PROCEEDINGS, Thirty-Ninth Workshop on Geothermal Reservoir Engineering

Stanford University, Stanford, California, February 24-26, 2014

SGP-TR-202

Hydrogeochemical characterization and the origin of hot springs in the Cidanau Geothermal

field, West Java, Indonesia

Boy Yoseph CSS SYAH ALAM1,2

, Ryuichi ITOI1,Sachihiro TAGUCHI

3, and RieYAMASHIRO

1

1 Department of Earth Resources Engineering, Faculty of Engineering, Kyushu University, Fukuoka 819-0395, Japan

2Facultyof Geology, Padjadjaran University, Bandung, West Java, Indonesia

3Faculty of Science, Fukuoka University, Fukuoka, Japan

boy_yoseph@mine.kyushu-u.ac.jp

Keywords: Cidanau geothermal field, hot spring, stable isotopes, geothermometry

ABSTRACT

The Cidanau geothermal field is situated in Banten province, West Java. Nine water samples from hot spring and well bore were

collected and analyzed for their chemical composition and stable isotopes of water. Hot springs are mainly discharging at the

northern foot and on the northern slope of Mt. Parakasak. On the basis of analysis the hot spring water was divided into three types:

HCO3, Cl-HCO3 and SO4 types. The stable isotope values for δ18O and δD range from -7.10/00 to -6.70/00 and from -430/00 to -410/00,

respectively. These are plotted on the meteoric water line, implying meteoric water origin. Na-K-Mg geothermometers shows

reservoir temperatures in the study areas vary from 220-2400C.

1. INTRODUCTION

Indonesia has many geothermal promising areas because of her tectonic setting. About 70 out of more than 200 geothermal

prospects in Indonesia were identified as high-temperature system by Sudarman et al. (2000). Tectonic setting in Java is controlled

by subduction between oceanic crust (Indo-Australia) and continent crust (Euresia). The study area located in the northern part of

Mt. Parakasak, Banten Province, West Java, Indonesia (Figure 1).Volcanoes in Java are dominantly characterized of andesitic type

and are related to stratovolcano-high terrain geothermal system (Utama et al., 2012). The understanding of hydrochemistry and

stable isotopes for identifying geothermal manifestation is required. The objective of this study is to investigate a geothermal

system in the Cidanau geothermal field based on the hydro-chemical and isotopes data from hot spring and well-bore.

2. GEOLOGICAL SETTING

There are two volcanoes in the study area: Mt. Karang and Mt.Parakasak. The highest point of Mt.Karang is 1778 m above sea

level. These mountains present cone shape with distinct break-in slopes. Thus, in accord with the underlying edifice, river courses

on the upper slopes have high gradients and narrow valley, the drainage pattern has developed radially. The edifice of Mt. Karang is

built on the caldera whose walls are present only in the north and east. The caldera bottom is partly covered by swamp and by

forest.

The oldest rocks in the study area are composed of (1) Rawa Danau sediment; (2) Mt. Karang pyroxene andesite-basalt; (3) Mt.

Karang andesitic-basaltic lava; (4) Mt. Karang tuff; (5) Mt. Karang volcanic breccia; (6) Mt. Parakasak lapili tuff; (7) Mt.

Parakasak volcanic breccia; (8) Mt. Kemuning andesitic-basaltic lava; (9) old volcanic tuff (tufa Banten); (10) weathered tuff, part

of the ancient volcanic lake product) (Figure2).

Normal faults with a strike of west-east and southwest-northeast control the appearance of springs in the study area. Herdianita and

Julinawati (2007) reported that there faults are extension of the Lampung fault system in south-east Sumatra.

3. SAMPLING LOCATION AND ANALYSIS METHODS

Eight hot spring (KR-5, 6, 7, 8, 9, 10, 37 and 43) and one wellbore (KR-45) samples were taken from northern part Mt. Parakasak.

