters (∆CCa) is calculated by thedifference between the Ca concentration measured in thegroundwater (in meq/L) and that expected from calcula-tion based on the dissolution of calcite. As illustrated in

Fig. 9a, the ∆Na versus ∆CCa concentrations in thegroundwaters exhibit a negative correlation with similarmagnitude of Na enrichment and Ca depletion. As smectitewidely occurs in the Yokohama well (Fig. 3b), it is attrib-uted to a cation exchange process between the Ca–HCO3groundwaters and Na–smectite in the marine deposits ofthe Kazusa Group by the reaction (4). Indeed, the Na–HCO3 groundwaters are supersaturated with respect toCa–smectite (Fig. 8b) and plot in the Ca–smectite regionof the (Na+/H+)–(Ca2+/H+2) activity diagram (Fig. 10).

CONCLUSIONS

Chemical and stable isotopic compositions of the riv-ers and groundwaters, mineral constituents of rock sam-ples from wells, and sulfur isotopic compositions ofanhydrite in the non-volcanic area of the Kanagawa andsoutheastern Yamanashi Prefectures were investigated toconstrain the chemical reaction and flow systems of thegroundwaters around the accreted IBM basin.

(1) Inside the accreted IBM basin where thegroundwaters are reserved in faults, the aquifer is re-charged by local rainwater and high density seawater. Theδ18O and δD values of the groundwaters confirm that thesulfate and high Na–Cl groundwaters are of meteoricwater and weakly altered fossil seawater origins, respec-tively. The δ34S value of anhydrite confirms that sulfur inthe SO4 rich groundwaters from the Tanzawa Mts. is ofanhydrite and gypsum origin. Considering water-mineralequilibria calculations for the sulfate-type groundwaters,they were formed by dissolution of the minerals in theGreen Tuff formations with rainwater infiltrated down-ward, followed by Ca2+ exchange with Na+ from the Na–smectite layer of the mixed-layer chlorite–smectite in theTanzawa Group. The Ca–Cl groundwaters were formedby mixing of fossil seawater with LMW, and Na+ ex-change on the Ca–smectite fraction of the aquifer.

(2) Outside the accreted IBM basin where thegroundwaters are reserved in permeable rocks of theMiura and Kazusa Groups, the shallow Na–HCO3groundwaters were formed by dissolution of authigeniccalcite in the Kazusa Group with LMW, and Ca2+ ex-change on Na–smectite in the Group. The deep moderateNa–Cl groundwaters were formed by mixing of the deepseated fossil seawater and the Na–HCO3 groundwater thatinfiltrated downward.

Acknowledgments—The authors thank Dr. M. Noto for sulfurisotopic analysis, and Dr. N. C. Sturchio for his constructivecomments on the manuscript.

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hydrochemistry of the groundwaters in the izu collision...

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