Geochemistry of Cold Springs in Geothermal Exploration Stage – Case Study of Candi Umbul Telomoyo, Central Java

Cold springs in geothermal fields are often overlooked during the exploration stage. The best practice of geothermal exploration suggests thermal manifestation as an indicator of a geothermal system in the subsurface. This common practice neglects cold springs as valuable information points related to the system. Thermal-contaminated cold springs can be helpful to indicate the presence of inferred geothermal activity below the surface during the exploration stage. It becomes important where thermal features are absent or limited, as in Candi Umbul Telomoyo. Candi Dukuh, Candi Umbul, and Pakis Dadu thermal springs are located at the periphery of the Telomoyo Volcanic Complex, at relatively low altitude. Those thermal springs are used to construct the existing conceptual models of the geothermal system. In this study, the authors tried to consider the presence of slightly acidic cold springs (pH 5.24-5.61). Located within the Suropati Depression in the North, and the flank of Mt. Telomoyo in the South, both Keningar and Sendang Ari Wulan cold springs are located at higher altitude with higher TDS and are enriched in Cl and SO4 compared to the others. These cold springs are associated with the argillic alteration zone and observed to have iron oxide deposition at the discharge area. Sendang Ari Wulan fluid is plotted at the same zone as the thermal springs on Na-Cl•SO4 facies of the Piper diagram, while Keningar fluid is plotted on HCO3-Ca•Mg facies are similar to other cold springs. Although Sendang Ari Wulan shows a better correlation to the thermal springs, based on Piper diagram, both Keningar and Sendang Ari Wulan cold springs are classified as HCO3-SO4 and SO4-HCO3-Cl. These observations show the possibility of contamination of the geothermal system occurring below the Keningar and Sendang Ari Wulan cold springs, which is higher than the thermal springs. This interpretation is supported by the anomaly of high Hg and CO2 surrounding the acidic Keningar and Sendang Ari Wulan cold springs, three temperature gradient wells that prove the presence of three times higher-than-normal geothermal gradient. The research concludes that cold springs data are indeed useful in aiding the interpretation, especially during the exploration stage.


Introduction
Candi Umbul Telomoyo geothermal working area is located approximately 42 km southwest of Semarang City, in Central Java.PT Geo Dipa Energi (GDE) is currently exploring the working area.Historically, the exploration activity was first conducted by PT.PERTAMINA (Persero) from 1988 to 1993, and later continued by the Geological Agency (PSDG) from 2010 to 2016.Several surveys were conducted by the Geological Agency, including geological, geochemical, geophysical, and two gradient temperature wells with a depth of 700m each.The assignment to PT GDE begins in 2017.In 2018-2019, exploration activity was restarted.In addition to this extensively studied geothermal field, this research aims to understand the role of the cold springs and their relationship to the thermal springs of the Candi Umbul Telomoyo geothermal system.
The presence of Candi Umbul, Pakis Dadu, and Candi Dukuh thermal springs as geothermal manifestations are not debatable.These thermal manifestations possessed relatively high Cl and HCO3 content and observable carbonate deposition, as the [1] reported.With the understanding from previously conducted studies, this research would like to provide a different view in terms of geochemistry analyses, especially on the role of the cold springs in identifying the geothermal system of Candi Umbul Telomoyo.
Compared to the other cold springs of lower elevation, Sendang Ari Wulan and Keningar cold springs have higher TDS and EC in addition to acidic pH.Both are associated with the argillic alteration zone and observed to have yellowish-brown iron oxide deposition at the discharge area.These physical and chemical characteristics are notable in cold springs that are thermally contaminated by geothermal fluid from the subsurface, as discussed previously by [2][3][4] [5] [6].These characteristics lead to the question of whether Sendang Ari Wulan and Keningar cold springs are thermally contaminated springs.Hence, triggered the authors to conduct this study.

