Vulnerability evaluation of a coastal plain sand aquifer with a case example from Calabar, southeastern Nigeria

Abstract

A point count index method using a well driller’s log and field measurements has been developed following the DRASTIC and SINTACS procedures to map and evaluate the vulnerability of a coastal plain aquifer to surface and near surface contamination. The input parameters with the acronym CALOD include clay layer thickness (C), aquifer media character (A), lateritic layer thickness (L), overlying layer character (O) and the depth to groundwater level (D). The CALOD vulnerability potential index (CALOD index) is computed as the sum of the products of weights and ratings assigned to each of the input parameters. The CALOD index, varying between 15 and 75, is divided into four classes: high (>60), high-medium, (40–60), low-medium (20–40) and low (<20). The CALOD index is then used to produce a vulnerability potential map for the area. From the map, areas of high, high-medium and medium-low are consistent with the upper gravelly aquiferous zone while areas of medium-low and low are restricted to the deeper lower sandy aquiferous layer. The most important parameters affecting groundwater vulnerability to pollution in coastal areas include saturated thickness of the aquifer, depth to groundwater level, lateritic layer thickness and the aquifer media character. The concentration of some chemical pollution indicators (electrical conductivity, K, NO₃, Cl and metal load) are relatively higher for the highly vulnerable shallower upper gravelly unit in comparison to the less vulnerable deeper sandy unit. This method is very suitable for coastal plain sand aquifers especially, where data is scare.