The study area for this research is located between latitudes -5.694167° and -5.744167° and longitudes 34.796944° and 34.882500° in the Manyoni district of central Tanzania. The Manyoni district is home to the Manyoni Uranium Project, which contains up to 11,146 metric tonnes of uranium resources in surficial-type deposits [12]. The study area, which is also around the Manyoni Project area, was identified by dispersion modelling of anticipated mine dust using the American Meteorological Society-Environmental Protection Agency Regulatory Model (AERMOD), as detailed in Lolila et al. [13]. The modelling process took into account factors like weather, topography, and dust emission rates [13]. The sampling points chosen in the study area were determined by Lolila et al. [14] in two ways. In the first way, stratified sampling coupled with systematic sampling was used in a region expected to receive mine-dust particles with mean annual ground level concentrations (AGLC) of PM₁₀ greater than or equal to 10% of the World Health Organisation (WHO) guideline limit of 20 µg/m³ [15]. In this way, 32 sampling points labelled S1–S32 were allocated to Site A, as shown in Figure 1. The 32 sampling points were in two different geological formations: the terrestrial coarse clastic sediments, laterite, and alterite, which made up 12.34% of the study area, and the gneiss-granite-migmatite complex, which made up 87.66% of the study area [14,16]. In the second way, judgmental sampling was used in an area expected to receive dust particles with mean AGLC of PM₁₀ greater than or equal to the WHO guideline limit of 20 µg/m³ [15]. In this way, 8 sampling points labelled H1–H8 were allocated at Site B, as shown in Figure 2. The 8 sampling points were within the same gneiss-granite-migmatite complex geology [14,16]. In both sites, measurements of ambient dose equivalent rates were determined, as detailed in the next section.

Figure 1
A map showing 32 sampling points, S1–S32, chosen for sampling at Site A.

Source: Google Earth

A Gamma-Scout portable radiation survey meter was used to measure ambient gamma dose equivalent rates in the air at 1 metre above the ground at forty (40) predetermined sampling points, shown in Figures 1 and 2, around the Manyoni uranium deposit. The survey meter was programmed to display a radiation dose rate reading every 2 seconds. According to the manufacturer's specifications, the survey meter was calibrated to measure radiation levels ranging from 0.01 μSv/h to 5,000 μSv/h. To evaluate its performance, the survey meter was further calibrated against a ¹³⁷Cs standard source at the Tanzania Atomic Energy Commission's Secondary Standard Dosimetry Laboratory.

Figure 2
A map showing 8 sampling points, H1–H8, chosen for sampling at Site B.

Source: Google Earth

Before taking measurements of the ambient dose equivalent rates (in µSv/h) at a sampling point, the calibrated survey meter was held 1 m above the ground, turned on, and allowed to stabilise for 1 minute. After stabilisation, a dose rate at a sampling point was read and recorded every 5 seconds. To improve statistics, twenty (20) readings of the ambient dose equivalent rates were taken at a sampling point, five in each of the four cardinal directions (north, east, south, and west), and the average value was determined to represent the ambient dose equivalent rate (in µSv/h) at the sampling point. All measurements were taken outdoors during the day in February 2022. The measurement procedure explained above was performed for all 40 sampling points: 32 (S1–S32) selected in Site A, and 8 (H1–H8) selected in Site B (Figures 1 and 2).

The mean and ranges of the outdoor ambient dose equivalent rates measured at 1 m above the ground for the selected locations in Site A are presented in Table 1. The mean of ambient dose equivalent rates varied from 0.08 to 0.69 µSv/h with an overall average value of 0.25 ± 0.03 µSv/h. It can be noted that the mean ambient dose rate was the highest at location S8 (0.69 µSv/h) and the lowest at location S1 (0.08 µSv/h) (Table 1). To illustrate the influence of geology on the spatial distribution of ambient dose equivalent rates, the mean dose rates for each sampling point were color-coded and superimposed on the geological map of the study area, as shown in Figure 3. From Figure 3 it can be noted that the ambient dose equivalent rates varied spatially, with relatively high values (> 0.40 µSv/h) found at locations S8, S14, and S9 within the gneiss-granite-migmatite complex geology. The presence of radioactively rich granite in the gneiss-granite-migmatite complex geology may account for the observed high dose rates. In the same geology, a series with locations S15, S18, S19, S24, S28, S30, S32, and S31 had dose rates ranging from 0.20 to 0.30 µSv/h. In addition, another series with locations S2, S3, S4, S5, S11, S23, S22, and S10 had dose rates ranging from 0.10 to 0.20 µSv/h. Figure 3 also shows that location S1 in the terrestrial coarse clastic sediments, laterite, and alterite geology had the lowest dose rate (< 0.10 µSv/h). In the same geology as S1, a series with locations S7, S13, and S16 had dose rates ranging from 0.20 to 0.30 µSv/h.

Table 1
Ambient dose equivalent rates at Site A.

Figure 3
Ambient dose equivalent rates superimposed on the geological map of Site A.

The results obtained from measurements of the ambient dose equivalent rates, due to gamma radiation, at locations H1–H8 at Site B are presented in Table 2. The mean ambient dose equivalent rates were found to be lowest in location H5 at 0.12 µSv/h and highest in location H1 at 0.34 µSv/h, with an overall average value of 0.23 ± 0.02 µSv/h. This average value is comparable to that found at Site A, which was 0.25 ± 0.03 µSv/h.

The spatial distribution of ambient dose equivalent rates at Site B was also visualised by color-coding the dose rates on a map. Figure 4 shows that at Site B, which has the same gneiss-granite-migmatite complex geology, the ambient dose rates were spatially variable. Locations H1 and H2 had relatively high dose rates (> 0.27 µSv/h) , while H5 had the lowest dose rate (< 0.17 µSv/h) (Figure 4).

Table 2
Ambient dose equivalent rates at Site B.

Figure 4
Distribution of ambient dose equivalent rates at Site B.

Table 3 compares the ambient dose equivalent rates found in the vicinity of the Manyoni uranium deposit (this study) to those found in studies carried out in other countries. It is clear that Manyoni's average dose rates for sites A and B were comparable to the average dose rate that [8] discovered in southwestern Nigeria. Some similarities between the geology of southwestern Nigeria, which ranges from sandstones and clayey gravels to quartz and migmatite gneiss complexes [8], and the geology of Manyoni, which ranges from gneiss-granite-migmatite complex to coarse clastic sediments, laterite, and alterite, could be responsible for the observed similarity in average dose rates. On the other hand, the Manyoni's average dose rates for sites A and B were higher than those found in Najaf City (Iraq) [9], Metro Manila (Philippines) [10], and Iran [11]. The observed variations in average dose rates could be explained by geological differences between the study areas; however, those studies did not report comprehensive geological formations of their study areas.

Table 3
Comparison of ambient dose equivalent rates from different countries.

*faridalolila855@gmail.com