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Origin of methane in high-arsenic groundwater of Taiwan – Evidence

from stable isotope analyses and radiocarbon dating

Tsung-Kwei Liu

a,*

, Kuan-Yu Chen

a,b

, Tsanyao Frank Yang

a

, Yue-Gau Chen

a

, Wen-Fu Chen

c

, Su-Chen Kang

a

,

Chih-Ping Lee

a

a

Department of Geosciences, National Taiwan University, Taipei, Taiwan, ROC

bEnergy and Environment Research Laboratories, Industrial Technology Research Institute, Hsinchu, Taiwan, ROC c

Institute of Hot Spring Industry, Chia Nan University of Pharmacy and Science, Tainan County, Taiwan, ROC

a r t i c l e

i n f o

Article history:

Received 29 August 2008

Received in revised form 12 June 2009 Accepted 25 June 2009

Keywords: Methane (CH4)

Dissolved inorganic carbon (DIC) High-arsenic groundwater Taiwan

Stable isotopes Radiocarbon dating

a b s t r a c t

Groundwaters in the confined aquifers of the Chianan and Ilan coastal plains of Taiwan are rich in dis-solved methane (CH4). Serious endemic ‘‘blackfoot disease”, which occurred in the Chianan plain,

espe-cially during AD1950-1970, has been demonstrated to have arisen from drinking highly reducing groundwater with abnormal arsenic and humic substance levels. In order to explore the origin of CH4

and its hydrological implications, stable carbon isotope ratios (d13C) and radiocarbon (14C) ages of

exsolved CH4, dissolved inorganic carbon (DIC), and sedimentary biogenic sediments from a total of 34

newly completed water wells at 16 sites were determined. The main results obtained are as follows: (1) The d13C

CH4 (65‰ to 75‰) values indicate that, except for one thermogenic sample

ðd13C

CH4¼ 38:2‰Þ from the Ilan plain, all CH4samples analyzed were produced via microbially mediated

CO2reduction. Many d13CDICvalues are considerably greater than 10‰ and even up to 10‰ due to

Rayleigh enrichment during CO2reduction. (2) Almost all the14C ages of CH4samples from the shallow

aquifer (I) (<60 m depth) are greater than the14C ages of coexisting DIC and sediments, suggesting the

presence of CH4from underlying aquifers. (3) The14C ages of coexisting CH4, DIC and sediments from

aquifer (II) of the Chianan plain are essentially equal, reflecting in-situ generation of CH4and DIC from

decomposition of sedimentary organic matter and sluggishness of the groundwater flow. On the other hand, both CH4and DIC from each individual well of the relatively deep aquifers (III) and (IV) in the

Chi-anan plain are remarkably younger than the deposition of their coexisting sediments, indicating that cur-rent groundwaters entered these two aquifers much later than the deposition of aquifer sediments. (4) Each CH4sample collected from the Ilan plain is older than coexisting DIC, which in turn is distinctly

older than the deposition of respective aquifer sediments, demonstrating the presence of much older CO2and CH4from underlying strata.

Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Considerable amounts of dissolved methane (CH4) are often

present in groundwaters pumped from wells tapping confined aquifers in the Chianan and Ilan alluvial coastal plains of SW and NE Taiwan, respectively (Fig. 1). These groundwaters are also char-acterized by high concentrations of dissolved humic substances and arsenic (Chen and Liu, 2007), both of which were considered to be responsible for endemic ‘‘blackfoot disease” through drinking of groundwater (Tseng et al., 1961; Lu, 1990). It has long been rec-ognized that occurrence of CH4 generally implies a considerably

reducing state in the geochemical environment (Stumm and

Mor-gan, 1996). In addition, strongly reducing waters tend to be con-taminated by heavy metals and natural discharges are likely to be minimal (Smedley and Kinniburgh, 2002; Gooddy and Darling, 2005). Previous studies of dissolved gas from the six water wells drilled by the Chinese Petroleum Corporation in the Chianan plain focused mainly on the potential for profitable production (Hsu, 1984), and the composition of the gas was shown to contain more than 90% CH4, up to 6% CO2, and a few percent N2. However, the

origins and source of CH4 in these aquifers have not been well

studied.

