Hydrochemical characteristics and geothermometry applications of thermal groundwater in northern Jinan, Shandong, China

Abstract

Carbonate rock thermal reservoirs are important geothermal resources but lack systematical study on a regional scale. This paper integrated hydrochemical and isotopic data to outline thermal water resources in carbonate rock aquifers as part of deep regional groundwater flow system. Hydrochemical characteristics of thermal groundwater in northern Jinan geothermal field, where Ordovician carbonate rocks are the main thermal reservoir, were examined and characterized by SO4–Ca, SO4–Ca·Na, SO4·Cl–Ca·Na and Cl·SO4–Na·Ca types. Hydrochemical evolution was revealed by analyzing correlation between different chemical constituents and their relative changes in various waters. Isotopic data allowed the origin of thermal water to be determined and presented different recharge elevations. Thermal groundwater was originated from precipitation in the northern piedmont of Tai Mountain with elevation of 708–1493m.a.s.l.. Reservoir temperature was estimated by chemical geothermometry and validated by fluid–mineral equilibria calculations. The feasibility of different geothermometers was verified, indicating that quartz geothermometers were more suitable for the geothermal system and yielded reservoir temperatures of 60–80°C. Finally, a conceptual hydrogeological model of the geothermal field was proposed: thermal groundwater was derived from regional flow system with high recharge areas, deep circulation depth and slow flow velocities, confirmed by its depleted stable isotopic contents, high chloride and TDS concentrations and long residence time. The improved understanding of this geothermal system is fundamental for the better management of this karst geothermal resource.