Geochemical characteristics of hot springs in Southern Thailand

Abstract

Hot water springs can be found in most regions of Thailand, from the north to the south, except in the northeast. There are around 120 hot springs altogether, with about 40 hot springs in Southern Thailand, and all are related to a non-volcanic geological setting. Most hot springs are mainly developed as tourist destinations, for local as well as for international visitors, often combined with spas or other health-related activities. However, for any further development, including geothermal energy, but not limited to, a better understanding of relationships between the geological environment and hot water characteristics is required. In this study, field investigations and laboratory measurements were carried out to identify the hydrogeochemical signatures of major hot springs in Southern Thailand, with locations in Krabi, Nakhon Si Thammarat, Phang Nga, Surat Thani, Chumphon, and Ranong Province. Standard and advanced geochemical analysis of water samples was done in laboratories in Thailand and in Germany; water samples were collected from natural hot springs, groundwater wells, and seawater sources along the eastern (Gulf of Thailand) and western coast (Andaman Sea) of Southern Thailand. Results show that hot springs discharge geothermal water with temperatures between 35.7 to 70.0 °C and mostly have an alkaline pH of around 6.6 to 8.1. Hot springs in Southern Thailand separated into two main systems which are hot spring in igneous area and in sedimentary area. The major cation and anion concentrations indicate that hot springs in the Southern Thailand can be separated and classified into four types: Na+-Cl-, Na+-Mg2+-HCO3-, Ca2+-HCO3-, and Ca2+-SO42- rich type. Carbonate components in the water originated from weathering and dissolution of carbonate and dolomite formations nearby hot springs. Moreover, elevated salinity concentrations (Na+, Cl-, Mg2+) in some of the hot springs can be correlated to seawater intrusion from the Andaman Sea in the west or the Gulf of Thailand in the east. Oxygen stable isotope data and CFC concentration display reveals that the pathway of geothermal water in Southern Thailand is mainly from old local precipitation water that accumulated in the system before the 1950s and then mixed with low proportion of recharged fresh meteoric water from the 2010s going downwards into the subsurface and heated up by heat sources and finally to be discharged at the subsurface as a hot spring.