Journal of Volcanology and Geothermal Research 229-230 (2012) 23–33

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Journal of Volcanology and Geothermal Research

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Water geochemistry and soil gas survey at Ungaran geothermal field,central Java, Indonesia

Nguyen Kim Phuong 1,2,⁎, Agung Harijoko 3, Ryuichi Itoi 1, Yamashiro Unoki 1

1 Department of Earth Resource Engineering, Faculty of Engineering, Kyushu University, 744 Motooka, Nishiku, Fukuoka 819-0395, Japan2 Department of GeoEnvironment, Faculty of Geology and Petroleum Engineering, Ho Chi Minh City University of Technology, 268 Ly Thuong Kiet street, Ward 14, District 10,Ho Chi Minh City, Viet Nam3 Department of Geological Engineering, Faculty of Engineering, Gadjah Mada University, Jl. Grafika 2, Yogyakarta (55281), Indonesia

⁎ Corresponding author at: Department of Earth ResEngineering, Kyushu University, 744, Motooka, NishikTel./fax: +81 92 802 3345.

E-mail addresses: nkphuong6280@yahoo.com, nkph(N.K. Phuong).

0377-0273/$ – see front matter © 2012 Elsevier B.V. Alldoi:10.1016/j.jvolgeores.2012.04.004

a b s t r a c t

a r t i c l e i n f o

Article history:Received 7 September 2011Accepted 3 April 2012Available online 11 April 2012

Keywords:Soil gasWater geochemistryUngaran geothermal fieldIndonesia

A soil gas survey for radon (Rn), thoron (Tn), CO2, and mercury (Hg), and the chemical analysis of hot springwaters, were undertaken in the Ungaran geothermal field, Central Java, Indonesia. The results of soil gas surveysindicate fault systems trending NNE–SSWandWNW–ESE. Particularly high CO2 concentrations (>20%), and highHg concentrations were detected in vicinity of the fumaroles. Emanometries of Rn, Tn and CO2 also conclusivelyidentified the presence of a fracture zone for the migration of geothermal fluid. The Hg results infer that the up-flow zone of high temperature geothermal fluids maybe located in the north of fumaroles in the Gedongsongoarea (near the collapse wall). Chemistry of thermal springs in the up-flow zone are acid (pH=4) and show aCa–Mg–SO4 composition. The thermal waters are mainly Ca–Mg–HCO3 and Ca–(Na)–SO4–HCO3 types near thefumarolic area and are mixed Na–(Ca)–Cl–(HCO3) waters in the south east of Gedongsongo. The δ18O (between−5.3 and−8.2‰) and δ (between−39 and−52‰) indicate that thewaters are essentiallymeteoric in origin. Aconceptual hydro-geochemical model of the Gedongsongo thermal waters based on the soil gas, isotope andchemical analytical results, was constructed.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Geothermal exploration began in Indonesia in 1970. More than200 geothermal systems with significant active surface manifesta-tions occur throughout Indonesia. Most of the geothermal systemsin Indonesia that have surface manifestations with fluid dischargesat boiling temperature occur in areas with Quaternary volcanismand active volcanoes, along well-defined volcanic arcs (Sudarmanet al., 2000; Hochstein and Sudarman, 2008). During the 1980s thePertamina Company explored a number of areas characterized bysignificant fumarolic activity on the flanks of inactive Holocene volca-noes. At Ungaran, Pertamina Co. conducted geophysical surveys usingseveral geophysical methods (resistivity, magnetotellurics, and aero-magnetic surveys) between 1985 and 1990 at Ungaran (Budiardjoet al., 1989).

The Ungaran volcano is located in the Central Java province about30 kmsouthwest of Semarang, Indonesia (Fig. 1), and is anundevelopedgeothermal prospect. There are two active fumaroles on the southern

ources Engineering, Faculty ofu, Fukuoka 819-0395, Japan.

uong@mine.kyushu-u.ac.jp

rights reserved.

flank of the dormant Ungaran volcano, about 2 km SW from its summit.The Gedongsongo area, on the southern flank of Ungaran volcano, ischaracterized by the presence of thermal manifestations such as fuma-roles, hot springs, acidic mud pools and hydrothermal alterationzones. Four wells were drilled down to a depth of 500 m around theGedongsongo area. These wells showed slightly anomalous tempera-ture (about 47 °C at 300 m depth) and temperature-gradient valuesnear the bottom (Hochstein and Sudarman, 2008). A low resistivityanomaly, about 30Ωm, associated with a deep geothermal reservoirwas inferred to occur beneath the Ungaran summit (Budiardjo et al.,1989). Further exploration was not conducted because about 90% ofthe prospect area is located in a protected forest with restricted access.

The soil gas method, i.e. the measurement of gas concentrations inthe soil, has been used to estimate the location of heat sources andtheir areal extension in geothermal prospecting (Varekamp andBuseck, 1984, 1986; Bertrami et al., 1990; Finlayson, 1992; Hernándezet al., 2000). The gases (Rn, Tn, CO2, and Hg) are assumed to be releasedfrom active geothermal systems at depth and then ascend throughoverlying rock formations and/or fracture zones. The high mobility ofthese gases makes them ideal pathfinders for concealed natural re-sources, as the gases produced and accumulated in geothermal reser-voirs can escape to the surface by migration along fractures and faults(Koga, 1982; Fridman, 1990). Soil gas surveys (Rn, Tn, CO2 and Hg)have regularly been used for the exploration of geothermal reservoirs(Varekamp and Buseck, 1984, 1986; Corazza et al., 1993), and for

Fig. 1. Location of the Ungaran volcano (solid red circle).

