Geochemical Journal, Vol. 42, pp. 61 to 74, 2008
Relationship between geological structure and helium isotopes in deep groundwater from the Osaka Basin: Application to deep groundwater hydrology NORITOSHI M ORIKAWA ,1* KOHEI KAZAHAYA ,1 HARUE MASUDA,2 M ICHIKO OHWADA,1 ATSUKO NAKAMA,1 KEISUKE NAGAO3 and H IROCHIKA S UMINO3 1
Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, AIST Tsukuba Central 7, Higashi 1-1-1, Tsukuba, 305-8567, Japan 2 Department of Geoscience, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan 3 Laboratory for Earthquake Chemistry, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan (Received April 7, 2007; Accepted October 12, 2007) The relationship between geological structure and helium isotopes is discussed for deep groundwaters from the Osaka sedimentary basin, southwest Japan, to understand dissolved He in groundwater for hydrological application. Although this area shows no Quaternary volcanic activity, nearly upper mantle-like 3He/ 4He ratio (1.1 × 10–5) has been observed in the area along the fault system where the basement rocks are outcropped. In contrast, deep groundwaters from the lowest part of the aquifers beneath the Osaka Basin showed a wide variation in 3He/ 4He ratio (0.27–8 × 10 –6), which seems to reflect the geological structure. The moderately high 3He/ 4He ratios, not as high as those in the mantle value, are due to a contribution of radiogenic 4He within the aquifer. The flux of mantle He through the Uemachi thrust fault into the aquifers in the east block (hanging wall of the fault) of the Osaka Basin will be smaller than those in the west block (foot wall), because mantle He through this fault encountered the aquifers in the west block and dissolved in this aquifer. The model for the Osaka Basin presented in this study implies that there are at least two different sources of He flux into an aquifer; mantle He flux through the fault system and crustal He from the underlying formation. The former should be spatially heterogeneous, while crustal 4He flux is rather spatially constant throughout the lowermost part of the Osaka Basin. Keywords: helium isotopes, noble gas, groundwater, hydrology, Osaka basin
1979). Many authors attributed these observations to external 4He flux from deep seated crust (e.g., Andrews, 1985; Torgersen and Clarke, 1985; Stute et al., 1992; Marty et al., 1993; Castro, 2004; Zhou and Ballentine, 2006). The validity of estimated groundwater residence time depends on the accurate estimation of 4He accumulation rates and some hydrological parameters such as porosity and thickness of aquifers (Weise and Moser, 1987; Stute et al., 1992; Morikawa et al., 2005). Ballentine et al. (2002) stated that the 4He accumulation rates are thought to be highly variable, from virtually no external 4He contribution required to 4He accumulation rates apparently exceeding the 4He flux from the whole continental crust. In contrast, Mazor and Nativ (1992) suggested that hydraulic calculations often underestimated groundwater ages due to the lack of hydraulic interconnection between wells from which data were applied. The radiometric dating methods such as 14C, 36Cl and 129I (Phillips and Castro, 2003 and references therein) have their specific dating ranges according to their half lives and usually require complex corrections for water-rock interactions. Therefore, there is no ideal groundwater dating method for both deep and old groundwater.
INTRODUCTION Noble gases in groundwater can be used as potential tracers in isotope hydrology. Among the noble gas isotopes, helium is most useful for investigating the groundwater flow regime, age of groundwater and its origin since 4 He is continuously produced in an aquifer and surrounding rocks by radioactive decay of uranium and thorium and dissolves into groundwater over geologic time spans. Helium-4 concentrations in many aquifers are observed to increase with the groundwater residence time estimated from other dating methods (e.g., Andrews and Lee, 1979; Torgersen and Clarke, 1985; Castro et al., 2000). However, the residence time deduced from He concentration assuming that 4He originated from an in situ produced component in an aquifer often yields very old ages compared with hydrodynamic age (Torgersen and Clarke, 1985; Marty et al., 1993) and with those from radiometric dating methods such as 14C (e.g., Andrews and Lee,
*Corresponding author (e-mail:
[email protected]) Copyright © 2008 by The Geochemical Society of Japan.
