Hydrogeological Characteristics of Aquifers in the Greater Accra Region

Abstract

The economic importance of groundwater in the Greater Accra Region cannot be overemphasized, since supply from boreholes and wells continues to be the most dependable alternative sources of water for most rural and urban communities in the region. Three different consolidated hard rocks underlie the region making it difficult to produce volumes of groundwater for supplies. Demand for portable water has led to the drilling of large numbers of boreholes. This study statistically assesses the general conditions of groundwater resources for successful exploitation, classify areas for prolific groundwater exploration and assess the quality of water in the region to be used for domestic purposes. Statistical approach was adopted to compare the variability and distribution of specific capacity and transmissivity values of existing boreholes in the various hydrogeological units. Hydrogeological units were classified using Krasny’s transmissivity classification in order to delineate prospective zones for groundwater exploration. The results from the Krasny’s transmissivity classification showed Transmissivity coefficient of 197m2/day, 197.3m2/day and 211.3m2/day for Dahomeyan, Granitoids and Togo hydrogeological units respectively. All the hydrogeological units belonged to the class of transmissivity magnitude class II which depict high transmissivity coefficient that suggests abstraction potential suitable for regional supply. The transmissivity indices are 6.04, 6.23 and 6.24 for Birimian Granitoids, Dahomeyan formation and Togo formation respectively. These categorised all the three hydrogeological units into moderate variation in a heterogeneous environment but the Togo formation being the most prolific. Based on WQI most of samples are suitable for domestic purpose except for few locations, which show values beyond the permissible limits that cannot be used without treatment. R and Q-mode hierarchical cluster analysis (HCA) are combined with factor analysis with principal components and varimax rotation, to determine field associations among the sample points, and their most possible sources of origin. R-mode and Q-mode HCA results showed linkages in general fields that suggest the varying geochemical sources in the three hydrogeological units. Physico-chemical parameters of the groundwater showed low pH values ranges suggesting acidic water in all the hydrogeological units. Nitrate values ranged between 0.0 and 2.73 mg/l which are within WHO standard guideline but very high in few samples suggesting pollution introduced by anthropogenic activities. Correlation analysis between the major ions and physical parameters showed positive correlation between TDS and Na, Mg, Ca that were significant at levels of significance above 0.5 for all the three hydrogeological units suggesting mineralisation through rock weathering processes. Piper Trilinear diagrams showed Na-Cl, Ca- Mg-Cl, and Ca–Mg–HCO3- are the dominant water type in the study area. Multivariate statistical methods employed to determine the factors that influenced hydrogeochemistry indicated factors including the dissolution of soluble minerals, evaporative enrichment as a result of the dry weather conditions and sea water intrusion or sea water spray resulting in high TDS values. Gibbs diagram results used to validate the results from multivariate analysis showed groundwater in the various hydrogeological units evolved from precipitation evaporation–crystallization and mainly rock mineral weathering.