Deep groundwater circulation in a syncline in Rucheng County, China

Abstract

Deep groundwater circulation is closely related to the earth surface processes of water, energy and matter, and imposes great influences on the earth critical zone. It is still unknown whether a deep groundwater circulation occurs in a syncline structure. Hydrogeochemical investigations of hot spring, cold spring and shallow groundwater were carried out in a syncline basin in Rucheng County, China. The water source, circulation depth, reaction temperatures, and mixing processes were evaluated by water chemistry and thermodynamic methods to reveal the pattern and controlling factors of the deep groundwater circulation. Results showed that two deep circulation groundwater systems with meteoric origin were developed in the syncline. The Tangkou system is related with the northwest wing of the syncline, the main fault F2, and the RS01 thermal spring. The Luoquan system is related with the southeast wing, the main fault F1, and the RS02 thermal spring. The main faults are water-blocking faults, separating the two systems. The mean elevations of recharge areas of the two systems were 1200m and 850m, respectively. The deep groundwater circulation depths of two systems reached 5.5 ± 0.5 and 3.6 ± 0.3 km, respectively. Within the geothermal reservoirs, deep circulating groundwater was heated up to 156°C for RS01 and 108°C for RS02. Before arriving at the ground, cold shallow groundwater mixed into the deep circulation systems. The mixing ratios of deep circulating groundwater were 14.8% at RS01 and 35.6% at RS02. The HCO3-Ca type deep circulating groundwater was slightly mineralized karst or karst-related water. Carbonate dissolution was the dominant water-rock interaction. The mixing obviously diluted the deep circulating groundwater, but didn’t alter its water type. In our case, the large-scale synclinal structure is favorable to deep groundwater circulation, which is closely related with the carbonate stratum and faults. The depth of the deep groundwater circulation can be determined by recharge area elevation and geothermal reservoir depth, which is a simple and effective method.