The Declining Middle Zone Aquifer of the Southwestern Chad Basin in the last Millenium.

Abstract

The Middle zone aquifer is the most extensive and most widely exploited of the three well known aquifers of the Chad Formation. This aquifer is confined by variable thickness of argillaceous deposits of 100 - 150 m thick. This led to the variable heads in boreholes drilled to this zone. Boreholes drilled from 1960 to 2010 shows a drastic decline in piezometric heads. The piezometric heads of over 20 m above ground level ( agl) observed in 1960 to 1.2 m agl in 2010 have declined over the last half century. In the present work the heads at 0.2 m/ year giving the present piezometric head at an average of 0.5 m agl. The lower heads in old boreholes are attributed due to siltation, encrustation and ageing of borehole. Furthermore, lack of present day recharge and poor borehole construction methods play a vital role in determining the piezometric head.