Occurrence and flow systems of the anticline-controlled thermal groundwater near Chongqing in eastern Sichuan Basin of China

A review and assessment of earlier studies shows that the thermal groundwater near Chongqing in the eastern Sichuan Basin of China has a unique occurrence called the 'basin-anticline outcropping' type. Its occurrence and emergence are strongly controlled by the nearly north-south trending anticlines. The basin-anticline outcropping type groundwater is similar to that of the basin type but also has the characteristics of the outcropping type because of the anticlines. The natural hot springs in the study area exist mainly in the outcropping areas of the carbonates, in the middle and the plunging ends of the anticlines where the topography was cut by rivers. They can also rise through the overlying sandstones and form up-flow springs. Geothermal wells tapping the carbonate reservoirs on the flanks of the anticlines also produce thermal groundwater. The groundwater flow can be divided into three levels: (1) shallow circulation system with groundwater of HCO3-Ca type and low TDS discharging through normal temperature springs, (2) middle circulation system with groundwater of SO4-Ca type and TDS of 2-3 g/L discharging through hot springs and (3) deep circulation system with groundwater of Cl-Na type and high TDS discharging through hot springs or wells.