61
The reliability of He dating method depends on the accurate estimates of the He accumulation rate, which is affected by the contribution of various sources of He. A number of 3He sources (atmosphere, 3H decay, in situ production from 6Li neutron reaction, mantle He) in groundwater show a wide variation in the 3He/4He ratio. It is well documented that the He isotopic ratio (3He/4He) in the Mid-Ocean Ridge basalt (MORB), which represents a mean upper mantle value, falls around a value of 1 × 10–5 (Ozima and Podosek, 2002). Typical crustal rock 3 He/4He production ratios have been estimated to be about 1 × 10–8 (Andrews, 1985), which is much lower than the mantle and atmospheric value. Therefore, the He isotopic ratio as well as its concentration reflects the nature of He dissolved in groundwater. In this study, we discuss the relationship between geological structure and He isotopes in deep groundwaters from the Osaka sedimentary basin to understand dissolved He in groundwater for hydrological use. The Osaka Basin is located on the northwest side of the Kii Peninsula, southwest Japan. The north border of the basin has unique characteristics of a high-temperature and -chlorine (up to 40,000 ppm) thermal water, so called Arima-type thermal brine (Matsubaya et al., 1973; Masuda et al., 1985, 1986). The 3He/4He ratios of hot spring gases in this region are exceptionally high, sometimes up to the MORB value, in spite of a fore-arc region with no Quaternary volcanism (e.g., Nagao et al., 1981; Sano and Wakita, 1985). Although many noble gas studies have been undertaken for hot spring gases in the vicinity of the Osaka Basin, they are focused on discussing the mechanism of mantle He release from fore-arc regions and their relation with the tectonics of the Kii Peninsula (Wakita et al., 1987; Okada et al., 1994; Matsumoto et al., 2003; Umeda et al., 2006). In this paper, we present the noble gas data dissolved in deep groundwaters from the Osaka Basin and discuss the distribution of the 3He/ 4He ratio in the shallower region with emphasis on the hydrological application of noble gases. Since geological and geophysical surveys were intensively carried out for constructing the underground geological structure of this basin, the Osaka Basin is suitable for studying the relationship between the geological structure and noble gases in groundwater. S AMPLING SITES The Osaka sedimentary basin is located on the northwest side of the Kii Peninsula, the fore-arc region of southwest Japan, where the Philippine Sea plate is subducting to the northwest under the Eurasian plate (e.g., Seno et al., 1993). This region has distinct structural and tectonic features characterized by high seismicity in the crust and subducting Philippine Sea slab (Ikeda et al., 2001). The
62
N. Morikawa et al.
tremor activity in the Kii Peninsula is related to the movement of aqueous fluid along the existing faults and/or newly created hydraulic fractures induced by fluid addition (Obara, 2002). The Osaka Basin is a tectonic subsidence basin, which consists of 500–2,000 m thick Late Pliocene to Pleistocene sediments, so called Osaka Group. This area is characterized by the alternative arrangement of basin and range, which is the topographic expression of the differential movements of the faulted blocks of granitic basement (Huzita, 1990). Within the studied area, mountainous ranges of the Cretaceous plutonic rocks and pre-Tertiary sedimentary rocks surround Tertiary sedimentary basins. The Plio–Pleistocene Osaka Group is found basically under alluvial plains. Based on lithology, the Osaka Group is divided into the Lower, Middle and Upper Subgroups (Huzita and Kasama, 1982). The lower subgroup consists of coarse sand and gravel of lacustrine facies, while the Middle and Upper Subgroups alternate between marine clays and non-marine sand/gravel beds. The granitic basement rocks of the Osaka Basin have been broken into two major blocks called West and East Osaka blocks by the Uemachi fault trending north to south. These two basement