Dickson Adomako

Estimating groundwater recharge from water isotope (δ2H, δ18O) depth profiles in the Densu River basin, Ghana

Abstract Accurate estimation of groundwater recharge is essential for the proper management of aquifers. A study of water isotope (δ2H, δ18O) depth profiles was carried out to estimate groundwater recharge in the Densu River basin in Ghana, at three chosen observation sites that differ in their altitude, geology, climate and vegetation. Water isotopes and water contents were analysed with depth to determine water flow in the unsaturated zone. The measured data showed isotope enrichment in the pore water near the soil surface due to evaporation. Seasonal variations in the isotope signal of the pore water were also observed to a depth of 2.75 m. Below that depth, the seasonal variation of the isotope signal was attenuated due to diffusion/dispersion and low water flow velocities. Groundwater recharge rates were determined by numerical modelling of the unsaturated water flow and water isotope transport. Different groundwater recharge rates were computed at the three observation sites and were found to vary between 94 and 182 mm/year (± max. 7%). Further, the approximate peak-shift method was applied to give information about groundwater recharge rates. Although this simple method neglects variations in flow conditions and only considers advective transport, it yielded mean groundwater recharge rates of 110–250 mm/year (± max. 30%), which were in the same order of magnitude as computed numerical modelling values. Integrating these site-specific groundwater recharge rates to the whole catchment indicates that more water is potentially renewed than consumed nowadays. With increases in population and irrigation, more clean water is required, and knowledge about groundwater recharge rates – essential for improving the groundwater management in the Densu River basin – can be easily obtained by measuring water isotope depth profiles and applying a simple peak-shift approach. Citation Adomako, D., Maloszewski, P., Stumpp, C., Osae, S. & Akiti, T. T. (2010) Estimating groundwater recharge from water isotope (δ2H, δ18O) depth profiles in the Densu River basin, Ghana. Hydrol. Sci. J. 55(8), 1405–1416.

Relationship between land use and groundwater quality in six districts in the eastern region of Ghana

The chemical quality of groundwater in six district of the eastern region beneath the different types of land use areas of Ghana was examined to evaluate the effects of human activities on groundwater. Analyses indicate that groundwater in the studied area is fresh and generally suitable for most uses. The groundwater is generally characterised by a chemical facies of Ca-HCO3−, Na-Cl and mixed Na–Ca-HCO3 types and is weakly mineralised. Anthropogenic disturbances have had and continue to have an impact on the aquatic ecosystem of Ghana. High concentration of Cl− and TDS were found in wells in high residential areas while the highest levels of Na, Ca, SO42− and NO3− were found in agricultural and high density residential areas. About 50% of boreholes sampled have elevated level of NO3−-N emanating from agricultural runoff.

Evaluating the suitability of groundwater for irrigational purposes in some selected districts of the Upper West region of Ghana

Groundwater is a very important asset to the people of the Upper West region of the Ghana where majority of them are farmers. Groundwater serves as the most reliable source of water for their domestic and agricultural activities. This study was aimed at assessing the suitability of groundwater for irrigational purposes in some selected communities of five districts where farming activities are very intensive. Twenty-three groundwater samples were collected and analysed for major anions and cations. Physicochemical parameters such as electrical conductivity (EC) and total dissolved solids (TDS) were also measured. From the results of the analyses and measurements, the suitability of the groundwater for irrigation were evaluated based on the TDS, EC, percentage sodium (%Na), sodium adsorption ratio (SAR), permeability index (PI), residual sodium carbonate (RSC), magnesium adsorption ratio (MAR), Kelly’s ratio (KR) and chloro-alkaline Indices (CAI). US salinity laboratory diagram and Wilcox diagrams were also applied. The EC results show that the groundwater in the study area can be classified as none and slight to moderate. According to the US salinity diagram, groundwater in the study area falls within the low salinity-low sodium hazard and medium salinity-low sodium hazard class. The %Na and the resulting Wilcox diagram also classify the groundwater as excellent to good and good to permissible. The groundwater in the study area is generally good for irrigation purposes. However, there are few instances which are problematic and would require special irrigation methods.