Geochemical distribution, fractionation, and sources of heavy metals in dammed-river sediments: the Longjiang River, Southern China

Abstract

In the present study, six sediment cores were collected from six river-dammed reservoirs to reveal the geochemical distribution of heavy metals (As, Cd, Pb, Sb, and Zn) in the Longjiang River, South China, which is highly impacted by nonferrous metal mining and smelting activities. The sediments were geochemically characterized, combining geochemical analysis, sequential extractions, and 210Pb chronology. The results indicated that the river sediments were severely polluted by heavy metals in the order of Cd > Zn ≈ Pb ≈ Sb > As. These heavy metals generally exhibited relatively low enrichment in the upstream sediments because of the limited anthropogenic impact, but their abundances drastically increased in the midstream sediments due to local smelting activities. In downstream sediments, the heavy metal concentrations (except for Cd) decreased, owing to the effect of dam interception and detrital inputs. Cadmium levels tended to increase in downstream sediments, which were attributed to the intensive discharge of Cd during the pollution event in 2012. The sedimentary records were traced back to 1985, and a significant decrease of heavy metal enrichments could be found after the year 2000, suggesting the enhancement of environmental management in this period. The statistical results indicated that local metal smelting and mining activities were the main anthropogenic contributors for the enrichment of heavy metals in the dammed-river sediments. High enrichment factor and non-residual fractions of heavy metals in local sediments may pose a direct threat to aquatic organisms. Cd presents significant danger because of its extreme enrichment and high labile fractions.