Mobility and source apportionment of As and heavy metals in the Taehwa River sediment, South Korea: anthropogenic and seasonal effects

The river sediment is a sink for heavy metals flowed into a river from natural and anthropogenic sources. It can be a potential pollutant source in varying environmental conditions. The Taehwa River runs through Ulsan City with different geological terrains and different land uses. Hence, research on the determination of various factors affecting accumulation and mobility changes in heavy metals in the river sediment is important. The present research investigated the mineralogical compositions of the Taewha River sediment in addition to the distribution, speciation, sources, and contamination level of As and heavy metals. The sediment showed different mineralogical changes associated with flowing distance, indicating the influence of country rocks, comprising igneous rocks in the upper stream region and sedimentary rocks in the lower stream region, on mineral composition. The total concentrations of As and heavy metals in the sediment exhibited the order of Zn > Pb > Cr > Cu > Ni > As > Cd. The overall concentrations of As and heavy metals increased in the downstream region, albeit with varying degrees. According to the Environmental Protection Agency guideline for sediment pollution, including the index of pollution intensity (IPOLL) and the potential ecological risk index (RI), the sediment in the sampling stations was discovered to be polluted to varying degrees from anthropogenic activities. An abrupt increase in Pb, Zn, and Cd concentrations was observed at Station 3 in summer and fall, which was linked to the increased clay mineral content caused by seasonal and lithological changes. The sources of this increase can be attributed to a nearby industrial complex or the oxidation of sulfide minerals, which could be related to an abandoned amethyst mine. Sequential extraction studies show that potential toxicity varies for each metal. By comparison, metals such as Cr, Ni, and Cu with higher percentages in exchangeable fractions and fractions bound to carbonates can be highly toxic. The statistical analysis indicates that two groups of metals, one including Zn, Cd, and Pb and another including Cr, Ni, As, and Cu, had distinct origins.