The well bore has a depth about 30m. Sampling points are mostly located in the elevation about 100 m a.s.l., except KR-43 at 253

m a.s.l. as shown in Figure3. Field survey was carried out during July-August 2012. Water samples were filtrated with 0.45 µm

membrane filter and stored in polyethylene bottles in situ. pH, temperature and electric conductivity (EC) were measured in the

field. Samples for SiO2 and cation analyses were acidified by adding 1 ml of 1N HCl to 10 ml solution. Cations (Li, Na, K, Mg and

Ca) and anions (F, Cl⁻ and SO42-) were analyzed using ion chromatography (Dionex ICS-90) and aluminum (Al), boron (B) using

ICP-AES (Vista-MPX). SiO2 was analyzed by the molybdate yellow method using spectrophotometer (Hitachi U-1100). HCO3-

was analyzed by the titration method with 0.1 M HCl soon after the sampling. Water isotopes (δ18O and δD) were determined using

the CO2–H2 equilibration method (Epstein and Mayeda, 1953). Then, the isotope ratios were measured using DELTA Plus mass-

spectrometer. These internal standards were calibrated using international reference materials V-SMOW and SLAP with analytical

precisions of ±0.10/00 for δ18O and ±10/00 for δD.

4. RESULT AND DISCUSSION

4.1 Chemical characterization

Table1 summarizes the results of chemical analysis of water samples from hot spring and well bore. pH values of water from hot

springs and well bore samples are generally of neutral or slightly acidic in the range from 4.3 to 7.4. In particular, the hot spring

water of KR-43 is acidic with pH 4.3 and the sampling location is higher than the others. Water temperature ranges from 32.2 to

610C. KR-37 has the lowest temperature, 32.20C, among others, may be due the water experiencing mixing with the water from

shallow aquifer. The electrical conductivity (EC) values vary in the range from 60 to 241 mS/m with average about 170 mS/m. The

chemical concentration of hot spring and well bore water sample have values range from 25 to 302 mg/l for Na+, from 7 to 51mg/l

for K+, from 22 to 112mg/l for Mg2+, from 55 to 250mg/l for Ca2+, from 9 to 419mg/l for Cl-, from 1 to 535mg/l for SO42- and from

0 to 631mg/l for HCO3-. Figure 4 shows ternary diagram for SO4 – HCO3 - Cl. This figure shows that water samples are divided

into three groups: Cl-HCO3, SO4 and HCO3. The Cl-HCO3 type of water is dominant (KR-6, 7, 8, 9, 10 and 45) and is discharging

at the northern foot of Mt. Parakasak, which elevations are about 100 m a.s.l. Cl rich waters among the Cl-HCO3 type (KR-6, 7 and

8) are discharging in the central part, and less Cl ones (KR-9, 10, and 45) are from the margin of the hot spring area. The higher Cl

content of these water indicates that water are come from deep reservoir at high temperatures with minimal groundwater mixing.

All water samples exhibit low SO4 concentrations, 0.93-25.4 mg/l, except KR-43, 535mg/l. Geothermal water in deep reservoirs

generally has low sulfide concentration because of reducing condition (Nicholson, 1993). The concentration of SO4 increases as a

result of oxidation of hydrogen sulfide when sulfide bearing fluids interact with oxygen-bearing groundwater at relatively shallow

depth. The Cl/SO4 ratio is generally higher in deep geothermal waters than in surface waters (Ellis and Mahon, 1977). In the study

area, several hot spring samples demonstrated a high Cl/SO4 values due to the occurrence of high Cl- concentrations such as 210-

419 mg/l. KR-43 of SO4 type occurs on the northern slope of Mt. Parakasak at 253 m a.s.l. The sample has relatively low Cl

concentration, 34mg/l, and high SO4, 535mg/l, with pH4.3, then it can be classified into steam-heated water. KR-5 and KR-37 are

of HCO3 types. KR-5 discharges at the margin and KR-37 at the center of the hot spring area. These waters may be formed near

the surface due to condensation of water vapor and mixed with either ground or surface water. This reason also reflection of the

KR-37 has lower temperature. Water of SO4 and HCO3 types, which react with H2S and CO2 gas near the surface through

oxidation, consequently forms H2SO4 and H2CO3 in the solution (Herdianita and Julinawati, 2007).

Figure 4: Ternary plot of hot springs and one wellbore showing Cl-SO4 –HCO3 compositions (Giggenbach, 1988).