blocks have been tilting separately, but both have continuously subsided with the rate of 0.7–0.2 m/ky since ca. 1.2 Ma (Uchiyama et al., 2001). The northeastern Osaka Basin (Kobe Basin) is bounded by the Awaji– Rokko fault system to the northwest, and divided by the northern branch of the Osaka-wan fault (Yokokura et al., 1996). There are a large number of active fault systems in the Cretaceous basement. The major tectonic divide in the Kii Peninsula is the Median Tectonic Line (MTL) which is a vertical right-lateral strike-slip fault with an E-W strike. SAMPLE C OLLECTION AND A NALYTICAL PROCEDURE Figure 1 shows the sampling points of deep groundwaters. As noble gases are highly volatile, it is important to avoid gas exchange between the water sample and the atmosphere during sampling. The samples for noble gas analyses were collected in annealed copper tubes (3/8 inch o.d., 30 cm length). The copper tubes were connected to sampling wells and flushed sample water thoroughly in order to remove air bubbles completely from a water sample, and were then tightly closed at both ends by customized steel clamps, which were equipped with 0.5 mm stainless steel spacers. Noble gases dissolved in water samples were extracted into glass bulbs following a procedure described in JeanBaptiste et al. (1992). The copper tubes were attached to the extraction line equipped with the thick glass flask and the glass bulb (Schott AR-Glas®) connected by a capillary tube (0.6 mm i.d., 40 mm length). The lines were
Fig. 1. Map of the studied area, showing the locations of sampling points and topographic contours at 100 m intervals. The samples from the Kobe Basin and Rokko mountains (A-9, -10, -37, C-36, E-40, EK-1, -2, -6, -7) are previously reported in Morikawa et al. (2005). The dashed line (A–A ′) is the cross section line of Fig. 6.
pumped down to 10–8 torr and isolated from the vacuum unit. The clamp on the copper tube was removed, and the tube was re-opened slightly to allow the water to flow down into the glass flask. The glass flask was placed in an ultra-sonic bath to facilitate the extraction of dissolved gases. After an equilibration period of 15 min, the AR glass bulb was immersed in liquid nitrogen. All gases are transport into the bulb by the water vapor streaming from the line to the glass bulb where it is frozen. In the early stage of this work in 2002, some water samples were collected into 150 ml Pyrex glass containers with vacuum cocks on both ends. Dissolved gases in the water samples were extracted by a vacuum line and were transferred into two breakable seals.
Gas samples were also collected by a displacement method using Pyrex glass reservoirs with vacuum-tight stopcocks at both ends. The gas samples were split into a few fractions in about 8 cm3 glass ampoules attached to an all-metal ultra-high vacuum line, and the pressure and temperature were measured before flame sealing. Noble gases in glass ampoules were attached to a noble gas purification line and purified by exposure to a Ti–Zr getter at about 800°C and two SAES Getters (GP50). The sample gases were then split into two fractions for He–Ne and Ar–Kr–Xe measurements. The He–Ne fraction was further purified using two charcoal finger held at liquid N2 temperature and a cryogenic sintered stainless steel trap at 20 K. Helium isotopic ratio and noble
Relationship between geological structure and He isotopes in deep groundwater
63
64
N. Morikawa et al.
2005/9/14 2005/9/19
2005/10/13
3 4
5
6
7
8
9
10
11
12 13
14
15
16 17
18
19
20
21
22 23
24
27
28
OSK05003
OSK05004
OSK05005
OSK05006
OSK05007
OSK05008
OSK05009
OSK05010 OSK05011
OSK05012 OSK06001
OSK06002
OSK06003
OSK06004
OSK06005
OSK06006
OSK06007
OSK06008
OSK06009
OSK06010 OSK06011
OSK06012
OSK06015
OSK06016
2006/11/9
2006/11/8
2006/11/7
2006/11/7 2006/11/7
2006/11/7
2006/11/7
2006/11/6
2006/11/6
2006/11/6
2006/11/6
2006/11/6
2006/11/6
2005/10/19 2006/11/6
2005/8/5
2005/8/5
2005/8/5
2005/8/5
2005/8/4
2005/8/4
2005/8/4 2005/8/4
(y/m/d)
1 2
OSK05001 OSK05002
Sampling date
Sample No.
