Uptake of trace elements by food crops grown within the Kilembe copper mine catchment, Western Uganda

Abstract

The mining and processing of copper from the Kilembe mine between 1956 and 1982 left behind millions of tons of cupriferous and cobalt ferrous mine tailings within the Kilembe mine catchment. Subsequent erosion and deposition of the tailings into adjacent areas led to increased concentrations of Cu, Co, Ni, Zn, and Pb in the catchment soils. The Kilembe catchment is utilised for subsistence farming, producing mainly food crops, but there are also a number of settlements in the contaminated area. A study was conducted in 2016 to establish the concentrations of trace elements in a range of food crops grown within the catchment. Samples of maize, bananas, cassava, sweet potatoes, ground nuts, amaranthus, onions, beans and yams were collected, washed and oven dried at 80 °C. The dried foods were finely ground, microwave- digested in nitric acid and analysed using inductively coupled plasma mass spectrometry (ICPMS). All the foods grown in contaminated soils showed instances of higher concentrations of Cu, Co, Ni, Zn, and in some cases Pb, compared with controls grown in uncontaminated soils. Amaranthus accumulated a range of trace elements with 26% of the samples exceeding EC thresholds for Cu in vegetables of 26 mg kg−1.Other crops with elemental concentrations exceeding recommended thresholds in some of the samples included beans (Zn), yams (Zn and Pb) and ground nuts (Zn). The concentrations of trace elements in onions, cassava, sweet potatoes, bananas and maize were not significantly different from controls. However, strong and positive correlations between the trace elements were found in beans, yams, amaranthus, maize and ground nuts, suggesting a common source of trace metals. There was strong evidence of soil dust retention on leaf vegetables (Amaranthus) despite washing. The accumulation of trace elements in the edible parts of vegetables and foods could have a direct impact on the health of local people, because the foods produced from gardens are mostly consumed locally.