THE USE OF STATISTICAL GRAIN-SIZE METHOD IN ANALYSING BOREHOLE AND EVALUATING AQUIFER PARAMETERS. A CASE STUDY OF AJALI SANDSTONE FORMATION, SOUTHEASTERN NIGERIA
Several challenges abound with the accurate estimation of aquifer parameters in Groundwater resource development. This paper describes the investigation on the aquifer (mainly Ajali Sandstone Formation) parameters in Nsukka area. The Ajali Sandstone Formation is among the principal sources of groundwater in the area. The investigated area lies within Longitudes 7o 2213011E and 7o 5813011E, and Latitude 6o 7313011N and 7o 0010011N, covering an area of about 482 square kilometres. Sixty (60) cuttings from nine (9) boreholes drilled in the area were analysed texturally using statistical grain size method.
Results show that the highly permeable members of Ajali Sandstone are devoid of surface waters and are seen to be friable, medium-grained, poorly sorted, and highly cross-bedded. Recharge is mainly through rainfall. The distribution of major geological units, well log data, static water level data, and surface features were found to have influenced groundwater occurrence and flow pattern in the study area. The lithological logs showed sandy aquifers separated by clayey aquitards, while stratigraphic analyses of well lithological logs revealed the presence of at least four layers of contrasting hydraulic conductivity, making the aquiferous Ajali Sandstone homogenous and anisotropic. Values of permeability (k) from 0.00753 to 0.01512 cm2, hydraulic conductivity (K) from 8.2886 to 16.6444 m/s, transmissivity (T) between 455.87 and 1002.48m2s-1, and specific yield from 2.35 to 4.76m3 were found to be consistently high, thus suggesting thick and prolific aquiferous zones.
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