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Journal of Volcanology and Geothermal Research 229-230 (2012) 23–33 Contents lists available at SciVerse ScienceDirect Journal of Volcanology and Geothermal Research journal homepage: www.elsevier.com/locate/jvolgeores 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 Department of Earth Resource Engineering, Faculty of Engineering, Kyushu University, 744 Motooka, Nishiku, Fukuoka 819-0395, Japan 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 Nam Department of Geological Engineering, Faculty of Engineering, Gadjah Mada University, Jl Grafika 2, Yogyakarta (55281), Indonesia a r t i c l e i n f o Article history: Received September 2011 Accepted April 2012 Available online 11 April 2012 Keywords: Soil gas Water geochemistry Ungaran geothermal field Indonesia a b s t r a c t A soil gas survey for radon (Rn), thoron (Tn), CO2, and mercury (Hg), and the chemical analysis of hot spring waters, were undertaken in the Ungaran geothermal field, Central Java, Indonesia The results of soil gas surveys indicate fault systems trending NNE–SSW and WNW–ESE Particularly high CO2 concentrations (>20%), and high Hg concentrations were detected in vicinity of the fumaroles Emanometries of Rn, Tn and CO2 also conclusively identified the presence of a fracture zone for the migration of geothermal fluid The Hg results infer that the upflow zone of high temperature geothermal fluids maybe located in the north of fumaroles in the Gedongsongo area (near the collapse wall) Chemistry of thermal springs in the up-flow zone are acid (pH= 4) and show a Ca–Mg–SO4 composition The thermal waters are mainly Ca–Mg–HCO3 and Ca–(Na)–SO4–HCO3 types near the fumarolic 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 the waters are essentially meteoric in origin A conceptual hydro-geochemical model of the Gedongsongo thermal waters based on the soil gas, isotope and chemical analytical results, was constructed © 2012 Elsevier B.V All rights reserved Introduction Geothermal exploration began in Indonesia in 1970 More than 200 geothermal systems with significant active surface manifestations occur throughout Indonesia Most of the geothermal systems in Indonesia that have surface manifestations with fluid discharges at boiling temperature occur in areas with Quaternary volcanism and active volcanoes, along well-defined volcanic arcs (Sudarman et al., 2000; Hochstein and Sudarman, 2008) During the 1980s the Pertamina Company explored a number of areas characterized by significant fumarolic activity on the flanks of inactive Holocene volcanoes At Ungaran, Pertamina Co conducted geophysical surveys using several geophysical methods (resistivity, magnetotellurics, and aeromagnetic surveys) between 1985 and 1990 at Ungaran (Budiardjo et al., 1989) The Ungaran volcano is located in the Central Java province about 30 km southwest of Semarang, Indonesia (Fig 1), and is an undeveloped geothermal prospect There are two active fumaroles on the southern ⁎ Corresponding author at: Department of Earth Resources Engineering, Faculty of Engineering, Kyushu University, 744, Motooka, Nishiku, Fukuoka 819-0395, Japan Tel./fax: +81 92 802 3345 E-mail addresses: nkphuong6280@yahoo.com, nkphuong@mine.kyushu-u.ac.jp (N.K Phuong) 0377-0273/$ – see front matter © 2012 Elsevier B.V All rights reserved doi:10.1016/j.jvolgeores.2012.04.004 flank of the dormant Ungaran volcano, about km SW from its summit The Gedongsongo area, on the southern flank of Ungaran volcano, is characterized by the presence of thermal manifestations such as fumaroles, hot springs, acidic mud pools and hydrothermal alteration zones Four wells were drilled down to a depth of 500 m around the Gedongsongo area These wells showed slightly anomalous temperature (about 47 °C at 300 m depth) and temperature-gradient values near the bottom (Hochstein and Sudarman, 2008) A low resistivity anomaly, about 30 Ωm, associated with a deep geothermal reservoir was inferred to occur beneath the Ungaran summit (Budiardjo et al., 1989) Further exploration was not conducted because about 90% of the prospect area is located in a protected forest with restricted access The soil gas method, i.e the measurement of gas concentrations in the soil, has been used to estimate the location of heat sources and their areal extension in geothermal prospecting (Varekamp and Buseck, 1984, 1986; Bertrami et al., 1990; Finlayson, 1992; Hernández et al., 2000) The gases (Rn, Tn, CO2, and Hg) are assumed to be released from active geothermal systems at depth and then ascend through overlying rock formations and/or fracture zones The high mobility of these gases makes them ideal pathfinders for concealed natural resources, as the gases produced and accumulated in geothermal reservoirs 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 24 N.K Phuong et al / Journal of Volcanology and Geothermal Research 229-230 (2012) 23–33 Fig Location of the Ungaran volcano (solid red circle) detecting and delineating faults