环境分析化学杂志

Seasonal and Spatial Dynamics of Physico-Chemical Characteristics As A Factor for Water Quality Deterioration In Legedadi Reservoir, Ethiopia

Abstract

Adane Sirage Ali, Demeke Kifle Tsega

The water quality of Legedadi Reservoir, which provides more than seventy five percent of the water supplies to Addis Ababa City, is deteriorating from time and time. The reservoir has suffered water quality problems for several decades due to unacceptable changes in the odor, color and other parameters. In this study the seasonal and spatial dynamics of physico-chemical parameters of Legedadi Reservoir were investigated at monthly intervals. The study was conducted from November to August at three study Sites (represented as Sites 1, 2 and 3) in the reservoir. All physicochemical parameters were measured using standard techniques in Addis Ababa Water and Sewerage Authority, Central Laboratory. The reservoir is characterized by unusually high turbidity throughout the study period (with most values >400 NTU) mainly due to sediment influx during run off and resuspension of sediment. Secchi depth (transparency) of the reservoir was extremely shallow and (always <10 cm) and showed both seasonal and spatial variations (7.6 ± 0.6, 8.3 ± 0.9 and 8.1 ± 0.6 cm at Sites 1, 2 and 3 respectively). The pH was around neutral (7.85 ± 0.23, 7.87 ± 0.26 and 7.69 ± 0.33 at Sites 1, 2 and 3 respectively) throughout the study period at all study Sites. Both TDS and conductivity varied temporally and spatially at all locations although the conductivity and TDS values were within the range that drinking water should have. The total alkalinity was found to be low (0.84 ± 0.28, 0.8 ± 0.32 and 0.87 ± 0.26 meq/L at Sites 1, 2 and 3 respectively) resulting in high sensitivity of the reservoir even to a small change of pH. Carbonate hardness varied both seasonally and spatially with a value of 48.6 ± 6.9, 45.6 ± 8.1 and 45.3 ± 7.3 mg/L at Sites 1, 2 and 3 respectively. With regard to conductivity, TDS, pH and hardness, although there were significant seasonal and spatial variations (p<0.05), the values were within the acceptable range. However, the reservoir is characterized by unusual values of turbidity and transparency, which could be the major cause to the deterioration of the water quality of the reservoir.