Mining-related contamination of surface water and sediments of the Kafue River drainage system in the Copperbelt district, Zambia: An example of a high neutralization capacity system
Abstract
Contamination of the Kafue River network in the Copperbelt, northern Zambia, was investigated using sampling and analyses of solid phases and water, speciation modeling, and multivariate statistics. Total metal contents in stream sediments show that the Kafue River and especially its tributaries downstream from the main contamination sources are highly enriched with respect to Cu and exceed the Canadian limit for freshwater sediments.
Results of sequential analyses of stream sediments revealed that the amounts of Cu, Co and Mn bound to extractable/carbonate, reducible (poorly crystalline Fe- and Mn oxides and hydroxides) and oxidizable (organic matter and sulfides) fractions are higher than in the residual (Aqua Regia) fraction. Compared to Cu, Co and Mn, Fe is bound predominantly to the residual fraction.
Values of pH in the Kafue River and its tributaries are alkaline in the contaminated area and concentrations of sulfate gradually increase downstream. Water balance based on sulfate indicates that inflow from the most contaminated tributaries is less than 5% of total discharge in the Kafue River.
There is a significant input of Cu and Co from multiple contamination sources close to the town of Chingola, located on the Mushishima tributary, but both metals are mostly bound to suspended particles, which settle in the Kafue River. Additional contaminant sources such as the Uchi mine tailings and Nkana smelter are located in the industrial area around Kitwe, but metals released from these sources are mostly found in suspension and are attenuated efficiently.
Low Fe/Cu ratios in suspension and mineralogical composition of stream sediments downstream of contaminant sources around Chingola indicate that native copper, Cu-sulfides, and Cu carbonates are present in stream sediments in addition to Cu bound to Fe(III) oxide and hydroxides.