Hydrogeological Characterization Of The White Volta River Basin Of Ghana

ABSTRACT

Fresh groundwater resource located in the shallow semi-confined aquifer system of the White Volta River Basin of Ghana contributes significantly to the livelihood of the communities in the basin. The objectives of this study were to evaluate the spatial variations in the hydraulic parameters of the shallow aquifer system of the White Volta River Basin of Ghana, develop a 3 - dimensional groundwater flow model for the basin and assess various scenarios of groundwater development and climatic variability on the shallow aquifers. Lithological data, groundwater hydraulic heads and relevant hydrogeological information were used to characterize and conceptualize the hydrogeological system of the shallow semi-confined aquifers. A groundwater flow simulation model was generated for steady state conditions using MODFLOW and MODPATH in the Groundwater Modelling System, GMS. Based on the borehole data and limited hydrogeological data, the domain was conceptualized as a single layer model. The top of the domain was conceptualized as semi-confining to mimic the prevailing conditions of limited direct vertical recharge from precipitation as recorded in previous studies. Confining conditions were ascribed to the bottom of the terrain to reflect the low hydraulic conductivities of the impervious rock and the confining clay layers that appear in places below the simulated aquifer. The vertical sides were conceptualized and simulated with the Robin’s boundary condition so that subsurface lateral flows would be adequately characterized. The resulting numerical model was calibrated using observed hydraulic head data from 20 monitoring wells in the area. The results of the calibrated groundwater flow model show an apparent dominance of local flow systems. This is probably attributed to the variations in the hydraulic conductivity field, topography, structural entities and drainage patterns. The study found that hydraulic conductivity ranged from a low of 5.51 m/day in the central and southeastern portions of the domain, specifically the Voltaian aquifers to 80 m/day in the northern and north-eastern part of the study, representing the Birimian Basement

iii

aquifers. A stochastic calibration approach was applied in MODFLOW to assess model non-uniqueness often associated with the uncertainties in aquifer hydraulic properties and recharge. Twenty solutions were simulated simultaneously and calibrated through the Stochastic Parameter Estimation (PEST). This approach enabled MODFLOW to produce a number of combinations of aquifer parameters and recharge. At calibration, statistical procedures were used to determine the extent to which the several solutions were different from each other. On the basis of this, the uniqueness of the simulation was assessed and the resulting scenario analyses were placed in the proper contextual framework. The results suggest that recharge rates range from 0.02 to 4% of the annual rainfall in the terrain appears to hold promise for large-scale groundwater development to support irrigation schemes. The stochastic model was used to simulate responses of the system to various scenarios of abstraction increment and recharge rates reduction. The simulation suggest that under the current recharge rates, the system can sustain increasing groundwater abstraction rates by up to 200% with minimal drawdown in the hydraulic head for the entire terrain. This suggests that groundwater can sustain future increased groundwater demands from population growth and industrialization. However, significant drawdowns will be expected in the wake of 80% reduction in recharge, due to intense climatic variability and change. This study strongly recommends the protection of some of the local groundwater recharge areas identified in the study and the promotion of artificial groundwater recharge through the development of dugouts to encourage recharge.