Figure 5: Ternary plot for relative concentration of Ci, Li and B

Figure 5 shows a ternary plot for the relative concentrations of Cl, Li and B. The result shows that Cl presents relatively high

concentration compared to Li and B. KR-10 is plotted close to the Cl corner, implying strong mixing effects with sea water of low

B/Cl ratio (Giggenbach, 1991). Hot spring sample KR-43 is plotted close to the B corner represent the absorption of high B/Cl

steam. Cl and B content reflect their abundance in the rock with which the water reacted rather than the maturity of the system. This

indicated the geothermal water could be considered as relatively young geothermal system (Gemici and Tarcan, 2002). Others hot

spring samples (KR-5, 6, 7, 8, 9, 37 and 45) are plotted closed to the crustal rocks, suggesting that its composition may be due to

simple rock leaching.

4.2 Stable Isotopes of δD and δ18O Isotope studies of the groundwater and surface water in the northern part of Mt. Karang has been carried out by Syah Alam et al.,

(2013). The result was concluded all water samples plotted closely to the global meteoric water line, implying that they are of

meteoric water origin as shown in figure6. In this figure also show the hot spring samples are plotted among the groundwater

samples, implying the water sources are similar. The δ18O values in hot spring and well bore have range from -7.10/00 to -6.70/00 and

δD from -430/00 to -410/00. The increase of δ18O is related to the δ18O exchanging between deeply circulating meteoric waters and

reservoir rocks in geothermal systems.

Figure 6: Stable isotopes of of δ18O versus δD

4.3 Chemical Geothermometers

Chemical geothermometers were used to calculate the reservoir temperature for the hot springs water samples in the study area.

Calculated temperatures with silica and Na/K geothermometers are summarized in Table3. The silica geothermometers provides

temperatures of 147-1720C by quartz (adiabatic), 154-1840C by quartz (conductive) and 129-1640C by calcedony.

Figure 7: Na-K-Mg ternary diagram (Giggenbach, 1988)

Figure 7 shows a plot of Na-K-Mg on ternary diagram developed by Giggenbach (1988). The results indicate that all data are

plotted in the immature water region and Mg corner. This indicates that none of these hot spring and well bore samples have

attained equilibrium conditions with the reservoir rock. However, all water samples are plotted in the region of immature waters.

Using extrapolation, the reservoir temperature was estimated to be 2200C-2400C. This suggests mixes of a deep water and shallow

groundwater.

5. CONCLUSIONS

A hot spring discharging along the slope of the Mt. Parakasak is acidic and SO4 type, however in the low elevation that have HCO3

and Cl-HCO3 types with neutral pH. All water samples plotted are deviated to the global meteoric water line, implying that they are

the meteoric water origin. Na-K-Mg geothermometers shows reservoir temperatures in the study areas vary from 220-2400C.

6. ACKNOWLEDGEMENTS

The authors thank to the Global-Center of Excellence in Novel Carbon Resource Sciences, Kyushu University for financial support

for this work, and Directorate General Higher of Education National Ministry of Education Indonesia for Scholarship Abroad. The

authors thank to Faculty of Geology, University of Padjadjaran and the Unpad team for their invaluable participation in this study.

7. REFERENCES

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Epsteins, S., and Mayeda, T.: Variations of the O18 content of waters from natural sources, Geochimica et Cosmochimica Acta, 4,

(1953), 213-24.

Fournier, R.O., and Rowe, J.J.: The solubility of amorphous silica in water at high temperatures and high pressures. Am. Min., 62,

(1977), 1052-1056.

Fournier, R.O., and Potter, R.W. II: Magnesium correction to Na-K-Ca chemical geothermometer, Geochimica et Cosmochimica

Acta, 43, (1979), 1543-1550.

Gemici, U., and Tarcan, G.: Distribution of boron in thermal waters of western Anatolia, Turkey, and examples of their

environmental impacts, Environmental Geology, 43, (2002), 87-98.

Giggenbach, W.F.: Geothermal solute equilibria. Derivation of Na-Ka-Mg-Ca geoindicators, Geochimica et Cosmochimica Acta

52, (1988), 2749-2765.

Giggenbach, W.F.: Chemical techniques in geothermal exploration, Application of Geochemistry in Geothermal Reservoir

Development (D’Amore F., Ed.), UNITAR/UNDP Center on Small Energy Resources, Rome, (1991), 19-144.

Herdianita, N.R., and Julinawati, T.: Hidrogeokimia air panasbumi daerah Cidanau dan sekitarnya, Anyer, Provinsi Banten

berdasarkan manifestasi permukaannya. Jurnal Geoaplika, 2, (2007), 105-119. (In Indonesian)

Nicholson, K.: Geothermal Fluids Chemistry and Exploration Techniques, Springer-Verlag, Germany, (1993).

