Hydrochemical characteristics, hydrochemical processes and recharge sources of the geothermal systems in Lanzhou City, northwestern China

Abstract

Lanzhou city has abundant geothermal resources in the sedimentary geothermal system. Based on water quality indicators from 9 geothermal water samples and 7 phreatic water samples, the hydrochemical properties of geothermal water were delineated and the hydrochemical processes and recharge sources of geothermal water were investigated. The principal component analysis (PCA) was used to determine the main physicochemical components of geothermal water and their influencing factors in the study area. Three principal components explaining 84.34% of the total variance were identified. The storage conditions and sources of geothermal water were analyzed using ionic ratios and isotopic analyses. The results demonstrated that geothermal waters are of Na-Cl and Na-SO4 facies, while the phreatic waters are of Na-SO4, Na-Cl, and Ca-HCO3 facies. Moreover, the halite and silicates dissolutions showed the greatest impact on geothermal and phreatic waters, respectively. On the other hand, the primary recharge source of geothermal water is ancient infiltration water, whereas the primary recharge source of phreatic water is a mixture of modern infiltration and geothermal waters. Although the geothermal water is stored in a relatively closed system, it may have a hydraulic connection with phreatic water through faults.