and fractures (Gregory and Durrance, 1985; Toutain and Baubron, 1999; Guerra and Lombardi, 2001; Walia et 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 radioactive noble gas that is soluble in water and decays by alpha emission The presence of Rn in geothermal areas is a function of the porosity and fracture distribution of the rocks in between the deep geothermal source and the surface, i.e., the pathway for uprising fluids (Koga, 1988) Radon gas surveys are widely used to monitor seismic 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 geothermal exploration techniques Van Kooten (1987), Lescinsky et al (1987) and Murray (1997) all found broad Hg anomalies outlining high-temperature thermal activity zones Soil Hg surveys has also been used to locate faults in volcanic and geothermal regions (Klusman and Landress, 1979; Cox and Cuff, 1980; Varekamp and Buseck, 1983) In the sub-surface, Hg is strongly partitioned into the ascending vapor and is transported to the surface as elemental Hg This vapor is absorbed onto organic matter and clay minerals in the shallow, low-temperature soil horizons, producing elevated (above 10 ppm) concentrations of Hg (Nicholson, 1993) Mercury is absorbed by the soil in anomalous concentrations relative to the surrounding areas (Lescinsky et al., 1987; Van Kooten, 1987) Mercury levels in soil are the result of accumulation and loss processes; consequently, soil gas mercury is a reliable indicator of geothermal fluid at depth (Koga, 1988) The present study aimed to i) delineate the up-flow zone of high temperature geothermal fluids at depth, fractures below the surface, in the Ungaran geothermal field using a soil gas survey; ii) characterize chemical characteristics of hot spring waters in the Ungaran geothermal field; and iii) develop a hydro-geochemical conceptual model of thermal water in the Ungaran geothermal field Geological setting Geothermal areas in Central Java, including the Ungaran volcano, are located in the Quaternary Volcanic Belt (Solo Zone) (Fig 1) This belt is located between the North Serayu Mountains and the Kendeng Zone and contains numerous Quaternary eruptive centers, including Dieng, 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 Ungaran volcanic system is primarily controlled by the Ungaran collapse structure that runs from west to southeast of the Ungaran volcano Old volcanic rocks of the pre-caldera formation are controlled by northwest– southwest and southeast–southwest fault systems The post-caldera volcanic rocks, however, not seem to be structurally controlled by the regional faulting system (Budiardjo et al., 1997) The precaldera volcanic rocks and the Tertiary marine sedimentary rocks N.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 et al., 1997) Van Bemmelen (1970) noted that there is a gradual development of volcanism along the transverse fault from north to south, starting in the north with the Oldest or Proto-Ungaran in the Lower Pleistocene and ending in the south with the very active Merapi volcano (Fig 1) Two generation of the Ungaran volcano (2050 m) were observed because of gravitational collapse The Oldest Ungaran deposits resulted from submarine activity Its basement is transitional beds, in which the facies changes from marine into fresh water deposits consisting of coarse polymictic conglomerates of the Lower Damar Beds After magma broke through the crust, the Oldest Ungaran volcano originated at the eastern end of the crest The coarse volcanic breccias of the Middle Damar Beds, and the coarse conglomerates, tuff-sandstones and blackclay of the Upper Damar Beds occur at the northern foot of the Oldest Ungaran volcano In the Upper Pleistocene, volcanic activity was wide-spread In the eastern part of the northern Serayu Range, volcanic activity built up the Old Ungaran volcano, which is the second generation of Ungaran volcano The breccias at its northern foot form the Notopuro Beds, which cover the breccias of the Oldest Ungaran in the Damar Beds with an angular unconformity After the early Pleistocene phase of volcanic growth, volcanic activity continued until the Holocene, building up the Young Ungaran volcano, which consists of pyroclastic flow deposits, pyroclastic lava and alluvial deposits The Ungaran geothermal system is associated with the Upper Quaternary volcanism of the Ungaran volcano The volcanic rocks are rich in alkali metals and are classified as trachyandesite to trachybasaltic andesite, primarily containing plagioclase, sanidine and cristobalite (Budiardjo et al., 1997; Kohno et al., 2005) Gedongsongo is the main geothermal area on the southern flank of the Ungaran volcano (Fig 2) The Gedongsongo area is characterized by the presence of fumaroles (90–110 °C), neutral pH bicarbonate warm/hot springs and diluted steam heated hot spring (22–80 °C) with underground temperatures of 20 °C to 82 °C measured from m depth According to Budiardjo et al (1997), the composition of thermal spring waters at Gedongsongo can be divided into two water