Establishment of baseline data for radioactivity and heavy metals to support best practice in uranium mining at Mkuju river basin

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University of Dar es Salaam
From past experience, today uranium mining requires precautions guided by regulations to minimize radioactivity and heavy metal pollution associated with the process. Verification of for compliance by the mining operation requires pollution levels above the baseline to be below specified standards. To achieve this, the baseline as distribution of natural background of quantities sensitive to pollution in the vicinity of the proposed uranium mining project had to be established. The AERMOD model used parameters associated with the source, meteorological, topographic as inputs to predict a study area of 1300 km2. To produce a baseline representative of such vast area required 1.3 x 105 sampling points. Due to absence of methods to reduce sampling points to manageable amount without affecting the quality of baseline data, one of the research tasks was to develop a method. This was done by using ambient dose rates to cluster areas with similar dose rates to form sampling blocks. The approach produced 42 blocks from which one composite sample was drawn. Radioactivity in of soil in the 42 locations was determined using HPGe detector, radioactivity in water using alpha spectrometer and heavy metals in soil and water were determined using EDXRF and ICPMS, respectively. All measurements were made during the dry season between May and September. Radioactivity concentrations (Bqkg-1) of elements intimately related to uranium ore deposits obtained from 42 locations which varied from minimum of 24.94 and maximum of 53.50 for 226Ra, and 20.86 to 47.14 for 232Th, could be used as baseline data. The radioactivity concentrations measured in water was used to establish concentration ratios of 234U/238U known to be sensitive to pollution and therefore can serve as base line data in the study area. The ratios were observed to vary from 1.07 to 1.37 for surface water and from 1 to 1.16 for ground water. Concentrations (mgkg-1) of As (1.31 ± 0.98) which was below the detection limit could be used as baseline data since their future detection would be interpreted as pollution. Pb and Cu with low concentrations of 2.2 to 39.1 and 3.2 to 15.4 in the top soil (0-15cm) and from 2.6 to 45.2 and 1.8 to 25.1 in the bottom soil (15-30 cm) respectively. Since these metals are trapped on the top soil, the ratio between the two soil layers is sensitive to atmospheric pollution and therefore the distribution of ratios in the study area could be used as baseline data. In conclusion, quantities found to be sensitive to pollution in the entire area and therefore could serve as baseline data were: radioactivity concentration in soil, ratio of 234U/238U in water, heavy metal concentrations of As and distribution of the ratios of Cu and Pb in the two layers of soil. However, we recommend that since the effects of seasons on the baseline data was not tested, further studies are necessary in order to gain full understanding of the influence of weather parameters on the baseline data and its replication during the dry season. However, these activities should be conducted before uranium mining commences.
Available in print form, East Africana Collection, Dr. Wilbert Chagula Library, Class mark (THS EAF QC795.32.S3B36)
Radioactivity, Heavy metals, Uranium mines and mining, Tanzania
Banzi, F. P. (2017) Establishment of baseline data for radioactivity and heavy metals to support best practice in uranium mining at Mkuju river basin, Doctoral dissertation, University of Dar es Salaam, Dar es Salaam.