24 N.K. Phuong et al. / Journal of Volcanology and Geothermal Research 229-230 (2012) 23–33

detecting and delineating faults and fractures (Gregory and Durrance,1985; Toutain and Baubron, 1999; Guerra and Lombardi, 2001; Waliaet al., 2005, 2008; Yang et al., 2005).

The enrichment of radon (Rn), carbon dioxide (CO2) and mercury(Hg) in soil or soil gas has been observed in several geothermal areas(e.g., Koga, 1982; Varekamp and Buseck, 1983; Chuaviroj et al., 1987;Lescinsky et al., 1987; Klusman, 1993; Murray, 1997). Radon is a ra-dioactive noble gas that is soluble in water and decays by alpha emis-sion. The presence of Rn in geothermal areas is a function of theporosity and fracture distribution of the rocks in between the deepgeothermal source and the surface, i.e., the pathway for uprisingfluids (Koga, 1988). Radon gas surveys are widely used to monitorseismic activity and to detect the locations of fractures and faults(Walia et al., 2005; Yang et al., 2005).

Soil and soil gas surveys of Hg have been successfully used as geo-thermal exploration techniques. Van Kooten (1987), Lescinsky et al.(1987) and Murray (1997) all found broad Hg anomalies outlininghigh-temperature thermal activity zones. Soil Hg surveys has alsobeen used to locate faults in volcanic and geothermal regions(Klusman and Landress, 1979; Cox and Cuff, 1980; Varekamp andBuseck, 1983). In the sub-surface, Hg is strongly partitioned into theascending vapor and is transported to the surface as elemental Hg.This vapor is absorbed onto organic matter and clay minerals in theshallow, low-temperature soil horizons, producing elevated (above10 ppm) concentrations of Hg (Nicholson, 1993). Mercury isabsorbed by the soil in anomalous concentrations relative to the sur-rounding areas (Lescinsky et al., 1987; Van Kooten, 1987). Mercury

levels in soil are the result of accumulation and loss processes; conse-quently, soil gas mercury is a reliable indicator of geothermal fluid atdepth (Koga, 1988).

The present study aimed to i) delineate the up-flow zone of hightemperature geothermal fluids at depth, fractures below the surface,in the Ungaran geothermal field using a soil gas survey; ii) character-ize chemical characteristics of hot spring waters in the Ungaran geo-thermal field; and iii) develop a hydro-geochemical conceptual modelof thermal water in the Ungaran geothermal field.

2. Geological setting

Geothermal areas in Central Java, including the Ungaran volcano,are located in the Quaternary Volcanic Belt (Solo Zone) (Fig. 1). Thisbelt is located between the North Serayu Mountains and the KendengZone and contains numerous Quaternary eruptive centers, includingDieng, Sindoro, Sumbing, Ungaran, Soropati, Telomoyo, Merapi,Muria, and Lawu (Van Bemmelen, 1970; Thanden et al., 1996)(Fig. 1). A structural analysis of this area revealed that the Ungaranvolcanic system is primarily controlled by the Ungaran collapse struc-ture that runs fromwest to southeast of the Ungaran volcano. Old vol-canic rocks of the pre-caldera formation are controlled by northwest–southwest and southeast–southwest fault systems. The post-calderavolcanic rocks, however, do not seem to be structurally controlledby the regional faulting system (Budiardjo et al., 1997). The pre-caldera volcanic rocks and the Tertiary marine sedimentary rocks

25N.K. Phuong et al. / Journal of Volcanology and Geothermal Research 229-230 (2012) 23–33

are inferred to be the main geothermal reservoir rocks (Budiardjo etal., 1997).

Van Bemmelen (1970) noted that there is a gradual development ofvolcanism along the transverse fault from north to south, starting in thenorth with the Oldest or Proto-Ungaran in the Lower Pleistocene andending in the south with the very active Merapi volcano (Fig. 1). Twogeneration of the Ungaran volcano (2050 m) were observed becauseof gravitational collapse. The Oldest Ungaran deposits resulted fromsubmarine activity. Its basement is transitional beds, in which the facieschanges from marine into fresh water deposits consisting of coarsepolymictic conglomerates of the Lower Damar Beds. After magmabroke through the crust, the Oldest Ungaran volcano originated at theeastern end of the crest. The coarse volcanic breccias of the MiddleDamar Beds, and the coarse conglomerates, tuff-sandstones and black-clay of the Upper Damar Beds occur at the northern foot of the OldestUngaran volcano. In the Upper Pleistocene, volcanic activity waswide-spread. In the eastern part of the northern Serayu Range, volcanicactivity built up the Old Ungaran volcano, which is the second genera-tion of Ungaran volcano. The breccias at its northern foot form theNotopuro Beds, which cover the breccias of the Oldest Ungaran in theDamar Beds with an angular unconformity. After the early Pleistocenephase of volcanic growth, volcanic activity continued until theHolocene,building up the Young Ungaran volcano, which consists of pyroclasticflow deposits, pyroclastic lava and alluvial deposits.

The Ungaran geothermal system is associated with the UpperQuaternary volcanism of the Ungaran volcano. The volcanic rocksare rich in alkali metals and are classified as trachyandesite to trachy-basaltic andesite, primarily containing plagioclase, sanidine and cris-tobalite (Budiardjo et al., 1997; Kohno et al., 2005). Gedongsongo isthe main geothermal area on the southern flank of the Ungaran volca-no (Fig. 2). The Gedongsongo area is characterized by the presence offumaroles (90–110 °C), neutral pH bicarbonate warm/hot springs anddiluted steam heated hot spring (22–80 °C) with underground tem-peratures of 20 °C to 82 °C measured from 1 m depth. According toBudiardjo et al. (1997), the composition of thermal spring waters atGedongsongo can be divided into two water types. The hot wateraround the fumarolic area originates as a steam heated meteoricwater characterized by low chloride content (similar to local surfacewater), high sulfate content (up to 1000 ppm), and low pH (up to5) while neutral bicarbonate or chloride waters are located atthe other areas. Based on the analysis of soil and rocks samples col-lected around the Ungaran volcano, Kohno et al. (2005) concluded

Fig. 2. Sampling sites and geological map of th

that quartz, halloysite and alunite are the main secondary mineralsfound in the hydrothermal alteration zones. Quartz is formed by thealteration of cristobalite from Ungaran rocks, while halloysite andalunite are minerals formed by alteration s due to acidic and low tem-perature hydrothermal waters.

