Spatial Patterns, Enrichment and Ecological Risk Screening of Trace Elements in River Sediments of a Historical Coal-Mining Area in Southwestern China

Metal mining affects an estimated 479,200 km of river channels worldwide, and contaminated sediments threaten aquatic ecosystems and human health long after mining ceases. This study examined 53 surface sediment samples from the Xiaozi River and six tributaries in a historical coal-mining area of Chongqing, southwestern China. As, Cd, Pb, Hg and Cr were evaluated using the geoaccumulation index, potential ecological risk assessment and freshwater sediment quality guidelines; differences among rivers were tested with the Kruskal–Wallis test, and sources were explored using APCS–MLR and PMF. Hg and Cd showed the greatest enrichment (mean geoaccumulation indices 1.19 and 0.35). Hg exceeded the background value at all sites and dominated potential ecological risk, accounting for 93.4% of the maximum risk index (914) in the Xibu River. Cr exceeded the probable effect level at 19 sites, whereas As and Hg showed localized exceedances. Concentrations of As, Cd, Pb and Cr differed significantly among rivers (p < 0.01), whereas those of Hg did not. Both receptor models identified Cr–Pb and As–Cd components, whereas Hg was associated mainly with a separate component. These findings identify Hg as the main management priority, together with tributary-specific monitoring of As, Cd and Cr.