Hydrogeological evaluation of geological formations in Ashanti Region, Ghana

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The execution of any hydrogeological project to exploit groundwater using boreholes requires prior knowledge on some of the hydrogeological parameters of the underlying geological formation at the planning stage for smooth and successful implementation of such project. These parameters include the yield from the aquifer, the expected final depth of the borehole, the expected overburden thickness, static water level, specific capacity and the quality of water. This study employed Geographical Information System to assess some of these parameters in the geological formations of Ashanti Region; namely the Birimian and its intrusive undifferentiated Granitoids, Tarkwaian and Voltaian, using data from 2788 drilled boreholes. The ordinary kriging interpolation method was used to produce maps of the airlift yield, overburden and final borehole depths, static water level, specific capacity and the occurrence of higher amounts of iron in the groundwater of the study area. The study results indicate that the Birimian is generally within the medium (30 – 60 l/min) and high (> 60 l/min) yield zones at a success rate of 90.6 % and average final depth of 52.5 m whilst the undifferentiated granitoids are, mostly, within the low yield (less than 30 l/min) zones with isolated high yield boreholes and have average borehole depth and success rate of 49.6 m and 72.3 % respectively. Aquifers within the Tarkwaian group are generally within the medium yield zone with a success rate of 79.5 %. The groups within the Voltaian formation –i.e. Kwahu, Oti Pendjari and Obosum– are classified mainly within the low yield zones with 53.5 %, 60.0 % and 66.7 % success rates respectively, but significant high yield zones occur within the sandstone formation underlying the westernmost part of the region. Also, about 21 % of successful boreholes had unsuitable water quality with high iron, nitrate, manganese and low pH being their main problems. About 11.7 % of these boreholes were located within the Birimian and intrusive undifferentiated granitoids and had high iron concentrations above the accepted guideline value for drinking water.
A thesis submitted to the Department of Geological Engineering, College of Engineering in partial fulfilment of the requirements for the degree of Master of Philosophy.