Dissolved methane in groundwater can be formed via bacterial reduction, or from thermogenic decomposition of organic matter at relatively high temperatures. Furthermore, the former type can proceed through either reduction of dissolved forms of CO2 (i.e.,

CO2+ 4H2?CH4+ 2H2O) or acetate fermentation (i.e., CH3COO+

H2O ? CH4+ HCO3) (Schoell, 1980, 1988; Oremland et al., 1982).

1367-9120/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.

doi:10.1016/j.jseaes.2009.06.009

*Corresponding author. Tel./fax: +886 2 2365 7380. E-mail address:liutk@ntu.edu.tw(T.-K. Liu).

Contents lists available atScienceDirect

Journal of Asian Earth Sciences

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / j s e a e s

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The best diagnostic tool for identifying the origin of CH4in the

groundwater environment appears to be the13C and2H

fraction-ation between coexisting CH4and CO2(Oremland, 1988; Coleman

et al., 1988; Whiticar et al., 1986; Whiticar, 1999; Lansdown et al., 1992; Aravena et al., 1995, 2003). It has long been recognized that the stable carbon isotope ratios (d13C) of thermogenic methane range from 30‰ to 50‰, and are mostly greater than 45‰ (Barker and Fritz, 1981; Tyler, 1991). By contrast, d13C values for

microbial CH4 are less than 50‰ and mostly fall in the range

50‰ to 65‰ for acetate fermentation pathway and 65‰ to 90‰ for CO2reduction pathway. In addition to the problem of

the origin of CH4, it is also interesting to know whether CH4was

formed in-situ. The purpose of this study is to verify the origin and source of CH4 associated with groundwater in the Chianan

and Ilan plains by using stable carbon isotopes and radiocarbon dating of CH4, DIC and shell (or plant) sediments collected from

34 recently drilled groundwater-monitoring wells. A d2H value

was measured for a CH4sample to further support its origin

iden-tified by d13C criterion.

2. Hydrogeological setting 2.1. Chianan plain

The Chianan plain is a part of the coastal plain of southwestern Taiwan, covering about 1100 km2with a N–S length of

approxi-mately 40 km and an average E–W width of 30 km. The ground surface slopes seaward with a very low gradient. It is bounded on the east by the western edge of the Western Foothills, which is mostly composed of siltstone and fine-grained sandstone of the late Tertiary epoch and provides the source rocks for the Chia-nan plain (Ho, 1975). Accordingly, the upper-most 300 m of which is mostly silt and clay, intercalated with thin lenticular fine-sand layers (Fig. 2a; Chen and Liu, 2007); these sediments

were deposited in mixed sedimentary environments, including la-goon, estuarine, shallow marine, and fluvial plains during late Pleistocene and Holocene. The average rate of sediment accumula-tion during Holocene reached a high value of 1 cm/yr in the coastal areas as determined by radiocarbon dating (Liu et al., 1997).

Based on grain-size of sediments, the upper 300-m thick stra-ta at each well locality are generally divided into four aquifers: I, II, III and IV from top to bottom. Unconfined aquifers are limited to the upper few meters of the plain, even at the eastern parts adjoin-ing the hills. Hydrogeological characteristics are consistent with the very low permeability shown in pumping tests and the occur-rence of saline water in the aquifers deposited in a marine environ-ment.Chen and Liu (2007)interpreted the high chlorinity for some wells in aquifer (I) as being due to strong evaporation in the coastal zone.

