Indonesian Journal of Geology, Vol. 8 No. 2 June 2013: 89-96

Hotwater Geochemistry for Interpreting The Condition of Geothermal Reservoir, Dieng Plateau Case, Banjarnegara- ,

Geokimia Air Panas untuk Menafsirkan Kondisi Reservoir Panasbumi, Kasus Dataran Tinggi Dieng, Kabupaten Banjarnegara-Wonosobo, Jawa Tengah

Y. Ramadhan1, K. Channel1, and N. R. Herdianita2

1Geological Engineering Programme, Bandung Institute of Technology, Jln. Ganesha 10 Bandung 2Research Group of Applied Geology, Bandung Institute of Technology, Jln. Ganesha 10 Bandung

Abstract The researched area, located in the Dieng Plateau, is included into the Holocene Dieng Volcanic Rock Unit. The regional structure in this area is originated from the major caldera with local fault having orientation of SE - NW. Surface manifestations found in the researched area are hot springs located in Bitingan, Sileri, Siglagah, Pulosari, Kaliputih, and Sikidang. Fumaroles occur in Candradimuka and Pagerkandang and mud pools are located in Sileri and Sikidang craters. Temperatures of the hot springs ranges from 43o to 61o C, pH of 6 - 7, and their conductivity are of 38-78 MeV. The type of hotwater is a mixture of bicarbonate, sulfate, and chloride sulfate deriving from condensation of steam. Based on a relative composition of Cl-Li-B, the hot water is originated from four different reservoirs with different rock associations, while their reservoir temperatures vary from 225o to 300o C. Keywords: geochemistry, water type, reservoir, subsurface temperature, Dieng

Abstrak Daerah penelitan yang terletak di Dataran Tinggi Dieng, termasuk ke dalam Satuan Batuan Gunungapi Dieng berumur Holosen. Daerah penelitian terletak di struktur kaldera dengan sesar penyerta berarah tenggara - barat laut. Manifestasi yang ditemukan di daerah penelitian adalah mata air panas yang berada di Bitingan, Sileri, Siglagah, Pulosari, Kaliputih, dan Sikidang. Fumarola muncul di Candradimuka dan Pagerkandang, serta kolam lumpur hadir di Kawah Sileri dan Kawah Sikidang. Mata air panas mempu- nyai temperatur 43o - 61o C, pH 6 - 7, dan konduktivitas 38 - 78 MeV. Studi geokimia menunjukkan bahwa air panas tersebut terdiri atas bikarbonat, sulfat, dan campuran klorida sulfat yang menandakan berasal dari kondensasi uap air. Berdasarkan kandungan relatif Cl-Li-B, air panas ini diduga berasal dari empat reservoir yang berbeda asosiasi batuannya dengan temperatur reservoir berkisar antara 225o dan 290o C. Kata kunci: geokimia, tipe air, reservoir, temperatur bawah permukaan, Dieng

Introduction The aim of this research is to find out the charac- teristics of the surface manifestations for modeling Administratively, most of Dieng Plateau is lo- the pattern of their subsurface geothermal fluid flow. cated in Banjarnegara Regency, and the rest is in The similar studies carried out by Pardyanto (1970) , Central Java Province, about and Condon et al. (1996) are used as references and 26 km in the north of Wonosobo City (Figure 1). the basic for this further research.

Manuscript received: November 22, 2010, final acceptance: June 11, 2013 Corresponding Author: [email protected]/[email protected] 89 90 Indonesian Journal of Geology, Vol. 8 No. 2 June 2013: 89-96

o o 109 39'00'’ E 109 56'00'’ E S S o o 7 09'00'’ 7 09'00'’ S S o o 7 16'00'’ 7 16'00'’ o o 109 39'00'’ E 109 56'00'’ E

LEGEND : N

Mud pond Bitingan hotspring Siglagah hotspring Kaliputih hotspring

0 2 km Fumarole Sileri hotspring Pulosari hotspring Sikidang hotspring

Figure 1. Locality map of the studied area and surface manifestation (image from http://maps.google.com, December 2009).

