Hydrochemical evolution of hydrothermal systems driven by seawater mixing in the southeastern coastal region of China: insights from water chemistry and isotopes

Abstract

Hydrothermal systems are extensively developed in the southeastern coastal region of China. However, their hydrochemical evolution following seawater mixing remains unclear. Based on hydrochemical and isotopic analyses (δ²H, δ¹⁸O, and δ³⁴S), this study investigates the origin of geothermal water, the degree of reservoir confinement, groundwater mobility, and the hydrochemical evolution induced by seawater mixing. According to the classification criteria of the United States Geological Survey (USGS), the geothermal waters were categorized into two types: coastal saline geothermal water (Group A, 7 samples near the coast) and hilly fresh geothermal water (Group B, 4 samples from inland hilly areas). Group A is recharged by both meteoric water and seawater, with seawater mixing ratios calculated from Cl⁻ and Br⁻ ranging from 2.61–72.97% and 2.10–88.24%, respectively. The two estimates are broadly consistent, although those based on Br⁻ exhibit a slightly higher maximum value. In contrast, Group B is recharged predominantly by meteoric water. Characteristic Na⁺/Cl⁻ and SO₄²⁻/Cl⁻ ratios suggest that the geothermal reservoirs of Group A are well confined with weak groundwater mobility, whereas those of Group B exhibit poor confinement and active groundwater mobility. Seawater mixing significantly enhances water–rock interactions: seawater mixing and high temperatures enhance silicate dissolution; cation exchange and chloritization alter the cation composition; microbial sulfate reduction modifies the sulfur isotopes and SO₄²⁻ concentration. This study reveals the hydrochemical evolution of coastal geothermal systems influenced by seawater mixing, providing insights for sustainable geothermal resource development.