Geology of Candi Umbul Telomoyo
Candi Umbul Telomoyo is situated in the middle part of the Quaternary Volcanic Zone of Java which generally extends in an E-W direction parallel with the subduction zone and occupies the Solo Depression Zone [7].The Kendeng Ridge bounds this zone to the north and the Southern Mountains to the south.The Quaternary volcanic arc of Java was formed by compressional forces associated with the Sunda orogeny due to orthogonal subduction of the Indo-Australian plate beneath the Eurasia plate in the late Neogene [8] [9].The major geological structures of Java are characterized by E-W Kendeng-Baribis thrust faults and major strike-slip faults of NE-SW Cimandiri and Muria-Kebumen Fault, and NW-SE Citanduy Fault [9].The NW volcanic alignment in Central Java may suggest the presence of an NW trending structure (i.e., Sumbing-Sindoro-Dieng and Merapi-Merbabu-Telomoyo-Ungaran), as shown in Figure 1.
Based on geological field mapping performed by GDE in 2019, the stratigraphy and volcanostratigraphy units of the Candi Umbul Telomoyo prospect area (Figure 2) from oldest to youngest are divided into: a. Middle Miocene Sediments and Volcanics of Kerek Formation (Tmk).Based on regional geological mapping conducted by [10], this formation is composed of interbedding of claystone, marl, tuffaceous, sandstone, conglomerate, volcanic breccia, and limestone.b.Middle Miocene-Pleistocene Sediments and Volcanics of Penyatan Formation (QTp).Referring to geological mapping conducted by [10]

Geothermal System of Candi Umbul Telomoyo
The geothermal system of Candi Umbul Telomoyo has been interpreted previously by several authors.
[15] defined that the system was associated with the recent volcanism of the andesitic-basaltic Telomoyo Volcanic Complex based on the 3G study conducted by the Geological Agency in 2010.The reservoir is defined as one with the upflow area to be within the caldera complex based on the argillic-advanced argillic alteration zones and a temperature core hole TSH-1.The outflows are towards Candi Umbul and Pakis Dadu thermal springs in the SW and Candi Dukuh thermal springs in the NE.[16] published the geochemical study of the thermal springs, and the conclusion of the system agrees with [15].On the other hand, [17] proposed a different model and concluded that there are two reservoirs in the area based on the fluid evolution study of CTL-1 and CTL-2 core samples of the Geological Agency, one is located beneath Candi Umbul and Pakis Dadu, while the other is beneath Candi Dukuh.Surface geological structures identified from geological survey conducted by [14] are dominated by NE-SW normal faults that intersected by NW-SE strike-slip faults and minor faults with an N-S trending.The existence of normal fault and the fault-intersection zone potentially enhance permeability in rocks.This permeability is associated with the Rawaboni Fault 1 that controlled surface alterations (Figure 3) and thermal manifestations of Candi Umbul, Pakis Dadu, and Candi Dukuh.Surface hydrothermal alterations in Candi Umbul Telomoyo are related to argillic alteration based on XRD analyses, which is  T10 cm is soil temperature at 10 cm depth.
Sedimentary and volcanic rock seems to host a geothermal reservoir in Candi Umbul Telomoyo [15].Tertiary sedimentary rock formation under the Quaternary volcanic, namely Penyatan Formation and Kerek Formation, could have contributed to the system as studied by [14].However, [16] stated that the reservoir rock is solely composed of pyroclastic rock.The authors find the possibility of sedimentary and volcanic rock units as reservoir rock instead of solely pyroclastic rock, with the heat source located beneath Mt.Kendil as the youngest volcanic in the Telomoyo-Andong Volcanic Complex.

Field Observation and Analyses
Three thermal manifestations have been reported in various publications (among others are [15][17] [18]), namely Candi Dukuh, Candi Umbul, and Pakis Dadu which surround Telomoyo-Andong Volcanic Complex at its foot (lower elevation, at 467-578 masl).These warm springs are generally neutral with temperature of 34.6-36.3°Cwith presence of gas bubbles and carbonate deposition.Candi Umbul and Pakis Dadu thermal springs are observed at paddy fields, while Candi Dukuh thermal springs are at the edge of Rawa Pening Lake.
The presence of slightly acidic cold springs in Keningar and Sendang Ari Wulan has never been reported.Both cold springs are located at 803-978 masl.The pH of Keningar and Sendang Ari Wulan are 5.61-5.24with the temperature of 23-25.3°Cand deposition of yellowish-brown iron oxide.Neutral cold springs are also observed, such as Soko and Muncul, with temperature of 20.7-23.8°C at the elevation of 473-719 masl.The distribution map of the springs is available in Figure 4, and the physical characteristics observation is available in Table 1.Yellowish-brown iron oxide deposits are observed in Suko, Keningar, and Sendang Ari Wulan cold springs, as shown in Figure 5.

