Contaminants such as heavy metals in rivers are partially retained in the sediments at the bottom as a result of the altered water regime within the cascade weirs. The associated heavy metals in the sediments can affect surface water quality and ecology for a long period. In May 2020, sediments were collected in a typical mountainous river (Liangtan River) with cascade weirs in the main urban area of Chongqing. The accumulation of sediments in each river section, the content of heavy metals, and other basic physicochemical indicators in the samples were monitored. The results showed that the average ω(As), ω(Cd), ω(Cu), ω(Hg), ω(Ni), and ω(Pb) in the sediments of the main stream of Liangtan River were (4.66±4.78), (0.361±0.256), (32.30±14.38), (0.069±0.039), (33.47±15.37), and (26.34±11.52) mg·kg-1, respectively. A large coefficient of spatial variation regarding the content of heavy metals in the sediments across the sampling sites was observed owing to the uneven spatial distribution of pollution sources and the destruction of river connectivity by cascade weirs. The modified geoaccumulation index (Im) showed that Cd was the most polluted heavy metal element in the sediments. Of the monitored river sections, 12.50% approached or were at moderate pollution levels, and these sections were mainly found in the upstream and downstream reaches of Liangtan River with relatively concentrated construction lands. The potential ecological risk assessment index method (RI) and the sediment quality guideline method (SQGs) showed that, in addition to Cd and Hg with a high pollution level and strong toxicity, Ni in the sediments also posed a potential threat to the ecological safety of surface water due to its high background content in the watershed. The total amounts of As, Cd, Cu, Hg, Ni, and Pb retained in the sediments by cascade weirs were estimated to be 446.10, 47.28, 4997.80, 10.81, 5135.68, and 4048.16 kg, respectively, in which Cd, Ni, and Pb were identified to be the major threats to the ecological safety of surface water over a long period. The upper reaches prior to the weirs with the most retained heavy metals and the easier formation of an anaerobic environment are suggested to be the key areas for investigation of the endogenous release of heavy metals. Source apportionment of heavy metals in river sediments through the combination of principal component analysis and cluster analysis revealed that Cd, Cu, and Pb mainly originated from residential/industrial point source pollution. Hg mainly originated from agricultural non-point source pollution, whereas As and Ni mainly originated from natural soil erosion.
Keywords: cascade weirs; ecological risk; heavy metals; mountainous urban river; sediment; source analysis.