Aquifers in the benin formation (miocene—recent), eastern Niger delta, Nigeria: Lithostratigraphy, hydraulics, and water quality

Abstract

A regional lithostratigraphic and hydraulic interpretation is presented for the upper 0–300 m of the Benin Formation where groundwater is abstracted in the Rivers State, Nigeria. The aquifers are predominantly sand beds with minor clays, lignite, and conglomerate intercalations. The sands are very fine to coarse grained, subangular to subrounded, poor to fairly well sorted and mostly lithic arenites. A maximum thickness greater than 50 m is developed in places and vertical stacking is common. Most of the conglomerate beds have a matrix support fabric and appear restricted to the east as the lignite beds. An east-west trending belt, about central to the state, seems to contain more clay interbeds. The transmissivity values for the aquifers range from 1.05 × 10−2 to 11.3 × 10−2 m2/sec, while the coefficient of storage varies between 1.07 × 10−4 and 3.53 × 10−4 and specific capacity values lie between 19.01 and 139.8 m3/h/m drawdown. These values suggest that the aquifers have very good capacity to transmit groundwater. The static water level map shows a north-to-south regional groundwater flow pattern except in the northeast (Imo River catchment area) where the flow is northeast to southwest. The groundwater quality is very good and compares favorably with WHO standards for drinking water. However, relatively high iron and chloride values are localized in time and space. Deposition of the aquifer materials is thought to have occurred in alluvial fan, fluvial channel, tidal channel, intertidal flat, beach, and related microenvironments.