2.2. Ilan plain

The Ilan plain is located in the western tip of the rifting back-arc basin of the Okinawa Trough. This plain is triangular in shape and bounded by low-grade metamorphic rocks of the Hsueshan Range and medium-high grade metamorphic rocks of the Central Range to the northwest and the south, respectively. The upper 120-m thick layer was deposited during Holocene and its average sedi-ment accumulation rate is similar to the Chianan plain, also reach-ing a high of 1 cm/yr. Grain-sizes of sediments in most parts of the plain decrease distinctly from gravel in the proximal part to silt/clay in the coastal area. A relatively shallow (<20 m depth) and thick (10 m) clayey layer acts as the upper-most aquitard overlying the confined aquifers in the middle and distal parts. The CH4samples analyzed were taken from the southern margin

of the plain, where the whole pile of strata drilled through for this study is basically composed of dark-gray clay intercalated with thin layers of silt and fine-grained sand (Fig. 2b).

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3. Samples and analytical methods

During the establishment of the groundwater-monitoring net-work of Taiwan, continuous sediment cores of up to 250 m or more in length were taken and up to four wells tapping individual aqui-fers were drilled at each monitoring site. Most of the water sam-ples were collected from the same wells as those used byChen and Liu (2007)for arsenic study. The water samples were ensured to be fresh and representative by pumping each well long enough to replace the free-standing water in the casing before taking sam-ples. About 50 l of each water sample was collected from these wells for precipitation of dissolved inorganic carbon (DIC) for14C

dating using the procedures described previously by Hackley et al. (1992)andLiu (1995a). Collection of exsolved gases for gas isotopic analyses was carried out using inverted glass bottles sub-merged in sample water at the discharge outlet of the flow cham-ber. The depositional ages of aquifer sediments corresponding to the depth of each well screen were either determined by14C dating

of biogenic (i.e., plant or shell) fragments from the sediment cores corresponding to well screens, or estimated by correlation of neighboring strata with known 14C ages (Chen and Liu, 2007).

The original CO2,if any, in the gas samples was eliminated by

bub-bling the gas through 0.5 M BaCl2solution under pH 13 to facilitate

carbonate (BaCO3) precipitation, and further passing through an

Fig. 2. Hydrogeological profiles of (a) the Chianan plain and (b) the Ilan plain. The strata within the drilling range are divided into four aquifers: (I), (II), (III), and (IV) from top to bottom. Location of profile lines is shown inFig. 1.

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ascarite column before any hydrocarbon gas was combusted and converted to CO2for14C dating and d13C determinations. The14C

activity was measured by the ultra-low level liquid scintillation counter, Quantulus 1220TM(Gupta and Polach, 1985). The maxi-mum determinable radiocarbon age, defined by the two above-background conventions described byStuiver and Polach (1977), is 50 Ka for our laboratory. The zinc technique (Coleman et al., 1988) was used to convert H2O resulting from combustion of a

CH4sample to H2 for d2H analyses. Stable isotope analyses were

performed on a triple-collecting mass spectrometer and are ex-pressed in the usual delta notation relative to the PDB (d13C) and

SMOW (d2H) standards. Analytical reproducibility was better than

0.1‰ for d13C and 3‰ for d2H. The14C results of DIC and CH4were

reported as conventional14C ages (Stuiver and Polach, 1977) for

comparison with depositional ages of aquifer sediments. 4. Results and discussion

The analytical results of d13C and14C ages for methane and

dis-solved inorganic carbon are listed inTable 1. In addition, strati-graphic 14C ages for each screen level are estimated by lateral

correlation and average depositional rates based on the age data ofChen and Liu (2007). It is evident that except for sample IL-01(IV) from the Ilan plain all other d13C values for CH

4samples fall

in the range 67‰ to 78‰, demonstrating that CH4was formed

via bacteria mediated CO2reduction (Fig. 3). This is further

sup-ported by the d2H (211.8‰) and d13C (65.31‰) values for

meth-ane from well CN-03(III) on the classification scheme ofColeman et al. (1995)and the generally positive correlation between the

14C

DICand14CCH4ages (Fig. 4). A general increasing trend of

14C DIC

Table 1

14C ages and d13C values for dissolved methane (CH

4) and inorganic carbon (DIC), and stratigraphic age of each well in the Chianan and Ilan plains.