types The hot water around the fumarolic area originates as a steam heated meteoric water characterized by low chloride content (similar to local surface water), high sulfate content (up to 1000 ppm), and low pH (up to 5) while neutral bicarbonate or chloride waters are located at the other areas Based on the analysis of soil and rocks samples collected around the Ungaran volcano, Kohno et al (2005) concluded 25 that quartz, halloysite and alunite are the main secondary minerals found in the hydrothermal alteration zones Quartz is formed by the alteration of cristobalite from Ungaran rocks, while halloysite and alunite are minerals formed by alteration s due to acidic and low temperature hydrothermal waters Sampling and measurement The study area is focused on the Gedongsongo area (Fig 2), which is the main geothermal prospect in the Ungaran area, located in the southern part of the Ungaran volcano Water samples (UW-1 to UW-7A and UW-7B) were collected around the Gedongsongo area where comprises a volcanic complex terrain at an altitude ranging from 1200 to 2000 m above sea level Others samples (UW-8A, UW8B and UW9), however, were collected at Kendalisodo area (approximately km far from the Gedongsongo area) with altitude at 600 m above sea level 3.1 Chemical analysis of water Water sampling was complemented by in situ measurement of pH, temperature and conductivity The water samples were filtered through 0.45 μm membrane 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 with mL of concentrated HCl Filtered, un-acidified samples were collected for anions (F, Cl, HCO3, SO4) analysis All water analyses were conducted at Kyushu University using standard methods Cations and anions were analyzed using ion chromatography (Dionex ICS-90) while boron (B) was analyzed using ICP-AES (Vista-MPX) SiO2 contents was determined by colorimetry and analyzed using a digital spectrophotometer (Hitachi U-1100) (APHA, 2005), while HCO3 was analyzed by titration with 0.1 M HCl The analytical error for techniques was ≤5% An aliquot of the water samples (20 mL) was collected and stored in sterile polyethylene bottles (HDPE) for stable isotope analysis Water isotopes (δ 18O and δD) were determined using the CO2–H2 equilibration method (Epstein and Mayeda, 1953) Then, the isotope ratios were measured using the DELTA Plus mass-spectrometer at Fukuoka University, Japan These internal standards were calibrated using international reference materials V-SMOW and SLAP with analytical precisions of ±0.1‰ for δ 18O and ±1‰ for δD Fig Sampling sites and geological map of the study area (UTM coordination system) 26 N.K Phuong et al / Journal of Volcanology and Geothermal Research 229-230 (2012) 23–33 Fig Location of soil gas and water samples around the fumarolic area (UTM coordination system) 3.2 Soil gas measurement Soil gas surveys for Rn, Tn, CO2, mercury in soil gas (Hgsoil-gas) and in soil samples (Hgsoil) were conducted in an area approximately 1.3 km north to south by 1.5 km west to east (Fig 3) The distance between measurement points varied from 50 to 150 m Soil gases were collected from a depth of 60 cm using a steel pipe (5 cm in diameter) inserted into the ground 3.2.1 Radon measurement The Rn and Tn concentrations were measured with a radon detector (RD-200, EDA Instruments Co Ltd.) The soil gas was circulated through the detector with an electrical pump for 10 s, replacing the air in the detection cell The Rn concentration was measured by an α-scintillation radon counter with the soil gas pumped directly into a scintillation chamber When the α-particles produced during radon decay impact the ZnS(Ag) layer in the scintillation counter, an energy pulse is created in the form of photons, measured by a photo-multiplier and a counter As both Rn and Tn decay by means of α-emission, the concentrations of Rn and Tn were calculated from three counts in each minute obtained for three sequential minutes 3.2.2 CO2 measurement To measure the CO2 concentration, 100 mL of soil gas was sampled from the stainless steel probe inserted into the ground using a stainless steel syringe, and the CO2 concentration was measured using an SA-type gas detector tube (Komyo-Kitagawa Instruments Co Ltd.) This gas detector works on the principles of chemical reaction and physical absorption and has ±1% analytical precision As the gas is entered into the detector tube, a constant color is produced, which varies in the length of discolored layer due to the reaction between the reagent and the CO2 The CO2 concentration can then be obtained directly by reading from the measuring scale on the tube or using a concentration 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, and the concentration in the ascending gas The Hgsoil measurement represents the concentration of Hg absorbed onto the surface of soil particle To measure Hg in soil gas, a pure gold wire (10 cm long, mm diameter, 1.5 g weight and 3.16 cm in the effective surface) was left in the hole for days after completing the CO2 measurement (Koga, 1982, 1988) After a week, the gold wire was removed from the hole and stored in a tightly sealed glass tube Soil samples were collected at 0.6 m depth in the hole and sealed in plastic bags The soil samples were then air-dried at room temperature for two weeks and ground with a mortar and a pestle after removing rock fragments and plant roots The mercury concentrations were determined in the laboratory by the cold vapor atomic absorption method using mercury analyzer SP-3 (Nippon Instruments Co., Japan) This equipment uses the heating-vaporization (700 °C) technique to liberate mercury present in the sample Results and discussions 4.1 Water chemistry and stable isotope compositions The results of the water analyses are given in Table 1, and show that 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 contents of the thermal waters ranged from 47 to 219 mg/L, while the EC values were generally between 36 and 561 μS/cm, except for relatively high values in UW 8A and 8B (up to 5300 μS/cm) Other major N.K Phuong et al / Journal of Volcanology and Geothermal Research 229-230 (2012) 23–33 27 Table Chemical composition (in mg/L) and δ18O and δD values of water samples in the Ungaran geothermal field ID UW-1 UW-2 UW-3 UW-4 UW-5a UW-6 UW-7A UW-7B UW-8A UW-8B UW-9a Temp (°C) pH 21.9 40.0 56.0 32.2 n.a 18.0 50.0 25.0 35.2 38.1 23.8 3.45 5.36 6.10 6.00 6.31 5.42 6.10 5.90 6.84 6.78 7.87 EC (μS/cm) HCO3− 561 368 333 297 36 177 491 164 4580 5210 513 50 59 200 465 100 107 501 496 1732 1824 351 F− Cl− SO42 − SiO2 Li+ Na+ NH4+ K+ Mg2 Ca2 + B δD (‰) δ18O (‰) 42.5 32.6 37.1 35.9 3.5 18.2 38.9 40.7 217.3 278.4 62.1 1.22 0.79 0.61 0.65 0.36 0.42 0.58 0.52 15.9 19.7 0.15 − 47 − 49 − 50 − 51 − 51 − 50 − 51 − 52 − 39 − 40 − 39 − 7.9 − 7.9 − 8.0 − 8.2 − 8.2 − 8.0 − 8.1 − 8.2 − 5.3 − 5.3 − 6.1 + 0.12 0.21 0.13 0.05 0.02 0.01 0.06 0.05 0.06 0.06 0.07 1.8 1.2 0.8 0.8 0.7 0.7 0.8 0.8 998 1088 7.2 247 136 31.8 2.6 3.5 50.3 3.4 2.9 0.2 0.1 4.4 58 109 86 82 23 51 93 89 92 95 51 1.6 1.1 0.3 0.1 0.1 0.2 0.5 0.4 4.4 5.6 0.5 34.3 25.3 14.1 10.7 2.3 6.8 11.8 12.3 700 746 23.2 0.43 0.64 0.45 0.49 0.02 0.04 0.65 0.53 16.1 18.0 0.29 15.0 8.6 7.9 5.5 1.2 3.1 6.4 6.0 44.2 47.1 2.4 13.4 10.3 15.1 14.7 0.7 5.6 19.8 17.6 117.7 126.0 26.9 n.a.: not analyzed a River water elements range from to 746 mg/L for Na, from 3.54 to 278 mg/L for Ca while concentrations of Mg are lower (b126 mg/L) The waters contain relatively low K concentrations (1.18–47.11 mg/L) For anions, the HCO3 concentration is relatively high (39–1824 mg/L) followed by SO4 (b246 mg/L) Chloride concentration is rather low except UW-8A and 8B are relatively high (about 1000 mg/L) The chemical compositions of the water samples are plotted on the Cl–SO4–HCO3 (Giggenbach, 1988) and Na–SO4–Mg diagrams shown in Fig The UW and UW samples are classified as acidsulfate waters, with high concentrations of SO4 (247 mg/L and 136 mg/L, respectively), but low concentrations of F (b0.25 mg/L) and Cl (below 1.5 mg/L) (Table 1) suggests that UW and UW have been steam heated, absorbing a gas phase enriched in Sbearing compounds The SO4 enrichment can be explained by the O2-driven oxidation of H2S to H2SO4 in oxygenated near surface groundwater (Henley and Stewart, 1983; Tassi et al., 2010; Joseph et al., 2011) Differences from the above samples, most samples are HCO3-dominated water, mostly Ca–HCO3 or Ca–Mg–HCO3 type (UW 3, 4, 5, 7A, 7B and 9) while UW 8A and 8B are of the Na–HCO3–Cl or Na–Ca–Cl–HCO3 type with much higher Na, Ca, HCO3, Cl and B concentrations than the other samples (Table 1) Water–rock interaction should 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 (Kohno et al., 2005) Table shows the molar ratios of some of the major components of thermal waters in the study area Increases in the Na/Cl and K/Cl ratios in thermal waters are likely to reflect reactions with feldspar or clay minerals These ratios can therefore be used as an independent indicator of residence time Thermal waters often follow a longer, deeper, regional flow path than non-thermal waters, and thus have much higher Na/Cl and K/Cl ratios than non-thermal waters (Han et al., 2010) The Na/K ratio is controlled by temperature dependent mineral–fluid equilibria (Koga, 1988; Gemini and Tarcan, 2002) The ratios of Na/K are large for all water samples, indicating Table Molar ratios of some major components of water samples in the Ungaran geothermal field Fig Chemical compositions (a) Cl–SO4–HCO3 and (b) SO4–Mg–Na of thermal waters in the Ungaran geothermal field ID Temp (°C) Na/Cl K/Cl Ca/Cl Na/K Na/Ca HCO3/SO4 Cl/B UW-1 UW-2 UW-3 UW-4 UW-5a UW-6 UW-7A UW-7B UW-8A UW-8B UW-9a 21.9 40.0 56.0 32.2 – 18.0 50.0 25.0 35.2 38.1 23.8 28.68 33.82 28.36 21.90 5.40 16.06 21.78 23.11 1.08 1.04 4.96 7.42 6.80 9.37 6.57 1.63 4.39 6.94 6.62 0.04 0.04 0.3 20.4 25.0 43.0 42.1 4.7 24.8 41.4 44.1 0.2 0.2 7.6 15.8 29.5 152 358 87.9 96.0 349 353 26.9 26.8 16.7 0.32 0.68 9.91 280 44.58 3.36 231.9 274 5.6 4.66 0.65 2.17 2.25 2.61 2.83 1.79 2.12 2.28 2.09 13,629 28,706 124.4 0.46 