Environmental radioactivity of groundwater and sediments in the Kuiseb and Okavango-Omatako basins in Namibia

Naturally occurring radionuclides of terrestial origin are ubiquitously present in the environment including groundwater and sediments and their concentrations vary depending on the geology and hence the type of soil. The objective of this study was to evaluate the radioactivity levels and health risk due to radionuclides in ground water and sediments samples collected from Kuiseb and Okavango-Omatako river basins in Namibia. The activity concentrations of radionuclides in groundwater and sediments were measured by a high purity germanium detector and PM1406 Food monitor. The activity concentrations of 238U, 232Th and 40K in sediment samples varied from 7 ± 0.001 Bq.kg−1 to 46 ± 0.001 Bq.kg−1, 8 ± 0.001 Bq.kg−1 to 70 ± 0.002 Bq.kg−1 and 345 ± 0.010 Bq.kg−1 to 776 ± 0.026 Bq.kg−1 respectively. To assess the radiological hazard of radioactivity present in the sediment samples, the absorbed dose (D), the annual effective dose equivalent (AEDE), Radium equivalent (Raeq) and external hazards (Hex) were calculated and these varied from 25.19 to 82.55 nGyh−1, 0.03 to 0.10 mSvy−1, 51.30 to 181.21 Bqkg−1 and 0.14 to 0.49 respectively. Similarly, the mass elemental concentration in all sediment samples collected from the two water basins ranged from 1 ± 0.001 to 4 ± 0.001 mg kg−1 for 238U, 2 ± 0.003 to 17 ± 0.002 mg kg−1 for 232Th and 1 ± 0.001 to 3 ± 0.026% for 40K while the activity concentrations in groundwater samples of 40K were below the detectable limits (BDL) for the two water basins. The Raeq values for sediment samples were lower than 370 Bqkg−1 which is equivalent to a gamma dose rate of 1.0 mSvy−1 - the dose prescribed by International Commission on Radiological Protection (ICRP) for members of the public and thus the use of sediments for construction purposes does not constitute any radiological health hazard to the general population in these areas. •The activity concentrations of 238U, 232Th and 40K lies within the safe limits.•The radiological indices for the studied samples are within the safe limits.•The risk due to radiation contamination in sediments and water samples is low.