International Journal of Modern Engineering Research (IJMER) www.ijmer.com Vol.3, Issue.1, Jan-Feb. 2013 pp-291-295 ISSN: 2249-6645 www.ijmer.com 291 | Page Ikechukwu Boniface Ijeh Department of Physics, College of Natural and Applied Sciences, Michael Okpara University of Agriculture, Umudike. PMB 7267, Umuahia, Abia State, Nigeria. ABSTRACT: Eight locations in the study area were investigated to obtain data on the depth to water table, net recharge, aquifer media, soil media, topography, impact of the vadose zone and hydraulic conductivity. These parameters are denoted by the acronym, DRASTIC, an empirical groundwater model that estimates groundwater contamination vulnerability of aquifer systems based on the hydrogeological settings of the area. This was used to develop a vulnerability map for the study area. The vulnerability map shows that areas within Benin Formation have generally moderate vulnerability to pollution. Keywords: Aquifer Media, DRASTIC, Net Recharge, Soil Media, Topography, Vulnerability, Water Table. I. INTRODUCTION The Imo River Basin is based on a bedrock of a sequence of sedimentary rocks of about 5480m thick and with ages ranging from Upper Cretaceous to Recent (Uma, 1986) 1 . It is known to contain several aquiferous units. The proportion of sandstones to shales varies from formation to formation and sometimes from place to place within the same formation. The interlayering of sandy and shaly units form complex aquifer systems that are respectively localized in the formations, such that it is almost impossible for aquifers to cross formation boundaries. This is because the regional stratigraphy and the general trends of the geologic formations feature sharp changes in lithology which prevent hydraulic continuity (Uma, 1989) 2 . The characteristics of these aquifers such as transmissivity, hydraulic conductivity and storage potentials are not clearly understood. Since the mid 1980’s, some researchers from the academia have carried out geological/geochemical investigations. Uma (1986) carried out a study on the ground resources of the Imo River Basin using hydro-geological data from existing boreholes. He concluded that three aquifer systems (shallow, middle and deep) exist in the area (Uma, 1989). Geophysical investigations on groundwater resources in the Imo River Basin were also carried out in different sections of the basin. While the contributions made by these workers are remarkable, more work still needs to be done, particularly in the area of vulnerability of the aquifer systems. The present study is aimed at investigating the vulnerability of the hydro- geological character in Ameki and Imo shale Formations to pollution. Vulnerability assessment is more meaningful in areas where water resources are under stress due to industrial or agricultural activities, as it can provide valuable information for planning prevention of further deterioration of the environment ( 3 Mendoza et al., 2006; 4 Antonakos et al., 2007). 1.1 The Study Area Figure 1 show the location map of the Imo River Basin where the study area is situated. The study area (Figure 2) lies between latitudes 5 o 05 ′ N and 5 o 37 ′ and longitudes 7 o 00 ′ and 7 o 30 ′ . The Benin Formation ( Miocene to Recent) covers more than half of the area of the Imo River Basin. It consists of sands, sandstones, and gravels, with intercalations of clay and sandy clay. The sands are fine-medium-coarse grained and poorly sorted ( 5 Whiteman, 1982; and 6 Uma and Egboka, 1985). Petrographic study on several thin sections ( 7 Onyeagocha, 1980) show that quartz makes up more than 95% of all grains, but 8 Asseez (1976) and 9 Avbovbo (1978) indicated a possible presence of more percentage of other skeletal materials including feldspar. This formation has very low dip to the south and south-west. The youngest deposits in the basin are alluvium of recent age found mainly at the estuary of the Imo River at the Atlantic Ocean and on the flood plains of the river. The Imo River Basin has a large amount of recharge; estimated at 2.5 billion m 3 per annum, coming mainly from direct infiltration of precipitation. Average annual rainfall is about 2000mm ( 10 Onwuegbuche, 1993). The Benin Formation is by far the most aquiferous unit, consisting mainly of massive continental sands, sandstones, and gravels. It has a very extensive deep unconfined aquifer which covers more than half of the Imo River Basin. The aquifer consists of thick complex interbedded units of fine, medium and coarse-grained quartz sands and gravels (Uma, 1989). Determination of the Vulnerability of Water Supply Aquifers in Parts of Imo River Basin, South-Eastern Nigeria: The Case of Benin Formation