The Impact of Sewage Effluent and Natural Self-purification in the Upper Chinyika River Below Hatcliffe Sewage Works, Harare.

Abstract

The impact of sewage effluent and natural self-purification in the upper Chinyika River was investigated during the period October 2004 to February 2005. The physico-chemical and river flow variables, and water samples, were colleted on monthly basis. The samples were analysed for total nitrogen, total phosphorus, chemical oxygen demand and total dissolved solids using the appropriate methods. Sediment samples were also collected once during the dry months and also once during the wet months and were analysed for total nitrogen, total phosphorus and heavy metals. The nutrient concentrations in the water column and loading levels in the sediments were high just below sewage outflow in to the river, generally decreasing with increasing distance from the point of sewage outflow because of self-purification. The nutrient concentration was high during the dry months with the highest mean values (N = 6.35; P = 4.01 mg l-1) being recorded in November and was low during the wet months with the lowest values (N = 1.33; P = 0.57 mg l-1) being recorded in January and December respectively, suggesting dilution effect. The nutrient load, on the other hand, was high during the wet months with the highest mean values (N = 8704.80 ; P = 2434.00 kg month-1) being recoded in January suggesting that organic matter was washed away from the catchment (diffuse inputs) in to the river channel resulting in high nutrient loading levels. The nutrient level in the sediments was high during the dry months (mean N = 1.01; mean P = 0.39 mg g-1 dry sediment) and low during the wet months (mean N = 0.37; mean P = 0.06 mg g-1 dry sediment) probably due to sediment re-suspension and the subsequent transportation because of storm action. There were no detectable temporal and spatial trends observed in heavy metal levels in the sediments. The other physico-chemical variables showed a general tendency of deteriorating just below sewage outflow and then improving with increasing distance downstream because of self-purification, except conductivity. High self-purification capacity was observed in the upper Chinyika River during the dry and wet months though it was generally lower in the latter coinciding with the observed riparian vegetation senescence, thus, emphasizing the importance of riparian vegetation in water quality monitoring in river channels. Conservation of riparian wetlands is, thus, central to sound watershed management. The capacity of rivers to purify themselves should be managed so that they can absorb pollution before discharging into lakes and reservoirs.