Hydrogeochemistry and geothermometry of the carbonate-evaporite aquifers controlled by deep-seated faults using major ions and environmental isotopes

Abstract

Carbonate aquifers probably constitute the most important thermal water resources in non-volcanic areas, and are characteristic of great energy potential and easier production as the most ideal targets for development in the future. The thermal waters in the aquifers within Southeast Chongqing (SEC) of China occur in an extensive region with an area of 198,000 km2, which are mainly hosted in the Cambrian and Ordovician carbonate-evaporite rocks. The thermal waters are mainly in pristine conditions and their genesis has not yet been fully understood. The occurrence of thermal waters in SEC is closely related to the presence of deep-seated faults which constitute pathways for the hydrothermal circulation. In this study, an investigation of water major ions and environmental isotopes from 14 hot springs and three drilled wells was carried out to examine their hydrogeochemical evolution and to estimate reservoir temperature. The thermal waters have outflow temperatures from 19 to 54.2 °C. They are divided into two hydrochemical groups with the Yushan Fault as a boundary: Group A, located to the west of the fault, are characterized by facies of chloride sodium waters with higher TDS; Group B, to the east of the fault, are more complicated sulfate-bicarbonate-chloride alkaline-earth waters with lower TDS. The hydrochemical processes such as dissolution of carbonate, gypsum and/or anhydrite, and halite, the common ion effect, and dedolomitization are evident based on the major ions and δ34SSO4 and δ18OSO4. The δ18O and δD compositions of the thermal waters suggest a meteoric recharge in a relatively wetter and colder climatic condition. The silica geothermometers show that the average reservoir temperatures are 63 ± 18 °C for Group A, and 82 ± 15 °C for Group B. The corresponding average reservoir depths are estimated to be 2.5 ± 0.7 km for Group A, and 3.2 ± 0.7 km for Group B. The corrected 14C ages of the thermal waters average at 14.8 ka BP, corresponding to the late Pleistocene. A conceptual circulation model is proposed for the thermal waters. Driven by gravity of topographic gradients, the percolating groundwater circulates at a specific depth, reaching a higher temperature and interacting with the host carbonate with imbedded evaporitic minerals (gypsum and/or anhydrite, and additional halite for Group A) to be saline deep-seated thermal fluids. The fluids rise along faults and emerge at the surface as thermal springs. The slow renewability rate of the thermal aquifers highlights the importance of assessing the resource before groundwater exploitation and extraction.