Geochemical Journal, Vol. 45, pp. 309 to 321, 2011
Hydrochemistry of the groundwaters in the Izu collision zone and 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 rock samples from wells, and δ34S values of anhydrite in the Izu collision zone and its adjacent eastern area, southern Kanto Plain, 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 calculated roughly 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 hydrogen
isotopic compositions reveal that the Ca·Na–SO4 groundwaters in the Tanzawa Mts. and the high Na–Cl groundwaters in the coastal area are of meteoric water and weakly altered fossil seawater origins, respectively. Sulfur in the SO4 rich groundwaters is derived from anhydrite and gypsum based on the sulfur isotopic compositions. The sulfate-type groundwaters were produced by the following process: the LMW infiltrated downward with dissolution of the sulfate 2+ minerals from hydrothermal veins in the Tanzawa Group, produced the Ca–SO4 groundwater as a result of Ca exchange partly on Na–smectite layer of mixed-layer chlorite–smectite in the sedimentary rocks of the Tanzawa Group. The Ca–Cl groundwaters in the eastern margin of the Tanzawa Mts. were produced by mixing of LMW with fossil seawater recharged from the surface of the coastal area, and Ca2+ exchange of the mixed-layer mineral in pyroclastic rocks of the Tanzawa Group.
Outside the accreted IBM basin, the Na–HCO3 groundwaters in the shallow aquifer were formed by dissolution of authigenic calcite with LMW, and Na+ exchange in the Kazusa Group. The moderate Na–Cl groundwaters in the deep
aquifer were formed by mixing of the deep seated fossil seawater with the Na–HCO3 waters in permeable sandstone and conglomerate of the Kazusa Group.
Keywords: Izu collision zone, groundwater, hydrochemistry, formation mechanism, recharge, anhydrite, cation exchange
tectonic difference between the two areas since the INTRODUCTION Miocene. Within non-volcanic area of the Kanagawa Prefecture, Drilling of thermal wells for hot-spring bathing pur- central Japan, the northern tip of the oceanic Philippine poses since the 1980’s was performed extensively on a Sea Plate is generally thought to have been colliding with deep thermal aquifer at the depths more than 1000 m in the continental Eurasian Plate at the northern margin of the non-volcanic area of the Kanagawa Prefecture. As a the Izu Peninsula. Intense Quaternary crustal movements, result, geological structure and hydrochemistry of the area such as active faulting with high slip-rates and rapid up- have been reported by many investigators (e.g., Imahashi lift or subsidence, occur along this inland plate boundary et al., 1996; Seki et al., 2001; Oyama et al., 1995; Ozawa (Yamazaki, 1992). The flow system of groundwaters in and Eto, 2005), while details of the water-rock reactions the accretion area of the Philippine Sea Plate may differ and flow systems of the groundwater in the area have not from those in the eastern non-volcanic area based on the been clearly described previously. In this paper, we present the results of chemical and stable isotopic (δD, δ18O, δ34S) compositions of rivers and groundwaters from *Corresponding author (e-mail: [email protected]) the wells in the non-volcanic area of the Kanagawa and Copyright © 2011 by The Geochemical Society of Japan. southeastern Yamanashi Prefectures, to constrain the flow
309 Kanto Mts N N
Sea of Japan
Study area Tono
Akiyamakawa fault ki m 500 –Aika Makime w fault a fault Doshigawa fault Pacific Ocean – Tanzawa Mts. Susugaya
1500 m
1000 m
3500 m
2000 m
2500 m Kannawa fault Tokyo Bay
Kozu fault – Mat 1000 m suda Subductio 1500 m Quaternary system 2000 m Volcanic rocks n in 15 Ma Neogene system Sag Sagami Bay Sedimentary rocks ami trough Volcanic rocks Granitic rocks Accreted IBM basin Palaeogene system 0 50 km Pre–Tertiary system
Fig. 1. Geological map of the southern Kanto Plain (after Hayashi et al., 2006). Thin-broken lines indicate counters of depth of upper boundaries of the pre-Neogene systems from sea level.
system of the groundwaters around the accretion area of Eto, 2005; Hayashi et al., 2006; Takahashi et al., 2006). the Philippine Sea Plate. The Kobotoke and Sagamiko Groups are correlated with the Shimanto Group. The Cretaceous Kobotoke Group composed of shale and sandstone is widely dis- OVERVIEW OF GEOLOGY tributed through the northwestern Tanzawa Mountains A simplified geological map of the southern Kanto (Mts.) to the Kanto Mts. The Paleaogene Sagamiko Group Plain located southwest of Tokyo, including the Kanagawa composed of sandstone, conglomerate, and mudstone and southeastern Yamanashi Prefectures, is shown in Fig. crops out in the southern side of the Kobotoke Group 1. The western part of the study area is topographically (Sakai, 1987) and occurs deeper than 1005 m below the situated at the Tanzawa Mountain land (maximum alti- surface at Ebina City (location 15 in Fig. 2; Ozawa and tude of 1673 m above sea level), and the eastern part at Eto, 2005). There are normal NE-SW trending faults, and the Sagamigawa low land (10 m above sea level), normal and reverse NW-SE trending faults in the Tanzawa Sagamihara platform (50 m above sea level), Tama hill Mts., and the Tonoki–Aikawa reverse fault runs along the (70 to 90 m above sea level), and Shimosueyoshi plat- boundary between the Sagamiko and Tanzawa Groups. form (40 to 60 m above sea level) in eastwardly order, The Miocene Tanzawa Group, which is equivalent to the while the Miura Peninsula is located in the southeastern Green Tuff formation, is mainly composed of dacitic part of the study area at the Miura hill (maximum alti- lapilli tuff, tuff-breccia, andesitic lapilli tuff and tude of 241 m above sea level). The geology of the study tuffaceous mudstone (Oka et al., 1979). It has been meta- area has been reported by many investigators, based on morphosed under conditions from zeolite to amphibolite geological and logging surveys obtained during drilling facies, which were produced by the intrusion of the of many deep thermal and seismic survey wells and seis- Tanzawa plutonic complex (K–Ar dating of 4.3 to 7.6 Ma; mic reflection surveys (e.g., Ishii, 1962; Omori et al., Kawano and Ueda, 1966) composed mainly of tonalite 1986; Suzuki, 2002; Kanagawa Pref., 2003; Ozawa and with minor gabbro (Seki et al., 1969). According to Zhang
310 Y. Muramatsu et al. N Tokyo Yamanashi Otsuki RTR
FTR Sagamihara 2 1 3 29 4 6 30 24 5 28 Yamanaka Ebina Yokohama 22 16 Lake 31 711 25 15 19 23 26 20 17 Shizuoka 21 Tokyo Bay 12 13 Hiratsuka Yokosuka Odawara Ca Na–SO4 18 Miura Na–SO4 Sagami Bay Peninsula Na–Cl Ca–Cl 27 14 TAF Na–HCO3 0 20 km Ca–HCO3
Fig. 2. Location of the water samples in the study area.
