Hydrogeophysical characterisation of shallow coastal aquifers in the Western Cape, South Africa
Coastal aquifers present a key groundwater resource for freshwater supply in many coastal zones of Africa, and its availability is largely driven by the physical hydrogeological properties. An understanding of the aquifer properties in coastal areas is fundamental in that these aquifers present unique resource largely controlled to a very large extent by its geological and hydrological features and process. This study thus analysed information of resistivity variation of formations, drilling samples, water levels and slug test data, in an attempt to characterise aquifers in the coastal region of the Heuningnes Catchment, Western Cape. This was in an effort to address the issue of limited knowledge on key hydrogeological properties of aquifers in coastal regions. Resistivity survey results indicated that the shallow aquifers in the study area were limited in extent, had a poor potential, with resistant layers occurring below shallow, high conductive formations. The long profile of the wellpoints revealed that the area is underlain by various layers of material of consolidated to unconsolidated form. The hard rocks formations are overlain by sandy materials of fluvial origin, and clay material with marine deposits (mollusc shells). This findings were in agreement with the results of the resistivity models. In addition, the findings indicated that the saturation thickness of the aquifers was also small. The findings of the study on aquifer properties indicated that, hydraulic conductivity (K), transmissivity (T) and borehole yields were generally poor and small. Estimates of K and T ranged from 0.0030 to 0.2856 m/day and from 0.0008 to10.993 m2/day accordingly, while average borehole yields were at 0.55 l/s, with productivity of the aquifers classified as low to moderate. These were indicative of a low permeability environment, with low yielding formations. An updated conceptual groundwater flow model developed in this study, revealed a rather compacted groundwater flow systems, in which local and intermediate flows were dominant. These findings in this study support the view that aquifers in coastal regions were predominantly characterised by shallow depth, patchy distribution and low potential.