Geophysical And Hydrogeological Characterisation Of The Nasia Basin, Northern Ghana

Abstract

ABSTRACT The development of groundwater resources across the Voltaian Sedimentary Basin (VSB) is constrained by lack of knowledge on the location and suitability of aquifers for borehole construction. The main objective of this study is to evaluate the hydrogeological characteristics of the Nasia Basin to help in delineating suitable locations for groundwater exploration. An integrated geophysical investigation involving resistivity survey using 2D Electrical Resistivity Tomography (ERT), Electromagnetic Survey with EM34-3 and wireline logging of boreholes were employed to determine the resistivity of the different type of lithology occurring in the area and for providing information about the lateral and vertical extent of weathering and fracturing in the subsurface. A total of 58 ERT profiles were run at selected locations and at existing boreholes to obtain information about resistivity in different depth intervals. Majority of the profiles were run across the general strike of rocks of the VSB in the study area with length being either 400 m or 800 m. Five EM-34 ground conductivity profiles of 500-1000 m were conducted on each of two approximately parallel South-North traverses (EM campaign 1) to record the conductivity in the various geological formations and to assess its possible variation related to major geological structures. In addition to the two-parallel profile (EM campaign 1), 12 other EM-34 profiles each 500 m long were carried out at six selected communities in the central part of the study area (EM campaign 2) with the objective of locating areas within the weathered zone capable of storing groundwater for small scale pilot irrigation from dug-wells. The conclusion of the integrated geophysical investigation is that combining

interpretations from various geophysical methods provides an improved characterisation of the hydrogeology. Secondly, hydrogeological investigations were conducted which involved analysis of pumping test results, evaluation of various methods of estimating aquifer transmissivity and assessment of the major factors controlling groundwater occurrence. Using remote sensing and geographical information system (GIS), thematic maps of slope, drainage density and lineament, geological formation were prepared. Other thematic maps such as static water level, regolith thickness, depth, recharge and transmissivity developed from kriging were incorporated into GIS. Finally, multi-criteria analysis and GIS techniques were used to integrate these thematic maps to delineate suitable zones to obtain a comprehensive groundwater potential map of the study area. The results also show that in certain locations such as portions of the Bimbilla formation, probability of obtaining aquifers is very low and therefore the target should be to locate thick regolith for groundwater storage. The results from the hydrogeological investigation indicate that cokriging gives better estimates of spatial aquifer transmissivity and therefore is a better approach considering the paucity of long duration pumping test data. Regression models and variographic analysis conducted confirmed the findings of previous researchers that groundwater within the study area is mainly structurally controlled and not by lithology. The groundwater potential map was classified into five zones that describe the potentiality of each cell in the study area for groundwater exploration. These classes are; very poor, poor, moderate, good and very good groundwater potential areas. It was found that 2% and 18% of the study area was classified as very good and good

potential areas respectively. These are areas found to be concentrated in the Kodjari formation southwest of the Panabako sandstone formation of the study area. About 38% of the study area was classified as moderate potential which is sparsely distributed across the study area and 41% represent poor potential for groundwater exploration occurring mainly in the Bimbilla formation. Less than 1% of the study area was classified as very low potential areas and also concentrated in southeast of the study area. The reliability of the groundwater potential map was tested against successful and dry boreholes and the results showed that generally, the majority of high- and low-yielding boreholes fall in areas predicted by the map. Furthermore, a sensitivity analysis was performed to study the effect of each parameter on the overall groundwater map using the effective weight. It was found that the transmissivity was the most effective among the parameters that have the greatest influence on groundwater occurrence in the study area which is consistent with literature. The integrated geophysical method coupled with the comprehensive groundwater map has provided better information about subsurface geology of the Nasia Basin which is critical for understanding the lithological character in terms of hydrogeological conditions. The general concept of interpreting low resistivity zones as the presence of a sub-vertical fracture zone and therefore regarded as a favorable site has been refuted. A more scientific approach of interpreting geophysical results has been proposed for exploration hydrogeologist in the area and in similar geology. The groundwater potential map further shows that the potential for high yielding boreholes is limited to about 20% of the study area. Therefore, other strategies to augment for increasing irrigation such as exploring the weathered zone for groundwater storage should be considered.