Modeling Depth as a Constraining Factor for Optimum Groundwater Yield - A case Study

Abstract

This research work focused on the contribution of depth to the yield of boreholes for water supply project in parts of Fadama Floodplain West Chad Basin, Northeastern Nigeria. An integrated method involving geoelectric and borehole lithological analysis and record of yields and aquifer hydraulic properties from boreholes was employed. Four distinct subsurface geologic layers and corresponding depths were delineated. These included the topsoil, alluvial sand, Chad Formation/weathered column and the bedrock. The thicknesses of the upper three layers were 0.4 – 6.7; 1.6 - 32.2 and 15.9 -168.6 m respectively. The depth to rockheads varied from 18.4 to 175.5 m.  The second layer presumably alluvium layer constitutes the main aquifer unit with optimum depth not greater than 30 m. However, within the upper 25 - 30 m, high groundwater yield of between 9 – 13.4 l/s (779.3 – 1163.8 m3/day ) with  hydraulic conductivity and transmissivity values of  6.7 to 1329.4 m/day and 122.7 to 10427.5 m3/day respectively were obtained. Beyond this depth, yields were wholly retarded between 2.0 l/s (172.8 m3/day) and 6.9 l/s (602.2 m3/day) arising from the significant contribution of the poor hydrogeological materials . It is, therefore, concluded that borehole depth model (25 - 30 m) and not those terminated at rock heads or within bedrock depression zones as in general case gave an optimum groundwater yield with a cumulative discharge capacity greater than 420 l/s (36,288 m3/day) expected from a battery of maximum of 42 boreholes in this part of the Chad Basin.

Keywords: Depth, Contribution, Yield, Borehole, Transmissivity, Permeable, Floodplain.