and Yoshimura (1997) and Zhang et al. (1997), the low Group is composed of tuffaceous sandstone and mudstone. grade metamorphic rocks of the southern Tanzawa Mts. The Miocene to early Pliocene Miura Group, composed can be divided into stilbite, laumontite, prehnite– of tuffaceous sandstone and mudstone, unconformably pumpellyite, epidote, epidote–amphibole and amphibole overlies the Hayama Group, and is exposed on the north- zones in increasing order of metamorphic grade. The ern, central and southern parts of the peninsula. There Miocene Aikawa Group equivalent to the Green Tuff for- are E-W trending normal faults in the peninsula (Omori mation, which is bordered on the Tanzawa Group by the et al., 1986). The Late Pliocene to Early Pleistocene Makime–Susugaya tectonic line, is mainly composed of Kazusa Group, composed of tuffaceous sandstone and andesitic lapilli tuff, mudstone and tuffaceous sandstone. sandy mudstone, unconformably lies on the Miura group. The Kannawa reverse fault runs along the boundary The Sagami Group consists of neritic, lacustrine and flu- between the Tanzawa and Ashigara Groups at the south- vial deposits. ern margin of the Tanzawa Mts. The Izu–Bonin–Mariana (IBM) arc of the Philippine Sea Plate is thought to be SAMPLING AND ANALYTICAL PROCEDURES bordered on the Honshu arc of the continental Eurasian Plate by the Kannawa and Kozu–Matsuda faults Cutting samples were collected at 5-m intervals from (Sugimura, 1972). The Pliocene–Pleistocene Ashigara the two wells (locations 6 and 16 in Fig. 2) for X-ray Group is mainly composed of sandstone, mudstone and powder diffraction analysis. We collected thirty-one sam- conglomerate. The accreted IBM basin (Soh et al., 1998) ples of river water and groundwater from wells drilled between the Tonoki–Aikawa fault and the Kannawa, for hot-spring bathing and domestic purposes in 2006– Kozu–Matsuda faults formed by accretion of the IBM arc 2007 (Fig. 2). Half of the groundwaters used for hot-spring crust onto the Honshu arc since middle Miocene in the bathing were collected from depths below 1000 m. Two western part of the field (Takahashi, 2006). kinds of the groundwaters from the different aquifers were Weathered pyroclastic air-fall deposits called the collected from both shallow and deep thermal wells at “Kanto Loam” cover the Sagami Group, in addition to three locations (three pairs of locations 3 and 4, 14 and the Miura, Kazusa and Sagami Groups in ascending or- 27, 16 and 24). der at the Yokohama and adjacent district (Mitsunashi and Groundwater samples from most wells were collected Kikuchi, 1982). Geology of the Miura Peninsula is di- after running for several minutes at the valve located near vided into the Hayama, Miura, Kazusa and Sagami Groups the well head, except for wells at a faucet of a bathtub or (Eto et al., 1998). The Early to Middle Miocene Hayama the entrance of a reserve tank. Temperature, conductivity
Hydrochemistry of groundwaters in the Izu collision zone 311 (a) Tsukui well (b) Yokohama well Primary minerals Secondary minerals Primary minerals Secondary minerals
site tz
uifer
Gypsum Ch/Smectite
Anhydrite
Magnetite Calcite
Quar Chlorite (Ch) Chlorite Laumontite Prehnite
Kaolinite Aquifer Plagioclase Aq Halloy Smectite
Lithology Depth (m) Depth Plagioclase
Lithology Quartz Depth (m) Depth 0 0
200 200
Loam
400 400
600 600
800 Group Tanzawa 800
Kazusa Group
1000 1000
1200 1200
TPC
1310 m G. Green tuff Quartz diorite Loam 1400
Mudstone Sandstone Sandstone/Mudstone Miura 1500 m Conglomerate Casing pipe
Fig. 3. Stratigraphy, aquifer and distribution of minerals with depth in the Tsukui (a) and Yokohama (b) wells. TPC, Tanzawa plutonic complex. Aquifer below depth of casing pipe is shown in the figure. Solid and dashed lines indicate that minerals are abundant and minor, respectively.
