Overlooked Combined Ecotoxicological Risk of Naturally Occurring Beryllium and Thallium in Sediments to Aquatic Biota: An SPI Model-Based Assessment in the Pearl River Estuary

Beryllium (Be) and thallium (Tl) are highly toxic, naturally occurring trace metals increasingly recognized as emerging contaminants in aquatic ecosystems. Sediments act as both a major sink and a potential source of these metals, influencing their bioavailability and ecological impact. However, their combined adverse biological effects on aquatic biota in natural sedimentary environments remain unknown. This study presents the first quantitative ecotoxicological risk assessment of Be and Tl in surface sediments using the SPI model, which integrates species sensitivity distribution, probabilistic risk assessment, and the inclusion-exclusion principle. Due to the lack of an established diffusive gradients in thin films protocol for Be and Tl, we employed a validated transformation model to convert weak acid-exchangeable sediment concentrations (mg/kg) into estimated aqueous-phase concentrations (mg/L). These were used as inputs for the SPI model. Sediment samples from the Pearl River Estuary showed a 37.05% probability of combined toxic effects, exceeding the 25% ecological risk threshold and indicating a mild ecotoxicological risk. This study introduces a novel framework for evaluating the bioavailability and joint risk of Be and Tl in sediments, providing new insights for estuarine ecological risk assessment and environmental policy development.