3. Sampling and measurement

The study area is focused on the Gedongsongo area (Fig. 2), whichis the main geothermal prospect in the Ungaran area, located inthe southern part of the Ungaran volcano. Water samples (UW-1 toUW-7A and UW-7B) were collected around the Gedongsongo areawhere comprises a volcanic complex terrain at an altitude rangingfrom 1200 to 2000 m above sea level. Others samples (UW-8A, UW-8B and UW9), however, were collected at Kendalisodo area (approxi-mately 8 km far from the Gedongsongo area) with altitude at 600 mabove sea level.

3.1. Chemical analysis of water

Water sampling was complemented by in situ measurement of pH,temperature and conductivity. Thewater sampleswere filtered through0.45 μmmembrane filters prior to storage in sterile polyethylene bottles(HDPE). Samples for cation (Li, NH4, Na, K, Mg and Ca) and silica (SiO2)analyses were collected in plastic bottles that had been acidified with1 mL of concentrated HCl. Filtered, un-acidified samples were collectedfor anions (F, Cl, HCO3, SO4) analysis. All water analyseswere conductedat Kyushu University using standard methods. Cations and anionswere analyzed using ion chromatography (Dionex ICS-90) while boron(B) was analyzed using ICP-AES (Vista-MPX). SiO2 contents was deter-mined by colorimetry and analyzed using a digital spectrophotometer(Hitachi U-1100) (APHA, 2005), while HCO3 was analyzed by titrationwith 0.1 M HCl. The analytical error for techniques was ≤5%.

An aliquot of the water samples (20 mL) was collected and storedin sterile polyethylene bottles (HDPE) for stable isotope analysis.Water isotopes (δ18O and δD) were determined using the CO2–H2

equilibration method (Epstein and Mayeda, 1953). Then, the isotoperatios were measured using the DELTA Plus mass-spectrometer atFukuoka University, Japan. These internal standards were calibratedusing international reference materials V-SMOW and SLAP withanalytical precisions of ±0.1‰ for δ18O and ±1‰ for δD.

e study area (UTM coordination system).

Fig. 3. Location of soil gas and water samples around the fumarolic area (UTM coordination system).

26 N.K. Phuong et al. / Journal of Volcanology and Geothermal Research 229-230 (2012) 23–33

3.2. Soil gas measurement

Soil gas surveys for Rn, Tn, CO2, mercury in soil gas (Hgsoil-gas) andin soil samples (Hgsoil) were conducted in an area approximately1.3 km north to south by 1.5 km west to east (Fig. 3). The distance be-tween measurement points varied from 50 to 150 m. Soil gases werecollected from a depth of 60 cm using a steel pipe (5 cm in diameter)inserted into the ground.

3.2.1. Radon measurementThe Rn and Tn concentrations were measured with a radon detec-

tor (RD-200, EDA Instruments Co. Ltd.). The soil gas was circulatedthrough the detector with an electrical pump for 10 s, replacing theair in the detection cell. The Rn concentration was measured by anα-scintillation radon counter with the soil gas pumped directly intoa scintillation chamber. When the α-particles produced duringradon decay impact the ZnS(Ag) layer in the scintillation counter,an energy pulse is created in the form of photons, measured by aphoto-multiplier and a counter. As both Rn and Tn decay by meansof α-emission, the concentrations of Rn and Tn were calculatedfrom three counts in each minute obtained for three sequentialminutes.

3.2.2. CO2 measurementTo measure the CO2 concentration, 100 mL of soil gas was sampled

from the stainless steel probe inserted into the ground using a stain-less steel syringe, and the CO2 concentration was measured using anSA-type gas detector tube (Komyo-Kitagawa Instruments Co. Ltd.).This gas detector works on the principles of chemical reaction andphysical absorption and has ±1% analytical precision. As the gas isentered into the detector tube, a constant color is produced, whichvaries in the length of discolored layer due to the reaction betweenthe reagent and the CO2. The CO2 concentration can then be obtained

directly by reading from the measuring scale on the tube or using aconcentration chart.

3.2.3. Mercury in soil gas (Hgsoil-gas) and in soil (Hgsoil)The Hg concentration was measured by the gold wire method,

which indicates both the Hg in soil gas (Hgsoil-gas) in the hole, andthe concentration in the ascending gas. The Hgsoil measurement rep-resents the concentration of Hg absorbed onto the surface of soil par-ticle. To measure Hg in soil gas, a pure gold wire (10 cm long, 1 mmdiameter, 1.5 g weight and 3.16 cm2 in the effective surface) wasleft in the hole for 7 days after completing the CO2 measurement(Koga, 1982, 1988). After a week, the gold wire was removed fromthe hole and stored in a tightly sealed glass tube.

Soil samples were collected at 0.6 m depth in the hole and sealedin plastic bags. The soil samples were then air-dried at room temper-ature for two weeks and ground with a mortar and a pestle after re-moving rock fragments and plant roots.

The mercury concentrations were determined in the laboratoryby the cold vapor atomic absorption method using mercury analyzerSP-3 (Nippon Instruments Co., Japan). This equipment uses theheating-vaporization (700 °C) technique to liberate mercury presentin the sample.

