Estimation of Radiological Effects and Modeling Radionuclide Transfer to Milk and Fish in Kisoro, South-western Uganda

Abstract

Exposure to natural radiation energy (ionozing radiation) is one of the primary causes of radiation ailments like cancer. Radioactivity is widely spread in the earth’s environment and it exists in various geological formations in soils, rocks, plants, water, air and in building materials. Uranium (238U), thorium (232Th) and potassium (40K) are Naturally Occurring Radionuclides (NORMs). Activity concentrations of 238U, 232Th, and 40K in pasture, milk, water and fish were determined using sodium iodide detector in order to estimate the radiological effects and model radionuclide transfer to milk and fish. The radiological effects that were determined were radium equivalent, annual effective dose equivalent, and excess lifetime cancer risk. Seventy nine (79) pasture samples were randomly collected from cattle farms in seven sub-counties where geological formation was due to volcanic eruption. The same was done for milk on the same farms. Milk samples were collected by manual milking technique by the farmer. A lactating cow from which milk was collected, was chosen at random in case there were more than one cows lactating on the same fame. Sixteen (16) fish samples randomly collected from landing sites of the four lava dammed lakes. All fish types were sampled during rainy and sunny days to cater for different variations of background radiation due to weather conditions. One hundred twenty one (121) water samples were collected from all the four larva dammed lakes by scooping using a jug. Water samples were collected along the lake shores since fish mainly go near the lake shores to find their food and water sampler was not needed since water was collected on the surface where temperature and depth are constant In milk and fish, average radium equivalent was 4.531 and 39.24 Bq l −1, the annual effective dose equivalent was 0.014 and 0.126 mSv y −1 and the excess lifetime cancer risk was 0.041 × 10−3 and 0.378 × 10−3, respectively. The quantity of milk to be consumed was estimated to be 0.105 litres per day (0.74 litres per week) in order to keep the radiological effects low. The radiological effects from consumption of fish were about 10 times higher than the safe values recommended by WHO and RPA hence, the safe consumption rate was estimated to be 0.02 kg (fresh weight) per day. Contribution to radium equivalent, annual effective dose equivalent, and excess lifetime cancer risk due to the radionuclide transfer from pasture to milk were estimated as 0.074 (Bq l−1), 0.002 (mSv y−1), and 6.1×10−6, respectively. These contributions of radiological effects account for about 1.5 % to the safe limit, hence causes a minimal radiation hazard. In fish, accumulation of 238U, from water to fish remained constant after five years, the transfer does not contribute to radiological effects. Therefore, for poverty reduction and improving food security, animal rearing and stocking of the lakes need to be encouraged. This study was limited by considering only milk and fish, and assuming equilibrium conditions of radionuclide transfer a similar study where the chemistry of the soils and water is considered, on foodstuffs like vegetables, beans, potatoes, maize, yams, bananas and drinking water, should be carried out.