The use of statistical grain-size method in analysing borehole and evaluating aquifer parameters. A case study of Ajali Sandstone formation, southeastern Nigeria

Abstract

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 7^o22¹30¹¹E and 7o 5813011E, and  Latitude 6^o 73¹30¹¹N and 7^o00¹00¹¹N, 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.

Keywords: Field Mapping -Borehole analysis -Textural analysis -Ajali Sandstone Aquifer -Nigeria