and pH were directly measured using a standard hand- water at room temperature. The aqueous SO4 was pre- held calibrated field meter. Total alkalinity was measured cipitated as BaSO4, and the sulfur isotope analysis was by potentiometric titration with sulphuric acid to a final performed by an IsoPrime-EA mass spectrometer using pH of 4.8. Chloride, SO4, Br, F, PO4, Na, K, Ca and Mg about 0.1 mg BaSO4 mixed with 10 mg of V2O5. The shift δ34 were measured by ion chromatography, and Al, SiO2 by of raw S values during measurement was corrected visible spectrophotometry. using measured values of the BaSO4 laboratory standard. Hydrogen and oxygen isotopic compositions, reported Sulfur isotopic composition is expressed in terms of δ34S in terms of δD and δ18O (‰), relative to V-SMOW stand- (‰), relative to the Canyon Diablo Troilite (CDT) stand- ard were measured by an IsoPrime-EA mass spectrometer ard. Analytical precision is ±0.2‰. connected on-line to a gas chromatograph. After decom- Activities of various ions and chemical equilibria for ° posing a sample by heating at 1050 C for H2 analysis and relevant minerals were calculated using the SOLVEQ 1260°C for CO analysis in an oxygen free environment, computer programs (Reed, 1982). the product gases are chromatographically separated, and afterwards introduced to the ion source of the mass RESULTS AND DISCUSSION spectrometer. Analytical precisions are ±0.1‰ for oxy- gen and ±1.0‰ for hydrogen. Sulfur isotopic composi- Secondary minerals in aquifer tion was analyzed for three groundwaters and one The aquifer is divided into fault and rock facies types anhydrite. Powdered anhydrite was dissolved into pure based on the geological structure (Ozawa and Eto, 2005).
312 Y. Muramatsu et al. )
S
δ
)(
O
8.69 9.95
9.31
9.60 9.69 1.35 9.24
9.07 9.96 +17.0 2.20
2.24
8.52 8.57 3.54 8.06 9.39
9.10
8.38 8.31 8.05
4.30 7.12
7.69 8.57
7.43
8.47 +21.7 6.75
0.98 δ
10.64
10.41
10.27 +19.1
−
−
− − − −
− − −
− −
−
−
− − − − − − −
− − −
− − − −
−
−
− −
)(
D
7.9 δ
69.2 53.5
56.5
60.4 58.6
56.8 11.1
62.8 55.9
62.5 10.9
62.8 51.4 65.8
55.9 50.1 26.7 53.0 58.4 47.8
30.7 50.7 49.0
40.6
45.1 52.4
44.4
53.7 40.3
10.3
−
−
−
− − − − −
− − −
− − −
− − − − − − −
− − −
−
−
−
− − −
−
2
W, River water. W,
44.5
21.3
33.3 17.7
21.7
38.5 12.3 26.2 88.8
18.6 79.3 22.9
24.1 18.3 73.8 17.9 47.1 14.6
73.8 36.2 44.0
49.8
36.7
79.3
17.5 23.1 15.3
17.7
SiO
4
12.3 50.9
<0.1
<0.1
<0.1 <0.1
<0.1 <0.1 <0.1 <0.1
<0.1 <0.1
<0.1 <0.1 <0.1 <0.1
espectively.
Br F PO
<0.1 <0.1 <0.1
<0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 0.2 <0.1
<0.1 <0.1 <0.1 8.4
<0.1 <0.1 <0.1
<0.1 <0.1 <0.1
<0.1 <0.1 <0.1 <0.1 <0.1 <0.1
<0.1 <0.1 <0.1
<0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 3.7 <0.1 <0.1 <0.1
<0.1 0.6
<0.1 <0.1 <0.1 <0.1 <0.1 <0.1
<0.1 <0.1 <0.1
<0.1 <0.1 <0.1
<0.1 <0.1 <0.1
3
TR, Rock facies type aquifer; R
363.0
260.0
171.0 370.0
561.0 1.6 689.0
HCO
4
<0.1
<0.1
<0.1
<0.1 <0.1
0.8 <0.1 2.0 <0.1 <0.1 <0.1
57.2 2410 15.6
17.6 38.7 114.0
23.2
153.5 678.6 22.0
6579
5204
2086
5001 202.4 13.0
8024 23.1 228.0 23.1 2.1
20097 2381 183.0 1.7
19514 1712 118.0 105.0
20419 1.9 201.0 25.3
19750 2690 150.0 67.5
<0.1
<0.1
<0.1
<0.1
<0.1
<0.1 5.2 <0.1 <0.1
<0.1
<0.1
<0.1
<0.1 <0.1
<0.1 <0.1
129.6
610.2
eserve tank and geothermal gradient calculated using it, r
<0.1
<0.1
<0.1 2.9 0.2 0.2
48.1 2.6 23.2 3.2 2.3 36.6 117.2 25.0
82.5
