Aquifer Characterisation and Numerical Modelling for Groundwater Resources Assessment in the Densu River Basin

Abstract

Groundwater plays a crucial role in the delivery of water for domestic, industrial, and agricultural uses for communities in and around the Densu River Basin. A detailed understanding of the hydrogeological properties, hydrochemistry, and groundwater dynamics is crucial for informed and practical sustainable management of the resource. Assessment of essential aquifer parameters, recharge, and hydrochemical data was conducted for a holistic understanding of the hydrogeology of the basin. The study adopted the water table fluctuation (WTF), chloride mass balance (CMB), and baseflow filters (BFF) to estimate groundwater recharge, which is essential for understanding groundwater demand versus availability. Assessment of recharge and essential hydrogeologic parameters provided new insights for the construction of a conceptual and 3D transient groundwater flow model. The model was calibrated using PEST for various parameters and pilot points to represent heterogeneity in the hydraulic conductivities of two hydrostratigraphic units representing the regolith and fractured bedrock, and used to assess the potential impacts of enhanced abstractions and reduced recharge on groundwater resources in the basin. The study revealed groundwater recharge in the basin ranges from 27 - 338 mm/year, representing 4 – 24% of the total annual rainfall, trending north to south in magnitude. The basin averages were estimated at 34, 150, and 80 mm/year, representing 3%, 15%, and 7% for CMB, WTF, and BFF, respectively, with an overall average of ~144 mm/year, representing ~ 15% of the rainfall. The CMB and BFF methods largely agree on basin average recharge, whereas the WTF methods generate much higher average recharge estimates. The CMB method revealed weaknesses in the lower bound estimates (underestimated) compared to the other methods. Although the major rainy season peaks around May/June in the basin, the highest groundwater recharge occurs in the peak of the minor rainy season in October, which is attributable to the 2.5 months lag time between rainfall and recharge. Within the basin, the study finds that rocks of the Birimian Supergroup are the most prolific, consistent with previous studies, with an average borehole yield of 84.48 m3/day, as compared to average yields of 79.92 m3/day and 66 m3/day, respectively, for rocks of the Voltaian Supergroup and granites of the Eburnean and Tamnean Plutonic Suites. The spatial variation of the major ions in the basin mimics the presumed groundwater flow regime, with the hydrochemistry increasingly getting mineralised as it flows from the upland high-elevation areas (recharge areas) in the northeast and southwest (characterised by Ca/Na-HCO3 facies), to discharge areas (characterised by Na-Cl) in low-elevation areas in the central and southern sections. The groundwater hydrochemistry is dominated by silicate mineral weathering and ion-exchange processes in the groundwater system. However, carbonate mineral dissolution, anthropogenic pollution, seawater intrusion, and dissolution of evaporite play some role, especially in the extreme south of the basin. The recharge and transition zones in the groundwater flow regime present the best water quality compared to the discharge zones. Further insights into the hydrogeology of the basin were gained through model calibration. The process resulted in a computed horizontal hydraulic conductivity range of 0 – 15 m/d, with an average of 0 – 4 m/day. The simulations revealed enhanced abstractions caused by demand from population growth at an annual rate of 3.6 % over a 20-year period could increase groundwater abstraction from a baseline rate of 10,765,365 m³/year to approximately 24,489,637 m³/year. This projected demand represents about 16% of the basin’s baseline annual groundwater recharge. Despite the increase, the simulations suggest minimal impact on overall groundwater volumes and flow patterns within the basin. However, coupling enhanced abstractions with reduced recharge rate caused a decline in groundwaterrecharge by 0.026%, a slight decline in groundwater levels, and a reduction in groundwater contribution to stream flow by 8% and 21% respectively, compared to the baseline period. The observations made in these forecasts suggest abstractions at the current rate over 20 years without changes to the recharge rate pose risks to stream flows and ecosystems, especially under drier climate conditions