A geochemical study of the Kafue River, Zambia

Hydrogeochemical processes and the impact from mining activities within the Kafue River basin, Zambia, have been studied during an annual cycle (March 1995-April 1996). Samples of dissolved (<0.45mm) and suspended (>0.45mm) solids have been collected on weekly, once every two weeks and monthly bases at several locations along the river. Additional samples of waste rock deposits, soil, sediment and interstitial pore water have been collected during the dry season. Furthermore, a toxicological test on caged threespot Tilapia (oreochromis andersonii) was performed during low water discharge. An important result from this study is that the element composition of the Kafue River clearly shows seasonal variations that are not related to dilution effects or to the mining activities within the area. Mineralised organic material and accumulated weathering products from the upper parts of the soil profile are washed out, into the river, during the rainy period. This resembles processes observed in boreal rivers during spring time. Within the mining area the concentrations of dissolved Co, Cu and S in the Kafue River are drastically elevated. Dissolved Cu show highest concentrations during the rainy season. This is in contrast to most other elements which are diluted by the rain water. High Cu-concentrations during the rainy season are probably due to a wash-out from spoil heaps deposited within the area. Mining-related contaminants have also been detected in the Kafue River upstream from the mining areas. This is probably caused by atmospherically deposited particles originating from the mine related processes. Downstream from the mining area the dissolved trace element concentrations are rapidly lowered. However, elevated concentrations of dissolved S can be traced all the way down to the confluence with the Zambezi River. Suspended particles in the river show high levels of several trace elements within the mining area. When lime is added to acidic mining effluents, secondary particles rich in Ca, S, Fe, Mn, Cu and Co are formed and discharged into the Kafue River. These secondary particles aggregate and settle on the river bed quite rapidly, where they may coat the benthic flora and fauna. Filter feeding and grazing organisms on the river bed may therefore incorporate high levels of metals from these particles. Due to redox processes, secondary particles rich in Fe and Mn oxyhydroxides are dissolved and mobilised when buried in the sediment. Consequently, dissolution and re-precipitation may enrich the concentrations of Fe, Mn and sorbed trace elements at the oxygenated sediment surface. During periods of increased water discharge secondary particles, enriched in elements from the mining effluents, are re-suspended and transported further downstream in the river system. Suspended concentrations of Co and Cu are generally higher in the Kafue River compared with the Zambezi River, all the way to the confluence with the Zambezi River. Elevated concentrations of Co can be found in the sediment about 100km downstream from the mining area, and elevated concentrations of Cu can be found about 300km downstream. The very high levels of Cu and Co in the sediment reflect primary sulphides, which have been discharged from the mining activities. These primary particles are inert and probably not toxic for biota. Toxic effects are more likely related to the secondary particles formed, because accumulated trace alements on the secondary Fe-Mn-rich particles are easily mobilised during early diagenetic reactions in the sediment.Godkänd; 2002; 20061109 (haneit