Sample sitea

Aquifer Well screen depth (m) CH4 DIC Estimated Stratigraphic14C age (yBP)

14 C age (yBP) d13C (‰ PDB) 14 C age (yBP) d13C (‰ PDB) CN-01 (24)b (II) 62–86 10010 ± 60 74.3 10050 ± 70 18.2 10000 CN-02 (27) (I) 32–50 6210 ± 60 76.6 5070 ± 50 4.5 7000–9000 CN-03 (22) (I) 32–50 14400 ± 80 74.3 9400 ± 50 43.5 7000–9000 (II) 104–119 17280 ± 100 75.8 17590 ± 140 1.4 17000 (III) 150–168 25730 ± 230 82.2 27800 ± 200 15.7 S35000 CN-04 (23) (II) 94–114 13500 ± 75 74.2 13200 ± 70 7.3 12000 (III) 170–182 13360 ± 80 74.1 13840 ± 90 0.2 S40000 (IV) 216–228 13520 ± 80 73.8 17300 ± 100 6.2 >40000 CN-05 (14) (I) 18–30 10650 ± 110 82.2 5390 ± 50 2.5 5100 (II) 114–126 14340 ± 100 68.1 14580 ± 200 8.0 12000 (III) 172–190 15820 ± 90 68.3 15070 ± 220 3.3 S40000 (IV) 241–250 12800 ± 60 69.5 18600 ± 250 9.8 >40000 CN-06 (17) (I) 26–44 10370 ± 60 71.2 1410 ± 40 8.8 7000–9000 (II) 98–110 11310 ± 70 70.9 10300 ± 150 4.8 12000 CN-07 (18) (II) 101–113 11180 ± 60 76.3 8270 ± 70 7.0 12000 CN-08 (15) (III) 230–239 30070 ± 280 78.2 22850 ± 200 21.4 >40000 (IV) 265–283 17490 ± 100 71.2 n.d.c 6.3 S40000 CN-09 (13) (II) 94–106 11820 ± 70 77.1 9600 ± 70 0.2 12000 (III) 121–133 12620 ± 70 69.3 13500 ± 140 9.9 13000 CN-10 (10) (III) 191–203 42250 ± 1100 80.8 37900 ± 700 10.3 >40000 CN-11 (7) (I) 18–33 9530 ± 70 76.9 5030 ± 80 9.9 5100 CN-12 (4) (IV) 215–227 33930 ± 450 84.2 31000 ± 450 13.0 >40000 CN-13 (32) (III) 176–188 19150 ± 100 73.8 18670 ± 100 0.8 S40000 (IV) 230–248 17720 ± 120 69.4 20150 ± 150 6.9 >40000 CN-14 (1) (III) 180–192 16680 ± 95 72.9 18300 ± 120 3.8 >40000 (IV) 233–251 23070 ± 150 68.1 31500 ± 700 5.2 >40000 IL-01 (I) 13–37 11770 ± 120 75.6 5120 ± 90 11.6 1000–4000 (II) 60–78 n.d. n.d. 11700 ± 70 8.0 6000 (III) 100–118 22000 ± 850 71.5 15350 ± 110 10.1 8500 (IV) 136–151 35700 ± 200 38.2 21700 ± 100 12.1 S10000 IL-02 (I) 3–15 n.d. n.d. 3510 ± 40 13.0 n.d. (II) 29–41 n.d. n.d. 14900 ± 100 9.0 n.d. (III) 64–82 21300 ± 1050 75.4 16900 ± 80 17.6 6000–8000 (IV) 152–176 23050 ± 820 78.4 19100 ± 140 12.1 15000–17000 a

CN, Chianan plain; IL, Ilan plain.

b

Number in the bracket denotes the well number used byChen and Liu (2007)for the identical well.

c

n.d., not determined.