g/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (
212.2 0.0 21.7 4.7
1371 54.0 61.2 5.9
11000 391.0 410.0 1387
25.2 39.0 12.3 12.9 2.4 5.0 15.7 65.0 58.0
15.2 4122 173.4 86.5 18.7
214.0 200.0 16.6 408.0 14.1 8.2 5.0 1569 9.0
276.0 336.4 24.6 588.8 6.1 4.0 9.9 2052 12.0
240.0 343.9 20.8 230.6 10.5 3.5 51.2 1323 5.0
8.2 18.3 21.6 3.7 30.6 5.0 4.0 5.7 34.4 61.0 7.8 10.3 2.6 0.5 14.4 2.9 3.6 0.7 6.2 49.0
8.1 11.2 4.3 0.5 17.4 2.6 4.4 1.7 9.4 47.0
6.4 0.2 0.1 0.1 0.1 0.1
8.4
8.2 15.7 37.8 6.9 16.4 2.5 4.4 10.5 20.2 50.0
HCa CaMgAlClSO pHECNaK
3
om the southern Kanto Plain
TG*
C/100 m) (mS/cm) (m
°
2
e; TG, Geothermal gradient; EC, Electric conductivity; FTR, Fault type aquifer; R
C) (
°
(34.7) (1.3) 9.6 2.4
(26.8) 9.7
(21.6) (1.0) 8.3
(21.2) (0.4) 9.3 (30.0) (1.0) 8.7
(39.1) (1.8) 7.6
(27.7) (2.0) 9.7 0.3
(41.0) (1.7) 8.0 6.0 (19.6) (1.3) 8.6 1.0
e measured at a faucet of bathtub or entrance of r e measured
Date WT*
6/18/2007 37.3 6.8 9.9 102.6 193.0 10.1 42.3 3.3 4.0 61.6 392.5 51.0 <0.1 1.9 1.5 48.0
6/18/2007 33.2 6.2 9.7 8.4 119.3 2.8 30.3 0.7 4.1 21.8 284.2 38.0 6/18/2007
6/18/2007 34.1 15.910/17/2006 17.2 9.9 0.2 49.3 7.6 88.3 45.2 14.3 10298 8.2 620.5 2.0 755.9 1150 3.6 24.2 194.6 34.0 <0.1
6/18/2007 30.3 4.9 9.5 5.0 86.3 2.8 12.6 1.8 3.3 15.8 190.8 23.0
5/10/2006 10/16/2006 32.0 2.0 8.0 3.3 725.1 22.2 68.8 1.0 0.1 1160 19.9 262.0 47.9
5/29/2006 36.2 4.1 9.1 12.2 954.0 7.8 1861 7.0
1
0 7/2/2007 15.2 0 7/26/2007 15.8
0 3/4/1988
0 7/2/2007 15.6
(m) (
1500 7/26/2007
1301 5/29/2006
1000 7/2/2007 28.7 1.3 9.6 291.0 300.5 8.8 397.5 7.7 3.5 777.7 669.8 35.0 <0.1 3.5
1301 10/17/2006 30.1 1.1 8.8 17.7 2437 85.8 2393 0.0
4
water*
Sea
groundwater
4
water
water
3
3
groundwater
SO
4
−
Parenthesis show water temperatur Parenthesis
HCO
SO
Cl groundwater Cl
HCO
Cl groundwater
3
Na
−
−
−
−
⋅
−
*
1 Tsuru FTR 2 Tsukui 1 FTR 1400 10/23/2006 31.0 1.1 9.7 1.6 277.2 9 Nakagawa 3 FTR 320
7 Nakagawa 1 FTR 286 8 Nakagawa 2 FTR 106
3 Minamitsuru 1 FTR 600 7/26/2007
4 Minamitsuru 1 FTR 1500 7/26/2007
6 Tsukui 2 FTR 1310 10/23/2006 37.8 1.7 9.1 3.6 548.8
5 Minamitsuru 2 FTR 1500 7/26/2007
Total depth. Total
Data from Imahashi et al. (1996). Data from
19 Isehara FTR 20 Hadano FTR 500
21 Hiratsuka FTR
10 Nakagawa 412 FTR 116 Odawara FTR 804
11 Nakagawa 513 FTR 300 Fujisawa FTR 1500 10/16/2006 33.4 1.2 7.4 43.7 10549 601.2 999.9 739.8
14 Miura 1 FTR 1500 10/16/2006 26.7 0.7 7.8 41.9 10843 470.9 454.4 376.2
22 Atsugi 1 FTR 200 7/2/2007 18.2 1.3 10.3 19.4 29.7 2.2 8.8 1.0 3.8 19.2 32.4 47.0 15 Ebina RTR 16 Yokohama 1 RTR 1500 5/15/2006 43.8 1.9 7.6 13.5 3196 106.4 200.7 29.9
30 Shiokawa 3 23 Atsugi 2 FTR 600 8/18/2006 31 Minamitsuru 2 24 Yokohama 1 RTR 300 5/15/2006 20.732 1.7 Rainfall 8.4 3.3 39.8 0 9.6 9/18/2006 20.8 4.9 29 Shiokawa 2 FTR 10 7/2/2007 15.8
17 Yokohama 2 RTR 1507 5/10/2006 25 Yokohama 2 RTR 300 18 Yokosuka RTR 802 26 Yokohama 3 RTR 200 5/15/2006 18.7 1.6 8.4 1.3 354.4 28.9 26.1 5.7 0.2 34.3 27 Miura 1 RTR 300 10/16/2006 17.9 0.8 8.6 0.4 69.4 10.5 12.9 3.3 0.2 59.1 14.6 126.0
28 Shiokawa 1
1 2, 4
No. Location RT Depth*
Ca
Na
Na
Ca
Ca
Na
Table 1. Chemical composition of water samples fr Table
RT, Aquifer type; WT, Discharge water temperatur Discharge Aquifer type; WT, RT, * *
*
Hydrochemistry of groundwaters in the Izu collision zone 313 Fig. 4. Trilinear diagram of the water samples. Numbers refer to location numbers in Table 1.
