Hydrochemical and environmental isotope study of the geothermal water in Mae Chan (North) and Ranong (South) geothermal areas in Thailand

The exploration and development of geothermal resources in Thailand have been investigated for electricity generation since 1946. In total, 112 hot springs are manifested which mostly scattered at the granite body from the north to south of Thailand. Fang and Mae Chan areas, located in the NE-SW Mae Chan fault in the northern part of the country, are among five highest potential hot springs. These areas have been continuously investigated and successfully installed the geothermal utilization facility. One the other hand, a few pieces of research have investigated low enthalpies (low temperature) hot springs such as hot springs along the Mae Chan fault and Ranong fault lines. Therefore, the hydrogeological cycle of these hot springs is still considered ambiguous as little knowledge about these systems is available. This Ph.D. dissertation integrates the applications of natural hydrogeochemical and isotopic composition in water and rock minerals to evaluate and improve the understanding of these hydrothermal systems. The results generally reveal that these thermal waters are originated from local meteoric water, are recharged from the high elevation, flows through weathered and fractured granite and heated at the shallow reservoir (Na-HCO3 water type). These thermal waters are reliable on the conservative K-Mg and chalcedony geothermometers. The REE concentrations in thermal water are chemically reduced by increasing temperature and pressure which has the fluoride concentration predominantly control the precipitation processes of LREE-fluoride complex during flow path in this system. The geological setting of both areas makes a difference in isotopic and chemical composition of thermal water. -In chapter 4, based on a case study in the Mae Chan faults zone, the thermal recharges from the high elevation of the mountainous areas ranging from 740 to 1,200 m AMSL. The conservative K-Mg and chalcedony subsystems state equilibrium temperatures ranging from 98 to 139 °C and provide the estimated reservoir depth ranging from 353 and 814 m (-405 and 150 m AMSL). The silica-enthalpy mixing diagram provides temperature of geothermal reservoir ranging from 129 to 168 °C and most of the samples have stream-loss during ascending to the surface and mixed with cool water from 35 to 81% by volume. During the flow part, the chemical concentration of F, Na+, K+, and SO42- could be developed from the granitic minerals (e.g., Na-feldspar) of the local batholith granite together with decreased Ca2+ and Mg2+. The Cl- is derived from precipitation. The d13C in thermal waters were mixed with three different types of carbon sources. The first group is characterized by an initial d13C that remains stable from the d13C surface water. The second group is probably mixed with local geologic gases. The third group probably mixed with biological carbon. The 14C dating is specifically reliable for the first group (without external carbon contamination) that has the residence time periods ranging from 12,760 to 18,000 BP. The mixing model results of Sr and 87Sr/86Sr ratio in thermal water provided differently in the percentage of rock contribution ranging from 73.11 to 86.31%. The hot springs closed to the fault, had 73.11% to 75.10% Sr with rock origin, while the hot springs located far from the fault core had 82.50 to 86.31% Sr with rock origin. This relation probably conducted with the influence of that porosity and permeability of the crystalline basement rock, which varies with the distance of the fault core from the hot springs. -In chapter 5, based on a case study in the Ranong faults zone, thermal water recharges from the high elevation of the mountainous areas ranging from 378 to 483 m AMSL. The conservative K-Mg and chalcedony subsystems state equilibrium temperatures ranging from 82 to 105 °C. Based on drilling information and Mg/k geothermometer, the estimated reservoir depth is in the range from 327 and 640 m (-286 to -612 m AMSL). The silica-enthalpy mixing diagram provides the temperature of the geothermal reservoir ranging from 130 to 199 °C and most of the samples have stream-loss during ascending to the surface and mixed with cool water from 70 to 90% by volume. The main source of carbon in the thermal water is the organic-rich soil and river sediment from the southwestern coast of the country. During the flow part, the chemical concentration of F, Na+, K+, SO42-, and Ca2+ could be developed from the granitic minerals (e.g., Na-feldspar) of the local batholith granite together with decreased and Mg2+. The Cl- is derived from precipitation. The corrected residence time from Pearson’s model ranges from 2,675 to 5,235 BP. The mixing model results of Sr, and 87Sr/86Sr ratio in thermal water provided the narrow range in percentage of rock contribution ranging from 88.31 to 86.21 suggesting that these hot springs have similar water-rock interaction and are isotopically controlled by the granitic rock. Theses differences in environment chemical and isotopic results can be explained by multi, small reservoirs, rather than by single, large scale reservoirs. Moreover, these can establish four conceptual models in Ranong geothermal area and five conceptual models in Mae Chan geothermal area which will be helpful for future water resources assessment and sustainable geothermal water management.2021-06-0