1Groundwater Quality: Ghana

BackgroundThe Republic of Ghana, bordered by the Gulf ofGuinea, Togo and the Cote dIvoire in West Africa,has a total land area of 238,540 sq km. Terraincomprises mostly low-lying plains, divisible into sixphysiographic regions:

i) the low-lying coastal plain (up to 132 m);ii) the Voltaian Basin (100-200 m), 103,600 sq km

in area and surrounded by highlands withoutward-facing escarpments. The Basinincorporates Lake Volta, occupying around8,500 sq km;

iii) the Voltaian Highlands rising to around 800 m,on the western border of the Voltaian Basin(Koforidua-Mpreaso-Mampong);

iv) The Akwapim-Togo Hill Range, up to 885 m,extends east of Accra, via Ho along the borderwith Togo;

v) the north and north-west plateau, 150-300 maltitude, existing north of the Black Voltathrough Bole, Wa, Bolgatanga and Gambaga;

vi) the south-west plateau, also 150-300m inaltitude with occasional hills up to 800 m,occurs south of the Black Volta and west ofthe Voltaian Highlands and includes Kumasiand Sunyani.

Climate is tropical and varies from warm andrelatively dry in the south-east coastal region, hotand humid in the south and south-west and hot andarid (Sahelian) in the north. Average annual rainfallvaries between 1600 mm in the south to 1000 mmin the north.

GeologyGeology is dominated by ancient crystalline rocks,incorporating mainly metamorphosed sedimentsand granite. Minor occurrences of more recentunconsolidated sands, clays and gravels arerestricted to small areas along the coast.

A simplified geological map of the country is shownin Figure 1. The north-west and south-west plateauxare composed of the oldest crystalline rocks of the

Birimian and Tarkwaian Formations, withcommon major intrusions of granite (granitoids).South-eastern Ghana, incorporating the Akwapim-Togo Hill Range, is also composed of a mixedsequence of ancient metamorphosed sedimentaryand igneous rocks. The Voltaian Basin, covering alarge part of central and eastern Ghana, and theVoltaian Highlands along its western margin arecomposed of consolidated sandstones, mudstonesand limestones of the Voltaian Formation. Theyounger sediments of the coastal basins arecomposed of river-borne, wind-blown, beach ormarine deposits of sand, clay and gravel, largelyunconsolidated and of mixed age.

The oldest rocks in the country have been variablymetamorphosed, contorted and intruded by graniticrocks since their formation. Major episodes ofmineralisation have also produced major mineralveins, many of which are gold-bearing and ofsignificant economic value. The most important

This is one of a series of information sheets prepared for each country in which WaterAid works. The sheetsaim to identify inorganic constituents of significant risk to health that may occur in groundwater in thecountry in question. The purpose of the sheets is to provide guidance to WaterAid Country Office staff ontargeting efforts on water-quality testing and to encourage further thinking in the organisation on water-quality issues.

Figure 1. Simplified geological map of Ghanashowing locations of BGS study areas

2gold-bearing areas occur within the non-granitoidBirimian rocks (Figure 1; Kesse, 1985).

Groundwater AvailabilityGroundwater flow is mostly restricted to joints andfractures within the crystalline rock formations.Borehole yields are therefore often limited. In someareas, a thick layer of weathered friable material(regolith) overlies the crystalline basement andprovides potential for increased groundwaterstorage. This weathered layer can be in excess of100 m thick in places, though is more typically inthe range 1-70 m thick (e.g. Asomaning, 1992).Groundwater supplies may be higher in the minorsedimentary deposits of the coastal basins.

In the tropical southern part of the country, hand-pump tubewells and dug wells are in commonusage, though surface water is still used in partswhere tubewells are absent or not working. In thearid north where surface water is more scarce, watersupply is dominantly from tubewells and dug wells.

Groundwater QualityOverviewGeology plays an important role in thedetermination of groundwater quality and potentialwater-quality problems. As the country isdominated by crystalline silicate rocks andweathered derivatives (regolith), groundwater ismainly of low salinity and commonly acidic incomposition (pH

3High concentrations of iron are not directlyproblematic for human health but may causeindirect problems because of abandonment ofaffected water sources due to unpleasant odour andtaste, in favour of surface waters which may becontaminated by harmful bacteria.

Manganese typically occurs in association with iron,though concentrations are much lower in Ghanaiangroundwaters. Few observed results exceed theWHO recommended guideline (0.5 mg/l, health-based limit). Results from two comprehensive BGSsurveys of groundwater (Obuasi, Ashanti Regionand Bolgatanga, Upper East Region) indicate thatrespectively 3% and 0% of potable groundwatersources exceeded the WHO guideline formanganese (Smedley et al., 1995).

FluorideThe dominant controls on fluoride occurrence ingroundwater are climate and geology. The regionsof Ghana most vulnerable to high fluorideconcentrations (and associated dental fluorosis) arethe arid zones of the north and areas where bedrockgeology is dominated by granite. The UpperRegions of Ghana are therefore considered to bethe most likely areas to experience potentialproblems. Concentrations of fluoride in excess of1.5 mg/l (up to 3.8 mg/l) have been observed inBolgatanga and Sekoti (Upper East Region,Figure 2) in close association with granitic rocktypes (Smedley et al., 1995). Occurrence of dentalfluorosis is common in these areas. Groundwatersin granitic rocks of the south-west plateau areconsidered to be less at risk because of higherrainfall and its diluting effect on groundwatercompositions.

Marked variations in fluoride concentration withdepth were observed in groundwaters from theproblem areas of Bolgatanga (e.g. the BongoGranite, Figure 2). Shallow groundwaters from dug

wells had significantly lower concentrations thantubewell waters as a result of dilution by recentrecharge.

ArsenicArsenic poses a potential problem locally inGhanaian groundwater because of the abundance ofarsenic-bearing minerals, particularly arsenopyrite,which occurs in close association with gold insulphide-mineralised veins. Surface waters close togold mines have been found to contain relativelyhigh concentrations (>0.01 mg/l, the WHOrecommended guideline) of arsenic in localisedareas. However, analysed groundwaters have notrevealed major arsenic problems. Of 78 potablegroundwater samples analysed by BGS in theObuasi gold-mining area, only 14 (18%) exceededthe WHO guideline (maximum concentration,0.064 mg/l). In the Bolgatanga area, where sulphidemineralisation is present but less prevalent, only 2samples out of 118 analysed (

4Obuasi area which ranged between 0.001 mg/l and0.15 mg/l (mean 0.015 mg/l). Populations drinkingwater with concentrations of 0.005 mg/l or less areconsidered potentially vulnerable to iodine-deficiency disorders (although dietary intakes arealso an important consideration). The UpperRegions (with Birimian rocks) are likely to be mostat risk from iodine-deficiency problems. SouthernGhana is less likely to be vulnerable.

NitrateConcentrations of nitrate (as N) in mostgroundwater analyses reported from Ghana arebelow the WHO guideline value of 10 mg/l. Almostall groundwater samples from the BGS surveys ofObuasi and Bolgatanga had NO3-N concentrationsless than 10 mg/l. Only a few shallow hand-dugwells from the Obuasi area had values in excess ofthis (observed range