Hydrogeochemical Characterization of the Rocks of the Accra Plains for a Radioactive Waste Repository

Abstract

The geological disposal of radioactive waste in well-engineered repositories in stable geological formations has been internationally accepted as a suitable option. The disposal system makes use of both man-made and natural barriers to contain and isolation the radionuclides. Precambrian crystalline rocks are regarded as one of the favourable host rocks for a radioactive waste repository. Hydrogeochemical characterization of the rocks of the Accra Plains, located in the south eastern part of Ghana underlain by crystalline gneisses and schists, has been carried out to determine its suitability for a radioactive waste repository. The mineralogical analyses of the rock and soil samples collected indicated that feldspars, hornblende, biotite, quartz with minor pyroxenes are the primary minerals with illite, kaolinite, montmorillonite and carbonates occurring as secondary minerals. The accessory minerals present include chlorite, epidote, magnetite, rutile and sphene. The iron rich minerals, magnetite, biotite, chlorite, and amphiboles will create a reducing environment required for a repository. The pH value of the groundwater at the Valley View University is lower than the recommended pH values (6-10) for a repository environment, making the site unsuitable for a radioactive waste repository. Calcium and magnesium ions which constitute 48% of the total cations in the groundwater will provide a stable chemical environment for a backfill and buffer materials. The groundwaters of the Accra Plains is composed of three (3) water types. Groundwater Type 1 is composed of Na-Mg-Ca-Cl-HCO3 and found along the foot of the Akwapim-Togo Mountains. Groundwater Type 2 is composed of Na-Mg-Ca-Cl and found in most parts of the central area of the Plains whilst groundwater type 3 occur along the coastal zone of the Plains and composed of Na-Ca-Cl. Silicate mineral weathering is the predominant influence on the composition of the groundwaters. Evaporative enrichment of recharging waters, ion exchange and reverse ion exchange also controls the chemical composition of the groundwater especially along the coastal areas of the Plain. The stability diagram shows that the chemistry of the groundwater favours the formation of kaolinite and montmorillonite. The long time stability of these minerals together with their ion exchange properties which can retard the migration of long-lived radionuclides makes them a useful backfill and or buffer material for a radioactive waste repository. The groundwater had an average isotopic value of –3.19‰ for δ18O and –14.36‰ for δ2H. The groundwaters at Valley View University are more depleted than the other groundwaters in the Plains suggesting that the groundwater was recharge at a high elevation. The Akwapim-Togo Mountains would be the source of these groundwaters, implying there are preferential channels or routes through which waters that are recharged on the Akwapim-Togo Mountains find their way to the university campus. This makes that area unsuitable for a waste repository. The activity concentration of 40K was found to be higher than the other naturally occurring radionuclides which can be attributed to the abundance of K-feldspar minerals in the geological formation. The highest radionuclide concentrations were measured in the rock samples collected from GAEC premises whereas the Krobo Mountains samples had the lowest radionuclide concentrations. The relative U enrichment in the GAEC samples suggests reducing condition and precipitation of UO2. The Ashiaman samples reflected high Th concentration indicating preferential removal of uranium by leaching through groundwater infiltration. The area between the Akwapim Togo and the Valley View University was found to be unsuitable for a radioactive waste repository due to preferential channels which will easily saturate the repository. Secondly the pH of the groundwaters were lower than the required pH conditions for a radioactive waste repository The presence of redox-sensitive minerals in the mafic gneisses which will ensure a reducing environment for a radioactive repository makes the hydrogeochemical conditions of the geological formation at Okwenya and the foothills of the Krobo Mountains are favourable for a repository. The presence of montmorillonite clay minerals in these areas will give the repository with a good buffer material which will be stable for a long period of time and also retard the migration of the long live radionuclides.