Fig. 3. Plot of d13

CDICvs. d13CCH4for groundwater from the Chianan and Ilan plains. The general ranges of d13

C values for methane generated by different pathways according to the classification scheme ofBarker and Fritz (1981)are also shown.

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and14C

CH4ages with well depths can also be found (Fig. 5). In

addi-tion to the characteristics menaddi-tioned above, we make the following relevant comments.

4.1. Chianan plain

It is worth pointing out that d13C

DICvalues for about 18 of the 25

water samples from the Chianan plain are considerably greater than 0‰ (up to 9.9‰), suggesting the occurrence of Rayleigh enrichment during CO2reduction as was invoked byNissenbaum

et al. (1972)to explain unusually heavy d13C values of interstitial

water CO2. According to the Rayleigh model, successive fractions

of CO2are reduced to produce12C-enriched CH4, leaving residual

CO213C-enriched. Similar enrichment in d13C (as high as +35‰)

was reported for DIC in groundwater with dissolved methane in the United States (Scott et al., 1994; Aravena et al., 2003) and Aus-tralia (Smith and Pallasser, 1996). In contrast, the very negative value for DIC (43.5‰) in CN-03 (I) suggests that methane oxida-tion occurred.

Comparisons of14C ages between CH

4and DIC, and between DIC

and sediment deposition for each well are shown inTable 2. In view of the probability of water mixing during pumping, wide span of well screen, and the uncertainty of depositional ages,14C ages of

CH4, DIC, and strata are arbitrarily defined as significantly different

if their difference is greater than 10 times that of the larger

Fig. 4. Plot of14C ages for dissolved methane (CH

4) and inorganic carbon (DIC) from

the Chinana (CN) and Ilan (IL) plains. (I), (II), (III), (IV): number of aquifers. A positive correlation between the ages of the two components can be found.

0 50 100 150 200 250 300 0 5 10 15 20 25 30 35 40 14

CDIC age (kaBP)

well depth (m) 0 50 100 150 200 250 300 0 5 10 15 20 25 30 35 40 45 14 CCH4age (kaBP) well depth (m) CN-10(II) CN-07(II) CN-12(IV) CN-14(III) CN-14(IV) CN-02(I) CN-03(I) CN-05(II) CN-06(I) CN-05(I) CN-05(IV) CN-06(II) CN-08(III) CN-08(IV) CN-09(II) CN-09(III) CN-11(I) CN-13(III) CN-13(IV) IL-01(I) IL-01(III) IL-01(IV) IL-02(III) IL-02(IV) CN-05(III) IL-01(II) IL-02(II)

(a)

(b)

CN-04(II) CN-01(II) CN-03(III) CN-04(III) CN-04(IV) CN-03(II) CN-04(IV) CN-11(I) IL-01(III) CN-09(II) CN-09(III) CN-02(I) CN-03(I) CN-05(I) CN-06(I) IL-02(I) IL-01(I) IL-02(III) CN-03(II) CN-04(II) CN-05(II) CN-06(II) CN-07(II) CN-13(III) CN-03(III) CN-04(III) CN-05(III) CN-05(IV) CN-08(III) CN-10(II) CN-12(IV) CN-13(IV) CN-14(III) CN-14(IV) IL-01(IV) IL-02(IV) CN-01(I)

Fig. 5. Plot of well depth vs.14

C age of (a) CH4and (b) DIC from the Chianan (CN)

and Ilan (IL) plains. A generally positive correlation can be found.

Table 2 Comparison of14

C ages between coexisting dissolved methane (CH4) and inorganic

carbon (DIC), and between DIC and strata from each individual well.

