In Zambia, one of the top producers of copper (Cu), cobalt (Co) and lead (Pb), it is common for wastewater from mining activities to flow into natural wetlands from sedimentation ponds, decant dams, tailing dams and slag heaps. While natural wetlands can attenuate the emission of pollutants into downstream water bodies, their capacity for this is limited as buffering capacity becomes depleted. High or long-term loading of biodegradable pollutants and nutrients, or accumulation of non-biodegradable pollutants like heavy metals, reduces the sequestration capacity of wetlands. This can lead to eutrophication, oxygen depletion and toxicity of downstream receiving waters. This research aims at heavy metal pollution in Zambian wetlands; the main aim is to investigate retention, distribution, partitioning and speciation, as well as effects of redox potential and pH, on Co, Cu and Pb that enter wetlands through wastewater.
The study will include: assessing the efficiency of natural and greenhouse-based model wetlands in the removal of the metals from wastewater; characterising the distribution of the metals into the water column, sediments and macrophytes in these wetlands; analysing the binding forms in the wetlands, and investigating the role in the retention of the metals with changing redox potential and pH; investigating the role played by organic matter in binding the metals in the redox potential range of 0 to 100 millivolts; evaluating the effect that drying up of a wetland contaminated with the metals has on the behaviour of the heavy metals after re-flooding; and modelling the results using Stella software. Field work will involve collection of water, sediment and macrophyte samples from two wetlands in Zambia, TD11 Wetland in Mufulira and Waterway Wetland from the Kabwe lead mine, the latter which has been closed since 1994 but scavenging activities from the area emit Pb into the wetland. The greenhouse work will include investigating the capacity of model wetlands to retain Co, Cu, and Pb when exposed to water containing the metals. The model wetlands will also be subjected to drying out to investigate whether sequestrated metals get flushed out on re-inundating wetlands. Samples of water, sediment and macrophyte will be collected from the inlet midsection and outlet of the models.
The laboratory work will consist of experiments on part of the sediment samples and analyses of all samples. Water samples from the inlet and outlet of the wetlands will be analysed for Co, Cu, and Pb to estimate wetland sequestration efficiency. Water, sediment and macrophyte samples from the inlet, midsection and outlet of the wetlands will be analysed for the distribution of Co, Cu, and Pb into the water column, sediments and macrophytes. Sediments from the field and model wetlands will be subjected to further laboratory investigations on the effect of changing redox potential, pH and organic matter content as well as the effect on metal mobility of re-wetting of wetlands after a period of drying out.