Characterization of geothermal water in the piedmont region of Qinling Mountains and Lantian-Bahe Group in Guanzhong Basin, China

The piedmont region in the Qinling Mountains (QL) is a recharge area for geothermal water in the Guanzhong Basin, and the Lantian-Bahe Group (LB) is the main mining layer for the geothermal water in the basin. This study focuses on the origin, classification and reservoir conditions of geothermal water in the QL and LB by combining geochemical and isotopic approaches and principal components analysis (PCA). QL water types are HCO₃–SO₄–Ca–Na, HCO₃–SO₄–Na–Ca and SO₄–HCO₃–Na, while SO₄–Cl–Na and Cl–Na are major types in BL. QL geothermal water is a cycling-type water and it receives a large amount of meteoric water in an open reservoir environment. The main and secondary sources of QL geothermal waters are modern infiltration water and ancient lixiviation water, respectively. LB geothermal water originates from ancient lixiviation water, modern infiltration water and residual sedimentary water in a semi-closed or closed reservoir environment. Both of these geothermal water bodies have a direct or indirect hydraulic connection with faults. The formation of QL geothermal water is dominated by vertical heat conduction, followed by horizontal convection, and the main geothermal formation model of the LB is vertical heat conduction. The obtained values are 20.3–97.6 °C and 75.7–116.6 °C for QL and LB, respectively, indicating low–medium temperature fluids.