Methodology pression. Generally, the studied area is mountainous and a plateau, having steep to medium steep slopes. Methods used in this study include field work, The stratigraphy of this area according to Condon et water sampling, laboratory analysis, and interpreta- al. (1996) is included into the Dieng Volcanic Rock tion. The water sampling followed the guidance of Unit consisting of andesite lava unit and quartz Nicholson’s (1993) and Standar Nasional andesite, as well as volcaniclastic rock. The rock (2004). Chemical analyses were applied to six hot silica content decreases from young to old unit. The water samples including pH when the temperature regional structure of the studied area has a caldera was 25o C, sixteen main anion analysis elements Cl-, form with local fault trending southeast - northwest. 2- - 2+ + + SO4 , and HCO3 , and cation like Ca , Na , K , and Mg2+. The analyses were also applied to neutral ele- Surface Manifestation ments like SiO2, NH3, and F, as well as contaminant Based on the observation in the field, there are elements commonly found in geothermal system eleven geothermal manifestations identified consist- like As3+ and B. Data conformity has been evaluated ing of six hotsprings, three fumaroles, and two mud using ion equilibrium. Furthermore, water sample ponds (Figure 1). was also analyzed to find out the content of oxygen isotope-18 (δ18O) and H-2 or deuterium (δD) based Hotwater Chemical Composition on mass spectrometer (MS). Measurements in the field and laboratory analyses upon six hotwater samples resulted in the temperature, pH, value of the hardness of the water

