Hydrogeochemical and isotopic insights into the genesis and mixing behaviors of geothermal water in a faults-controlled geothermal field on Tibetan Plateau

Hydro-geothermal systems are usually governed by faults on their genesis, distribution, behaviors. A faults-controlled hydro-geothermal field on Tibetan Plateau is investigated to get insights into the regulation of faults on the output features and discharge regimes of thermal groundwater with the aid of hydrochemistry, environmental isotopes (2H, 18O, 3H, 14C) and topographical, hydrological and hydrogeological analysis. Thermal groundwater in Xihai geothermal field has lower pH (5.97–7.38) than the local nonthermal water (6.47–8.13). It originates from the meteoric water of northeastern mountainous area with the elevation of 3988–5110 m, and percolates downwards along the NE-trending primary fault F5 with the circulation depth of 3270–4855 m. The deep cycling groundwater is heated by the terrestrial heat flow with reservoir temperature of 134.95–201.09°C. Thermal groundwater has the residence time up to more than 20 ka beneath, leading to sufficient water-rock interactions and higher TDS (818.00–886.60 mg/L) and much saltier hydrochemical facies (HCO3–Mg·Ca) than the local nonthermal water. Faults and fissures developed in the discharge area of geothermal field provide favorable conditions for nonthermal water infiltrating downwards and mixing with the upwelling thermal groundwater. The ratio of nonthermal water mixed-in reaches to 59.5–85.6% for thermal groundwater in the discharge area of Xihai geothermal field and presents a gradual increase along the upward flow path of thermal groundwater or with the increase of distance to the primary faults. These large ratio of nonthermal water mixed-in would cool the thermal groundwater and change its output physicochemical and isotopic features, and lead to underestimation of geothermal potential and hydro-geothermal behaviors based on the apparent data of sampled thermal groundwaters from the discharge area. Thus, the 'cover' effects of nonthermal water mixed-in should be concerned for geothermal fields that with well-developed faults and fissures in the discharge area.