4. Results and discussions

4.1. Water chemistry and stable isotope compositions

The results of the water analyses are given in Table 1, and showthat the water temperatures ranged from 18 °C (UW 6) to 56 °C(UW 3), while pH values were in the range 3.45–7.87. The SiO2 con-tents of the thermal waters ranged from 47 to 219 mg/L, while theEC values were generally between 36 and 561 μS/cm, except for rela-tively high values in UW 8A and 8B (up to 5300 μS/cm). Other major

Table 1Chemical composition (in mg/L) and δ18O and δD values of water samples in the Ungaran geothermal field.

ID Temp pH EC HCO3− F− Cl− SO4

2− SiO2 Li+ Na+ NH4+ K+ Mg2

+Ca2+ B δD δ18O

(°C) (μS/cm) (‰) (‰)

UW-1 21.9 3.45 561 50 0.12 1.8 247 58 1.6 34.3 0.43 15.0 13.4 42.5 1.22 −47 −7.9UW-2 40.0 5.36 368 59 0.21 1.2 136 109 1.1 25.3 0.64 8.6 10.3 32.6 0.79 −49 −7.9UW-3 56.0 6.10 333 200 0.13 0.8 31.8 86 0.3 14.1 0.45 7.9 15.1 37.1 0.61 −50 −8.0UW-4 32.2 6.00 297 465 0.05 0.8 2.6 82 0.1 10.7 0.49 5.5 14.7 35.9 0.65 −51 −8.2UW-5a n.a. 6.31 36 100 0.02 0.7 3.5 23 0.1 2.3 0.02 1.2 0.7 3.5 0.36 −51 −8.2UW-6 18.0 5.42 177 107 0.01 0.7 50.3 51 0.2 6.8 0.04 3.1 5.6 18.2 0.42 −50 −8.0UW-7A 50.0 6.10 491 501 0.06 0.8 3.4 93 0.5 11.8 0.65 6.4 19.8 38.9 0.58 −51 −8.1UW-7B 25.0 5.90 164 496 0.05 0.8 2.9 89 0.4 12.3 0.53 6.0 17.6 40.7 0.52 −52 −8.2UW-8A 35.2 6.84 4580 1732 0.06 998 0.2 92 4.4 700 16.1 44.2 117.7 217.3 15.9 −39 −5.3UW-8B 38.1 6.78 5210 1824 0.06 1088 0.1 95 5.6 746 18.0 47.1 126.0 278.4 19.7 −40 −5.3UW-9a 23.8 7.87 513 351 0.07 7.2 4.4 51 0.5 23.2 0.29 2.4 26.9 62.1 0.15 −39 −6.1

n.a.: not analyzed.a River water.

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elements range from 2 to 746 mg/L for Na, from 3.54 to 278 mg/L forCa while concentrations of Mg are lower (b126 mg/L). The waterscontain relatively low K concentrations (1.18–47.11 mg/L). For anions,

Fig. 4. Chemical compositions (a) Cl–SO4–HCO3 and (b) SO4–Mg–Na of thermal watersin the Ungaran geothermal field.

the HCO3 concentration is relatively high (39–1824 mg/L) followed bySO4 (b246 mg/L). Chloride concentration is rather low except UW-8Aand 8B are relatively high (about 1000 mg/L).

The chemical compositions of the water samples are plotted onthe Cl–SO4–HCO3 (Giggenbach, 1988) and Na–SO4–Mg diagramsshown in Fig. 4. The UW 1 and UW 2 samples are classified as acid-sulfate waters, with high concentrations of SO4 (247 mg/L and136 mg/L, respectively), but low concentrations of F (b0.25 mg/L)and Cl (below 1.5 mg/L) (Table 1) suggests that UW 1 and UW 2have been steam heated, absorbing a gas phase enriched in S-bearing compounds. The SO4 enrichment can be explained by theO2-driven oxidation of H2S to H2SO4 in oxygenated near surfacegroundwater (Henley and Stewart, 1983; Tassi et al., 2010; Josephet al., 2011). Differences from the above samples, most samples areHCO3-dominated water, mostly Ca–HCO3 or Ca–Mg–HCO3 type (UW3, 4, 5, 7A, 7B and 9) while UW 8A and 8B are of the Na–HCO3–Cl orNa–Ca–Cl–HCO3 type with much higher Na, Ca, HCO3, Cl and B con-centrations than the other samples (Table 1). Water–rock interactionshould be a source of sodium and chloride in UW 8A and 8B.

The minerals in the volcanic rocks primarily consist of plagioclase,sanidine, and cristobalite with some biotite and hornblende (Kohnoet al., 2005). Table 2 shows the molar ratios of some of the majorcomponents of thermal waters in the study area. Increases in theNa/Cl and K/Cl ratios in thermal waters are likely to reflect reactionswith feldspar or clay minerals. These ratios can therefore be used asan independent indicator of residence time. Thermal waters often fol-low a longer, deeper, regional flow path than non-thermal waters,and thus have much higher Na/Cl and K/Cl ratios than non-thermalwaters (Han et al., 2010). The Na/K ratio is controlled by temperaturedependent mineral–fluid equilibria (Koga, 1988; Gemini and Tarcan,2002). The ratios of Na/K are large for all water samples, indicating

Table 2Molar ratios of some major components of water samples in the Ungaran geothermalfield.

