Assessment of Public Exposure to Naturally Occurring Radioactive Materials from Mining and Mineral Processing Activities of Tarkwa Goldmine in Ghana

Mining has been identified as one of the potential sources of exposure to naturally occurring radioactive materials (NORM). However, mining companies are not being regulated for NORM in Ghana. Whilst the developed countries have identified NORM as potential problems and measures are being taken to address the issues, very little is being done in the developing countries. However, most of the NORM industries such as mining and mineral processing are located in developing countries such as Ghana. Currently, there are over two hundred (200) registered mining companies operating small, medium and large scale mining in Ghana. Tarkwa Goldmine is one of the largest gold mining companies in Ghana and has been in operation for the past 200 years with no data on radioactivity levels. The mine currently undertakes only surface mining and the process produces large volumes of tailings and waste that may contain NORM. Some of the NORM are soluble in water and have the tendency to leach into water bodies and farm lands. These studies have been carried out to determine the exposure of the public to NORM from processing of gold ore at the Tarkwa Goldmine in Ghana. Direct gamma spectrometry and neutron activation analysis (NAA) techniques were used to analyse for U/Th series and K-40 in soil, rock, water, food and particulate (dust) samples from the mining environment. The mean activity concentrations measured for 238U, 232Th and 40K in the soil/rock samples were 15.2 Bq/kg, 26.9 Bq/kg and 157.1 Bq/kg respectively. For the water samples the mean activity concentrations were 0.54 Bq/L, 0.41 Bq/L and 7.76 Bq/L for 226Ra, 232Th and 40K respectively. The mean activity concentrations of 226Ra, 232Th and 40K in the food samples were 0.18, 0.14 and 45.00 Bq/kg respectively. The mean activity concentrations measured in the dust samples were 4.90 and 2.75 µBq/m3 for 238U and 232Th respectively. The total annual effective dose to the public was estimated to be 0.74 mSv. The results in this study compared well with typical world average values. The results indicate an insignificant exposure of the public to technologically enhanced NORMS from the activities of the Goldmine. The radiological hazard due to226Ra, 232Th and 40K were carried out. The radium equivalent activity (Raeq) and the calculated external and internal hazard indices, the absorbed dose rates and the corresponding annual effective dose were estimated in the soil and rock materials that might possibly be used as building materials. The results obtained in this study shows insignificant radiological hazards for the materials considered for use as construction materials for dwellings by the inhabitants in the study area. The results obtained in this study also shows that the background radiation levels are within the natural limits and compared well with similar studies for other countries. The study assessed the concentration of U, Th and K as well as other trace metals, anions and the physical parameters in water and soil samples in the goldmine and its surrounding areas. The mean concentrations of the U, Th and K were 0.020, 0.029 and 1.19 mg/L. The concentration of U, Th and K were variable in soil and rock samples taken from different locations in the study area with mean values varying in a range of 0.2 to 1.8 µg/g, 0.9 to 2.6 µg/g and 7037 to 71360 µg/g respectively. The concentrations of U, Th and K are comparable to world average values of similar studies. The calculated Th/U ratios show that there has not been significant fractionation during weathering of the radioelements with a mean value of 2.5. The concentrations of the other trace metals, anions and the physical parameters are within the WHO guideline levels in drinking water. The mean values of the gross-α and gross-β activity concentrations were 0.012 and 0.137 Bq/L which are also below the WHO recommended guideline values for drinking water.

Source: KNUST