Groundwater Quality Characterisation of Selected Hand-dug Wells and Geological Implications in the Assin North Municipality, Ghana

Abstract

Water from ten hand-dug wells from Assin North Municipality in Ghana were analysed at Ghana Water Company Limited Quality Assurance Laboratory, Ho in Ghana for various physico-chemical parameters. The analyses used standard methods with chemicals of AR grade; pH was measured with the Horiba Compact B-122 and Inolab 7300 Conductivity/TDS portable meter. Colour, turbidity, aluminium, copper, sulphate and total iron were analysed by spectrophotometry using Hach DR/2500 following standard methods. Flame photometer was used for determination of metal ions Na+, K+ and Ca2+. Silver nitrate method was used to estimate chloride, sulphate was determined by turbidimetric method. Total hardness was calculated by complexometric titration using EDTA. The results of the analyses show pH 6.0 to 7.2 with a mean of 6.5, conductivity from 300 to 800 μS/cm, TDS range from 90 to 400 mg/l. Piper Trilinear diagram revealed three main water types - calcium bicarbonate (Ca+Mg-HCO3) possibly sourced from Ca-plagioclase, biotite and amphibole; sodium chloride (Na-Cl) from Na-plagioclase, muscovite and chlorite; and mixed water types which may be due to the combined effect of Birimian metasedimentary rocks, Belt granitoid and pegmatites that released into the water through cation exchange and accounted for Na+ in the Na-Cl water type. Chloride in a few hand-dug wells may be linked to mineralogy crystallised from marine deposited sediments and infiltration of rainwater along fractures in the rocks. Hence most of the water samples have Na/Cl ratios<0.86. The rainwater in the area with weak acidity possibly infiltrated into the soil to cause appreciable concentration of HCO3- in the studied water. The total coliform values in water were less harmful as there was no faecal coliforms, though total microbial values in hand-dug wells X4 and X10 with total coliform values of 2.2 and 5.1 MPN/100ml respectively exceed the WHO guidelines. Hand-dug wells X4 and X6 in the study probably ended in aquifers in the weathered zone with the rest of the wells in fresh fractured horizons of the granitoids. Na/Cl ratios>1 (for water samples X1, X7 and X8) might have been due to contamination from anthropogenic sources. Pearson correlation indicates strong and positive correlation of TDS<Mg< Na with conductivity. All the water samples were safe for human consumption.