Integrated geophysical and geochemical investigations on the high-salinity geothermal waters of the Khlong Thom Hot Spring tourist attraction in Krabi, southern Thailand

The Krabi saline hot spring (Khlong Thom Hot Spring) located in southern Thailand has been recognized as a unique geothermal spring attraction due to its high-salinity geothermal groundwater. Studies have been performed to understand the mechanisms of saltwater intrusion into the geothermal aquifers. However, little is known about how the high-salinity groundwater affects the local geological structures, geological materials, and groundwater aquifers. Therefore, we performed integrated geological well measurements, geophysical electrical resistivity tomography (ERT) surveys, and geochemical analyses for major ions (cations and anions) and geothermometers (silica and cation) to investigate the local geological settings and the sources of salinity in the Krabi saline geothermal system. The water samples from the natural hot spring pools and geothermal wells were uniformly of the Cl--Na+ type with high total dissolved solids (TDS) contents ranging from 12,663 to 15,180 mg/L, but the nongeothermal water was considered to be shallow groundwater of the Na+-HCO3- type with lower TDS contents. Once the trapped saltwater intruded into the wetlands (salt-marshes), its chemical composition was modified by the freshwater/groundwater mixing processes active in the development of shallow geothermal reservoirs. However, a silica-quartz geothermometer was utilized, and the calculated reservoir temperatures ranged from 83 to 86 degrees C, unlike the cation and silica-chalcedony geothermometers, which were not likely to be applicable for estimating reservoir temperatures in this geothermal system. Moreover, the ion ratios indicated that the water-rock interactions occurring within the geothermal reservoir, mostly related to its nonequilibrium state, might have little effect on the quality of the geothermal groundwater. ERT interpretation was carried out to compare the characteristics of the geological materials obtained directly from the geothermal wells with their apparent resistivity values corresponding to the obtained 2D-ERT profiles. Low resistivity values, ranging from 1 to 10 ohm-m, may have arisen from the dominating influence of the freshly mixed saltwater, which mainly originated via saltwater intrusion into the shallow aquifer on the study site. Therefore, 2D-ERT can be highly useful for detecting changes in thickness and detecting the presence of local faults in the Krabi saline geothermal field. In comparison, geothermal water rose to the surface due to local faults and this finding was well associated with a tectonic junction of the Krabi geological setting. These research findings suggest that governmental agencies should consider combining the results from academic geoscience studies with business objectives to develop a sustainable developmental plan for the geothermal spring attractions of Thailand.