This paper is the result of environmental geological survey engineering.

Objective Clarifying the seasonal mechanisms by which exogenous inputs influence the accumulation and environmental risk of heavy metals in sediments of the Liujiang River Basin is of great significance for strengthening the seasonal prevention and control of heavy metal pollution in the basin.

Methods Seasonal sampling and investigation of sediments in the Liujiang River Basin were conducted to identify the seasonal variations in heavy metal content and speciation. The enrichment factor (EF) and risk assessment code (RAC) were used to evaluate the enrichment degree and environmental risk levels of heavy metals. Correlation analysis was applied to reveal the seasonal differences in the impact of exogenous inputs on heavy metal speciation distribution and environmental risk.

Results The content of heavy metals in the sediments was higher in the rainy season than in the dry season. In both seasons, heavy metals were primarily present in the residual fraction. From the rainy to dry season, the proportion of the residual fraction increased for all heavy metals, while the exchangeable and carbonate-bound fractions increased for some heavy metals. In contrast, the reducible and oxidizable fractions decreased for most heavy metals. Except Cd and Zn, the EF values of all other heavy metals were below 1.5, indicating no significant enrichment and limited anthropogenic influence. Correlation analysis showed that exogenous inputs during the rainy season were primarily in the residual and reducible fractions, while during the dry season, they were more enriched in the oxidizable, exchangeable, and carbonate-bound fractions. After the rainy season, the RAC values of Cd, Zn, Cu, and As decreased significantly. Cd consistently posed a very high risk (RAC>50%), while Cr and Zn fell into the medium-risk category (10%<RAC<30%). The correlation between RAC and EF further indicated that exogenous inputs significantly influenced the environmental risk of heavy metals during the rainy season.

Conclusions Exogenous inputs are a key factor affecting the seasonal variation in heavy metal accumulation and speciation in sediments of the Liujiang River Basin. Strengthening the management of exogenous inputs during the rainy season plays a crucial role in ensuring the ecological environmental security of the basin.