Geochemical processes influencing As distribution in waters associated with volcanic-hydrothermal systems: A case study of Coconucos volcanic chain-Puracé volcano, Colombia

Abstract

This study mainly combines hydrogeochemical and geochemical modelling approaches to elucidate the roles of mineral equilibria, water-rock interaction and fluid mixing processes, in controlling arsenic (As) mobility in eight thermal springs of the Coconucos volcanic chain - Puracé volcano in Colombia. Three distinct water groups were identified based on physico-chemical parameters and chemical composition. Group 1, near-neutral SO₄-Cl-HCO₃ waters with temperatures ranging from 54.8 to 73.3 °C, showed moderate As concentrations of 3.4 and 126.2 ppb. Waters of Group 1 are the only ones partially equilibrated, with reservoir temperatures from 118 to 226 °C estimated through solute geothermometers. Geochemical equilibrium modelling performed using PHREECQ software, allowed us to propose that As concentrations could be controlled by iron (oxyhydr)oxides at the outlet temperature and by the precipitation of sulfide minerals under higher H₂S conditions. Group 2, acid-SO₄ waters (pH 1.7–2.3) with the lowest temperatures (23.8–44.7 °C), displays As concentrations from 2.1 to 57.5 ppb and near isochemical dissolution of local andesite rock. Low As in most Group 2 waters are likely due to the dilution caused by mixing thermal water with cold water. PHREEQC simulation reveals Fe and Cu sulfides oversaturation at elevated H₂S, potentially contributing to As depletion in waters. Group 3, acid SO₄-Cl waters with pH from 2.3 to 3.6 and the highest temperatures (80.4–81.6 °C), contained the highest As concentrations (380.8–987.5 ppb). Hypotheses for such enrichment are intense water-rock interaction, As-rich mineral dissolution, and minimal secondary mineral precipitation, as evidenced by PHREEQC saturation index results.