Geochemical behavior of thallium shifts across seasons in a granite weathering river

Thallium (Tl), a highly toxic trace metal, poses severe risks to water ecosystems and human health, particularly in granite weathering regions with elevated Tl geological backgrounds. This study investigates the migration, enrichment characteristics, and ecological risks of Tl. The results demonstrate that the river's Ca-HCO₃ hydrogeochemical type reflects the silicate weathering dominance. Tl is the primary element found to exceed standards in the local water source. Its presence is primarily attributed to natural weathering processes in the region and exhibits significant seasonal variations. During the wet season, Tl is predominantly governed by the decomposition of Tl-rich clay minerals influenced by surface runoff. During the dry season, Tl primarily originates from the weathering of Tl-rich minerals (such as potassium feldspar and mica) and the ion exchange processes of clay minerals facilitated by water-rock interactions. Tl plays a predominant role in the ecological risk assessment of surface water. During the wet season, moderate risks are observed in both the upstream and downstream sections of Huashan River. Consequently, it is imperative for stakeholders and decision-makers to implement sustainable agricultural practices, restore degraded hilly vegetation, mitigate soil erosion, periodically dredge riverbed sediments, and enhance water quality monitoring efforts. These measures will help alleviate the current situation and safeguard the drinking water safety of local residents.