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Nigeria, Edo State, Benin City

Adverse effects of waste dumps on groundwater quality in Benin and environs

The project area covers Benin City and its environs in Edo State , South-South Nigeria. The area lies from Latitudes 60 15' to 60 27' N, and from Longitude 5030' to 5 045'; and covers an area of 610Km2. The area falls within 1:100,000 Sheet 298 (Benin City).\r\n\r\nThe purpose of the project is to study the effects of waste dumps on the quality of surface and groundwater in Benin City and environs. The methods adopted were geological mapping, hydrogeological mapping, soil sampling, geophysical studies and laboratory analysis of water samples. \r\n\r\nThe area is underlain by the Benin Formation. The formation is recognized by upper horizon of reddish brown to yellowish brown sand, and at the base by yellowish to white loose gravely to pebbly sand with discontinuous beds of clay. Its maximum thickness is about 1829m, and about 800m in Benin City. The formation has a single aquifer system and it is highly permeable, porous and prolific in water yield. \r\n\r\nA total of one hundred and two (102) samples were collected and analyzed for the study. Results for physiochemical parameters showed that the pH ranges from 5.79 to 7.26 with the highest value recorded in the rain-water and the lowest in the borehole water. This indicates that the waters in the area are moderately acidic to neutral. Conductivity ranges from 5µS/cm to 663 µS/cm. TDS values range from 2mg/l to 330mg/l. The values for conductivity corresponded to the values for TDS. The colour of the river waters were pale yellow, while those of the rain, borehole and well waters were colourless. The pale yellow colour of the river waters indicates impurities which could be from waste dumps. Chemically, all the values of the major cations and anions fell within the World Health Organization (WHO) permissible limits for potable water. \r\n\r\nThe dominant cation is calcium while the dominant anion is chloride. Concentration of Nitrate was observed in the waters with relative high values in the well waters. This is because effluent easily attenuates before reaching the deep aquifers. Similarly, coliforms were detected in the water samples except in rain-water. The presence of nitrate and coliform in water indicates some measure of contamination. Concentrations of some trace elements were above the WHO (2006) permissible limits. The trace elements include Fe, AS, Cd, Mn, and Pb. The high values of these trace elements in the waters indicate some measure of contamination. This contamination could be as a result of leachate from waste dumps in the area. The leachate from the waste dumps infiltrates into the groundwater as a result of the porous and permeable nature of the sandstone underlying the area.\r\n\r\nIt was however recommended that surface and groundwater monitoring should be done regularly in the area and that the dumpsites should be converted to concrete landfills for easy control of leachate infiltration into the surface and ground waters in the area.\r\n

Inserted in the reports are the following maps (1:50,000): Location map, Digital Elevation Model map, Geological map, and Water Table Elevation Contour map.