Evaluation of Groundwater Potentials of the Calabar Coastal Aquifers

Abstract

This article presents the results of groundwater site evaluation scheme and quality assessment of coastal aquifers in Calabar, South-eastern Nigeria based on ground water potential index (GWPI) scale, developed for this study. The GWPI consists of ten input parameters, namely: lithofacies (L), aquifer thickness (b), transmissivity (T), storativity (S), specific capacity (SC), static water level (SWL), formation resistivity (FR), chloride (Cl) contents, total dissolved solids (TDS) and Escherichia coli (E-coli). The groundwater potential index (GWPI) is computed as the sum of the products of weights and ratings assigned to each of the input parameters. The GWPI index varying between 20 and 60, is divided into three classes: high (> 40), medium (30-40), and low (< 30). The GWPI index, is then used to demarcate the study area into three hydrogeologic ground water potential zones. These are: (i) Northern zone 1 (transitional, low GWPI) (ii) Central zone 2 (coastal plain sands, high GWPI) (iii) Southern zone 3 (coastal alluvium, medium GWPI). The central zone 2 has the highest GWPI rating. The implication of this rating is that the aquifers in the central and southern zones 2 and 3 are more prolific water bearing than the transitional zone 1 that lies between the Coastal Plain Sands and the argillaceous sediments of the Calabar Flank. This is in agreement with the mean specific capacity (SC) and transmissivity (T) recorded for the central (355.6 m super( 3)/d/m, 2640 m super( 2)/d); southern (150.0 m super( 3)/d/m; 2150 m super( 2)/d) and northern (52.1 m super( 3)/d/m, 750 m super( 2)/d) zones respectively. Lithofacies, saturated thickness of the aquifer, static water level, transmissivity and storativity are the most important parameters which influence ground water availability in the study area. The GWPI results further reveal that E-coli (3-50 counts/100ml), chloride (Cl super( -)) (2.5-21.0 mg/l) and static water level (SWL) (2.3-28.7 m) remain the most significant parameters that contribute to groundwater pollution particularly in the southern zone of the study area. In the near future, water quality in aquifer will be affected due to poor management of human waste-disposal/salt water intrusion, thereby limiting the availability of potable water for domestic and industrial uses.