Groundwater pollution vulnerability and groundwater protection strategy for the Owerri area, southeastern Nigeria

The vulnerability to pollution of the Owerri regional water supply in, Nigeria, was assessed as the basis for proposing an appropriate strategy for protecting the groundwater resources. The vulnerability assessment was accomplished using the LeGrand, GOD, SIGA, and DRASTlC computer models. The model techniques generally involve parameter rating and point-count systems, based on evaluation of various hydrogeological parameters in relation to their capacity to influence the flow of contaminants in the groundwater system. The Owerri area is generally flat and groundwater recharge is relatively high. The area is underlain by predominantly sandy strata in the north, which grade into gravelly sequences towards the southwest. The southeastern part of the area is underlain in part by a thick clayey faces that thins and interfingers into the gravelly strata. Effective hydraulic conductivity ranges from 5.6 x 10(-9) to 1.44 x 10(-3) m s(-1), the higher value being in the coarse sand and gravel units. The depth to the water table is about 60 m in the north decreasing southward to less than 20 m; the hydraulic-head gradient is 9-22%. The southwestern part of the Owerri area including the Owerri conurbation has a high vulnerability to groundwater pollution and therefore waste disposal sites should not be located there. The northern and southeastern areas have moderate and low vulnerabilities, respectively, and carefully selected and engineered sites within those areas could be considered for future waste disposal. Policies should be instituted to facilitate the closing of pollution sources in the area of high vulnerability such as open disposal pits, leaking petrol stations, effluent discharge from industries, and use of pit latrines, and to locate future disposal site in areas of lower vulnerability.