ID Temp Na/Cl K/Cl Ca/Cl Na/K Na/Ca HCO3/SO4 Cl/B(°C)

UW-1 21.9 28.68 7.42 20.4 15.8 0.32 2.17 0.46UW-2 40.0 33.82 6.80 25.0 29.5 0.68 2.25 0.44UW-3 56.0 28.36 9.37 43.0 152 9.91 2.61 0.38UW-4 32.2 21.90 6.57 42.1 358 280 2.83 0.35UW-5a – 5.40 1.63 4.7 87.9 44.58 1.79 0.56UW-6 18.0 16.06 4.39 24.8 96.0 3.36 2.12 0.47UW-7A 50.0 21.78 6.94 41.4 349 231.9 2.28 0.44UW-7B 25.0 23.11 6.62 44.1 353 274 2.09 0.48UW-8A 35.2 1.08 0.04 0.2 26.9 5.6 13,629 19.1UW-8B 38.1 1.04 0.04 0.2 26.8 4.66 28,706 16.8UW-9a 23.8 4.96 0.3 7.6 16.7 0.65 124.4 14.6

n.a.: not analyzed.a River water.

Fig. 5. Binary diagram of Li vs Cl and B vs Cl.

Table 3Estimated temperature (in °C) for thermal water in the Ungaran geothermal filed usingsilica geothermometers.

ID Measuredtemperature

Estimated temperature(°C)

(°C)TQz a TQz b TC c T d

UW 1 21.9 109 109 80 110UW 2 40.0 142 137 116 143UW 3 56.0 129 126 101 129UW 4 32.2 127 124 99 127UW 6 18.0 102 103 73 103UW 7A 50.0 133 129 106 133UW 7B 25.0 131 127 103 131UW 8A 35.2 132 129 105 132UW 8B 38.1 134 130 107 134UW 9* 23.8 102 103 72 103

a Quartz — no steam loss from Fournier (1983).b Quartz — maximum steam loss at 100 °C from Fournier (1983).c Chalcedony from Fournier (1983).d From Fournier and Potter (1982).

28 N.K. Phuong et al. / Journal of Volcanology and Geothermal Research 229-230 (2012) 23–33

that the temperature of geothermal reservoir is not probably too high.This is in agreement with general increase in Na/K ratios of thermalwater with decreasing reservoir temperature (Ellis and Mahon,1967; Koga, 1988; Cortecci et al., 2005). Based on relatively lowNa/K ratios (b15, Table 2) water of springs at the Gedongsongo areathat have reached the surface rapidly and are therefore associatedwith up-flow structures or permeable zones while higher Na/K ratios(>15, Table 2) are indicative of lateral flows which may undergonenear-surface reactions and conductive cooling (Nicholson, 1993;Cortecci et al., 2005; Di Napoli et al., 2009). Similarly, high Na/Ca ra-tios are also indicative of direct feeding from a geothermal reservoirand less groundwater contribution, while the HCO3/SO4 ratio can beused as an indicator of flow direction (Table 2). The Na/Ca valuesfor deep well thermal waters are very high (>50), while for coldgroundwater this ratio is around 0.25. Low Na/K and Na/Ca ratiosare found in thermal waters in the north of the fumarole (UW 1 andUW 2) while to the south of the fumarole, the thermal waters havehigh Na/K and Na/Ca ratios and increasing HCO3/SO4 ratios (Table 2).Therefore, we can infer that thermal waters in the north of the fumaroleare associatedwith up-flow zones, while thermalwaters to the south ofthe fumarole are associated with lateral flow.

Fig. 6. δD vs δ18O composition of thermal waters in the Ungaran geothermal field.

The behavior of conservative components useful in the delineationof formation processes of waters, involving Cl, Li and B, is investigatedin Fig. 5. As pointed out above, Li is the alkali element least affected bysecondary absorption processes. Li is also released during water–rockinteractions and remains largely in solution (Giggenbach and Soto,1992; Mainza, 2006; Tassi et al., 2010). Boron and Cl− are not readilyincorporated into secondary, alteration minerals, so they can beconsidered conservative chemical species (Seyfried et al., 1984;Nicholson, 1993; Tassi et al., 2010). Boron may have several origins.It may be leached from sedimentary rocks; due to its volatility inhigh temperature steam, it may also be introduced with any hightemperature vapor phase absorbed into water. Moreover, the B con-tent of thermal fluids is likely during the early heating up stages.Therefore, fluids from older hydrothermal systems can be expectedto be depleted in B while the converse holds for younger hydrother-mal systems (Mainza, 2006). It is, however, striking both Cl and B isadding to the Li containing solutions in proportions close to those incrustal rocks. For the UW 8A and 8B, it can be elucidated that dissolu-tion of an averaged andesitic–rhyolitic rock, followed by exchangewith secondary minerals or interaction with gases (Fig. 5). Moreover,water rock interaction can be postulated by Cl/B ratio. Ellis andMahon (1967) found that in areas where andesitic or rhyolitic rockspredominate, Cl/B ratios are often between 10 and 30. The Cl/B ratios

Fig. 7. Na–K–Mg ternary diagram for thermal waters in the Ungaran geothermal field(Giggenbach, 1988).

29N.K. Phuong et al. / Journal of Volcanology and Geothermal Research 229-230 (2012) 23–33

of the UW 8A and 8B are from 14 to 19 (Table 2), thus, it is suggestedthat high concentration of Na, Cl, Li and B is originated from water–rock interaction.

The results of the stable isotope analysis in the Gedongsongo areaare given in Table 1 and plotted in the δD vs δ18O diagram (Fig. 6).Stable isotope compositions of meteoric water from coastal Jakarta(Gat and Gonfiantini, 1981) were used as reference data (δD=8.05δ18O+16.48) (Fig. 6). This line does not deviate significantly fromthe global meteoric water line defined by Craig (1961). In Fig. 6, allsamples from Gedongsongo plot along the meteoric water line, sug-gesting that the thermal waters are of meteoric origin. Compared to

Table 4Soil gas concentrations of Rn, Tn, CO2 and Hgsoil-gas and Hgsoil in the Gedongsongo area.

