Geochemical and isotope characterization of geothermal spring waters in Sri Lanka: Evidence for steeper than expected geothermal gradients
Graphical abstract
Highlights
► We studied geochemical and isotope compositions of geothermal and natural springs. ► Geothermometric calculation indicated mean reservoir temperatures of about 120 °C. ► Hot water springs are recharged by local meteoric rain. ► During subsurface circulations, water is heated by a steep geothermal gradient.
Introduction
Although geothermal systems are commonly associated with volcanic activities or recent uplifts (Diamond and Harris, 2000), several hot water springs with low to intermediate temperatures can be found in the Proterozoic crust of Sri Lanka where recent volcanic or tectonic activities are unknown. Most of these wells discharge hot water as natural springs. In addition, an artesian hot water well occurs in one location. Measured surface temperatures of the geothermal springs in Sri Lanka ranged between 34 and 55 °C (Dissanayake and Jayasena, 1988). Recently some of these springs have fallen dry or were inundated in man-made reservoirs and nowadays only seven geothermal springs occur in the high-grade metamorphic terrain of Sri Lanka. Only few such geothermal springs were reported in high grade terrains such as in India and Canada (Chandrasekharam and Antu, 1995, Jessop et al., 1991).
In recent years, these geothermal springs raised much interest for potential development of geothermal exploitation for green energy. Despite this interest, hardly any work has been published on the geothermal springs in Sri Lanka, except for the work of Dissanayake and Jayasena (1988) and Senaratne and Chandima (2011). These workers found that subsurface temperatures of some of the thermal springs are around 143 °C. These temperatures raise speculation about economic use and with this about the origin of the waters. The localization of heat sources need further investigation as they are located in a Precambrian high grade metamorphic terrain that is untypical for thermal springs. Recently, Senaratne and Chandima (2011) discussed the origin of the Marangala geothermal spring (Fig. 1) with structural and surface geological evidences. These authors outlined the mechanisms of the artesian type of well in the area. Although the hot springs of Sri Lanka are known since centuries, the origin and the structure of these hydrothermal systems are still not well understood. The main objective of this work was therefore to characterize geothermal springs in Sri Lanka by chemical and water isotope methods. Comparison with nearby low-temperature groundwaters and with precipitation helped to trace hydrogeological flow paths and to understand the recharge sources as well as to narrow down potential heating mechanisms.
With this, the present study interprets the first set of geochemical data and water stable isotope values (expressed as δ2HH2O, δ18OH2O) that were obtained from the geothermal spring systems from the high-grade metamorphic terrain. These parameters have become important tools in hydrogeology and have widely been used as natural tracers (Clark and Fritz, 1997). For instance, they were applied in geothermal systems with different origins (Delalande et al., 2011, Han et al., 2010, Pasvanoğlu and Chandrasekharam, 2011).
Section snippets
Geological and hydrological setting
The Precambrian basement of Sri Lanka is a small fragment of the Gondwana supercontinent, over 90% of which is made up of amphibolite to granulite grade metamorphic rocks. The remainder is covered by sedimentary sequences that belong to Jurassic, Miocene and Holocene periods (Fig. 1). The Proterozoic crust of Sri Lanka is divided into three major lithological units based on geochronologic, petrologic and geochemical data. These are known as the Highland Complex (HC), the Vijayan Complex (VC)
Materials and methods
Anions, cations and isotope ratios of water (expressed as δ18OH2O and δ2HH2O) from seven hot water springs, three cold-water springs, and four deep wells were collected for this study (Table 1). Out of these, one cold-water spring is located close to the hot water springs and all other cold groundwater-sampling sites were selected from the nearest possible location (<100 m) to the sampled thermal springs. The Nelumwewa geothermal spring (NW in Fig. 1) is located in a lake and the cold-water
Geochemistry of thermal waters
Descriptions of sampling locations and geochemical results of thermal and non-geothermal waters are listed in Table 1, Table 2. The discharge temperature of geothermal waters varied from 39.1 °C (Rankihiriya) to 62.2 °C (Nelumwewa) while their pH varied from 5.7 (Kanniyai) to 8.0 (Kapurella). The electric conductivity (EC) of geothermal waters varied from 532 μS/cm (Kanniyai) to 7360 μS/cm (Mahapelessa) while it ranged from 354 μS/cm (Bibile) to 7290 μS/cm (Mahapelessa) in non-geothermal groundwaters
Origin of the geothermal springs
The hydrogeochemical and isotope (δ18OH2O, δ2HH2O) characteristics of geothermal and non-geothermal waters from the Precambrian metamorphic terrains of Sri Lanka helped to narrow down the origin of the geothermal water. Various geothermometers were applied to explore deep reservoir temperatures. However, remarkable similarities of geochemical and isotope compositions of geothermal waters with those of the local non-geothermal waters indicates similar sources of both water types. Based on
Conclusions
Analyses and comparison of hydrogeochemical and isotopic characteristics of waters from thermal springs and non-thermal groundwater collected close to thermal springs reveal that both waters are hydrogeochemically and isotopically similar in composition. Stable Isotope data suggest that the origin of both thermal and non-geothermal water recharge is almost all exclusively precipitation. Low concentrations of trace elements in both type of water also provide evidences for limited rock–water
Acknowledgements
The Authors are grateful to the Alexander von Humboldt (AvH) Foundation, Germany for financial support in the form of fellowship offered to Rohana Chandrajith. We are also indebted to Sonja Konrad, Lars Wunder, Silke Meyer, Veith Becker and colleagues and friends of the GeoZentrum Nordbayern Erlangen and the Chair of Applied Geology for their help during laboratory work. We also thank Thusitha Nimalasiri and N. Sooriyaarachchi from the Institute of Fundamental Studies, Kandy, Sri Lanka for
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