(CaCO3) and sixteen elements as shown in Tables Research Results and Discussion 1 and 2, as well as deuterium isotope analysis (δD) and oxygen-18 (δ18O) as shown in Table 2. Geological Condition According to Van Bemmelen (1949) physio- Characteristics of The Hotwaters graphically the researched area is included into the Of six hotwater samples studied, generally South Serayu Zone which is a southern mountain de- their temperature is between 44o - 65o C, pH in the Hotwater Geochemistry for Interpreting The Condition of Geothermal Reservoir, Dieng Plateau Case, 91 Banjarnegara-Wonosobo Regency, Central Java (Y. Ramadhan et al.) large, coming out of cracks, there out of cracks, coming large, large, coming out of cracks, there coming large, 2 2 2 Manifestation type and Description in broad 2 Consisting of five hotsprings, coming out of cracks, no bubbles, colourless, odourless, tasteless, no surface deposit Hotspring coming out of cracks, no bubbles, colourless, odourless, tasteless, no surface deposit Hotspring formed through depression system, no bubbles, colourless, odourless, tasteless, no surface deposit Hotwater pond, 1.05 m pond, 12.5 m Hotwater are bubbles, a bit sulphur smell, there is deposit around the pond Hotspring, coming out of cracks, no bubbles, sulphur smell there is surface deposit and a bit muddy, Fumaroles, white smokes, thundering sound, no bubbles sound, no bubbles, Fumaroles, white smokes, thundering a bit sulphur smell, there is deposit Fumaroles, white smokes, thundering sound, there are bubbles, strong sulphur smell, there is deposit are there smell, strong sulphur Mud pond, muddy water, area the is sulphur deposit, smokes, there white bubbles, about 100 m are there smell, strong sulphur Mud pond, muddy water, 25 about is sulphur deposit, smokes, there bubbles, white m are bubbles, a bit sulphur smell, surface deposit n/a n/a n/a n/a n/a n/a n/a n/a 0.47 0.07 0.01 (l/dt) Debit - n/a n/a n/a n/a n/a 50.6 48.5 71.5 35.0 78.8 54.8 tivity (MeV) Conduc n/a n/a n/a n/a n/a pH 6.20 6.77 6.30 7.09 6.14 6.57 C o 61 70 88 59 65 61 44 43 94* 93* 94* T 53’00 E o 53’59.1’’ 53’26.5’’ 51’25.3’’ 54’53.2’’ 53’24.8’’ 53’0.48’’ 54’3.54’’ 52’21.6’’ 54’21.72’’ 43’49.68’’ o o o o o o o o o o 109 109 109 109 109 109 109 109 109 109 109 Coordinate S 11’28.5’’ 11’47.9’’ 11’10.6’’ 11’42.3’’ 11’06.6’’ 11’42.3’’ 13’11.6’’ 13’09.4’’ 11’27.18’’ 13’36.54’’ 13’54.48’’ o o o o o o o o o o o 7 7 7 7 7 7 7 7 7 7 7 Date of Sampling 9 Juni 2009 9 Juni 2009 9 Juni 2009 9 Juni 2009 9 Juni 2009 8 Juni 2009 8 Juni 2009 8 Juni 2009 8 Juni 2009 8 Juni 2009 9 Juni 2009 n/a n/a n/a No Slr-01 Bit-06 Pul-01 Klp-01 Pgd-06 Pgd-14 Pgd-07 Skd-09 Sample Location Sileri Siglagah Pulosari Sikidang Sipandu Pager Kandang Candradimuka Sileri Sikidang Bitingan Kaliputih 1 2 3 4 5 6 7 8 9 11 10 No. Table 1. Characteristics of Surface Manifestation Table 92 Indonesian Journal of Geology, Vol. 8 No. 2 June 2013: 89-96 5 9 9 3 4 11 (%) Ion Balance O 18 δ -7.62 ± 0.2 -6.98 ± 0.4 -7.59 ± 0.5 -6.47 ± 0.2 -5.03 ± 0.1 -2.46 ± 0.4 δD -46.97 ± 0.3 -49.37 ± 0.8 -49.57 ± 1.0 -46.97 ± 0.2 -30.87 ± 0.4 -38.97 ± 0.9 + Li 0.03 0.05 0.00 0.02 0.88 0.01 3+ As 0.01 0.01 0.09 1.73 0.00 0.01 2 total 0.00 0.00 0.00 1.73 0.00 5.19 CO - 3 0.00 21.74 118.36 243.30 231.90 634.09 HCO 2 SiO 38.94 41.09 44.06 46.34 46.34 58.68 4 0.83 1.94 0.04 0.95 5.27 NH 0.005 B 1.38 3.84 0.32 0.87 1.19 0.69 O) in ‰ 18 Mn 0.07 0.50 0.40 0.86 0.05 0.61 Fe 0.01 0.12 0.34 0.68 0.08 11.44 + K 28.20 21.30 21.10 52.00 41.50 51.60 + Na 71.30 61.69 17.43 76.23 321.30 203.40 2- 4 SO 11.10 59.50 76.80 28.70 214.0 862.00 F 1.39 1.15 1.17 5.96 1.20 6.82 - Cl 7.42 17.96 69.22 289.0 27.48 484.50 2+ Mg 23.38 58.97 17.04 63.35 58.94 125.50 2+ 4.03 Ca 11.28 13.69 14.50 19.33 43.50 3 124 277 106 358 527 351 CaCO 693 849 283 1218 2790 1832 DHL (uS/cm) c) 0 pH 7.99 8.37 8.27 4.54 8.36 3.88 (lab .25 Sileri Pulosari Bitingan Siglagah Sikidang Kaliputih Location 1 2 3 4 5 6 No Table 2. Water Geochemical Analysis (in mg/kg), Isotope Deuterium (δD), and Oxygen-18 (δ Geochemical Water 2. Table Hotwater Geochemistry for Interpreting The Condition of Geothermal Reservoir, Dieng Plateau Case, 93 Banjarnegara-Wonosobo Regency, Central Java (Y. Ramadhan et al.) field is between 6 - 7 (neutral) and the result of hotsprings were formed in shallow areas as the laboratory analysis is between 3 - 8 (acids - bases). result of CO2 gas absorbsion and steam heated wa- Measured conductivity ranges from 35 - 78 Mev, ter condensation. Sulfate hotwater coming out of and the measured hardness of the water (CaCO3) Sikidang hotspring shows that it was formed in the based on laboratory analysis is between 106 - 527 shallowest part caused by steam heated water into mg/kg. meteoric water or as the result of H2S oxidation in