ID Temp. at 60 cm depth Rn Tn(°C) (cpm) (cpm)

UG-1 51.5 601 1272UG-2 24.5 498 1678UG-3 19.8 157 593UG-4 18.3 44 493UG-5 19.0 251 971UG-6 20.2 223 914UG-7 23.9 266 843UG-8 18.4 5.21 486UG-9 19.1 83.7 429UG-10 19.0 107 352UG-11 20.5 105 695UG-12 23.0 22.4 177UG-13 22.2 51.5 351UG-14 20.7 0.29 235UG-15 19.8 28.3 209UG-16 24.9 81 336UG-17 29.5 230 857UG-18 19.3 63.1 312UG-19 19.8 40.7 773UG-20 20.9 58.7 533UG-21 18.8 176 676UG-22 19.0 31 298UG-23 19.6 6.42 144UG-24 18.4 153 505UG-25 18.8 161 593UG-26 19.5 58.4 396UG-27 20.2 235 450UG-28 20.1 70 302UG-29 22.0 73.3 467UG-30 20.3 141 967UG-31 20.7 40.4 220UG-32 24 358 848UG-33 22 24.3 266UG-34 19 22.8 214UG-35 18.5 18.6 223UG-36 20 19.3 266UG-37 22.5 0.37 117UG-38 23.5 2.26 35.7UG-39 22.4 17.8 101UG-40 22.6 14.2 83.8UG-41 21.1 12.6 179UG-42 23.5 3.19 62.8UG-43 24.1 91.7 434UG-44 22.6 6.19 67.8UG-45 22.8 10.5 72.5UG-46 21.6 9.85 182UG-47 18.2 377 931UG-48 17.8 26.6 237.UG-49 18.1 13.3 161UG-50 18.5 38.6 228UG-51 20.4 123 606UG-52 20.1 86.4 408UG-53 20 74.1 351UG-54 22.8 1.74 251UG-55 20.3 60.3 394UG-56 20.5 12.2 419UG-57 21.6 81.4 339UG-58 20 150 484UG-59 19.4 47 215

the others samples, increase in δD values of UW 8A, 8B and UW 9are results of altitude affect. The UW 8A, B and 9 were located atarea whose altitude (about 600 m a.s.l.) is relatively lower than theGedongsongo area. Moreover, the thermal waters from UW 8A and8B show positively shift of δ18O which caused by a reaction with rock.

The estimated reservoir temperatures for the Ungaran geother-mal field using silica geothermometers (Fournier and Potter, 1982;Fournier, 1983) are listed in Table 3. Chalcedony geothermometersindicate lower temperatures (72 °C–116 °C) than quartz geother-mometers (102 °C–142 °C). The Na–K–Mg1/2 triangle proposed byGiggenbach (1988) is shown in Fig. 7. All of the thermal waters

CO2 Rn/Tn Hgsoil-gas Hgsoil(%) (ng) (ppm)

>20 0.472 142.3 1.90>20 0.297 83.3 0.31>20 0.264 41.3 0.69

0.9 0.089 46.7 0.230.6 0.134 37.9 2.80

>20 0.229 104.9 2.21>20 0.315 61.1 0.79

1.5 0.011 104.9 0.139.0 0.195 93.3 0.095.0 0.304 37.2 0.121.0 0.151 11.0 0.010.7 0.127 7.0 1.921.8 0.147 52.4 0.130.6 0.001 13.5 0.010.8 0.136 21.1 0.622.4 0.241 104.9 2.34

>20 0.269 57.6 8.065.0 0.202 15.6 0.53

>20 0.053 66.5 21.569.5 0.110 79.1 13.28

>20 0.260 104.9 20.560.8 0.104 26.3 0.430.4 0.045 29.0 0.63

14 0.303 78.2 0.26>20 0.272 35.7 0.09

0.25 0.148 104.9 9.229.0 0.523 50.2 1.581.1 0.231 89.2 2.921.4 0.157 36.7 2.540.3 0.146 24.9 0.011.9 0.184 1.3 0.20

>20 0.422 38.7 0.440.7 0.091 1.4 0.021.0 0.107 3.0 0.010.5 0.083 7.9 0.691 0.072 1.5 1.540.55 0.003 3.1 0.310.3 0.063 1.8 0.010.3 0.176 1.3 0.010.3 0.169 1.8 0.000.28 0.070 2.3 0.370.29 0.051 3.1 0.330.5 0.211 3.8 0.380.2 0.091 3.1 0.110.4 0.145 3.1 0.020.7 0.054 1.1 0.01

>20 0.406 5.9 00.4 0.112 3.0 0.070.35 0.083 3.0 00.38 0.169 5.4 0

16.8 0.202 6.2 011.0 0.212 2.3 01.5 0.211 30.6 0.030.5 0.007 30.5 0.000.8 0.153 4.1 0.110.6 0.029 2.8 0.590.3 0.240 5.4 0.144.0 0.309 45.4 1.330.6 0.218 12.8 0.02

Fig. 8. Cumulative frequency diagrams of Rn (a), Tn (b), CO2 (c) Hgsoil-gas (d) and Hgsoil (e) in the Gedongsongo area. Three populations: low (I); high (II); and anomalous (III) areshown.

30 N.K. Phuong et al. / Journal of Volcanology and Geothermal Research 229-230 (2012) 23–33

from the Gedongsongo area are classified as immature waters (locat-ed to the Mg apex), so the use of chemical geothermometers for esti-mating subsurface temperatures is not appropriate for this system.The use of silica (quartz-no steam loss) geothermometers may there-fore be acceptable for estimating reservoir temperatures of theUngaran geothermal field. However, estimating reservoir tempera-tures are rather low because maybe part of SiO2 precipitated duringstorage.

4.2. Soil gas survey

4.2.1. Statistical interpretation of soil gas dataThe soil gas measurements for all samples were conducted within

one or two days to minimize the influence of changes in meteorolog-ical conditions on the soil gas compositions. Table 4 shows the soilgas concentrations of all samples collected in the Gedongsongo area(Fig. 3).