The fault type aquifer (FTR) where the groundwater flows In contrast, the rock facies type aquifer (RTR), where along faults is distributed in the accreted IBM basin from the groundwater flows horizontally along permeable lay- the Tanzawa Mts. to the southern Miura Peninsula (Fig. ers such as sandstone and basal conglomerate in the 2). The Miocene sedimentary and plutonic rocks are dis- Kazusa and Miura Groups (Ozawa and Eto, 2005), is dis- rupted by northeast-trending normal faults and northwest- tributed in the eastern area adjacent to the accreted IBM trending normal and reverse faults in the Tanzawa Mts. basin from the Yokohama district to the northern Miura (Oki et al., 1967; Omori et al., 1986). Figure 3a shows Peninsula. Figure 3b shows the distribution of primary the distribution of primary and secondary minerals with and secondary minerals with depth in the Yokohama well depth in the Tsukui well (location 6 in Fig. 2) located the (location 16 in Fig. 2) as a representative well in the RTR western Tanzawa Mts. as a representative well at the FTR area. The groundwater is reserved in the basal conglom- area. The groundwater is reserved near the intersection erate layer of the Kazusa Group from 1215 to 1270 m of these faults in the Tanzawa plutonic complex at 1290 depth (Ozawa and Eto, 2005). Kaolinite, smectite and m depth (Oyama et al., 1995). Anhydrite and gypsum calcite occur widely as minor secondary minerals in the occur widely as minor secondary minerals accompanied Kazusa and Miura Groups. with laumontite and prehnite in the Tanzawa Group and Tanzawa plutonic complex. Clay mineral is a random Groundwater quality and its geographical distribution mixed-layer chlorite/smectite, which is identified by a The analytical results and the seawater composition broad peak, non-integral spacing of 001 reflection and a from the Kamogawa offshore, Boso Peninsula, Chiba systematic response of d001 to the air-dried and ethylene– (Imahashi et al., 1996) are listed in Table 1. The chemi- glycol treatments. cal compositions of the waters described in terms of rela-
314 Y. Muramatsu et al. tive concentrations of ions allow us to distinguish the 20 following types of waters (Fig. 4): (1) Sulfate-type groundwaters: Compositions vary 0 12 from Ca·Na–SO to Na–SO . The Ca·Na–SO waters 21 4 4 4 13,14 found in the western Tanzawa Mts. show temperatures –20 ° 15 from 21.2 to 37.8 C, pH values between 8.3 and 9.7 and 16 sulfate concentrations between 117 and 2410 mg/L. The –40 Na–SO waters from shallow depths in the Nakagawa 4 –60 district, central Tanzawa Mts., show high pH values be- Ca Na–SO4 Na–HCO3 tween 9.5 and 9.9, and sulfate concentration between 65 Na–SO4 Ca–HCO3 –80 Na–Cl Seawater and 393 mg/L. LMWL Ca–Cl Fossil seawater (2) Chloride-type groundwaters: Compositions vary –100 from Na–Cl to Ca–Cl. The seawater like high Na–Cl wa- –16 –14 –12 –10 –8 –6 –4 –20246 ters with Cl concentrations of approximately 20 g/L and temperatures from 17.2 to 33.4°C are found in the coastal area bordering Sagami Bay to southwestern Tokyo Bay. 20 The moderate Na–Cl waters with Cl concentrations from 1160 to 6579 mg/L and temperatures from 32.0 to 43.8°C 0 21 13 12 are found in the Yokohama district (including Ebina City) 14 –20 ML1 and Miura Peninsula. The Ca–Cl waters with moderate 16 15 17 Cl concentrations from 778 to 8024 mg/L are found in 18 –40 ML2 the eastern margin of the Tanzawa Mts. and Hiratsuka 27 20 ML3 district. –60 19
(3) Bicarbonate-type waters: Compositions vary from Ca Na–SO4 Na–HCO3 Na–HCO3 to Ca–HCO3, and location 22 belonging to Na– Na–SO4 Ca–HCO3 –80 Na–Cl Seawater HCO3·SO4·Cl water is included in this type. The Na– Ca–Cl Fossil seawater HCO3 waters found in the shallow depths of the Yokohama –100 district and Miura Peninsula show relatively low tempera- 0 5000 10000 15000 20000 25000 tures from 17.9 to 20.7°C and narrow pH values between 8.4 and 8.6. They are also found in the shallow to middle Cl (mg/L) depths of the eastern margin of the Tanzawa Mts. having Fig. 5. Relationships between δ18O and δD values and between temperatures from 18.2 to 27.7°C and high pH values Cl concentrations (mg/L) and δD values of the water samples. δ between 9.7 and 10.3. In contrast, the dilute Ca–HCO3 The solid line (LMWL) is the local meteoric water line ( D = waters have low temperatures from 15.2 to 15.8°C and 8δ18O + 16). The ML1 and ML3 show the mixing of fossil pH values between 7.8 and 8.2 in the river waters and seawater and meteoric water for the Na–Cl groundwaters of groundwater above 10 m depth in the Tanzawa Mts. They the Yokohama district and Miura Peninsula, respectively, and the ML2 shows the mixing of these waters for the Ca–Cl are the most widespread water-type discharged from shal- groundwaters of the eastern margin of the Tanzawa Mts. and low wells and springs in the Kanagawa Prefecture Hiratsuka district. (Ishizaka et al., 1993).