0.44 0.38 0.35 0.56 0.47 0.44 0.48 19.1 16.8 14.6 n.a.: not analyzed a River water 28 N.K Phuong et al / Journal of Volcanology and Geothermal Research 229-230 (2012) 23–33 Table Estimated temperature (in °C) for thermal water in the Ungaran geothermal filed using silica geothermometers ID UW UW UW UW UW UW UW UW UW UW a b c d Fig Binary diagram of Li vs Cl and B vs Cl that the temperature of geothermal reservoir is not probably too high This is in agreement with general increase in Na/K ratios of thermal water with decreasing reservoir temperature (Ellis and Mahon, 1967; Koga, 1988; Cortecci et al., 2005) Based on relatively low Na/K ratios (b15, Table 2) water of springs at the Gedongsongo area that have reached the surface rapidly and are therefore associated with up-flow structures or permeable zones while higher Na/K ratios (>15, Table 2) are indicative of lateral flows which may undergone near-surface reactions and conductive cooling (Nicholson, 1993; Cortecci et al., 2005; Di Napoli et al., 2009) Similarly, high Na/Ca ratios are also indicative of direct feeding from a geothermal reservoir and less groundwater contribution, while the HCO3/SO4 ratio can be used as an indicator of flow direction (Table 2) The Na/Ca values for deep well thermal waters are very high (>50), while for cold groundwater this ratio is around 0.25 Low Na/K and Na/Ca ratios are found in thermal waters in the north of the fumarole (UW and UW 2) while to the south of the fumarole, the thermal waters have high 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 fumarole are associated with up-flow zones, while thermal waters to the south of the fumarole are associated with lateral flow Fig δD vs δ18O composition of thermal waters in the Ungaran geothermal field 7A 7B 8A 8B 9* Measured temperature (°C) Estimated temperature (°C) 21.9 40.0 56.0 32.2 18.0 50.0 25.0 35.2 38.1 23.8 109 142 129 127 102 133 131 132 134 102 TQz a TQz 109 137 126 124 103 129 127 129 130 103 b TC c 80 116 101 99 73 106 103 105 107 72 T d 110 143 129 127 103 133 131 132 134 103 Quartz — no steam loss from Fournier (1983) Quartz — maximum steam loss at 100 °C from Fournier (1983) Chalcedony from Fournier (1983) From Fournier and Potter (1982) The behavior of conservative components useful in the delineation of formation processes of waters, involving Cl, Li and B, is investigated in Fig As pointed out above, Li is the alkali element least affected by secondary absorption processes Li is also released during water–rock interactions and remains largely in solution (Giggenbach and Soto, 1992; Mainza, 2006; Tassi et al., 2010) Boron and Cl − are not readily incorporated into secondary, alteration minerals, so they can be considered 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 in high temperature steam, it may also be introduced with any high temperature vapor phase absorbed into water Moreover, the B content of thermal fluids is likely during the early heating up stages Therefore, fluids from older hydrothermal systems can be expected to be depleted in B while the converse holds for younger hydrothermal systems (Mainza, 2006) It is, however, striking both Cl and B is adding to the Li containing solutions in proportions close to those in crustal rocks For the UW 8A and 8B, it can be elucidated that dissolution of an averaged andesitic–rhyolitic rock, followed by exchange with secondary minerals or interaction with gases (Fig 5) Moreover, water rock interaction can be postulated by Cl/B ratio Ellis and Mahon (1967) found that in areas where andesitic or rhyolitic rocks predominate, Cl/B ratios are often between 10 and 30 The Cl/B ratios Fig Na–K–Mg ternary diagram for thermal waters in the Ungaran geothermal field (Giggenbach, 1988) N.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 suggested that high concentration of Na, Cl, Li and B is originated from water– rock interaction The results of the stable isotope analysis in the Gedongsongo area are given in Table and plotted in the δD vs δ 18O diagram (Fig 6) Stable isotope compositions of meteoric water from coastal Jakarta (Gat and Gonfiantini, 1981) were used as reference data (δD = 8.05 δ 18O + 16.48) (Fig 6) This line does not deviate significantly from the global meteoric water line defined by Craig (1961) In Fig 6, all samples from Gedongsongo plot along the meteoric water line, suggesting that the thermal waters are of meteoric origin Compared to 29 the others samples, increase in δD values of UW 8A, 8B and UW are results of altitude affect The UW 8A, B and were located at area whose altitude (about 600 m a.s.l.) is relatively lower than the Gedongsongo area Moreover, the thermal waters from UW 8A and 8B show positively shift of δ 18O which caused by a reaction with rock The estimated reservoir temperatures for the Ungaran geothermal field using silica geothermometers (Fournier and Potter, 1982; Fournier, 1983) are listed in Table Chalcedony geothermometers indicate lower temperatures (72 °C–116 °C) than quartz geothermometers (102 °C–142 °C) The Na–K–Mg 1/2 triangle proposed by Giggenbach (1988) is shown in Fig All of the thermal waters Table Soil gas concentrations of Rn, Tn, CO2 and Hgsoil-gas and Hgsoil in the Gedongsongo area ID Temp at 60 cm depth (°C) Rn (cpm) Tn (cpm) CO2 (%) Rn/Tn Hgsoil-gas (ng) Hgsoil (ppm) UG-1 UG-2 UG-3 UG-4 UG-5 UG-6 UG-7 UG-8 UG-9 UG-10 UG-11 UG-12 UG-13 UG-14 UG-15 UG-16 UG-17 UG-18 UG-19 UG-20 UG-21 UG-22 UG-23 UG-24 UG-25 UG-26 UG-27 UG-28 UG-29 UG-30 UG-31 UG-32 UG-33 UG-34 UG-35 UG-36 UG-37 UG-38 UG-39 UG-40 UG-41 UG-42 UG-43 UG-44 UG-45 UG-46 UG-47 UG-48 UG-49 UG-50 UG-51 UG-52 UG-53 UG-54 UG-55 UG-56 UG-57 UG-58 UG-59 51.5 24.5 19.8 18.3 19.0 20.2 23.9 18.4 19.1 19.0 20.5 23.0 22.2 20.7 19.8 24.9 29.5 19.3 19.8 20.9 18.8 19.0 19.6 18.4 18.8 19.5 20.2 20.1 22.0 20.3 20.7 24 22 19 18.5 20 22.5 23.5 22.4 22.6 21.1 23.5 24.1 22.6 22.8 21.6 18.2 17.8 18.1 18.5 20.4 20.1 20 22.8 20.3 20.5 21.6 20 19.4 601 498 157 44 251 223 266 5.21 83.7 107 105 22.4 51.5 0.29 28.3 81 230 63.1 40.7 58.7 176 31 6.42 153 161 58.4 235 70 73.3 141 40.4 358 24.3 22.8 18.6 19.3 0.37 2.26 17.8 14.2 12.6 3.19 91.7 6.19 10.5 9.85 377 26.6 13.3 38.6 123 86.4 74.1 1.74 60.3 12.2 81.4 150 47 1272 1678 593 493 971 914 843 486 429 352 695 177 351 235 209 336 857 312 773 533 676 298 144 505 593 396 450 302 467 967 220 848 266 214 223 266 117 35.7 101 83.8 179 62.8 434 67.8 72.5 182 931 237 161 228 606 408 351 251 394 419 339 484 215 > 20 > 20 > 20 0.9 0.6 > 20 > 20 1.5 9.0 5.0 1.0 0.7 1.8 0.6 0.8 2.4 > 20 5.0 > 20 9.5 > 20 0.8 0.4 14 > 20 0.25 9.0 1.1 1.4 0.3 1.9 > 20 0.7 1.0 0.5 0.55 0.3 0.3 0.3 0.28 0.29 0.5 0.2 0.4 0.7 > 20 0.4 0.35 0.38 16.8 11.0 1.5 0.5 0.8 0.6 0.3 4.0 0.6 0.472 0.297 0.264 0.089 0.134 0.229 0.315 0.011 0.195 0.304 0.151 0.127 0.147 0.001 0.136 0.241 0.269 0.202 0.053 0.110 0.260 0.104 0.045 0.303 0.272 0.148 0.523 0.231 0.157 0.146 0.184 0.422 0.091 0.107 0.083 0.072 0.003 0.063 0.176 0.169 0.070 0.051 0.211 0.091 0.145 0.054 0.406 0.112 0.083 0.169 0.202 0.212 0.211 0.007 0.153 0.029 0.240 0.309 0.218 142.3 83.3 41.3 46.7 37.9 104.9 61.1 104.9 93.3 37.2 11.0 7.0 52.4 13.5 21.1 104.9 57.6 15.6 66.5 79.1 104.9 26.3 29.0 78.2 35.7 104.9 50.2 89.2 36.7 24.9 1.3 38.7 1.4 3.0 7.9 1.5 3.1 1.8 1.3 1.8 2.3 3.1 3.8 3.1 3.1 1.1 5.9 3.0 3.0 5.4 6.2 2.3 30.6 30.5 4.1 2.8 5.4 45.4 12.8 1.90 0.31 0.69 0.23 2.80 2.21 0.79 0.13 0.09 0.12 0.01 1.92 0.13 0.01 0.62 2.34 8.06 0.53 21.56 13.28 20.56 0.43 0.63 0.26 0.09 9.22 1.58 2.92 2.54 0.01 0.20 0.44 0.02 0.01 0.69 1.54 0.31 0.01 0.01 0.00 0.37 0.33 0.38 0.11 0.02 0.01 0.07 0 0 0.03 0.00 0.11 0.59 0.14 1.33 0.02 30 N.K Phuong et al / Journal of Volcanology and Geothermal Research 229-230 (2012) 23–33 Fig 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) are shown from the Gedongsongo area are classified as immature waters (located to the Mg apex), so the use of chemical geothermometers for estimating subsurface temperatures is not appropriate for this system The use of silica (quartz-no steam loss) geothermometers may therefore be acceptable for estimating reservoir temperatures of the Ungaran geothermal field However, estimating reservoir temperatures are rather low because maybe part of SiO2 precipitated during storage 4.2 Soil gas survey 4.2.1 Statistical interpretation of soil gas data The soil gas measurements for all samples were conducted within one or two days to minimize the influence of changes in meteorological conditions on the soil gas compositions Table shows the soil gas concentrations of all samples collected in the Gedongsongo area (Fig 3) Threshold values, used to recognize anomalous concentrations in the soil gas data, were calculated using the geometric mean plus one 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 considered anomalous In geochemical exploration, cumulative frequency diagrams are used for the determination of low (background) range, anomalous samples or recognition of multiple populations in log- normally distributed data (Lepeltier, 1969; Varekamp and Buseck, 1983; Lescinsky et al., 1987; Klusman, 1993) Individual populations were separated by visual assessment using the procedure outlined by Lepeltier (1969) The geometric mean, m, was read at the 50th percentile; and the coefficient of deviation, σ, representing the spread in the data, is the logarithm of the ratio of the value one standard deviation from the geometric mean over the geometric mean Thus, soil gas data from the study area are classified into three populations as low or background (I), high (II), and anomalous values (III) as shown in cumulative frequency diagrams (Fig and Table 5) High soil gas concentrations were found over broad areas, while anomalous concentrations were identified in the north of the fumarole in the Gedongsongo area Areas located at the east and south of the fumarole had generally low soil gas concentrations 4.2.2 Spatial distribution of soil gas data and recognition of trace of fault/fracture To characterize the study area, the spatial distribution of soil gas data was interpreted using