Quality of natural waters in carbonate aquifers at a densely populated Odukpani area of cross river state, Southern Nigeria

Abstract

Abstract The study area, Odukpani is underlain by cretaceous sediments of the Calabar Flank. The main aquifers are fractured black shales that are inter-bedded with calcareous sandstones. The aquifers range from confined to unconfined. Water samples were taken from boreholes and shallow hand dug wells for chemical characterization with standard laboratory techniques. Results from the chemical analysis show that the waters are mostly alkaline with the exception of two mildly acidic samples. They depicted low electrical conductivity values and a total dissolved solids range from 13.1 to 341 mg/l which indicate fresh water. Cations levels occur in the order, Ca2+ ˃ Na+ ˃ Mg2+ ˃ K+ ˃ Fe2+ while anions occur as the following order, HCO3− ˃ SO42− ˃ Cl− ˃ CO3− ˃ NO3−. Piper trilinear and stiff plots show that Ca2+ – Na+ – HCO3− is dominant water type. The waters are supersaturated with respect to calcite in five of the samples. The rest have calcite under-saturation. Areas of calcite super-saturations are likely undergoing precipitation of carbonate minerals in the aquifer matrices. Precipitation and dissolution of carbonate minerals within the aquifers are likely contributing to the modification of flow characteristics of the aquifer. Comparison of the water chemistry with standards set by regulatory bodies indicates that the waters are of good quality.