Sample name
34°37′29″
34°52′16″
34°25′22″
34°27′43″ 34°28′08″
34°50′07″
34°50′06″
34°31′22″
34°27′47″
34°22′21″
34°22′22″
34°40′55″
34°47′53″
34°47′47″
34°39′10″
34°39′27″ 34°42′37″
34°41′11″ 34°32′50″
34°41′24″
34°40′00″
34°28′38″
34°34′51″
34°44′22″
34°50′26″ 34°47′47″
Latitude
135°31′42″
135°35′31″
135°34′35″
135°35′52″ 135°35′01″
135°28′20″
135°28′19″
135°34′15″
135°35′08″
135°27′08″
135°27′08″
135°37′10″
135°29′40″
135°26′43″ 135°33′28″
135°30′55″
135°35′01″
135°27′40″
135°33′18″
135°33′04″
135°27′49″
135°23′37″
135°32′11″
135°35′32″
135°33′58″ 135°35′08″
Longitude
48.1
18.9
17.6
9.5 7.0
6.3
6.1
21.1
24.3
8.6 8.2
8.5
8.3
6.2
7.0
6.9
8.0
7.1
7.0
6.9 7.0
7.1
6.9
8.1
8.2
8.0
6.9
6.8
7.5 7.4 6.0
pH
24.7
19.9
23.6
19.4
21.4
26.3
42.6
25.1
50.7 27.7
35.1
46.5
50.5
38.0
36.0
47.0
38.0
43.5
30.5
29.1 24.5
(°C)
Temp. He
110
1.34 ± 0.01 0.83 ± 0.01
91.8 275 10.4 46.9 11.3 8.78 47.8 192 4.47 22.2 11.6
1.35 ± 0.02 1.92 ± 0.02 5.36 ± 0.10
1.20 ± 0.02 3.51 ± 0.05 6.21 ± 0.15 250−351 1097−1291
594−696
n.k. 297−402
706−857
706−857
488−603
1.16 ± 0.02
0.24 ± 0.02
4.81 ± 0.04
1.03 ± 0.02
3.61 ± 0.03
81.0
0.55 ± 0.01
n.k.(c) 1067−1187, 1358−1635 n.k. 126−358
60.7 108
180 191
336
76.8 583
0.81 ± 0.01
2.83 ± 0.03 0.71 ± 0.01
3.02 ± 0.03 1.59 ± 0.02 0.59 ± 0.01
0.80 ± 0.01
46.8
1900
0.27 ± 0.01 3.44 ± 0.03 4.84 ± 0.04
(10 −14)
3
2490 41.8 189 7650
(10 −6)
He/ 4He
1.03 ± 0.01(b) 1.21 ± 0.01
3
508−646
813−951
577−648 1336−1492
851−1249
691−795 910−1063
908−930
730−957 885−1010
882−985
555−665 555−585 1015−1195
(m)
Depth of the sampling point(a)
Table 1(a). Noble gas concentrations and 3He/ 4He ratios in groundwaters from the Osaka Basin He
0.72 91.2 10.0
7.29 13.6 39.9
1.94 13.0 11.0
67.8 143
147
85.9 134
67.5
305
211
95.5 193
56.7
82.6
393
2400 34.6 697 2230
(10 −8)
4
Ne Ar
(10 −6)
36
0.073 2.77 1.44
1.53 1.65 0.295
0.241 2.25 1.56
0.147 0.058
1.76
0.034
0.137
0.481 0.037
0.200
1.86 0.125
1.97
1.03 2.05
1.39
0.731 1.82 0.348
0.0412 1.39 0.501
0.887 0.901 0.165
0.0583 1.47 1.00
0.531 0.0754 0.0454
0.0835
0.0783
0.566 0.0465
0.357
1.38 0.165
1.40
0.956 1.45
1.56
0.801 1.09 0.441
(cm STP/gH2O)
3
(10 −7)
20
Kr
0.146 5.35 1.67
3.31 3.38 0.679
5.63 3.86
0.165
2.22 0.278 0.158
0.587
0.405
2.78 0.247
1.96
5.58 0.956
5.58
4.18 5.71
4.76
3.09 4.04 2.20
(10 −8)
84
Xe
0.142 3.70 1.03
2.23 2.23 0.545
3.92 2.69
0.122
1.79 0.204 0.0363
0.733
0.452
2.17 0.287
1.76
3.88 0.926
3.78
3.06 3.81
