OCCURRENCE OF GROUNDWATER IN THE VARIOUS ROCK TYPES IN GHANA

Below is a summary of the average yield that may be obtained when groundwater is abstracted in the following rock units:

Birimian System and Associated Granitic Intrusives:

Generally strongly foliated and jointed; considerable water may percolate through the joints, fractures where they outcrop or lie near the surface. Boreholes tapping the Lower Birimian in West, Central and Ashanti Regions have an average yield of about 2000 – 36000 liters/hour. Boreholes drilled in the Upper Birimian in West Region have 900 – 2000 liters/hour. The associated Granitic rocks have secondary porosity. In the Wa District, the zone of weathering can be as high as 140m thick. Successful boreholes averaged 5000 liters/hour with average depth of borehole in Northern and Upper Regions being 35m. Boreholes in Winneba granite have low yields, with average yield of 400 liters/hour. Boreholes in Kumasi Granitic batholiths are higher than in Wa and Winneba. The average yield is 9000 liters/hour. The chemical analysis show the groundwater is usable for all purposes, with an average hardness of 100 mg/l and Fe and Mn of 0.3 mg/l.

Tarkwaian System, Togo Series and Buem Formation:

They are lithologically similar and have similar water bearing properties. They all consist of arenaceous, pebbly sediments which have been indurated, folded and faulted with high grades of metamorphism in places. Rocks themselves are impervious but contain opening along joint, bedding and cleavage planes. These openings can be good suppliers of groundwater. The average yield from boreholes is about 9000 liters/hour with an average depth of 60m. Chemical analysis shows that the water can be used for most purposes. The concentration of Cl is 16 mg/l and Fe and Mn is 40 mg/l.

Dahomeyan System:

They are generally massive and have few joints or fractures. The silicic gneiss commonly weathers to slightly permeable clayey sands and the mafic gneiss weathers to impermeable calcareous clay. The impervious and massive nature limits the available groundwater that can be developed. Electrical methods can be used to locate few weathered fracture zones which will yield water supplies to borehole. Successful boreholes yield 500 – 11000 liters/hour. The water is unsafe for domestic, municipal and industrial purposes. Fe concentration is 0.3 – 4.2 mg/l, pH is slightly acidic, Cl is 30 – 195 mg/l.

Voltaian System:

They are generally well consolidated and not permeable. Sandstones, quartzites and arkoses produce sandy superficial deposits by weathering and are themselves well jointed in many places. In northern Ghana, boreholes have average depth of 100m and average yield of 3600 liters/hour. They have TDS of 1500 mg/l. In Kete Krachi, the boreholes have higher yields than any other location in the system with an average yield of 8700 liters/hour.

Coastal Basins

Consists of the Accraian Series, Sekondi Series and Amisian Formation. The boreholes have an average yield of 3900 liters/hour.

Tano & Keta Basins:

Groundwater in Tano Basin are obtained mainly in the upper 100m of section. The boreholes have an average yield of 1200 liters/hour and the water is suitable for most purposes and with an average chlorine concentration of 10 – 25 mg/l.

The Keta Basin has a wide distribution of high chlorine in groundwater. 43 boreholes in Keta Basin have an average yield of 1300 liters/hour. Boreholes tapping limestone aquifers along the coast have an average yield of 21000 liters/hour. The water is generally of usable quality, a TDS of 1000 mg/l, hardness of 250 mg/l, Fe less than 1 mg/l and pH slightly alkaline.

Alluvial Areas:

They are locally and relatively thick deposits of permeable water – bearing alluvium, present in larger streams such as the Volta River and its tributaries. The alluvial deposits have not yet been developed to any significant extent for water supply in Ghana, but they have great potential. Shallow, vertical or horizontal wells can induce river recharge and supply moderate water for municipal, irrigation and industrial purposes.

In all of the above, a detailed geophysical investigation has to be carried out to determine an appropriate location for a borehole to be drilled.

HYDROGEOLOGICAL MAP FOR URBAN PLANNING OF GREATER ACCRA METROPOLITAN AREA