GROUNDWATER RECHARGE AND HYDROCHEMICAL STUDIES AT ATANKWIDI CATCHMENT

Abstract

The rate of groundwater recharge and the processes that affect groundwater quality at the Atankwidi Catchment were investigated in this study. The rate of recharge was estimated using Water Balance Method (WBM) and Chloride Mass Balance (CMB) method and compared. The rate of recharge by the WBM was 6% whiles point recharge by CMB method averaged 5.7% and ranged from 1.6 to 16% of mean annual rainfall. Hydrochemical analysis of groundwater showed varied concentrations of cations and anions in the order Na+ > Ca2+> Mg2+> K+ and HCO3 - >Cl- > SO4 2- > NO3 - respectively. Geochemical assessment of major ions using bi-plots suggested that, the occurrence of these ions in groundwater were controlled by mainly dissolution of silicate minerals and ion exchange reactions. This was supported by principal component analysis (PCA) of major ions and Gibbs diagram which suggested that the groundwater quality in the Atankwidi Catchment was mainly controlled by rock water interactions and to some lesser extent anthropogenic activities probably through application of both organic and inorganic fertilisers. Hierarchical cluster analysis of the major ions classified the groundwater into three groups. The groups showed different degree of rock water interactions. The major distinguishing factor among the groups was the total dissolved solids which generally increased from group one to group three. Piper diagram also classified the groundwater into three main hydrochemicalfacies in the order of Na-Ca-HCO3, Na-Ca-Mg-HCO3 and Na-HCO3. The occurrence of these hydrochemicalfacies supported the PCA and Gibbs diagram. The concentrations of elements such as As, Cd, Co, Cr, Cu, Fe, Mn, Ni, Zn and Pb were generally within the WHO guidelines for drinking-water. Groundwater quality was also found to be suitable for drinking based on the water quality index. The assessment of groundwater quality based on salinity index, United States Salinit Laboratory (USSL) diagram, sodium percentage and Wilcox diagram showed that groundwater from the Atankwidi Catchment was suitable for irrigation on wide range of soils. However, problems associated with magnesium hazard and residual sodium carbonate is likely to limit its use.