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Spatial and seasonal variation of major cation and selected trace metal ion concentrations in the Okavango-Maunachira-Khwai channels of the Okavango Delta

2005; Volume: 37; Linguagem: Inglês

0525-5090

Wellington Masamba, A. Muzila,

Water Quality and Pollution Assessment

The concentration of the metal ions Na, K, Ca, Mg, Zn, Cu, Mn, Co, Fe, Cr, Ni, and Pb were determined in water samples of the Okavango Delta's panhandle Maunachira-Khwai water system during periods of high (June 2004) and low (November 2004) water flow. The concentrations of Na (1.67 to 12.77 mg/L), K (1.37 to 5.80 mg/L), Ca (2.46 to 6.98 mg/L) and Mg (0.69 to 2.83 mg/L) increased with distance from Mohembo indicating concentration of these metal ions by evaporation. Cu (up to 0.028 mg/1), Co (up to 0.053 mg /L), Fe (0.010 to 0.60 mg/L), Ni (up to 0.052mg/L), Mn (up to 0.044 mg/L) and Pb (0.017 to 0.28 mg/L) generally decreased in concentration with increase in distance from Mohembo, indicating that the Delta acts as a filter for these ions. Zn (0.049 to 0.101 mg/L) had lower concentrations at Mohembo than the other sampling sites, indicating solubilisation of previously deposited metals ions, or presence of Zn within the Delta. Chromium was not detected except at Seronga (0.026 mg/L) during periods of high water flow Introduction The sustainability of the Okavango Delta depends primarily on the quantity and quality of water that comes in from its catchment in Angola and Namibia. However, very few water quality studies have been carried out on the Okavango Delta, presumably because the Okavango basin is considered to have little industrial, agricultural or other human developments that could lead to pollution. Major ion chemistry of the lower Boro River showed that the waters could be characterised as calcium-sodium-bicarbonate, with moderate alkalinity and moderate to high amounts of silica (Sawula and Martins, 1991). The metal ions determined in this study were Ca, Mg, Na and K. In another study, dissolved trace metals in the Boro River were found to have concentrations in the sub-|ig/l level for transitional metals except Fe, Mn and Zn (Sawula et al, 1990). A water monitoring programme of the Okavango Delta reported, among other water quality parameters, the mean concentrations of Ca, Mg, K, Fe, Mn and Al (Cronberg et al, 1995). Water samples from areas around the Upper Panhandle, Lower Panhandle, NorthWestern Moremi Game Reserve and South East of Chief's Island were analysed for Ca, Mg, Na, K, Fe and Mn as part of a rapid biological assessment of the aquatic ecosystems of the Okavango Delta (Ashton et al, 2003). In most of these studies, analysis was done for only a few metal ions, and the emphasis was on the Jao/Boro River system (Warmeant, 1997). It is however known that, generally, there is a gradient of increasing concentration towards the outlet as well as the edges of the Delta. Metal contamination of aquatic ecosystems is important because some metals are toxic to plants and/or animals. For example, Pb and Cr (as Cr6* compounds) are toxic to humans. Lead can cause several unwanted effects, such as disruption of the biosynthesis of haemoglobin 1. Authors: Harry Oppenheimer Okavango Research Centre, University of Botswana, Private Bag 285, Maun.Tel: 267 6861833. Email: wmasamba@orc.ub.bw