Hydrochemistry of the Ayensu River Basin in the Central Region of Ghana

Abstract

The focus of this work was to use the hydrochemical and isotopic composition of groundwater and surface water resources to establish the hydrochemistry, recharge mechanism, and interrelationship between surface and groundwater resources in the Ayensu River Basin. The methodology consisted of sampling and laboratory analysis of both groundwater and surface water resources. Na'" and K+ were determined using flame photometer, Ca2+ and Mg2+ by EDTA titration and the trace metals were determined using atomic absorption spectrometer. The anions were analysed using Dionex ICX- 90 Ion Chromatography system. Analysis of the hydrochemical survey data from the study area revealed that the groundwater is mildly acidic (4.77 - 7.8). Four boreholes at Gyangyanadze, Brahabekume, Topiase and Aduafokwa had their Total Dissolved So~ds (TDS) values above lO00mg/L. lbree major water types have been delineated using the phase diagrams. These are Na - CI, Ca-Mg - CI and Ca - Mg - SO.'" waters. Chloride ion (CI-) concentrations of 10 boreholes exceeded the permissible levels of 250mg/L for Cl- ion and 7% of the groundwater samples had their N03- concentrations exceeding the WHO limit. Approximately 43% and 57% of the samples had the AI" and Fe'" concentrations respectively, above the WHO maximum acceptable limits and 25% of the water samples had their Cd2 " concentration above the WHO limit. The groundwater chemistry is controlled mainly by rock weathering and to a very small extent by evaporation during both the dry and rainy seasons. From the stable isotope data for deuterium (S2H %0) and Oxygen - 18 (S"O %0) it is concluded that recharge to the aquifer system in the Ayensu River Basin mainly come from local rainfall.