Mixing of fossil seawater with local meteoric water The stable isotopic compositions of the groundwaters –7.0‰) from the Adachi well drilled to the Kazusa Group are shown in Table 1. The δ18O values range from –10.64 around the northern end of Tokyo Bay where a large quan- to –0.98‰, and δD values from –69.2 to –7.9‰. Because tity of fossil seawater is reserved in the non-volcanic cen- the groundwaters except for six chloride-types plot close tral Kanto Plain (Muramatsu et al., 2008). to the local meteoric water line (LMWL; δD = 8δ18O +16) Dilution of the chloride-type waters is confirmed by in the δ18O versus δD diagram (Fig. 5a), they are of me- the plots of δD versus Cl (Fig. 5b) and δ18O versus Cl. teoric origin. High Na–Cl groundwaters (locations 12, 13 The moderate Na–Cl groundwaters (locations 15 to 17) and 14) with Cl concentrations of approximately 20 g/L from the Yokohama district (including Ebina City) in the have δ18O and δD values slightly lower than those of RTR area are plotted on the mixing line (ML 1) of the seawater, suggesting that they are fossil seawaters with a high Na–Cl groundwaters (locations 12 and 13) and the weak water-rock reaction. The δ18O and δD values of the LMW (δ18O = –6.68‰ and δD = –42.3‰), confirming high Na–Cl groundwater from the Odawara well (loca- that they have been formed by mixing of these waters tion 12) are similar to those (δ18O = –2.4 and δD = (Fig. 5b). Based on the same reason, the moderate Ca–Cl
Hydrochemistry of groundwaters in the Izu collision zone 315 N N
m RTR Tokyo 1 Yamanashi 0.71
Yamanashi Tokyo 1500 m 1000 Otsuki RTR
Sagamihara 1 FTR FTR m 500 0.70 (1.3) 1.1 0.01 (1.0) 0.00 0.00 1.7 2 1.7 (0.4) 0.00 0.00 0.53 (1.0) 1.5 0.00 0.27 Yamanaka 6.8 1.3 Ebina 1.9 Yokohama 0.33 0.00 0.5 Lake 4.9 6.2 (1.8) (1.3) 1.5 15.9 1.3 (2.0) 0.04 1.6 4.1 0.25 0.11 (1.7) 1.5 Tokyo Bay 1.1 0.01 Shizuoka 0.2 Shizuoka 0.41 Tokyo Bay Yokosuka 1.2 1.02 0.99 Odawara 0.06 2.1 0.5 Ca Na–SO4 Sagami Bay Na–SO4 0.8 Ca Na–SO4 Sagami Bay 1.5 1.03 Na–Cl 0.9 Na–Cl TAF Ca–Cl 0 20 km TAF Ca–Cl 0 20 km Na–HCO3 Na–HCO3
Fig. 6. Two-dimensional geothermal gradient distribution. FTR Fig. 7. Two-dimensional seawater fraction distribution of the and RTR show fault type and rock facies type aquifers, respec- deep groundwater samples. *1Data from Muramatsu et al. tively. Parenthesis shows geothermal gradient calculated us- (2008). *2Data from Awaya et al. (2002). Dashed line shows ing water temperature measured at a faucet of bathtub or en- the counter line of basal boundaries of the Kazusa Group trance of reserve tank. (Suzuki, 2002). Depths are shown from sea level. FTR and RTR show fault type and rock facies type aquifers, respectively.
groundwaters (locations 19 and 20) from the eastern mar- bottomhole depth (Muramatsu et al., 2008). The tempera- gin of the Tanzawa Mts. and Hiratsuka district in the FTR ture gradient below 900 m depth calculated from the equi- area seems to have been formed by mixing (ML2) be- librium temperature estimated from the borehole tempera- tween the high Na–Cl groundwaters (locations 12 and 13) ture logging data using the conventional Horner method and the LMW (δ18O = –8.64‰ and δD = –53.7‰; Fig. (Fertl and Wichmann, 1977) is approximately 2.5°C/100 5b). m, whereas the gradient estimated from the discharge tem- The water circulation system in the Miura Peninsula perature is 1.9°C/100 m (Table 1). The small temperature is independent from that of its adjacent Yokohama dis- difference (0.6°C/100 m) reveals that the geothermal gra- trict. As the moderate Na–Cl groundwater from the dient calculated by using water temperature measurements Yokosuka well (location 18) is perfectly plotted on the is possible to use roughly for characterization of mixing line (ML3) of the high Na–Cl groundwater from geothermal activity. Assuming that surface and the Miura well (location 14) and Na–HCO3 groundwater bottomhole temperatures are equal to the mean tempera- from the Miura well (location 27), it seems to have been ture (15.6°C) of the river water (locations 28, 30, 31) and formed by mixing of the fossil seawater and the LMW the discharge temperature in a well, respectively, infiltrating to depth at the Miura hill. geothermal gradients of the study wells are roughly shown in Table 1. Recharge system estimated from geothermal gradient and Compared with the geothermal gradients (1.6 to 2.1°C/ seawater fraction 100 m) of the RTR area, the FTR area has low values As the deep fluid temperature gradually decreases with from 0.2 to 1.3°C/100 m except for some wells including upward flow in a well due to heat loss, the aquifer tem- the Nakagawa (Fig. 6) as also reported by Kikugawa et perature corresponding to the deep fluid temperature at a al. (2007), indicating that recharge water has regionally feed point is higher than the discharge water tempera- reached to the depths in this area. The lowest gradient ture. Because of this phenomenon, several chemical (0.2°C/100 m) for the Odawara well (location 12) can be geothermometers are usually used to estimate the aquifer attributed to the down flow of seawater from the surface. temperature of geothermal fields (Fournier, 1991). How- The Holocene sediments composed of the permeable al- ever, the aquifer temperature at a feed point is reflected luvial gravels are distributed over the full depth, and prob- in discharge temperature better than the silica ably continue to 1400 to 1800 m depth in the well geothermometer in the non-volcanic central Kanto Plain (Kanagawa Pref., 2002; Itadera et al., 2004). Consider- (Muramatsu et al., 2008). Based on the borehole tempera- ing the stratigraphic data, recharge must have been per- ture logging of the Yokohama well (location 16), the tem- formed by downward plunging of cold high density fos- perature with a conductive profile is 52°C at 1500 m sil seawater along the Kozu–Matsuda fault (Fig. 1) in the