Threshold values, used to recognize anomalous concentrations inthe soil gas data, were calculated using the geometric mean plusone standard deviation (Lepeltier, 1969; Klusman and Landress,1979; Varekamp and Buseck, 1983; Lescinsky et al., 1987; Klusman,1993). Samples with concentrations above this threshold are consid-ered anomalous. In geochemical exploration, cumulative frequencydiagrams are used for the determination of low (background) range,anomalous samples or recognition of multiple populations in log-

Table 5Distribution of soil gas into three populations (low, high and anomalous).

Classification Radon T(cpm) (c

Geometric mean (m) 98Standard deviation (σ) 123Low (concentrationbσ) b123 b

High (σbconcentrationbσ+m) 123–221Anomalous (σ+mbconcentrationbσ+2 ⁎m) 221–318

normally distributed data (Lepeltier, 1969; Varekamp and Buseck,1983; Lescinsky et al., 1987; Klusman, 1993). Individual populationswere separated by visual assessment using the procedure outlinedby Lepeltier (1969). The geometric mean,m, was read at the 50th per-centile; and the coefficient of deviation, σ, representing the spread inthe data, is the logarithm of the ratio of the value one standarddeviation from the geometric mean over the geometric mean. Thus,soil gas data from the study area are classified into three populationsas low or background (I), high (II), and anomalous values (III) asshown in cumulative frequency diagrams (Fig. 8 and Table 5). Highsoil gas concentrations were found over broad areas, while anoma-lous concentrations were identified in the north of the fumarole inthe Gedongsongo area. Areas located at the east and south of the fu-marole had generally low soil gas concentrations.

4.2.2. Spatial distribution of soil gas data and recognition of traceof fault/fracture

To characterize the study area, the spatial distribution of soil gasdata was interpreted using contour maps (Figs. 9, 10 and 11) thatwere produced using the kriging method with interpolation basedon a linear variogram model provided by the Surfer software.

Fig. 9 shows contour maps of the Rn concentration and the Rn andTn ratio. The Rn results show that the high and anomalous concentra-tions occur in the northern parts of the surveyed area (200 m fromthe fumarole) (Fig. 9a and Table 5). Anomalously high radon values

horon CO2 Hgsoil-gas Hgsoilpm) (%) (ng) (ppm)

450 5.4 34 1.7320 7.7 37 4.3320 b7.7 b37 b4.3320–770 7.7–13.1 37–71 4.3–6.0770–1266 13.1–20 71–105 6.0–7.8

Fig. 9. Contour map of (a) radon concentration and (b) radon to thoron concentrationratio.

Fig. 11. Contour maps of (a) Hgsoil-gas and (b) Hgsoil concentration.

31N.K. Phuong et al. / Journal of Volcanology and Geothermal Research 229-230 (2012) 23–33

could be indicative of enhanced permeability where Rn-222 rapidlymi-grates to the surface before disintegrating into daughter products. How-ever, the release of radon is dependent on other factors, including the

Fig. 10. Contour maps of CO2 concentration.

degree of rock fractures and the ability of the groundwater to permeatethrough such rocks. Percolating ground water transports radon fromfractured porous rocks by preventing diffusion. As described above,the radon will partition into the steam phase and be transported tohigher elevations through permeable zones. A NNE–SSW alignment ofRn anomalies, associated with the fault in this area, can be observed.Other areas west of the fumarole especially on a WNW–ESE alignmentalso have relatively high Rn values, while low Rn values are observedin most of the remaining survey area.

Many studies have been published on the feasibility of using Rnand CO2 measurements to detect active structures such as fracturesand faults (King, 1980; Koga, 1988; Etiope and Lombardi, 1995;Giammanco, et al., 1998; Fu et al., 2005; Yang et al., 2005; Lan et al.,2007). Faults favor gas transport because they increase rock permeabil-ity, helping the gas ascending to the surface. Furthermore, gases from adeep source can migrate upward through faults where the gas flow isdriven by advection. As Tn has a short half-life, 55 s, its concentrationin counts per minute (cpm) decreases quickly during 3 min of sequen-tial measurement. However, Rn has a half-life of 3.8 days and can betransported in fractures for a considerable distance. To detect the pres-ence of fracture or fault systems connecting the deep zone to the sur-face, the Rn/Tn concentration ratio can be a suitable indicator. Fig. 9bshows the Rn/Tn ratio contour map. High Rn/Tn ratios (>0.4) arefound mainly about 200 m to the north and 250–300 m to the southeast of the fumaroles. Zones with a high Rn/Tn ratio not only indicate

Fig. 12. Simplified conceptual hydro-geochemical model of the Ungaran geothermalfield.

32 N.K. Phuong et al. / Journal of Volcanology and Geothermal Research 229-230 (2012) 23–33

the presence of a fault/fracture zone but also indicate the extending offault/fracture from deep zone to the surface.

Fig. 10 is a contour map of the CO2 concentration. The CO2 mapclearly shows a close relationship with the Rn, and Rn/Tn maps. It isrelevant that in the Gedongsongo, CO2 values greater than 20% areconsidered anomalous. The high and anomalous CO2 values (>10%)were detected around the fumaroles and 250–300 m to south ofthe fumarole while low CO2 values are mainly located in the east(Fig. 10). Zones with anomalous and high CO2 concentrations trendfrom NNE–SSW and WNW–ESE, and these can be postulated as faultlocations. It is interesting to note that areas with high Rn/Tn ratios andanomalous CO2 concentration occur at the same location. This align-ment, trending NNE–SSW andWNW–ESE (Figs. 9b and 10) can be pos-tulated as a fault in this area. The NNE–SSW alignment of soil gasanomalies agrees with topographic and geologic data of a fault zone(Van Bemmelen, 1970; Thanden et al., 1996), while the WNW–ESEalignment may be another fault zones, unknown prior to the soil gassurvey.

