Hydrogeochemistry and Impacts of Climate Change on Groundwater Recharge: Case Study of the Crystalline Basement Aquifer of Northern Ghana

Abstract

Northern Ghana is mostly underlain by a crystalline rock aquifer. Like in many arid and semi-arid areas, surface water is largely unavailable in the prolonged dry season. Therefore groundwater appears to be a good alternative source for domestic and agricultural water supply. However, crystalline basement rocks are characterised by a poor water storage and low water yield. The water quality is also usually poor due to elevated content in some minerals like fluoride. Further, climate change may worsen the situation groundwater since recharge is fairly related to rainfall in the area. Therefore this work was done to provide decision support for the sustainable development and management of groundwater resources in Northern Ghana. The current work first studied the water quality of both surface and groundwater in a part of the crystalline basement aquifer of Northern Ghana which covers 25000 km2 and used five different methods to estimate present day recharge, namely the baseflow (BF) recession analysis, the Chloride Mass Balance (CMB) method, the Water Table Fluctuation (WTF) method, the model based method using the distributed WetSpass model and the simple water balance method. The study revealed that groundwater chemistry is mainly controlled by cation exchange and silicate weathering processes and Ca-Mg-HCO3 is the major water type, regardless of aquifer geology. The recession curve analysis results in recharge ranging from 29 mm to 68 mm representing 3% and 9% of annual rainfall in 2004 and 2005, respectively; with average value of 49 mm (5%) for the period 2003-2008. The estimated annual baseflow based on the six methods ranged from 19.1 mm to 69.8 mm or from 2% to 7% of the annual rainfall. Groundwater recharge estimated by the WTF and CMB methods ranges from 68 mm (6%) to 163 mm (17%) with a mean value of 10% and from 21 mm (2.1%) to 191 mm (19.3%) with average value of 81 mm (8.1%), respectively. The recharge estimated by the WetSpass model, for the period lasting from 2003 to 2008, ranged from 519 mm to 756 mm, representing 68% and 70% of the annual reainfall during 2005 and 2007, respectively. The Average recharge estimated using the simple water balance method for the period 1976-2005 fell within the range of 283 mm (26.3%) to 330 mm (29.2%) with average of 300 mm (28%). Compared to the recharge rate of 300 mm during the baseline period (1976-2005), the ensemble mean recharge is projected to increase in periods 2011-2040, 2041-2070 and 2071-2100 by a rate ranging from 23% to 44%, from 29% to 55% and from 33% to 55%, respectively. This work suggested the use of dynamical downscaling methods for comparison purpose and the the coupling of the WetSpass model with groundwater flow model in order to improve the results.