contour maps (Figs 9, 10 and 11) that were produced using the kriging method with interpolation based on a linear variogram model provided by the Surfer software Fig shows contour maps of the Rn concentration and the Rn and Tn ratio The Rn results show that the high and anomalous concentrations occur in the northern parts of the surveyed area (200 m from the fumarole) (Fig 9a and Table 5) Anomalously high radon values Table Distribution of soil gas into three populations (low, high and anomalous) Classification Radon (cpm) Thoron (cpm) CO2 (%) Hgsoil-gas (ng) Hgsoil (ppm) Geometric mean (m) Standard deviation (σ) Low (concentration b σ) High (σ b concentration b σ + m) Anomalous (σ + m b concentration b σ + ⁎ m) 98 123 b 123 123–221 221–318 450 320 b 320 320–770 770–1266 5.4 7.7 b7.7 7.7–13.1 13.1–20 34 37 b37 37–71 71–105 1.7 4.3 b 4.3 4.3–6.0 6.0–7.8 N.K Phuong et al / Journal of Volcanology and Geothermal Research 229-230 (2012) 23–33 31 Fig Contour map of (a) radon concentration and (b) radon to thoron concentration ratio Fig 11 Contour maps of (a) Hgsoil-gas and (b) Hgsoil concentration could be indicative of enhanced permeability where Rn-222 rapidly migrates to the surface before disintegrating into daughter products However, the release of radon is dependent on other factors, including the degree of rock fractures and the ability of the ground water to permeate through such rocks Percolating ground water transports radon from fractured porous rocks by preventing diffusion As described above, the radon will partition into the steam phase and be transported to higher elevations through permeable zones A NNE–SSW alignment of Rn anomalies, associated with the fault in this area, can be observed Other areas west of the fumarole especially on a WNW–ESE alignment also have relatively high Rn values, while low Rn values are observed in most of the remaining survey area Many studies have been published on the feasibility of using Rn and CO2 measurements to detect active structures such as fractures and 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 permeability, helping the gas ascending to the surface Furthermore, gases from a deep source can migrate upward through faults where the gas flow is driven by advection As Tn has a short half-life, 55 s, its concentration in counts per minute (cpm) decreases quickly during of sequential measurement However, Rn has a half-life of 3.8 days and can be transported in fractures for a considerable distance To detect the presence of fracture or fault systems connecting the deep zone to the surface, the Rn/Tn concentration ratio can be a suitable indicator Fig 9b shows the Rn/Tn ratio contour map High Rn/Tn ratios (>0.4) are found mainly about 200 m to the north and 250–300 m to the south east of the fumaroles Zones with a high Rn/Tn ratio not only indicate Fig 10 Contour maps of CO2 concentration 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 of fault/fracture from deep zone to the surface Fig 10 is a contour map of the CO2 concentration The CO2 map clearly shows a close relationship with the Rn, and Rn/Tn maps It is relevant that in the Gedongsongo, CO2 values greater than 20% are considered anomalous The high and anomalous CO2 values (>10%) were detected around the fumaroles and 250–300 m to south of the fumarole while low CO2 values are mainly located in the east (Fig 10) Zones with anomalous and high CO2 concentrations trend from NNE–SSW and WNW–ESE, and these can be postulated as fault locations It is interesting to note that areas with high Rn/Tn ratios and anomalous CO2 concentration occur at the same location This alignment, trending NNE–SSW and WNW–ESE (Figs 9b and 10) can be postulated as a fault in this area The NNE–SSW alignment of soil gas anomalies agrees with topographic and geologic data of a fault zone (Van Bemmelen, 1970; Thanden et al., 1996), while the WNW–ESE alignment may be another fault zones, unknown prior to the soil gas survey The combined CO2 concentrations and soil temperatures (Table 4) are correlated to the north of the fumarole This observation may identify the displacement of a high temperature heat source or an up-flow zone of high temperature geothermal fluid (Lan et al., 2007) The interpretation of Hg survey results will provide more evidence on this potential up-flow zone 4.2.3 Mercury as feature of geothermal activities The 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 shows a contour map of Hgsoil-gas The high concentration zone represents Hgsoil-gas concentrations above 40 ng, and extends from 0.7 km north to south by km west to east from the fumarole This supports the suggestion that geothermal activity is widespread around the fumarole zone Absorbed Hg on the soil surface is difficult to desorb except after heating at high temperatures Thus, the resulting mercury anomalies are 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 Hg anomalies occur when geothermal fluids escape from a deep reservoir and migrate to shallow levels Anomalous Hgsoil-gas