The combined CO2 concentrations and soil temperatures (Table 4)are correlated to the north of the fumarole. This observation mayidentify the displacement of a high temperature heat source or anup-flow zone of high temperature geothermal fluid (Lan et al.,2007). The interpretation of Hg survey results will provide more evi-dence on this potential up-flow zone.

4.2.3. Mercury as feature of geothermal activitiesThe Hg concentrations have a wide range from 1.1 to 142.3 ng and

0.01 to 21.6 ppm for Hg Hgsoil-gas and Hgsoil, respectively. Fig. 11a showsa contour map of Hgsoil-gas. The high concentration zone representsHgsoil-gas concentrations above 40 ng, and extends from 0.7 km northto south by 1 kmwest to east from the fumarole. This supports the sug-gestion that geothermal activity is widespread around the fumarolezone. Absorbed Hg on the soil surface is difficult to desorb except afterheating at high temperatures. Thus, the resulting mercury anomaliesare related to the temperature of the ascending geothermal fluids,which act as a carrier while also providing the migration pathways.Varekamp and Buseck (1983) and Murray (1997) concluded that Hganomalies occur when geothermal fluids escape from a deep reservoirand migrate to shallow levels. Anomalous Hgsoil-gas values were foundin the north of the fumarole which consistent with the location of theanomalous Rn, Tn, CO2 and temperature values. The chemistry of springwaters in this area is acid-sulfate type, indicating that a component ofthese fluids has condensed from H2S-rich vapor phase. Mercury isstrongly partitioned into the vapor phase, so steam escaping towardsthe surface will be enriched in Hg, and acid hot-springs and fumaroleswill display strong Hg anomalies. We, therefore, postulate that upwell-ing and subsequent boiling occurs beneath the area to the north of thefumarole. The soil gas contours appear to define the NNE–SSW andWNW–ESE alignments, which are thought to represent fault/fracturezones. Non-thermal waters or a mixture of thermal and non-thermalwaters are found in the south of fumarole and this area coincides withlow Hg concentrations.

Mercury enrichment in soils is a dynamic process, as re-volatilization and biogenic uptake with subsequent volatilization(Varekamp and Buseck, 1983) will continuously remove Hg fromthe soil. A steady-state will occur after an initial period of non-equilibrium. The continuous Hg loss processmeans that old and currentthermal activity can be distinguished (Koga, 1982, 1988; Varekamp andBuseck, 1983). The Hg results in soil and in soil gas are not always ingood agreement because Hgsoil-gas indicates current geothermal activitywhile Hgsoil shows the history of geothermal activity up to the present(Koga, 1982, 1988). Fig. 11b shows high Hgsoil concentrations are locat-ed in the east of the fumarolic area, and we infer that was an ancientgeothermal zone. Varekamp and Buseck (1983) have documentedcases where active geothermal zones are enriched in Hgsoil-gas but lackHgsoil.

4.3. Conceptual hydro-geochemical model of the Ungaran geothermalfield

From this study, a hydro-geochemical model of the Gedongsongothermal waters has been developed and is shown in Fig. 12.

This model shows the up-flow of high temperature geothermalfluid located in the north of the fumarole. The anomalous Hg valuesoccur in the vapor-dominated part of the system, which is explainedby the strong partitioning of Hg into the vapor phase during boiling.Deep geothermal fluids are present below this area, and circulatethrough the fault and fracture zones with some portion of the fluidsdischarging at the surface. Based on the geochemical and isotopicdata, the thermal springs at Gedongsongo are of meteoric origin.The meteoric waters percolate through the fault systems into themixing zone where they are heated by deep geothermal fluids and as-cend to the surface along the NNE–SSW and WNW–ESE fault. In theshallow zone, CO2 and H2S rich steam rises off the thermal waters,which can lead to formation of sulfate-rich waters, while some ofthe ascending thermal waters mix with cold water and rise alongthe WNW–ESE fault. The areas with low soil gas concentrationsshow that the boundary of the limited geothermal system is in thenorthern section of the fumarole field at the Gedongsongo area.

5. Conclusions

The chemistry of the thermalwaters discharged in the Gedongsongoarea indicates steam that heated acid-sulfate waters (Ca–(Na)–Mg–SO4) are present in the north of the fumarole while mixed bicarbonate(Ca–Mg–HCO3) and bicarbonate–chloride (Na–HCO3–Cl or Na–Ca–Cl–HCO3) water is present in the south and southeast of the Gedongsongoarea. The compositions of the thermal waters reveal that they have notreached chemical equilibrium with the host rocks. The reservoir tem-peratures estimated using silica geothermometers is from102 °C to142 °C.

A soil gas survey was also conducted at the Ungaran geothermalfield and has provided an overview of soil gas distribution patternsproduced by the underlying system. The soil gas contour mapsshow that, Rn, and CO2 are reliable and sensitive indicators for tracing

33N.K. Phuong et al. / Journal of Volcanology and Geothermal Research 229-230 (2012) 23–33

faults. In this study, high gas concentrations were identified along thefaults trending NNE–SSW and WNW–ESE which act as conduits forgeothermal fluid and soil gas. From the anomalous Hg concentrations,we inferred that the up-flow zone of high temperature geothermalfluid is in the north of the fumarole.

Acknowledgments

This study was supported by AUN/SEED-Net program, JICA(Japanese International Cooperation Academic) and Gadjah MadaUniversity, Indonesia. The authors thank Prof. Sachihiro Taguchi ofFukuoka University for his help to analyze stable isotope of watersamples.

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