Outcrop observations and borehole litho logs have been used to show that contrary to previous
opinion, an approximately 25-30m thick alluvial terrace deposit underlies Asaba the
administrative headquarter of Delta State, Nigeria. This superficial deposit masks the underlying
Oligocene Ogwashi-Asaba Formation. A new map showing the approximate areal distribution of
this superficial deposit is presented. The alluvial terrace deposit in combination with the upper
horizon of the Ogwashi-Asaba Formation is the first and preferred aquifer exploited by hundreds
of shallow water supply boreholes. The second aquifer horizon of the Ogwashi-Asaba Formation
occurs at greater depths and contains iron rich water. It is also more difficult to drill into due to
loss of circulation problems. It is shown that the west to east regional flow regime in the first
aquifer has been distorted in the West End area of the city due to heavy groundwater abstraction
such that flow direction is now east to west and the River Niger could be directly recharging the
aquifer in this area with an implied risk of contamination. Management of the aquifer is
recommended.