316 Y. Muramatsu et al. Odawara district. 10 Ca Na–SO The fraction of seawater in mixing water of seawater (a) Anhydrite 4 Na–SO4 and LMW is given by: 5 Na–Cl Ca–Cl Supersaturated Na–HCO3 0 mm− Ca–HCO3 f = Cl,, sample Cl fresh .1() sea − mmCl,, sea Cl fresh –5 Log Q
–10 Where, fsea is the fraction of seawater in the mixed water, mCl is the Cl concentration, and the subscripts sample, Undersaturated sea and fresh represent the groundwater, the seawater and –15 LMW as end-members, respectively. The m and Cl,sea –20 mCl,fresh are derived from the data of the Kamogawa off- shore, Boso Peninsula (Imahashi et al., 1996) and 20 (b) Ca–smectite Ca Na–SO4
Matsudo City, Chiba (Muramatsu et al., 2008), respec- Na–SO4 tively. A contour map of the seawater fraction in the 15 Na–Cl Supersaturated Ca–Cl groundwaters from the wells deeper than 500 m is pre- Na–HCO 10 3 sented in Fig. 7. Within the FTR area, the waters with Ca–HCO3 highest seawater fractions of above 0.9 are located in the 5
coastal area bordering on the Sagami bay to the southern Log Q Miura Peninsula, while its fractions are nearly zero in 0 the Tanzawa Mts. In conclusion, the aquifer may be re- charged by high density seawater and local rainwater in –5 Undersaturated the accreted IBM basin. The geothermal gradient has exceptionally high val- –10 ° ues from 4.9 to 15.9 C/100 m for the Nakagawa wells 10 20 30 40 50 60 (locations 7 to 11) in the accreted IBM basin, probably suggesting the residual heat source from the Tanzawa plu- tonic complex (Oki et al., 1967). Slightly high value for Fig. 8. Water temperature versus log activity product for the Tsukui well (location 6; 1.7°C/100 m) must be also anhydrite (a) and Ca–smectite (b). interpreted by the same reason because of the occurrence of the plutonic rock in the deeper part of the well (Fig. 3a). As illustrated in Fig. 6, the geothermal gradients rang- Formation mechanisms of three groundwaters ing from 1.6 to 2.1°C/100 m in the RTR area are similar Because many groundwaters are derived from mixing to those (2.0 to 2.2°C/100 m) in the non-volcanic area, of seawater with LMW as mentioned above, an evalua- Kanto Plain (Uchida et al., 2002). According to tion of the extent of M element relative to a theoretical Muramatsu et al. (2008), a large quantity of fossil seawater encroachment can be obtained through calcula- seawater is reserved in the Kazusa Group around the tion of ∆M indices: northern end of Tokyo Bay. Adding chemical composi- ∆ × tions of the high Na–Cl waters from three thermal wells [M] = [M] – [M/Cl]sea [Cl] (2) around the Tokyo–Kanagawa prefectural border (Awaya et al., 2002; Muramatsu et al., 2008), the seawater frac- where ∆[M] is the difference between the M concentra- tion in the waters is inclined to increase from southwest tion measured in groundwater (in meq/L) and that ex- to northeast within the RTR area in the Kazusa Group pected from calculation based on the seawater M/Cl ra- (Fig. 7). Considering increase of the depth of a basal tio. δ34 boundary of the Kazusa Group in the direction of the Sulfate-type groundwaters The S values of three SO4 northern end of Tokyo Bay (Suzuki, 2002; Odawara, rich waters in the Tanzawa Mts. ranges from +17.0 to 2– 2008), the LMW recharged probably from the direction +21.7‰ (Table 1), confirming that sulfur in SO4 de- of the Tanzawa Mts. seems to flow along permeable rocks rives from dissolutions of anhydrite and gypsum from in the Kazusa Group northeastward, then gradually mix, hydrothermal veins in the Tanzawa Group because of its in variable proportions, with the deep seated fossil similar value to anhydrite cuttings from the Tsukui well seawater in the Kazusa Group (Omori et al., 1986; (+20.0‰). The water temperatures versus activity prod- Muramatsu et al., 2008). ucts for anhydrite are plotted in Fig. 8a, and a similar
Hydrochemistry of groundwaters in the Izu collision zone 317 0 20 (a) –5 Ca–smectite 18 –10 Ca (meq/L) ) C +2
H 16 –15 /a Na–smectite 2+ Ca
Ca and Ca Na–SO4 A –20 Na–SO4 14
Log (a Ca Na–SO4 Na–HCO3 –25 Na–SO4 Na–Cl 0 5 10 15 20 25 12 Ca–Cl
Na (meq/L) Na–HCO3 Ca–HCO 140 3 10 120 (b) 0 2 4 6 8 10 12 14 16
100 Log (aNa+/aH+)
80 Fig. 10. Stability fields of Ca–smectite and Na–smectite as a function of log (a +/a +) and log (a 2+/a +2) at 25 and 60°C, 60 Na H Ca H Ca (meq/L)
A and 1 atmosphere. Thermodynamic data are from Helgeson 40 (1969).
