Physicochemical analysis of roof runoff in the Obuasi area

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The quality of water in the Obuasi area is poor due to the mining activities and ore treatment methods which is very common in the area. As a result, harvesting roof runoffs has become an alternative source of water for domestic purposes. It is therefore important to ascertain the quality of this source of drinking water. 75 roof runoffs samples from three selected areas (Wawasi, Ramia and Antobuasi) in the Obuasi area were sampled during five rain events between May and August2007 from Aluminium, Aluzinc, Asbestos, Clay tile roofs and one collected directly from the sky to serve as control. The samples were analysed for the following physical parameters vis: pH, Alkalinity, Electrical conductivity, Total Dissolved Solids, Total Suspended Solids, Turbidity. Anions such as; Sulphates, Chlorides, Nitrites, Phosphates were also analysed. The concentrations of six trace metals were also determined. Metals were fractionated into dissolved and particulate fractions. Metals analysed include; Iron, Lead: Zinc, Aluminium, Chromium and Cadmium using Varian 220 Atomic Absorption Spectrophotorneter. It was observed that the general quality of the water sampled from roofing material was seriously affected by the type of roofing material used with respect to the sampling area. It was also observed that most of the metals occurred in the particulate form in nature and that contributed to over 60% of the total metal concentration. Except for pH, turbidity, Pb, and Cr all the parameters recorded values that were below the WHO guidelines for drinking water. pH had a good correlation with Fe and Zn but a poor correlation with Pb, Al, Cr and Cd. Mean values obtained for aluminium roofing materials were 0.12 mg/L, 0.04 mg/L, 0.75 mg/L, 0.42 mg/L and 0.05 mg/L respectively for Pb, Fe, Zn, Al and Cr. Asbestos roofing materials recorded mean values of 0.2 mg/L, 0.05 mg/L, 0.38 mg/L, 0.06 mg/L, 0.14 mg/L respectively for Pb, Fe, Zn, Al and Cr, Clay tiles roofing materials recorded mean values of 0.24 mg/L , 0.07 mg/L, 0.15 mg/L, 0.06 mg/L, 0.26 mg/L respectively for Pb, Fe, Zn, Al and Cr. Aluzinc roofing materials recorded mean values of 0.17 mg/L, 0.03 mg/L, 0.59 mg/L , 0.26 mg/L , 0.09 mg/L for Pb, Fe, Zn, Al and Cr. Values however recorded for cadmium were below detectable limits. The concentrations of Pb were high in samples from asbestos and clay tiles roofing materials than that of the metal sheets. The orders in which the roofs are liable of releasing metals into the runoffs are: Cr (ceramic > asbestos > metal sheet), and Zn and Al (metal sheet > asbestos > ceramic tiles). Asbestos and clay tiles roofing materials pose more environmental risk than other roofs investigated in this study, hence should not be used to harvest rainwater for domestic purposes. It can therefore be advised to use metal sheets to harvest rainwater since it contains less contaminant. Generally roof runoffs could be recommended for washing utensils, flushing toilets laundry but not drinking and possibly for cooking.
A Thesis Submitted to the Department of Chemistry, Kwame Nkrumah University of Science and Technology in partial fulfillment of the requirements for the Degree of Master of Science (Environmental Chemistry), 2008