Hydrogeochemical Characteristics and Formation Mechanism of Piedmont Karst Geothermal Water In The Northern Edge of The Luzhong Uplift

The karst thermal reservoirs in the northern edge of the Luzhong Uplift in Shandong Province are rich in low– and medium–temperature geothermal resources, which are characterized by a large water yield and easy recharge. Thus, we can study the hydrogeochemical characteristics of the geothermal water to analyze the formation mechanism of the geothermal resources, which is of great significance in promoting the effective development and utilization of the geothermal resources. In this study, 32 geothermal wells in the geothermal area in the northern edge of the Luzhong Uplift are studied, and the Piper diagram, Schoeller diagram, ion component ratio characteristics, isotopic characteristics, Na–K–Mg ternary diagram, mineral saturation index and SiO2 geothermal temperature scale are used to analyze the recharge source, water-rock interactions, transport pathway, and cyclic evolution characteristics of the geothermal water. The results show that in the direction of the groundwater runoff, the hydrochemical types of the karst groundwater and geothermal water are completely different, the total dissolved solids (TDS) content of the karst water gradually increases and the HCO3 content decreases. The same section of the geothermal field has a similar hydrochemical composition and the same hydrochemical type within the same geothermal field. It is determined that there is not only a conduction–type geothermal system but also a convection–conduction–type geothermal system in the study area by calculating the thermal storage temperature and the circulation depth of the geothermal water. In addition, it is further determined and verified that the geothermal water was directly or indirectly derived from the infiltration and recharge of Late Pleistocene paleo–atmospheric precipitation in the northern foothills of Mount Tai and was formed after the atmospheric precipitation flowed along the rock orientation and stratum slope and heated by the terrestrial heat flow through deep circulation.