The Origin And Hydrochemical Evolution Of Groundwater In The Lake Bosumtwi Area, Ghana

ABSTRACT

Lake Bosumtwi is an important natural inland freshwater meteorite crater lake due to its scientific and socio-economic importance to both local and international communities. Although groundwater has been the main source of water supply for people living around the lake and visitors/tourists, very little work has been conducted with regard to the quality of the groundwater delivered by the aquifers within the lake basin. These aquifers are made up of the metasediments of the Birimian Supergroup and boreholes that are mostly fitted with pumps tap the groundwater. A combination of conventional graphical methods, multivariate statistical and mass balance models have been applied to surface water in Lake Bosumtwi and groundwater hydrochemical and stable isotope (δ2H and δ18O) data from Birimian aquifers around the lake. The objective was to contribute to and improve the understanding of the hydrochemistry of the lake water and groundwater in aquifers around the lake and also to understand the relationship between these two reservoirs. Results indicate that groundwater is of good to excellent quality for domestic use and is generally suitable for irrigation in comparison to the lake water that has high salinity and high sodicity and is, therefore, not suitable for irrigation. Hydrochemical and isotopic (i.e., δ2H and δ 18O) compositions of the groundwater and lake water suggest that there is no apparent incipient hydraulic relationship, which benefits the main aquifer system in terms of recharge from the lake. However, the reverse process, whereby the lake receives contribution from the aquifers through subsurface flow cannot be discounted on the basis of the data from this research. iii Recharge of the aquifers appears to occur on hilltops where the water is characteristically acidic with low level of mineralization, suggesting short residence time. The groundwater within the basin has been recharged by recent meteoric water that has undergone evaporative enrichment prior to recharge. Evaporative loss in the range of 45% to 51% has been estimated for rainwater available for infiltration and subsequent recharge. The lake water is considerably enriched relative to the heavier isotopes of hydrogen and oxygen as a result of severe evaporation over the open lake surface. Estimated evaporative loss over the lake surface is about 82% resulting in the concentration of the univalent cations in the Lake. Q-mode hierarchical cluster analysis (HCA) of the hydrochemical data employed to examine the spatial distribution of the investigated samples resulted in four spatial clusters (C1 to C4). Conventional graphical plots of the hydrochemical compositions of these clusters combined with mass balance hydrochemical modelling suggested that the groundwater has evolved from Na-Mg rich water types located on hill tops to Ca-Mg rich hydrochemical facies at the lower reaches of the crater. The relevant reactive minerals within the aquifers and the geochemical processes that control the hydrochemical evolution of the groundwater and lake water were also determined. Rmode principal component analyses (PCA) of the hydrochemical data suggested dissolution of aluminosilicates followed by carbonate mineral weathering and finally anthropogenic activities as the principal processes that influenced the hydrochemistry. Mineral stability diagrams suggested kaolinite as the most stable clay mineral phase in the groundwater system thus the groundwater in the area is generally at the intermediate stage in the evolution processes. In addition to dissolution of silicates, the chemical composition of the lake appears to have been influenced by evaporation and consequent saturation of carbonate minerals. The research has demonstrated that an understanding of the hydrogeochemistry and the relationship between the lake and groundwater is important for environmental management.