Aquifer Sample sitea 14

C ageb CH4 DIC Strata (I) CN-02 > = CN-03 > = CN-05 > = CN-06 > = CN-11 > = (II) CN-01 = = CN-03 = = CN-04 = = CN-05 = = CN-06 = = CN-07 > < CN-09 > < (III) CN-03 = < CN-04 = < CN-05 = < CN-08 > < CN-09 = = CN-10 = < CN-13 = < CN-14 = < (IV) CN-04 < < CN-05 < < CN-12 = < CN-13 < < CN-14 < < (I) IL-01 > > (II) IL-01 (n.d.)c > (III) IL-01 > > IL-02 > > (IV) IL-01 > > IL-02 > >

aCN, Chianan plain; IL, Ilan plain. b

Two ages are defined as equal if their difference is less than 10 times of the larger standard error (r) of the two ages. Otherwise, the greater (>) or less (<) symbol is used.

c

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standard deviation of two ages. Otherwise, two ages are considered to be essentially equal. Accordingly, it is evident that the14C age of

DIC is essentially the same as that of aquifer sediments from each individual shallow well of aquifer (I) (<60 m depth) in the Chianan plain. By contrast, the14C ages of CH

4from all aquifer (I) wells are

significantly greater than those of coexisting DIC, which in turn are essentially coeval with coexisting sediments except CN-02(I) and CN-06(I), indicating the presence of CH4from the underlying

aqui-fers. It is interesting to note that seeps of older methane from underlying sediments are commonly found in some other coastal waters and shallow aquifers (e.g.Laier et al., 1996; Laier, 2003). The relatively shallow and up-gradient CN-02(I) and CN-06(I) have remarkably younger 14CDIC ages than their respective sediment

depositional ages, very probably due to mixing of younger CO2

from overlying layers.

Although CH4 ages for samples CN-07(II) and CN-09(II) are

slightly older than their respective DIC ages, all the other14C ages

of CH4, DIC and aquifer sediments for individual wells in aquifer (II)

are essentially equal. This demonstrates that dissolved methane

and DIC of aquifer (II) were formed in-situ via bacterial degrada-tion of sedimentary organic matter within the aquifer and implies that the groundwater flow rate is very slow.

Like aquifer (II), all the14C ages of CH

4from aquifer (III) (except

sample CN-08) are also essentially the same as those of their coex-isting DIC. In contrast to aquifer (II), however, both CH4and DIC

from each individual well of aquifer (III) (except CN-09) and aqui-fer (IV) are remarkably younger than their respective sediment depositional ages. Obviously, current groundwaters in aquifer (III) and (IV) were recharged much later than the deposition of their sediments. Note that the sediments of aquifers (III) and (IV) were deposited during the interval between 20 and 90 KaBP (Liu et al., 1997; Chen, 2008) when the eustatic sea-level was much lower than present and hydraulic gradients were much larger caus-ing faster discharge of groundwater. The CH4from CN-08(III) is

sig-nificantly older than its coexisting DIC and the CH4 from the

underlying CN-08 (IV).Hackley et al (1992)also noted that CH4

in their study is generally older than DIC. They interpreted this as resulting from the fact that CO2from degradation of

sedimen-tary organic matter can react to form CH4before equilibration with

DIC. We believe the same effect might have occurred in CN-08(III). Except for sample CN-12(IV), the14C ages for DIC samples from

CN-04, -05, -13, and -14 wells tapping the deepest aquifer (IV) are significantly older than those of their respective CH4(Fig. 4),

prob-ably due to dissolution of carbonate sediments. Moreover, CH4

samples from CN-05(IV) and 08(IV) are much younger than those from their respective overlying wells in aquifer (III), suggesting that some portions of aquifer (IV) have faster groundwater dis-charge than their overlying aquifer (III).

