Screening effects of heavy metals in urban rivers within plains: implications for ecological risk assessment

The objective of this study was to investigate the spatiotemporal distribution patterns of two common heavy metals, Cd and Pb, in urban rivers in plains, and analyze the impact of weak hydrodynamics on the transport of heavy metals, and guide their ecological risk assessments in these regions. Two field surveys (wet and dry seasons) were conducted at a total of 36 sites in the tributaries of Gehu Lake, located in a plain region in China. The European Community Bureau of Reference (BCR) extraction method was employed to analyze the components of Cd and Pb. The Nemello index and ecological risk index were calculated to assess their pollution levels and ecological risks. Cd primarily accumulated at the river mouths, while Pb was predominantly concentrated near the discharge sources. The mobile fractions of Cd were more likely to be released and migrate downstream, and thus the total Cd content demonstrated a significantly negative correlation with these mobile forms (p < 0.05). In contrast, although Pb had a greater proportion of mobile fractions, they were readily re-adsorbed onto particles and settled near the source. The source area displayed notable pollution with Pb, whereas the downstream river mouth area posed a high risk of Cd pollution. The results indicated that the weak river hydrodynamics within plains amplify the impact of heavy metal mobility on their behaviors, producing a “screening effect” on Pb and Cd and resulting in distinct distribution patterns in sediments. These findings can guide the ecological risk assessment of heavy metals in aquatic ecosystems within plains.