Lithology controls on the mixing behavior and discharge regime of thermal groundwater in the Bogexi geothermal field on Tibetan Plateau

Geothermal springs within the same geothermal system are naturally considered with similar output manifes-tation, but usually distinct. A low temperature geothermal spring group (Bogexi) on eastern Tibetan Plateau was investigated , compared with the boiling geothermal spring group (Rekeng) in the same fault zone to get insights into the reasons behind. The Bogexi geothermal springs have quite lower output temperature (25.0-65.3 degrees C) than the boiling Rekeng geothermal springs (84.5-90.0 degrees C). Although all these two geothermal spring groups have the same hydrochemical facies of HCO3-Na, the Bogexi geothermal springs present lower concentration of Na+ and trace elements. The geothermal waters of Bogexi were originated from the precipi-tation and glacier/snow meltwater of the eastern Haizi mountains with the recharge elevation of 4,400-4,700 m. These recharged waters infiltrate into the geothermal system that with the reservoir temperature of 160-180 degrees C and are heated by tectonic deformation heat in the depth of 3,100-3,400 m. Lithology determines the fissures development in the shallow and furtherly controls the mixing behaviors and discharge characteristics of geothermal springs in the discharge area. As a result, the thermal groundwaters of Bogexi are mixed with great quantity of shallow cycling cold water with the mixing proportion averaging at 73-76 % during the ascending process, leading to their lower output temperatures and distinct hydrochemical and isotopic features from the boiling Rekeng geothermal springs. A conceptual model was proposed to portray the genesis of low temperature geothermal springs in high temperature geothermal system. The recharged water percolates downward and is heated in the deep thermal reservoir. Heated waters are enriched of chemical elements during this process and then flow upwards under the driving of hydraulic pressure. While the lithology determines the circulation of cold water in the discharge area. Karst lithology, usually with developed fissures and channels, can provide more favorable conditions for cold water cycling in the shallow part than other lithology. This leads to considerable cold water mixed into the upward-flowing geothermal groundwater , differs these geothermal springs from other ones in the same fault zone in output temperature and hydrochemcial features. This research can improve the understanding of formation mechanism of output discrepancies of various springs within one geothermal system.