Delineation of preferential flow pathways in a tropical crystalline rock aquifer in Tarkwa, Ghana using integrated hydrogeophysical methods

Abstract

In Ghana, crystalline rock aquifers with secondary hydraulic features and preferential flow pathways serve as very important aquifers. Protecting and managing these geological porous media require identifying preferential flow pathways and hydraulic characteristics. In this study, preferential flow pathways and hydraulic parameters were characterised for two boreholes (Hilly and Valley) located in Tarkwa, Ghana via the integrated use of borehole dilution testing (BDT), slug testing, and geological mapping. The geological fieldwork mapped a relatively sparsely fractured Sandstone Unit overlying a heavily foliated and fissile Phyllite Unit. Geology influenced groundwater flow in boreholes. The BDT confirmed the geology by showing a stratified flow system with preferential flow pathways. The Hilly/recharge area borehole shows relatively low downward flow, whereas the Valley/transition area borehole is dominated by the concentrated diffuse horizontal flow. These flow patterns are in agreement with the borehole locations and their dominant geologies. The estimated hydraulic conductivity, transmissivity, and storage coefficient ranged between 5.1 × 10−4–7.7 × 10−2 m/d, 4.0 × 10−2–3.2 m2/d, and 10−7–10−5, respectively. The cheap integrated hydrogeophysical methods used in this study are applicable for characterising, protecting, and managing other crystalline aquifers in the West African sub-region and other tropical terrains. HIGHLIGHTS Hydrogeophysical characterisation of preferential flow pathways and hydraulic properties for a crystalline aquifer.; Estimated hydraulic parameters fall within the values derived for well-to-well characterisations in similar hydrogeological terrain.; First-time use of borehole dilution testing (BDT) in Ghana and West Africa.; Recommended for use in similar aquifer terrains due to its cheapness and analytical simplicity.;

Keywords

• borehole dilution testing (bdt)
• crystalline rock aquifers
• delineation
• hydrogeophysics
• preferential flow pathways