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

Abstract

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.