Mapping of NORMs and Heavy Metals in Central District (Botswana) : Evaluation of Long-term Impact of Mining Activities on Water Quality of Letsibogo Dam

Abstract

The study was carried out in the central district of Botswana around Letsibogo dam and the surrounding communities of the new uranium mine. Mining has been identified as one of the main potential sources of exposure to naturally occurring radioactive materials (NORM) and heavy metals. All human activities within this area may lead to the rise of anthropogenic pollutants. NORMs and heavy metals in environmental samples are of crucial importance in the case of radiological impact studies in any environmental compartment. For this reason, valuable information is needed for the determination of NORMs and heavy metals in environmental samples to ascertain the level of natural and/or man-made radioactivity from a particular area and this requires accurate measurement techniques. The study was conducted with the aim of providing the baseline information on natural radioactivity and heavy metals in the central district of Botswana by evaluation of the long-term impact of mining activities on the water quality of Letsibogo dam and generate baseline data on environmental parameters that might affect radiological and toxicological health-related issues towards humans. In this investigation, identification and quantification of natural radioactivity and heavy metals in environmental samples were evaluated through the use of gamma spectrometry for the identification of the most likely nuclides that contribute to the activity of NORM-nuclides in environmental soil, sediments and vegetable samples, ICP-MS for the identification of heavy metals in all environmental samples and LSC for the quantitative determination of the gross ?- and ?-activities in water samples. The results were used to map the current level of NORMs and heavy metals in the study area. Some evaluations are made to the potential radiological and chemical hazard that the NORM-nuclides and heavy metals may impose on people living in the study area. The calculated absorbed dose values (DR) in soil samples ranged from 23.5 ± 1.2 to 103 ± 6 nGy/h, with an average value of 62.3 ± 2.41 nGy/h, which is comparable to the worldwide average value of 59 nGy/h. The radium equivalent (Raeq) calculated varied from 41.0 ± 2.1 to 224 ± 11 Bq/kg with an average value of 134 ± 5 Bq/kg, which is well below the permissible limit of 370 Bq/kg. The average values of external hazard index (Hex) and the annual effective dose equivalent (AEDE) for soil samples were found to be 0.360 ± 0.014 and 0.080 ± 0.003 mSv/y respectively, which are both below the permissible limit of 1 mSv/y. For foodstuff, the results revealed that the levels of radioactivity in almost food samples are insignificant and will not pose any radiological hazard from consumption except for ²³²Th and ⁴⁰K which indicated elevated values in vegetable samples that are above the world average value of 290 μSv/y. The cancer risk for people living in the study area, as a result of heavy metal in soil, water, foodstuff and dust was also evaluated. For non-carcinogenic risk, the HI values were found to be 1.5, 27.5 and 1.5 for As, Cr and Cu respectively. These values are greater than 1, which indicate a potential health risk of As, Cr and Cu to the residents of the study area. The ingestion pathway was the greatest contributor to non-carcinogenic risk with an HI value of 27.5 driven by Cr in food samples. For carcinogenic risk, the ingestion pathway was found to be the greatest contributor. Cr was observed to be the major contributor to the risk with a total cancer risk value of 4.1 x 10⁻², which is greater than the maximum permissible limit of 1 x 10⁻⁴, indicating a potentially large carcinogenic risk. The gross-α and gross-β activity concentrations in the water samples were also evaluated. The results of this study show that all values of the gross-α and gross-β activity are below the WHO recommended guideline values of 0.5 and 1 Bq/l respectively. The results of this study will be used as a baseline for the surveillance of any possible change in the future.