Assessment of sediment and water quality in the Owabi reservoir

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The Owabi reservoir is one of the major sources of drinking water for the Kumasi metropolis and its surrounding communities. The streams serving the Owabi reservoir have been encroached upon with various human activities due to high population increase within the catchment area of the reservoir, resulting in the pollution of the water. This study sought to assess the extent of pollution of sediment and water from the Owabi reservoir, measure the chemical forms or speciation of the heavy metals and determine their bioavailability. Ten water and sediment samples were collected from ten different sites within the Owabi reservoir. The water samples were analyzed for pollution indicators using standard methods from the American Public Health Association (APHA). Parameters such as temperature, pH, conductivity, hardness and nutrients were all within the acceptable limits of the World Health Organization (WHO). Levels of lead, arsenic, colour, turbidity, total suspended solids, faecal coliform and E. coli were above the acceptable limits of the World Health Organization (WHO). These make the water in the reservoir unsafe for domestic use without prior treatment or purification. Correlation analysis showed that arsenic correlated significantly with calcium, pH and conductivity, zinc also correlated significantly with fluoride, lead correlated significantly with magnesium and copper also correlated significantly with zinc. Factor analysis showed that agricultural discharge, domestic waste water discharge and the presence of decaying plant and other materials all contributed to the pollution of the water. The sediments were found to be acidic. The total metal content of Fe, Pb, Cu and Zn were also determined. The geoaccumlation index (Igeo) was also used to assess metal pollution in sediments. The sediment was found to be moderately polluted with respect to Pb and Zn and unpolluted with respect to Cu and Fe. Canonical correlation was carried out to establish the importance of pH, organic matter content (OMC), cation exchange capacity (CEC), and electrical conductivity (EC) in co precipitation and release of metals. pH was more important in co precipitating soluble Zn. Co precipitating lead and iron will more likely be associated with EC while co precipitating Zn and Cu will more likely be associated with CEC. Factor analysis was carried out on the heavy metals to identify the sites that were significantly polluted by the heavy metals. Sites Sl, S2 and S3 were found to be significantly polluted. Samples from these sites were subjected to a five step sequential extraction and the results showed that Zn and Pb were potentially bioavailable in the sediments. To understand the risk of the metals to the sediment dwelling organisms the data were compared with the Sediment Quality Values (SQV) using screening quick reference tables (SQUIRT). The comparison showed that Pb and Zn were above the probable effects level (PEL).
A thesis submitted to the Department of Environmental Science, Kwame Nkrumah University of Science and Technology in partial fulfillment of the requirements of Master of Science, October-2010