5.09
2.59 2.57 1.80
(10 −9)
132
He/ 20N e
0.987 3.29 0.694
0.477 0.827 13.5
0.577 0.705
0.806
8.35 46.1 246
62.7 390
63.4 181
105
5.12 154
2.88
4.03
38.2
160
329 1.90 200
4
Relationship between geological structure and He isotopes in deep groundwater
65
2001/12/12 2001/12/12 2002/8/27 2002/8/27 2002/8/29 2002/8/29 2002/2/9 2002/2/10 2002/10/7
K14
K20
K60
K68
K77
A-9(e) A-10(e) EK-1(e) EK-2(e) EK-6(e) EK-7(e) A-37(e)
C-36(e) E-40(e)
KNK02021C
KNK02022C
KNK02042C
KNK02034C
KNK02035C
KOB01036 KOB01037 KOB02001 KOB02003 KOB02005 KOB02006 KNK02037A
KNK02036C KNK02040E
2002/2/9
2002/2/11
2002/2/7
2002/2/7
2002/2/5
34°47′38″ 34°48′03″ 34°42′37″
34°41′46″ 34°41′46″ 34°40′24″ 34°42′24″ 34°42′39″ 34°43′20″
34°50′37″ 34°41′21″
34°23′43″ 34°51′05″
34°22′01″
34°24′50″
34°20′15″ 34°22′17″
Latitude
135°11′54″ 135°11′55″ 135°10′33″ 135°13′24″ 135°14′34″ 135°18′50″ 135°15′11″ 135°13′02″ 135°22′45″
135°21′43″
135°11′20″
135°18′18″
135°35′50″
135°36′06″
135°32′10″
135°22′39″ 135°24′22″
Longitude
15 15
24.5 41.0 29.6 35.9 31.8 43.1 13.4 31.3 41.4
45.8
28.0
23.8
15.6
15.0
24.9
18.4 10.5
(°C)
Temp.
6.5
7.7 4.6 6.3
7.7 8.1 7.2 7.1
6.5
8.2
6.1
6.8 8.1
9.3
7.9
9.2 7.6
pH
0 300 1500
727 1006 602 700 600 1000
1008
7 1200
n.k.
532 350
n.k. 275
(m)
Depth of the sampling point(a) He
1.38 1.38 6.27 5.18
250
865 4210 4350
813 3070 6860 1370
1380
82.4
1.83 ± 0.04 3.60 ± 0.07 3.78 ± 0.10 5.30 ± 0.13 7.73 ± 0.12 7.43 ± 0.18 5.69 ± 0.15 10.74 ± 0.07 9.18 ± 0.05 4.29 ± 0.15
172
4.39 ± 0.04 6.17 ± 0.13
56.9 2690 4600
533
3.03 ± 0.05 6.96 ± 0.06 2.12 ± 0.05 6.70 ± 0.04
(10 −14)
3
62.7 117
(10 −6)
He/ 4He
0.63 ± 0.04
3
He
4.56 3.77
58.2
152 392 474
215 579 888 184
383
27.9 44.9
76.5 26.8 402 1050
99.5 38.8
(10 −8)
4
Ne Ar
(10 −6)
36
1.74 1.42
0.762 0.639 1.49 0.041
0.597 0.998 1.44 0.466
0.623
0.852 1.70
2.82 1.55
1.77 1.81
2.05 1.83
18G K19G K228G
OSK06006(a) KNK02024C(b)
1.16 0.912
n.a. n.a. n.a. n.a.
2006/11/6 2002/2/7 2002/10/7
(y/m/d)
Sampling date
34°25′22″ 34°25′22″
34°40′00″
Latitude
135°27′49″ 135°34′35″ 135°34′35″
Longitude
3
7.96 ± 0.07(c) 7.44 ± 0.09 7.77 ± 0.10
He/ 4He (10−6) 3
3.45 1.71 1.25
He (10−5)
He
4.34 2.30 1.61
4
Ne
36
Ar
0.0571 0.339 0.222
(ppm) 0.00758 0.174 0.0994
20
Kr
0.00271 0.00905 0.00630
84
Xe
0.000318 0.000240 0.000386
132
(b)
4
572 13.2 16.2
He/ 20N e
4.52 3.51
n.a. n.a. n.a. n.a.
n.a. n.a.
n.a. n.a.
n.a.
n.a. n.a.
n.a. n.a.
n.a. n.a.
n.a. n.a.
n.a. n.a.