values were found in the north of the fumarole which consistent with the location of the anomalous Rn, Tn, CO2 and temperature values The chemistry of spring waters in this area is acid-sulfate type, indicating that a component of these fluids has condensed from H2S-rich vapor phase Mercury is strongly partitioned into the vapor phase, so steam escaping towards the surface will be enriched in Hg, and acid hot-springs and fumaroles will display strong Hg anomalies We, therefore, postulate that upwelling and subsequent boiling occurs beneath the area to the north of the fumarole The soil gas contours appear to define the NNE–SSW and WNW–ESE alignments, which are thought to represent fault/fracture zones Non-thermal waters or a mixture of thermal and non-thermal waters are found in the south of fumarole and this area coincides with low Hg concentrations Mercury enrichment in soils is a dynamic process, as revolatilization and biogenic uptake with subsequent volatilization (Varekamp and Buseck, 1983) will continuously remove Hg from the soil A steady-state will occur after an initial period of nonequilibrium The continuous Hg loss process means that old and current thermal activity can be distinguished (Koga, 1982, 1988; Varekamp and Buseck, 1983) The Hg results in soil and in soil gas are not always in good agreement because Hgsoil-gas indicates current geothermal activity while Hgsoil shows the history of geothermal activity up to the present (Koga, 1982, 1988) Fig 11b shows high Hgsoil concentrations are located in the east of the fumarolic area, and we infer that was an ancient geothermal zone Varekamp and Buseck (1983) have documented cases where active geothermal zones are enriched in Hgsoil-gas but lack Hgsoil Fig 12 Simplified conceptual hydro-geochemical model of the Ungaran geothermal field 4.3 Conceptual hydro-geochemical model of the Ungaran geothermal field From this study, a hydro-geochemical model of the Gedongsongo thermal waters has been developed and is shown in Fig 12 This model shows the up-flow of high temperature geothermal fluid located in the north of the fumarole The anomalous Hg values occur in the vapor-dominated part of the system, which is explained by the strong partitioning of Hg into the vapor phase during boiling Deep geothermal fluids are present below this area, and circulate through the fault and fracture zones with some portion of the fluids discharging at the surface Based on the geochemical and isotopic data, the thermal springs at Gedongsongo are of meteoric origin The meteoric waters percolate through the fault systems into the mixing zone where they are heated by deep geothermal fluids and ascend to the surface along the NNE–SSW and WNW–ESE fault In the shallow zone, CO2 and H2S rich steam rises off the thermal waters, which can lead to formation of sulfate-rich waters, while some of the ascending thermal waters mix with cold water and rise along the WNW–ESE fault The areas with low soil gas concentrations show that the boundary of the limited geothermal system is in the northern section of the fumarole field at the Gedongsongo area Conclusions The chemistry of the thermal waters discharged in the Gedongsongo area 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 Gedongsongo area The compositions of the thermal waters reveal that they have not reached chemical equilibrium with the host rocks The reservoir temperatures estimated using silica geothermometers is from102 °C to 142 °C A soil gas survey was also conducted at the Ungaran geothermal field and has provided an overview of soil gas distribution patterns produced by the underlying system The soil gas contour maps show that, Rn, and CO2 are reliable and sensitive indicators for tracing N.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 the faults trending NNE–SSW and WNW–ESE which act as conduits for geothermal fluid and soil gas From the anomalous Hg concentrations, we inferred that the up-flow zone of high temperature geothermal fluid is in the north of the fumarole Acknowledgments This study was supported by AUN/SEED-Net program, JICA (Japanese International Cooperation Academic) and Gadjah Mada University, Indonesia The authors thank Prof Sachihiro Taguchi of Fukuoka University for his help to analyze stable isotope of water samples References APHA, 2005 Standard Methods for the Examination 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Volcanology and Geothermal Research 229-230 (2012) 23–33 Fig Location of soil gas and water samples around the fumarolic area (UTM coordination system) 3.2 Soil gas measurement Soil gas surveys for... estimating reservoir temperatures are rather low because maybe part of SiO2 precipitated during storage 4.2 Soil gas survey 4.2.1 Statistical interpretation of soil gas data The soil gas measurements for... generally low soil gas concentrations 4.2.2 Spatial distribution of soil gas data and recognition of trace of fault/fracture To characterize the study area, the spatial distribution of soil gas data was