Result and Discussion
To understand the role of the cold springs and their relationship to the thermal springs and the geothermal system, field mapping and observation is conducted in Candi Umbul Telomoyo.The assessment of thermal and cold springs includes geochemical data analyses, springs characteristic distribution analyses, hydrogeology, and integrated with surface geology information.The datasets were analysed for liquid analyses (anion and cation), gas analyses, and stable isotopes ( 18 O and 2 H).The geochemical analyses include comparing cation and anion composition through Scholler's and Piper's diagrams for facies and SO4-HCO3-Cl ternary diagram for determining the fluid type and calculating solute geothermometer for estimating the reservoir temperature.The geochemistry analysis result is further correlated with the distribution of Hg and CO2 anomaly of soil gas and the result of temperature core holes previously drilled by the Geological Agency and Pertamina.
Sendang Ari Wulan and Keningar acidic cold springs are located within the Telomoyo Volcanic Complex and possess elevated SO4, even higher than the thermal springs.The SO4 content of the thermal springs are 0.2-0.211mg/kg, meanwhile the slightly acidic cold springs are 106-119 mg/kg.Correlating to the location, these cold springs have higher altitude than the thermal springs.Comparison from Scholler's Diagram (Figure 6 (a)) shows that the enrichment of Na and Cl in Sendang Ari Wulan has a similar pattern with the thermal springs, while Keningar follows the pattern of the cold springs.Identification of the facies and fluid type of the cold springs are done through Piper's plot and Cl-SO4-HCO3 diagram, the result is available in Figure 6 (b) and (c).These cold springs, however, are still dominated by meteoric water as indicated by stable isotope (δ 18 O and δ 2 H), and the Na-K-Mg diagram.The thermal springs have slightly higher Mg (14-33 mg/kg) than the cold springs (8-12 mg/kg).
The Schoeller's diagram in Figure 6 (a) also highlighted the anomaly of generally elevated HCO3 distribution and the spatial distribution map of the fluid type is available in Figure 6 (c).According to [1] and [17], the HCO3 content of Candi Dukuh samples indicates that the fluid was formed on the lower slopes of the geothermal system.Candi Umbul and Pakis Dadu are of Cl-HCO3 type, which indicated that the water derived directly from the depths and carried dissolved chemical compounds as a result of the interaction of hot fluids with rocks at depth.[16] interpreted that the thermal springs of Candi Dukuh and Candi Umbul are of the HCO 3 type, originating from the steam condensation process below the water table.With the possibility of sedimentary rock contribution in the reservoir, enrichment of B, HCO3, Ca, and Mg is expected in the reservoir fluid, alongside Na and Cl [19][20][21] [22].Following up on the hypotheses, HCO3-type is considered as a background since all samples of thermal and cold springs are characterized by this type.Aside from the enrichment from sedimentary rock, the increase of HCO3 content is potentially resulted by the geothermal system discharging at the outflow zones (as typically occur in high-terrain volcanic systems).This is reflected by the thermal springs Candi Dukuh, Candi Umbul and Pakis Dadu at the lower elevations.Similar with the relationship of Cimanggu and Rancawalini hot springs as the outflow of the Patuha Geothermal System [23][24] and Cangar, Coban, and Padusan hot springs as the outflow of Arjuno-Welirang Geothermal System [25].
The observation from the analyses and the role of each cold spring with respect to the concept of a geothermal system is discussed below.From the NE to the SW:

Muncul
Muncul cold spring is located at the NE of the working area.This spring pH is neutral and has a flow rate of 3 L/s and of HCO3 -Ca•Mg facies.Despite having a slightly higher composition of SO4 (2.16 mg/kg) compared to the composition in the thermal springs, the other cation and anion have the lowest concentration.Thus, making this cold spring the benchmark of uncontaminated spring.

Sendang Ari Wulan
Sendang Ari Wulan is located on the northern slope of Mt Telomoyo.It is a slightly acidic cold spring with the highest elevation, highest flow rate (5 L/s), and observable deposition of iron oxide.SiO2, Ca, and Fe in this cold spring are higher than the thermal springs.Sendang Ari Wulan is plotted at the same zone as the thermal springs on the Piper diagram, which has Na-Cl•SO4 facies.

