Geothermal hydrohemistry classification and genesis identification of Qicun using SOM-Weighted FCM uncertainty clustering

To investigate the chemical characteristics, control factors, and genesis mechanisms of geothermal fluids in the Qicun geothermal area, 24 geothermal water samples collected from May to June 2024 were analyzed. Piper ternary diagrams, Gibbs diagrams, and ion endpoint diagrams were employed to characterize the geothermal hydrochemistry, while the entropy-weighted water quality index method was used to evaluate water quality. Simultaneously, a combined approach of Self-Organizing Map (SOM) and weighted Fuzzy Compound Membership Function (FCM) clustering was employed to identify hydrochemical types. Cluster stability was verified through Bootstrap resampling, revealing the primary sources of geothermal water constituents, influencing factors on water quality, and the genesis mechanisms of geothermal fluids. The results indicate that the geothermal water in Qicun exhibits weak alkalinity, with the predominant hydrochemical types being SO₄·Cl-Na, HCO₃-Ca and HCO₃-Ca·Mg. Overall groundwater quality is favorable, with Class I and Class II water samples accounting for 91.67% of the total. However, localized areas show nitrate (NO₃⁻) concentrations exceeding regulatory standards. SOM-weighted FCM classified water samples into three categories: C1 influenced by shallow recharge, C2 representing a transitional type affected by endpoint mixing and moderate water–rock interaction, and C3 representing deep-circulating, highly evolved endpoints. Bootstrap results indicated overall cluster stability (mean 0.86), identifying 17 core wells and 1 transitional well. The hydrochemical characteristics of geothermal water in the study area are influenced by the mixing of deep geothermal water with shallow groundwater. Water-rock interaction is the primary influencing factor for deep geothermal water, with silicate mineral dissolution being the predominant type. This study provides a basis for the sustainable development and utilization of hydrothermal geothermal resources in the Qicun area, as well as for water quality protection.