Analysis result in Table 2 shows that ionic equi- oxidation zone forming H2SO4. Water mixed with librium has the value of 4 - 11 %. Ionic equilibrium sulfate chloride coming out of Pulosari hotspring of lack to 5 % is in Kaliputih, Bitingan, and Sikidang, is estimated to come from the mix of reservoir wa- while the rest is more than 5 %. The analytical result ter with condensate from the steam, and chloride is said to be proper if the ionic equilibrium is lack bicarbonate hotwater coming out at Kaliputih was or more than 5 %. This analytical result is from the formed in the condition of mix of meteoric steam hotwater coming out of Kaliputih, Bitingan, and heated water. Sikidang. Nevertheless, it does not mean that the re- sult of hotwater analysis having ionic equilibrium of Hotwater Sources and Reservoirs more than 5 % is not proper for interpretation. High Based on diagram Cl-Li-B in Figure 3, it is ionic equilibrium is also influenced by the type and estimated that hotwater coming out in the studied process undergone by the water (Nicholson, 1993). area is from four different sources or reservoirs. It is estimated the value of ionic equilibrium of more The first reservoir is the one that formed Pulosari than 5 % because it is mixed up with meteoric water hotspring. This hotspring has the comparison of B/ or surrounding rocks. Cl of less than 0.01. This shows the influence of a

Based on diagram Cl-HCO3-SO4, hotsprings volcanomagmatic process. The second reservoir in the studied area can be classified into several comes out as Kaliputih hotspring. This hotspring types (Figure 2). Bicarbonate water comes out of has the comparison of less than 0.02 which shows hotsprings in Bitingan, Sileri, and Siglagah. These the influence of a volcanomagmatic process but less

Legend: Cl Hotspring

Deepreservoir Bitingan

Sileri

Siglagah

Pulosari

Kaliputih

Sikidang Direct convention of dissolved CO to HCO

2

3

olcanic waters V

Steam heated water SO 4 HCO3

Figure 2 Diagram Cl-HCO3-SO4, showing hotwater which generally is of bicarbonate type. 94 Indonesian Journal of Geology, Vol. 8 No. 2 June 2013: 89-96

Legend: Cl/100 Hotspring

Bitingan .001 .01 Sileri

.002 .02 Siglagah B/CI Li/CI Pulosari

.005 .05

Steam heating Kaliputih

Sikidang .01 0.1

.05 Rock .5

2Li 1.0 0.5 0.2 0.1 .05 .02 .01 B/5

Li/B

Figure 3. Diagram Cl-Li-B showing four different hotwater reservoirs.

Legend: -25 Hotspring Bitingan -30 18 MWL : δD = 8.δ O +10 Sileri

-35 Siglagah

-40 Pulosari 50 Kaliputih 0 SMOW

δ D (%) -45

-50 18 O +10 Sikidang

δ D (%) : δD = 8.δ -50 -100 MWL Fluida magmatik -150

-55 -200 -20 -15 -10 -5 -0 -5 10 δ18O (%) -60 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 δ18O (%)