Keningar
Keningar is a slightly acidic cold spring, located within the Suropati Depression at 803 masl.Iron oxide deposition is observable.SiO2, Fe and Cl in this cold spring are lower than the thermal springs, but SO4 and Ca concentrations are higher.Keningar has HCO3 -Ca•Mg facies, similar to the other cold springs including Rawa Pening.Keningar cold spring is classified as HCO3-SO4, with SO4 content of 119 mg/kg.

Suko
Suko cold spring is located between Candi Umbul and Pakis Dadu thermal springs at the SW and the Suropati Depression.The elevation is relatively in the middle, at 719 masl.Similar to Keningar and Muncul cold springs, Suko is of HCO3 -Ca•Mg facies.The composition of HCO3 in this spring is heavily enriched (290 mg/kg), slightly higher than Candi Umbul thermal spring (275 mg/kg) and the highest among all cold springs.Fe is even higher in the Suko cold spring (almost 10 mg/kg) compared to the thermal springs and the deposition of iron oxide is observable.Suko cold spring has a flowrate of 3 L/s.When confirmed with respect to the location, the Suko cold spring may have an association with the NE-SW Grabag normal fault.

Role of the cold springs to the system
A comparison of Fe, SO 4 , Cl, HCO 3 and the flowrate of these cold springs and the thermal springs were observed to understand the role of the cold springs in the system.The HCO3 in the higher elevation acidic cold springs is 80-170 mg/kg, the mid-elevation Suko cold spring is 290 mg/kg, and the lowest elevation thermal springs are 270-410 mg/kg.
On the contrary, SO4 shows a drastic decreasing pattern from the acidic cold springs (106-119 mg/kg), and to the springs outside the depression (0-0.2 mg/kg).Chloride (Cl) shows the highest content at the thermal springs in the lower elevation (170-520 mg/kg), followed by the acidic cold springs (2-97 mg/kg), and the least is in Suko cold spring (2 mg/kg).
These observations support the interpretation that contamination from the geothermal system may occur below these cold springs since SO4 and Cl may occur due to the condensation of geothermal gases into near-surface, oxygenated groundwater [19].Referring to the distribution map in Figure 4, and the observation above, Suko cold spring may express the presence of HCO3-type water flow towards Candi Umbul and Pakis Dadu in the subsurface.This flow is possibly intersected by the Grabag Fault and diluted by the high volume of groundwater and/or meteoric water as it flows to the surface, forming the spring, as indicated by the enrichment of Fe.
In geothermal systems with close volcanic-magmatic association and located along convergent plate boundaries, the deep, magmatic heat source may add acid gases like HCl, HF, SO2, H2S and CO2 as well as some 'andesitic' water [27], accompanied by volatile species such as H3BO3, As and Hg [19].When steam separation takes place, the more soluble gases (HCl, HF, CO2, H2S, and NH3) are retained in part in the liquid phase [28].These gases in liquid form will take place as Cl, F, B, SO4, HCO3, NH4, etc.Meanwhile, the less soluble gases (e.g., Hg, N2, H2, CH4, and CO) enter preferentially the vapor phase [19] [28].Since these gases are often enriched and stored in soil, some of these gases are able to be pathfinders for geothermal system, such as CO2 and Hg [19][29] [30].
Analyses of soil and soil gas were conducted by the Geological Agency in 2010.The agency collected Hg and CO2 samples, and measured soil pH and temperature from 144 stations.The anomalies are mostly distributed around the Suropati Depression and Rawa Pening.The anomaly delineation of high Hg and CO 2, and low pH are available in Figure 7.At the depth of 1 (one) meter, both Keningar and Sendang Ari Wulan areas are associated with high Hg and CO2, and low pH anomalies.As reported by the agency [1], the lowest soil pH was measured in Keningar area (4.80) with an alteration zone, acidic cold spring, and yellow-brown iron oxide deposition.High Hg anomaly (>251 ppb) was also reported in the Keningar area, with the addition of Sendang Ari Wulan area, associated with alteration zone, acidic cold spring, yellow-brown iron oxide deposition and the older temperature gradient well drilled by Pertamina in 1993 (as reported by [1]).
In 2012, Nevada Bureau of Mines and Geology (through the publication of [31]) introduced the idea that the geothermometer estimates from thermally contaminated cold springs can sometimes be a valuable exploration tool to detect traces of geothermal activity.In this study, the authors experimented with quartz conductive geothermometer by [32], Na/Li for sedimentary setting by [33] and Na-K-Ca for samples containing excess Ca by [34], as presented in Table 2.The results are lower than the estimated temperature of Na/K geothermometers, as published by [15][16] [17].The geothermometer of the cold springs suggests a reservoir temperature of 175-221°C.There are two gradient wells in the eastern area of Keningar named CTL-1 and CTL-2 and another one of TSH-1 is in the western area of Sendang Ari Wulan cold spring [1][35] [36].The Geological Agency drilled these gradient wells in 2016 and Pertamina in 1993.Information regarding the locations, the depth of these gradient thermal wells, bottom hole temperature, and the thermal gradient is available in Figure 7.In summary, the gradient wells temperature of Candi Umbul Telomoyo resulted in thermal gradient ranging from 9.6 o C to 11.3 o C /100m.This information suggests a higher thermal gradient, three times higher than the normal gradient.The gradient well suggests a shallower temperature result in the vicinity of the Sendang Ari Wulan area.Knowing there are anomalies of high Hg and CO2, low pH, and elevated geothermal gradient underneath both Keningar and Sendang Ari Wulan cold springs, this possibly supports the interpretation of geothermal fluid presence in the Candi Umbul Telomoyo prospect.In addition, Mt.Kendil is a geological feature considered the youngest volcanic in the Candi Umbul Telomoyo as shown in Figure 8.