Same sample location as Sample No. 18. Same sample location as Sample OSK06012 (No. 24) in Table 1(a). (c) Error are 1 σ and include a statistical error of an individual measurement of the samples and an error of correction factor determined by standard gases.
(a)
KNK02041E(b)
Sample No.
Sample name
Table 1(b). Noble gas concentrations and 3He/ 4He ratios in gas samples from the Osaka Basin
(b)
Kr
(10 −8)
84
n.a. n.a.
n.a.
n.a. n.a.
n.a. n.a.
n.a. n.a.
n.a.
n.a. (d)
(cm STP/gH2O)
3
(10 −7)
20
The depth of the screen from the ground surface, i.e., the depth of the groundwater comes from. The depth of the borehole for KNK and KOB-series samples. Error are 1σ and include a statistical error of an individual measurement of the samples and an error of correction factor determined by standard gases. (c) Not known. (d) Not analyzed. (e) Data are from Morikawa et al. (2005). (f) Air-saturated water (ASW) and sea water (SW) at 1 atm pressure and a temperature of 15° C (Benson and Krause, 1980; Smith and Kennedy, 1983).
(a)
ASW (f) SW (f)
2002/2/10
K13
KNK02012C
2002/2/4 2002/2/4
(y/m/d)
K1 K2
KNK02006C KNK02007C
Sampling date
Sample No.
Sample name Xe
3.04 2.40
n.a. n.a. n.a. n.a.
n.a. n.a.
n.a. n.a.
n.a.
n.a. n.a.
n.a. n.a.
n.a. n.a.
n.a. n.a.
(10 −9)
132
61.5 39.6
36.1 58.0
61.4
3.27 2.65
4.32 1.48 14.2 67.8
4.84 2.12
He/ 20N e
0.261 0.265
20.1 61.4 31.8 143
4
RESULTS AND DISCUSSION Abundances of 20Ne, 36Ar, 84Kr and 132Xe Total amount of each noble gas in groundwater can be expressed as follows; NG = NGatm + NGea + NGmantle + NG cr
(1)
where NG atm and NGea are the amounts of each noble gas resulted from equilibrium solubility with the atmosphere and from air itself because of excess air during the recharge process (e.g., Heaton and Vogel, 1981), respectively. The NGmantle and NGcr are the amount of the components from the mantle and crust, respectively. Since a large amount of heavy noble gases dissolved in groundwater generally originates from the atmosphere (NGatm and NGea), both NGmantle and NGcr will be negligible when we discuss the abundance of heavy noble gases for the purpose of the following discussion. Air contamination during the sampling procedure enhances the amount of all noble gases. Depletion of noble gases in thermal water was also observed and considered to be a result of a boiling process and associated phase separation between
66
N. Morikawa et al.
0.010 0.009
50
0.008 0.007
50 10
Xe /36Ar
0.006
132
gas abundances were measured with a noble gas mass spectrometer, model MM-5400 (Micromass), which was installed at the Geological Survey of Japan (GSJ) in 2003. Neon was released from the cryogenic trap at 50 K. Before introducing the Ne sample, 40Ar peaks were checked by a quadrupole mass spectrometer. Since there were no significant peaks at mass 40, the contribution of Ar++ peak on 20Ne + mass was negligible for calculated Ne abundances. After He and Ne measurements and evacuation of the sample, the Ar–Kr–Xe fraction was expanded to the purification line and introduced into the mass spectrometer. For the isotope discrimination correction, HESJ (Helium Standard of Japan) with R/Ra of 20.63 ± 0.10 (Matsuda et al., 2002) was used as standard. The 3 He/4He ratio of the HESJ at GSJ, which was determined by using atmospheric helium as a primary standard, is 20.75 ± 0.21 R/Ra. This value is within a recommended value. From the repeated analyses of air saturated water (ASW) at GSJ, the reproducibility of 3He/ 4He and noble gas abundances are about 3% except 132Xe for 6% (n = 10, 1σ ). Total blanks which includes extraction procedure and hot blanks in purification line were 2, 0.8, 0.6,
