Release of Cu and Cr from Sediment by Seasonal Fluctuation of Rivers: A Case Study of the Xiaoqing River

Sediment serves as a vital role in the contamination of rivers with potentially toxic metals(PTMs), particularly in regions with seasonal variations in river flow. The alternating wet and dry conditions during different seasons significantly affect the release of PTMs from sediments. Our study focused on the Xiaoqing River in northern China, a representative seasonal river. In a laboratory setting, we simulated and analyzed the physicochemical properties of sediments under varying wet and dry conditions, and focused on the release behavior of internal PTMs, namely Cu and Cr. The results indicate that under alternating wet and dry conditions, pH and oxidation–reduction potential (ORP) remained relatively stable, but there is a significant change in dissolved organic matter (DOM). Conversely, constant humidity conditions led to notable effects on pH and ORP, showing a strong correlation with the release of Cu and Cr. Fluorescence properties analysis revealed the interconversion of DOM components in sediments during the release process, especially under constant humidity conditions. Over time, humic substances in DOM increased, while protein-like substances decrease. In both scenarios, Cu and Cr concentrations in sediments decrease, but under alternating wet and dry conditions, the Cu contents of A1, A2 and A3 decreased by 19.08%, 11.99% and 15.75%, respectively, and the Cr contents decreased by 23.75%, 29.17% and 26.69%, respectively. the percentage decreases in Cu and Cr were significantly greater, PTMs release intensified, posing higher environmental risks. This condition notably affected Cu chemical forms, whereas constant humidity impacts both Cu and Cr. This study will be conducted under varying conditions of dry–wet alternation frequency and cycles, ultimately serving as a theoretical foundation and technical guide for future endeavors in water pollution control and environmental protection of the river.