Assessment of Ground Water Vulnerability and its Application to the Development of Protection Strategy for the Water Supply Aquifer in Owerri, Southeastern Nigeria

Pollution vulnerability of theOwerri regional water supply aquifer wasevaluated as a basis for developing appropriateprotection strategy for the groundwaterresource. The assessment was accomplished usingLegrand, GOD, Siga and DRASTIC models.Techniques of the models generally involvedparameters rating and point count systems, whichare based on the evaluation of various parameterin relation to their capacity for enhancing orattenuating contaminants in the groundwatersystem. Field and laboratory evaluations of theparameters indicate that the Owerri areagenerally occupies a nearly, flat topographywith a relatively high groundwater recharge. Thearea is underlain by predominantly sandyfacies in the Northern area which grades intogravelly sequences towards the southwest. TheSoutheastern area is distinguished by thickclayey facies that thin westwards towards theOwerri metropolis. Effective hydraulicconductivity (Kz) in the downward directionranges from 1.44 × 10^-3 to 5.6 × 10^-9 m s^-1; with the upper limits reflecting coarsesands and gravelly units. The amount of clay andclay-size particles in the sandy and gravellyunits is negligible, suggesting that thesorptive capacity of the units is low. Depth towater table decreases southwards while hydraulichead gradients vary between 0.09 and 0.22.Groundwater occurs in unconfined conditions inmost places except in the southeastern zonewhere it is semi-confined due to the presence ofa clayey unit. The groundwater vulnerabilitymap developed on the basis of the models andseveral other thematic maps shows that theOwerri metropolis and the southwest area ofOwerri have high vulnerability, indicatinggroundwater pollution. The existing wastedisposal sites in these sub-areas should beabandoned and rehabilitated to forstall furtherpollution of the groundwater system. Areas tothe North and Southeast of Owerri have moderateand low vulnerabilities, respectively,indicating the relatively lower sensitivity ofthe groundwater system in the sub-areas tocontamination. The lower sensitivity couldfurther be matched with properly engineeredsanitary landfills in the event of choice ofsites, as an additional protective strategy forthe groundwater system.