The genesis of saline geothermal groundwater in the coastal area of Guangdong Province: Insight from hydrochemical and isotopic analysis

Abstract

Saline geothermal groundwater is widely distributed in the coastal region of Guangdong Province, China. The present study employed hydrogeochemical (physical parameters, ionic chemistry, and trace elements) and isotopes (H, O, C, and B) analyses to delineate the causes and evolution processes of saline thermal groundwater in different aqueous environments from the coastal region (geothermal groundwater, cold groundwater, seawater, river water, and rainwater). The results showed that meteoric water was the primary origin of the geothermal groundwater. Based on the hydrogeochemical characteristics and TDS values, the geothermal groundwater can be divided into two groups. Group A was mainly Cl-type with TDS values ranging from 1961 to 7752 mg/L and was located around the coastal area. Group B was mainly HCO3-type, which is located in the inland area. The hydrogeochemical differences between group A and group B are primarily caused by groundwater-rock interactions and mixing with seawater. Reservoir temperatures were calculated as 127 to 163 °C for group A and 81 to 140 °C for group B by a quartz geothermometer without steam loss. Besides, the circulation depth of group A (3007 to 4031 m) was much deeper than that of group B (711 to 3965 m) and cold groundwater (394 to 1656 m). The analysis of carbon isotopes (δ13C and 14C) revealed that the circulation times of group A were older than that of group B. Furthermore, seawater intrusion had a significant influence on group A, and the proportions of seawater in group A calculated by Cl (4.90 % to 49.60 %) were closed to that by Br (7.21 % to 53.81 %).