Rusmana, E. Suwitodirdjo, K., and Suharsono.: Geology of the Serang Quadrangle, Jawa, Scale 1: 100,000, explanatory notes and

geological map, Geological Research and Development Centre, Bandung, (1991), P.19.

Santosa, S.: Geology of the Anyer Quadrangle, West Java, Scale 1:100,000, explanatory notes and geological map, Geological

Research and Development Centre, Bandung, (1991), P. 32.

Sudarman, S., Suroto., Pudyastuti, K., and Aspiyo, S.: Geothermal development progress in Indonesia: Country update 1995-2000,

Proceedings, World Geothermal Congress, Kyushu-Tohoku, Japan (2000).

Syah Alam, B.Y.C.S.S., Itoi, R., Taguchi, S., and Yamashiro, R.: Recharge source and groundwater flow system in Mt. Karang,

West Java, Indonesia based on the stable isotopes, Proceeding, International Symposium on Earth Sciences and Technology

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Figure1. Study area about 160 Km from Jakarta (Source: Google earth 2013)

Figure 2: Geological map of Mt. Karang area showing the lithological unit (1 Rawa Danau sediment; 2 Mt. Karang

pyroxene andesite-basalt; 3 Mt. Karang andesitic-basaltic lava; 4 Mt. Karang tuff; 5 Mt. Karang volcanic breccia; 6 Mt.

Parakasak lapili tuff; 7 Mt. Parakasak volcanic breccia; 8 Mt. Kemuning andesitic-basaltic lava; 9 Old volcanic tuff (tufa

Banten); 10 weathered tuff, part of the ancient volcanic lake product) and modified from Rusmana and Santosa (1991)

Figure 3: Sampling location

Table1

Field measurement and chemical analysis

Table 2

Reservoir temperatures (⁰C) calculated using chemical geothermometers

Sample Id KR-

5

KR-

6

KR-

7

KR-

8

KR-

9

KR-

10

KR-

37

KR-

43

KR-

45

Measured water temperature 61.1 54.6 53.9 55.5 42.1 51.6 32.2 50.3 47.6

Geothermometers Calculated reservoir temperature (⁰C) References

SiO₂ Quartz adiabatic 172 164 164 165 158 162 147 166 157 Fournier (1977)

SiO₂ Quartz conductive 184 175 175 177 167 172 154 177 167 Fournier (1977)

SiO2 (Chalcedony) 164 153 153 155 144 150 129 156 143 Fournier (1977)

Sample Id Elevation Temp. pH EC Li+

Na+

K+

Mg²+

Ca²+

Cl-

SO4 ²-

HCO3-

SiO₂ B F δD δ18

O

(m asl) (⁰C) (mS/m)

KR-5 107 61.1 6.7 91.4 0.10 102 25.8 21.9 57.7 18.5 23.8 495 213 0.35 0.6 -41.7 -6.7

KR-6 98 54.6 6.5 217 1.55 302 49.8 29.9 69.6 419 7.00 499 185 4.81 0.5 -41.4 -6.8

KR-7 98 53.9 7.4 190 1.39 292 50.7 31.1 74.0 351 3.52 532 185 3.33 0.3 -41.0 -6.8

KR-8 98 55.5 7.4 200 1.43 293 49.8 30.3 70.7 368 1.89 537 190 2.49 0.5 -41.2 -6.8

KR-9 96 42.1 6.5 180 1.29 263 50.8 31.0 74.7 287 0.93 622 164 0.51 0.5 -41.5 -6.9

KR-10 96 51.6 7.1 159 0 214 39.4 24.7 55.5 210 25.4 531 178 0.88 0.6 -41.4 -6.8

KR-37 104 32.2 6.2 60.2 0.01 25.3 6.95 28.5 64.8 8.95 17.7 377 133 0 0.2 -42.9 -7.0

KR-43 253 50.3 4.3 241 0.08 157 26.2 112 250 34.1 535 0.00 192 0.16 0.3 -42.5 -7.1

KR-45 96 47.6 6.9 195 1.33 266 46.6 29.7 77.2 297 1.57 631 163 0.47 0.4 -41.4 -6.8

(mg/l) (‰)