A step towards the protection and management of the Shallow Aquifer of the Keta Basin, in Ghana West Africa: an initial physico-chemical characterisation

The Coastal Unconfined Shallow Sandy Aquifer of the Keta Basin, made up of Quaternary gravel, sand and clay and Neogenic Limonic deposits is the most economically accessible aquifer in Southern Volta, Ghana, West Africa. Water from the aquifer supports domestic supply and vegetable farming, which is the main stay of the area’s economy (Nerquaye-Tetteh 1993; Helstrup et al. 2007; Yidana et al. 2007). Despite the importance of this shallow aquifer, it is vulnerable to contamination from saline intrusion and agricultural activity in the area (Gill 1969; Nerquaye-Tetteh 1993; Kortatsi 1994; Kortatsi et al. 2005; Helstrup 2006). Protecting and managing this aquifer effectively will require the appreciation of the flow regime and dynamics via the collection of hydrogeological information such as geochemical properties and their variation over the years, hydraulic gradient, flow direction, well density, and their abstraction rates. However, this data is non-existent. This work set out to collect these basic hydrogeological information. This work was done via: geochemical sampling and analyses of thirty-five (35) wells for facies discrimination; the hydraulic head mapping of forty-five (45) wells for flow direction mapping and hydraulic gradient distribution, and particle size distribution testing of sampled aquifer material for the hydraulic conductivity distribution. The geochemical datasets showed: neutral and well buffered water groundwater; nitrates occurring in all the samples, with [NO₃^-] ranging between 0.35 – 25.3 mg/L, indicative of possible human influence on groundwater in the area; four (4)  main water types from the analyses as: Na-Cl, Ca-(HCO₃)₂, Na-HCO₃,  and Ca-Cl₂ with percentage dominances of 47, 41, 9 and 3% respectively. Na-Cl and Na-HCO₃ waters characterised by very high SECs and  found in farm wells located near the coast and lagoons suggest saline intrusion (due to heavy pumping on farms) from the sea and lagoon. The central part of the area, has fresh water which with the Ca-(HCO₃)₂ water type, indicative of natural rock weathering processes and flow dynamics. Analysing the irrigation water use parameters from the geochemical showed that the water in the area was suitable with respect to residual sodium carbonate (RSC) and magnesium absorption ratio (MAR), whereas waters mostly affected by saline intrusion did not meet the  sodium percentage (Na%), sodium absorption ratio (SAR) and Kelly’s ratio (KR). Heavy  groundwater abstraction without regulation is fingered for causing saline intrusion in the area because of reduction of groundwater levels. The hydraulic gradient in the area mimics that of the natural ground level, with relatively gentle slope of 0.002, with the dominant groundwater flow direction of north to south. This work is novel as it sets the tone for the first-ever initial hydrogeological characterisation of the aquifer, whose state can be continuously monitored for advising the Government, and the Water, Agricultural and Health Directorates of the Municipal Assembly for the regulation of agriculture and abstraction in the area, so as to protect the aquifer and human health.