Hydrogeochemistry and geothermometry of geothermal waters from the Pearl River Delta region, South China

Abstract

The Pearl River Delta (PRD) region, located in the southern edge of South China Foldbelt, is one of the best known geothermal areas in south China. This study focuses on the hydrogeochemical features and reservoir temperatures of the geothermal waters in the PRD region, which can provide useful information and overall assessments on the geothermal potential. 24 geothermal well waters and 8 hot spring waters were collected from the inland and coastal areas of the PRD region. Hydrochemical and isotopic analysis (δ18O, δ2H, 3H), empirical chemical geothermometer, mineral saturation induce, and silica-enthalpy mixing model have been applied in the PRD thermal waters. The hydrochemical facies of inland geothermal waters are of the bicarbonate-dominated and sulfate-dominated type, and the coastal waters are of chloride-dominated type. Chemical compositions of the geothermal waters reveal strong fluid-rock interactions, the common ion effect, mixture between seawater and thermal waters in coastal waters, and mixing process between cold groundwater and thermal waters in inland waters. Hydrochemical results and enriched heavy isotopes suggest that coastal waters are recharged by local precipitation and seawater. δ18O and δ2H compositions of the inland waters plot along the Local Meteoric Water Line (LMWL), indicating the recharge of local precipitation. The activities of tritium verify that inland and coastal waters have modern meteoric components. Calculations of chalcedony geothermometers, mineral saturation states, and silica-enthalpy models provide the most reliable estimations of reservoir temperature: 90-154 °C for inland thermal waters, 104-156 °C for coastal thermal waters, likely indicating low-moderate temperature geothermal resources in the PRD region.