Assessment of radioactivity levels and radiological risks of chemical fertilizers in Kampala City, Uganda

Abstract

The agricultural sector relies heavily on fertilizers to boost crop productivity, driving global growth in the fertilizer industry. Phosphate fertilizers, used worldwide at a rate exceeding 30 million tons annually, enhance yields, and restore soil. In Uganda, agricultural households using fertilizers apply an average of 46.9 kg/acre in the first season and 36.3 kg/acre in the second season, with urea and diammonium phosphate (DAP) dominating the market while NPK and calcium ammonium nitrate (CAN) are actively traded. Although essential for agricultural productivity and food security, chemical fertilizers may contain radioactive elements like thorium, uranium, and their decay products, which may accumulate in soil, crops, and the food chain, posing health risks. This study assessed radioactivity levels and radiological risks of chemical fertilizers available in Kampala, Uganda, focusing on their impact on farmers, workers, and the public. Samples were analyzed using gamma spectrometry (NaI detector, MAESTRO software). High activity levels of Ra-226, Th-232, and K-40 were found in some DAP and NPK fertilizers. The highest mean levels were 67.02 Bq/kg (Ra-226) and 28.18 Bq/kg (Th-232) in DAP, and 589.26 Bq/kg (K-40) in NPK, with Ra-226 and K-40 exceeding global limits. The absorbed dose remained below the global limit of 59 nGy/h for each fertilizer brand, but the outdoor annual effective dose (AED) in some cases exceeded the limit of 0.07 mSv/y and varied from 0.01 to 0.09 mSv/y. The mean radium equivalent activity was below 370 Bq/kg for all the fertilizer brands, with internal and external hazard indices under unity. A strong correlation was observed between radiological parameters, with some fertilizers showing slightly elevated radionuclide levels and radiological risks. DAP with high Ra-226 and NPK with elevated K-40 require careful handling to minimize radiation exposure for farm workers, employees, and the public. A high-resolution detector, such as a high-purity germanium detector, is recommended for detecting low-activity radionuclides from fertilizers and for a more precise spectral analysis.