4.2. Ilan plain

The CH4of IL-01(IV) has a much older14C age than its coexisting

DIC. In addition, it is of thermogenic origin as is shown by the d13C

CH4of 38.2‰, very probably reflecting the thermocatalytic

ef-fect of shallow and young intrusive igneous rocks under the plain (Liu, 1995b; Yang et al., 2005). This interpretation is further sup-ported by the much higher CO2content of 16 vol%, as compared

with <1 vol% in other wells, although no higher hydrocarbon (e.g. ethane) was found. It is worth pointing out that the well screen of IL-01(IV) is close to the underlying metamorphic basement. Be-sides, weak-alkaline carbonic acid springs, often with CO2gas

bub-bles, are common in and around the near-by metamorphic mountain range.

Except for IL-01(IV), all CH4samples from the Ilan plain have

d13C values in the range of 71‰ to 78‰, suggesting that they originated from CO2reduction via microbial decomposition. These

microbial CH4 gases also have much older 14C ages than their

accompanying DIC, indicating a mixture of older CH4 from the

underlying strata. Moreover, both CH4and DIC of each well are

dis-tinctly older than the depositional age of sediments at depths cor-responding to each individual well screen. The most likely cause for the occurrence of much older CH4 and DIC is that the CH4

and DIC in these aquifers is mixing by CO2and CH4from the

under-lying strata. 5. Conclusions

The stable carbon isotope ratios (d13C) and 14C ages for

dis-solved methane and inorganic carbon from a total of 34 groundwa-ter samples, in addition to depositional ages of sediments, at 16 sites in the Chianan and Ilan plain of Taiwan were measured. The results show that, except for one thermogenic CH4 sample from

the Ilan plain, all other CH4samples were produced via microbially

mediated CO2 reduction. Most d13CDIC values are considerably

greater than 10‰ and even up to 10‰ due to Rayleigh enrich-ment during CO2reduction. Almost all the14C ages of CH4samples

from the shallow aquifer (I) (<60 m depth) of the Chianan plain are greater than the14C ages of coexisting DIC and sediment

deposi-tion, suggesting the presence of CH4that has migrated from the

underlying aquifers. In contrast, 14C ages of coexisting CH 4, DIC,

and deposition of sediments for aquifer (II) in the Chianan plain are essentially equal, reflecting in-situ generation of CH4and DIC

and sluggishness of the groundwater flow. Moreover, both CH4

and DIC from each individual well of aquifers (III) and (IV) in the Chianan plain are remarkably younger than the respective deposi-tional ages of these aquifers. Obviously, these groundwaters re-charged much later than the deposition of aquifer sediments. However, the age of methane depends on the age of the organic matter source as well as groundwater flow. Finally, the CH4of each

well in the Ilan plain is distinctly older than coexisting DIC, which in turn is much older than the depositional age of aquifer sedi-ments, demonstrating dissolution of much older CO2and CH4from

underlying strata. Acknowledgements

The authors thank Drs. B.M. Jahn, T. Laier and an anonymous re-viewer for valuable comments and suggestions. We are grateful to Dr. Chao-Li Liu for technical instructions and measuring stable hydrogen isotopes. Thanks are also due to the Central Geological Survey, Ministry of Economic Affairs (ROC) for providing sediment core samples and related geological information, and the Ground-water Exploitation and Conservation Center of Taiwan Sugar Com-pany for helping in water sampling. We also wish to thank Su-Chin Kang and Chun-Yen Chou for their laboratory assistance. This study is supported financially by National Science Council, Republic of China (Grant: NSC95-2116-M-002-017-MY3).

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數據

Fig. 1. Location of wells sampled and the simplified geologic provinces around (a) the Chianan plain, and (b) the Ilan plain in Taiwan
Fig. 3. Plot of d 13 C DIC vs. d 13 C CH 4 for groundwater from the Chianan and Ilan plains.
Fig. 5. Plot of well depth vs. 14 C age of (a) CH 4 and (b) DIC from the Chianan (CN) and Ilan (IL) plains

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