20 Ca–Cl 0 ∆ where A[Ca] is the difference between the enriched Ca –140 –120 –100 –80 –60 –40 –20 0 concentration with respect to seawater in groundwater (in Na (meq/L) meq/L) and that expected from calculation based on anhydrite Ca/SO ratio (1.0) using the enriched SO con- ∆ ∆ 4 4 Fig. 9. Na versus CCa concentrations in the Na–HCO3 centration with respect to seawater. As illustrated in Fig. ∆ groundwaters (a), and ACa in the sulfate groundwaters (a) 9a, the values of ∆Na versus ∆ Ca in the sulfate-type and Ca–Cl groundwaters (b). See text for ∆Na, ∆ Ca and ∆ Ca A C A groundwaters exhibit a significant negative correlation calculations. with similar magnitude (stoichiometrically balanced) of Na enrichment and Ca depletion. Based on the wide dis- tribution of mixed-layer chlorite–smectite in the Tsukui result is also obtained for gypsum. The sulfate-type and well (Fig. 3a) and at the surface of southern Tanzawa Mts. dilute Ca–HCO3 waters are saturated and slightly (Zhang and Yoshimura, 1997; Zhang et al., 1997), the undersaturated with the minerals, respectively. The re- most probable explanation for the Ca depletion of the sult reveals that the chemical composition of the shallow sulfate-type groundwaters is Ca2+ exchange with Na+ from dilute Ca–HCO3 waters recharged during infiltration along Na–smectite layer of the mineral in the sedimentary rocks faults into the deeper level of the Tanzawa Group in the of the Tanzawa Group by the following reaction: Tanzawa Mts. must be modified by dissolution of 2+ anhydrite and gypsum in the Tanzawa Group and Tanzawa 6Na0.33Al2.33Si3.67O10(OH)2 + Ca → + plutonic complex (Fig. 3a) to produce the sulfate-type 6Ca0.167Al2.33Si3.67O10(OH)2 + 2Na . (4) groundwaters saturated with these minerals. In spite of dissolution of anhydrite and gypsum, the This hypothesis is supported by the mineral equilibrium Ca/SO4 mole ratios (0.2 to 0.6) in the sulfate-type calculation results; the sulfate-type groundwaters are su- groundwaters are significantly lower than that of anhydrite persaturated with respect to Ca–smectite (Fig. 8b), and + + 2+ +2 (1.0). Assuming that Ca and SO4 in the groundwaters were plot in the Ca–smectite region of the (Na /H )–(Ca /H ) derived from dissolution of sulfate minerals, the depleted activity diagram (Fig. 10). Ca concentration is calculated by the following equation: Chloride-type groundwaters Within the high Na–Cl groundwaters from three wells, the chemical composi- ∆ ∆ × ∆ A[Ca] = [Ca] – [Ca/SO4]anhydrite [SO4] (3) tion of groundwater from the Miura well (location 14) is similar to that of the fossil seawater from the Funabashi
318 Y. Muramatsu et al. ∆ ∆ well (Muramatsu et al., 2008) which is characterized by Fig. 9a, the Na versus CCa concentrations in the large depletions in SO4 and Mg as compared with the groundwaters exhibit a negative correlation with similar chemical composition of seawater (Table 1). These de- magnitude of Na enrichment and Ca depletion. As smectite pletions may be attributed to bacterial sulfate reduction widely occurs in the Yokohama well (Fig. 3b), it is attrib- (Kashiwagi et al., 2006; Muramatsu et al., 2010) and Mg- uted to a cation exchange process between the Ca–HCO3 chloritization of initial neritic deposits in the Kazusa groundwaters and Na–smectite in the marine deposits of Group (Sudo, 1967), respectively. To the contrary, Na+ the Kazusa Group by the reaction (4). Indeed, the Na– + 2+ exchange with K and Ca from Na–smectite may have HCO3 groundwaters are supersaturated with respect to led to increase of K and Ca concentrations in the ground- Ca–smectite (Fig. 8b) and plot in the Ca–smectite region water from the Odawara and Fujisawa wells (locations of the (Na+/H+)–(Ca2+/H+2) activity diagram (Fig. 10). 12 and 13). The water especially from the Odawara well must be the youngest weakly altered fossil seawater in CONCLUSIONS the high Na–Cl groundwaters because it has the nearest δ18O and δD values to seawater (Fig. 6a), along with a Chemical and stable isotopic compositions of the riv- down flow profile of the borehole temperature and low ers and groundwaters, mineral constituents of rock sam- water temperature (17.2°C). ples from wells, and sulfur isotopic compositions of ∆ The ACa values of three Ca–Cl groundwaters indi- anhydrite in the non-volcanic area of the Kanagawa and cate an inverse linear correlation with ∆Na values, hav- southeastern Yamanashi Prefectures were investigated to ing a similar magnitude of Ca enrichment and Na deple- constrain the chemical reaction and flow systems of the tion (Fig. 9b). Because they have been formed by mixing groundwaters around the accreted IBM basin. of the high Na–Cl groundwaters (locations 12 and 13) (1) Inside the accreted IBM basin where the and the LMW as previously mentioned, it confirms that groundwaters are reserved in faults, the aquifer is re- the LMW was mixed with seawater intruded downward charged by local rainwater and high density seawater. The along N-S trending faults (Fig. 1) from the coastal area δ18O and δD values of the groundwaters confirm that the bordering on the Hiratsuka district. Afterwards, Na+ in sulfate and high Na–Cl groundwaters are of meteoric the produced moderate Na–Cl water exchanged with Ca– water and weakly altered fossil seawater origins, respec- smectite layer of the mixed-layer chlorite–smectite in the tively. The δ34S value of anhydrite confirms that sulfur in pyroclastic rocks of the Tanzawa Group (Ishizaka et al., the SO4 rich groundwaters from the Tanzawa Mts. is of 1986) by an inverse of the reaction (4). This is supported anhydrite and gypsum origin. Considering water-mineral by the results of a column experiment with diluted equilibria calculations for the sulfate-type groundwaters, seawater displacing fresh water (Beekman and Appelo, they were formed by dissolution of the minerals in the 1990). That is, Ca concentration in the groundwater has Green Tuff formations with rainwater infiltrated down- been increased by Ca2+ exchange with injected Na+ and ward, followed by Ca2+ exchange with Na+ from the Na– the salinity effect, where cations must balance the in- smectite layer of the mixed-layer chlorite–smectite in the creased Cl– during displacement of the fresh water by the Tanzawa Group. The Ca–Cl groundwaters were formed brackish water. This process has been observed in sev- by mixing of fossil seawater with LMW, and Na+ ex- eral clay bearing coastal aquifers affected by seawater change on the Ca–smectite fraction of the aquifer. intrusion (Hafi, 1998; Gimenez et al., 2001; Vengosh et (2) Outside the accreted IBM basin where the al., 2002; Bianchini et al., 2005). groundwaters are reserved in permeable rocks of the Bicarbonate-type waters The dilute Ca–HCO3 waters that Miura and Kazusa Groups, the shallow Na–HCO3 occur in rivers and a domestic groundwater are plotted groundwaters were formed by dissolution of authigenic 2+ close to the mHCO3 = 2 mCa in the plot of Ca versus HCO3 calcite in the Kazusa Group with LMW, and Ca ex- concentrations, interpreted by dissolution of authigenic change on Na–smectite in the Group. 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