Formation of an up-flow hot spring of SO4-Ca type in the sandstone aquiclude near an anticline

Abstract The formation of springs is vital for understanding the subsurface transport of substances and energy in groundwater circulation. The Xiannvshan hot spring in the sandstone aquiclude near the Tongluoxia anticline in northeast Sichuan Basin of China is an up-flow spring discharging thermal groundwater from the underlying carbonate aquifer of the basin-anticline controlled type. This rarely-encountered up-flow spring neither issues directly from an aquifer, nor is controlled by a fault. The spring water is a very hard (hardness of 1050.069-1835.697 mg/L) and brackish hot spring water (TDS of 1650-2802 mg/L) of SO4-Ca type and low temperature (34-38°C) issuing from a neutral and reduction subsurface environment, and is characterized by high ratios of γNa/γCl (average 1.354) and low γCa/γSO4 (average 0.779), and high concentrations of Sr (7.62-17.183 mg/L) and F (1.87-8.15 mg/L). The ΣMREE of the hot water samples are greater than ΣLREE, and positive Eu and negative Ce anomalies exist. The dissolved constituents in the hot water are derived from the dissolution of calcite, dolomite, anhydrite, gypsum, polyhalite, halite, albite and K-feldspar and small amount of sylvite, epsomite, thenardite, langbeinite, celestite, barite, strontianite, magnesite, and fluorite in the carbonate aquifer, and the dissolution of sulphates prevails in the groundwater system. Hydrochemical similarity is found among the water samples from the two springs’ vents and the flowing borehole near the vents. Faster groundwater circulation caused by the flowing hot water borehole encourages the dissolution of minerals in the carbonate aquifer and slightly increases the TDS and temperature of the hot water from the flowing borehole.