Cold – water springs as integral part of geothermal systems: an example from Sri Lanka

Abstract

Hot water springs are typical surface manifestations of a geothermal system, especially in low enthalpy, amagmatic systems. In many of these systems, numerous cold-water springs are often associated with hot water springs. The smaller number of hot water springs and the wider spatial distribution between them make it difficult to perform a comprehensive study of such a geothermal system. In this case, associated cold water springs can be of particular help in understanding the hydrogeological setting of the geothermal system which is vital information for any future geothermal exploration programs. There are 9 known hot springs in Sri Lanka, however they are spread over a larger area of the eastern lowlands of the country. On the other hand, there are over 225 known cold-water springs distributed among the hot water springs, making Sri Lanka a perfect location for a case study. Most hot water springs are located at a great distance from their recharge zone (~ 50-100 km). With the exception of very few springs, cold water springs have short recharge to discharge distances (< 25 km). Geochemical and isotopic studies of the hot springs and the nearby cold springs show that both kinds are of the same origin and recharge at similar altitudes (> 600m). The electrical resistivity of cold-water springs is comparatively higher than that of rain and fresh surface water, but lower than that of hot water springs. This suggests that these cold spring waters also travel longer through the fault/fracture network through which hot spring waters circulate from recharge zones to discharge zones. These observations of cold-water springs show that they are also part of the geothermal system in Sri Lanka and reveal important information about the fluid flow paths of the geothermal system. The results and observations of the present study highlight the importance of cold-water springs in understanding the hydrogeological setting of a geothermal system. In addition, the new knowledge will be of significant benefit to future geothermal exploration programs, particularly in systems with a smaller number of hot water springs spread over a larger area.