Analysis Of Groundwater Abstraction Scenarios In The Kwahu Afram Plains South District, Ghana - Application Of Numerical Groundwater Flow Modelling Technique

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2016-07

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University of Ghana

Abstract

The groundwater flow system in the Kwahu Afram Plains South District was simulated and calibrated under steady-state conditions. The main objective of the research was to estimate the spatial distribution of a key aquifer hydraulic parameter (the hydraulic conductivity) within an estimated range of groundwater recharge in the area and then proceed to assess the responses of the aquifer system to stresses of abstraction and reduction in recharge that may be occasioned by climate change and other stresses. A conceptual model of the domain was developed by building a hydrostratigraphy from borehole logs and defining lateral domain boundaries. The digital conceptual framework was developed using the Map module in the Groundwater Modelling System. The United States Geological Survey Modular Finite Difference code, MODFLOW-2000 in GMS 10.1 was used to simulate the numerical modelling and scenario analysis. The results of the study suggest six main lithostratigraphic units from the well log section and these units are laterite, shale, siltstone, sandstone, conglomerate and quartzite from the bottom to the top. The information gathered from the drill logs, the pseudo-sections and the geology of the area suggests the aquifers were probably encountered within the depth of (24 m – 153 m). Aquifer thicknesses vary widely in space and may be as a result of different degrees of fracturing and weathering in the area. The average aquifer thickness is about 20 m with fractured sandstone being the dominant aquifer type found in the area. Thirty (30) boreholes were carefully selected and used for this research with their depths ranging within 24m to 153m. A groundwater flow model for the area indicates general flow from the central portions of the area to the outer areas where it discharges into the Volta Lake at some places. The study finds that the hydraulic conductivity field is quite heterogeneous and has values ranging between 2.0 and 98.0 m/day. The groundwater recharge from the calibrated steady-state model was estimated to range between 0.00365 m/year and 0.1022 m/year with an average of 0.2701 m/year representing 13 % and 16 % with an average of 14.5 % of the annual precipitation of 1.10 m of annual rainfall in the area. The highest recharge appears to occur in the southern portion. The rest of the terrain has low recharge rates. From the water table budget, the source of recharge to the groundwater resource is sub-surface inflows. When the initial abstraction rates were increased up to about 50 % and beyond, dry cells were observed around some of the wells. However, if the current abstraction rate is increased by 50 % up to 200 %, the system will record drastic changes in the potential field.

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Thesis (MPhil)

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Groundwater, Abstraction, Scenarios, Kwahu Afram Plains, Numerical, Ghana

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