Groundwater evolution in the Voltaian Basin, Ghana —an application of multivariate statistical analyses to hydrochemical data

Abstract

Multivariate statistical methods were applied to hydrochemical data obtained from various field sampling campaigns in order to highlight the major groundwater types and the evolution of groundwater from recharge areas to discharge areas in the groundwater flow regime. Q-mode hierarchical cluster analysis distinguished four major hydrochemical types: Ca-HCO3 low salin-ity fresh groundwater in the south of the basin, through Na-HCO3 fresh low to intermediate sa-linity groundwater types, to saline-brackish Na- Cl groundwater types in the north of the basin. The concentrations of the major hydrochemical parameters also increase from south to north, suggesting that groundwater evolves from ap-parent recharge areas in the south to saline- brackish Na-Cl groundwater types in the north. On the basis of the observed variation in the hydrochemical data, this study hypothesizes that the southern parts of the basin are the ma-jor recharge areas in the basin, whereas the northern sections are discharge locations in the groundwater flow regime. R-mode factor analy-sis was then applied to the data to differentiate the various factors responsible for the observed hydrochemical variations. Four factors were distinguished: dissolution of chlorides of the major ions, silicate mineral weathering and or-ganic matter oxidation, dissolution of kieserite group of minerals, and anthropogenic conta- mination. Of all these factors, silicate mineral weathering is the most pervasive throughout the study area. This is apparently due to the presence of silicate minerals almost everywhere in the terrain. This analysis was determined by means of factor scores computed from the factor loadings and the standardized dataset through regression. The resulting refined factor scores were then interpolated by ordinary kriging. The dissolution of the chlorides and kieserite group of minerals are limited to locations where such minerals are present in the lithology. Anthro-pogenic contamination of the aquifers occurs almost everywhere within the basin but the ef-fects are generally low.