Trends of Cu and Co contaminated sediment dispersal along the Kafue River, the Zambian Copperbelt

Abstract

Rivers are important pathways for metal(loid) contamination in mining areas. The Kafue River flows through the Zambian Copperbelt with nearly a hundred-year history of Cu and Co mining. Nine cores down to 2.5 m deep were drilled in fluvial sediments along a 750-km long river section and analysed for grain size, elemental composition, and mineralogy using extraction tests and scanning electron microscopy with an aim to investigate the spatial and temporal contamination patterns. The silty and fine- to coarse grained sandy sediments were deposited in natural levees, semi-abandoned meanders, and channel bars. They are heavily polluted by Cu reaching ore grade (up to 1.9 %) and Co, with UCC-based enrichment factors (EF) as high as 704 and 60, respectively, which is more than an order of magnitude higher than the most severely impacted large European rivers, and slightly polluted by Pb, As, U and other elements. The main carriers of Cu are malachite, pseudomalachite, secondary Fe oxyhydroxides (ferrihydrite, goethite), sulphides, and Cu-bearing grains of kaolinite, chlorite, and mica. This Cu contamination is traceable (EF up to 13) in sediments 750 km downstream of the Kafue River inflow to the Copperbelt. The contaminated sediments are ~100 cm to ~220 cm thick and their average sediment accumulation rates ~12 to ~27 mm/yr indicating that the contaminated suspended particulate matter can be rapidly deposited along the river, representing a potential environmental hazard. Arsenic and Pb can be scavenged to Fe oxyhydroxides and accumulate in capillary fringe above groundwater level during early diagenesis.