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

•Saline thermal groundwater was widely distributed in the coastal area of Guangdong.•Water-rock interaction and seawater intrusion modify groundwater hydrochemistry.•Boron and its isotope were used to illustrate the mixing process.•Seawater intrusion has a great influence on saline thermal groundwater.