Figure 4. A graph showing the relationship between hotwater stable isotope-18 (δ18O) and δD in the studied area. Hotwater Geochemistry for Interpreting The Condition of Geothermal Reservoir, Dieng Plateau Case, 95 Banjarnegara-Wonosobo Regency, Central Java (Y. Ramadhan et al.) dominant than that of Pulosari hotspring. The third This is also shown by the type of Sikidang hotspring reservoir comes out as Bitingan, Sileri, and Siglagah which is sulfate water. hotsprings that are included into one group. This shows that there is a similarity in steam heating Estimated Subsurface Temperature having the comparison of Li/Cl of less than 0.001 The estimated subsurface temperatures were and B/Cl between 0.05 and 0.1. The forth reser- calculated using geothermometer method of Na-K- voir comes out as Sikidang hotspring having the Ca compared with fumarol manifestations available comparison of B/Cl less than 0.03. The hotspring in the studied area. Geothermometer of Na-K-Ca reservoir differences are also in the hydrology pat- was used to calculate the reservoir temperatures at tern and geological unit. the studied area, because hotwater in this area has interacted with surrounding rocks and has a high Characteristics of Stable Isotope δ18O and δD Ca content. The hotwater used for geothermometer The relationship graph between stable isotope calculation is from Pulosari that has a mix type of δD and δ18O in Figure 4 shows that the content of sulfate chloride with pH value. Based on the geother- hotwater isotope δD and δ18O in the studied area is mometer calculation of Na-K-Ca, hotwater reservoirs generally like that of the content of global mete- in the studied area have a temperature of 295o C. oric water stable isotope. A bit displacement of the As a comparison, manifestation as dry fumarol content of stable isotope between meteoric water with dry characteristics and thundering sound is and hotwater shows that geothermal system has available in Pagerkandang and Sipandu fumaroles. interacted with surrounding rocks and has reached According to Hochstein and Browne (2000) dry an equilibrium (Nicholson, 1993). Figure 4 shows fumarol show the geothermal reservoir of ≥ 225o C. that Kaliputih hotspring has δD and δ18O of less than Therefore, it can be interpreted that Pagerkandang those other areas’. This shows that there are differ- and Sipandu geothermals have reservoir tempera- ent infiltration areas with other hotsprings. Sikidang tures of ≥ 225o C. Thus, it can be said that the geo- hotspring which has δD and δ18O shows that it has thermal temperature of the studied area is above 225o been steam heated or evaporation near the surface. C, and may be it reaches 300o C (Figure 5).

1 cm

Mt. Pagerkandang Mt. Merdada 500 m 2300 Sikidang Bitingan

1900

1500 o o Reservoir t res = 225-295 C Reservoir t res = 225-295 C N S

Legend:

Steam/hot water Geyser Fumaroles Geyser Mud pools

Figure 5. Tentative sketch model of Dieng geothermal trending north - south, the geology is based on Condon et al. (1996) and field observation. 96 Indonesian Journal of Geology, Vol. 8 No. 2 June 2013: 89-96

Conclusion Acknowledgments---This paper was presented on the 39th IAGI Annual Meeting, in Lombok, on 2010. The authors The studied area is located in the South Serayu would like to thank the reviewers for their comments improv- ing the quality of this paper. area as a depression of a mountain and a high. It lies at the Dieng caldera depression which generally consists of lava andesite. Geochemical study shows that in this area there Refferences are four reservoirs which can be divided based on Condon W.H., Pardiyanto L., Ketner K.R., Amin T.C., hotwater origin. The Bitingan, Sileri, and Siglagah Gafoer S., and Samodra H. 1996. Geological Map of areas are included into bicarbonate hotwater type. Banjarnegara dan Quadrangles, Central Pulosari hotwater is included into chloride sulfate Java. Geological Research and Development Centre, hotwater. Kaliputih is included into chloride bi- Bandung. carbonate hotwater, whilst Sikidang is included Google map, 2009. http://maps.google.com. [Desember dan Januari 2010]. into sulfate hotwater. The reservoir of Pulosari and Nicholson, K.N., 1993. Geothermal Fluids, Chemistry Kaliputih hotsprings is estimated to be influenced and Exploration Techniques. Springer- Verlag, Berlin by a volcanomagmatic process, but it comes from Heidelberg, 263pp. meteoric water that has undergone heating with- Pardyanto, L., 1970. The Geology of the Dieng area Central out mixing with magmatic water based on stable Java, A report based mainly on photo-interpretation and isotope. literature study. Vulcanology Report. Standard Nasional Indonesia, 2004. Air dan air limbah. SNI Based on the subsurface calculation, the tem- 06-6989.11-2004 Badan Standardisasi Nasional. o o peratures of studied area are between 225 - 295 C. Van Bemmelen, R.W., 1949. The Geology of Indonesia. Sileri and Sikidang is the potential area for a Vol.1A. The Hague, Government Printing Office, Jakarta. further study, this area is chosen based on the appear- ance of fumaroles which have high temperature and spring with bicarbonate type showing heated surface water with high surface temperature.