Conclusion
These cold springs data are indeed useful in aiding a deeper comprehension of the system during the exploration stage, with HCO3 concentration ranging from 80-170 mg/kg in higher elevation acidic cold springs, whereas SO4 concentration decrease from acidic cold springs and to springs beyond the depression.The Cl concentrations are highest in lower elevation thermal springs, followed by acidic cold springs (2-97 mg/kg) and lowest in Suko cold spring (2 mg/kg).These findings imply that contamination from the geothermal system may occur below these cold springs, as SO4 and Cl may develop as a result of geothermal gas condensation into near-surface, oxygenated groundwater.Based on the heavily enriched concentration of Fe and HCO3, the outflow may flow towards the Candi Umbul and Pakis Dadu thermal springs and intersected by the Grabag fault, resulting at the presence of Suko cold spring on the surface.
Soil and soil gas analyses identified elevated Hg and CO2 concentration, as well as low pH levels, in the Keningar and Sendang Ari Wulan areas.Both are accompanied by the presence of acidic cold spring, and iron oxide deposits.The cold springs geothermometer predicts a reservoir temperature of 175-221°C, supported by three gradient wells in Keningar and Sendang Ari Wulan area resulting in a thermal gradient ranging from 9.6°C to 11.3°C /100m.This data points to a larger thermal gradient, three times that of the typical gradient.In conclusion, the acid Cl-SO4 spring (Sendang Ari Wulan) and SO4-rich spring (Keningar) with the association of volcanic crater and argillic-advanced argillic alteration may indicate the presence of thermal contamination of high-temperature fluid (possible geothermal system?).-----------------
halloysite, kaolinite, smectite, and chlorite.This surface argillic alteration contributes to the clay cap of the geothermal system in Candi Umbul Telomoyo.

Figure 4 . 7 Figure 5 .
Figure 4. Sampling locations of cold and thermal springs.The cold springs are situated at a high elevation contrary to the thermal spring.

Figure 6 .
Figure 6.(a) Scholler's Diagram of thermal and cold springs of Candi Umbul Telomoyo.All samples are generally elevated in HCO3 content.(b) Piper's Diagram of s thermal and cold springs in Candi Umbul Telomoyo.(c) Cl-SO4-HCO3 diagram of samples in Candi Umbul Telomoyo (left), and the distribution of water type (right).HCO3-type is distributed in all samples.

Figure 7 .Figure 8 .
Figure 7.The anomaly of Hg, CO2, and low pH in soil (left), and the result of gradient temperature wells (right) within the Suropati Depression.The cross-section of the wells is at the bottom of the picture.

Table 2 .
Geothermometer calculation experiment for cold springs, in comparison to the thermal springs (in °C).Red -thermal springs; Blue -thermally contaminated cold spring; Black -cold spring.