Unmasking the Drought-Driven Toxicity Surge in Karst Wetlands: A Mechanistic Coupling of PHREEQC and Biotic Ligand Modeling

Freshwater wetlands are increasingly vulnerable to hydrological alterations driven by climate change, which can fundamentally shift the biogeochemical cycling of heavy metals. This study investigates the potential mobilization and toxicity of Cadmium (Cd) and Zinc (Zn) in the karstic Caohai Wetland under simulated drought and anoxic rewetting scenarios. By coupling PHREEQC geochemical modeling with the Biotic Ligand Model (BLM), we moved beyond total metal concentrations to assess bioavailable fractions and site-specific toxicity. Our simulations reveal that while carbonate minerals (Calcite, Dolomite) remain supersaturated, promoting natural attenuation, metal-specific phases like Otavite remain undersaturated, keeping Cd in the dissolved phase. Crucially, the BLM analysis indicates that drought-induced evapoconcentration significantly exacerbates Cd toxicity, with Toxic Units (TU) escalating from ۵.۹ in baseline conditions to ۱۶.۴ under drought stress. Conversely, Zn poses a negligible risk across all scenarios due to competitive binding with cations. These findings underscore the necessity of mechanistic models in predicting the "pulse effects" of climate extremes on wetland ecological security.

کلیدواژه ها:

Heavy metals ، Caohai Wetland ، PHREEQC ، Biotic Ligand Model ، Climate change ، Ecological risk ، Metal speciation