The Kamukuywa River’s coordinates are around 34.97˚ West and 0.7781˚ North. The Kamukuywa River basin in Western Kenya has the following coordinates: latitudes 1˚30'N and 0˚30'S, longitudes 34˚00'E and 35˚45'E. The 120-mile-long Kamukywa River starts in the Mt. Elgon hills before emptying into Nzoia River. The Kamukywa River is formed of intrusive and volcanic rocks. The majority of the River is made up of sedimentary rocks, with limestone, weathering rocks and rare granite intrusion formations adding to its character (See Figure 1Figure 1).

A total of fifteen (15) sediment spots were chosen at random along the river Kamukuywa during the rainy season. At each sampling point, a 1 × 1 m plot with a depth of 1 m was set up, with GPS coordinates recorded. Figure 1Figure 1 shows the locations of the sampling sites. One sample was collected at each point along the River Kamukuywa. Each sample, weighing between 0.5 and 1.0 kg, was collected at different points along the river. Following careful mixing, each of these samples was packed in polythene bags and transported to South Eastern University’s physics laboratory for additional analysis.

To achieve a constant weight, the fifteen (15) samples were air dried for two weeks before being oven-dried for eight hours at 80˚C. To ensure consistency, dry samples were crushed, ground, and sieved to a particle size of 150 - 200 µm. To ensure secular equilibrium between ²²⁶Ra and ²³²Th and their daughter nuclides for further laboratory analysis, 500 g with a density of 780 kg/m³ of the homogenized samples were weighed on a weighing balance and stored in a Marinelli container for 30 days [30]. Additionally, an empty Marinelli container filled with deionized water was utilized to assess the lab’s background gamma radiation levels under comparable environmental conditions. The activity concentration was then determined by subtracting the resulting background value. In addition to being evaluated alongside the samples, Certified Reference Materials IAEA-447 was used to ensure the correctness of the gamma spectroscopy for the studies.

Figure 1. Map of the study area.

The levels of ²²⁶Ra, ²³²Th and ⁴⁰K natural radioactivity were measured using a Gamma-ray Spectrometer equipped with a NaI(Tl) detector. Each sediment sample was placed on a radiation detector and counted for eight to twelve hours after achieving equilibrium. A lead shield and a support table assembly support the spectrometer, which is enclosed within a cylindrical multilayer graded shield. It was operated at a high voltage of 3500 keV, with a 75% relative efficiency and an energy resolution full width at half maximum (FWHM) of 1.91 keV at 1332.5 keV for Co-60. Spectra Line-GP software was used to process the collected data. A Mixed Nuclide BB-3591 gamma reference calibration source operating in the 60 - 3500 keV range was utilized to calibrate energy and efficiency. Elements from this source included Cs-137, Am-241, Co-60, Y-88, Co-57, Sn-113, and Ce-139. The activity concentration of ²³²Th was calculated using the mean gamma-ray peak energies of ²¹²Pb (238.6 keV) and ²²⁸Ac (911.1 keV), while the activity concentration of ²³⁸U was calculated using the peak energies of ²¹⁴Pb (351.9 keV) and ²¹⁴Bi (609.3 keV). The energy of the 1460.8 keV spectral line was used to calculate the activity concentration of ⁴⁰K [8].

Activity concentration Calculations of number of counts per second for the photo peak and activity concentrations of each detected radionuclides was based on the concept secular equilibrium being. The activity concentration in Bq·kg⁻¹ (A) in the samples was obtained by using Equation (1) [10]

where, N s is net counts per second (CPS) = (sample CPS – background CPS), ε is the abundance of the γ-line in a radionuclide, η is the measured efficiency for each gamma-line observed and the mass of the sample in kilograms is denoted by m.

Regardless of the temperature and pressure in situ, the RHP is a scalar petro physical property. According to [22], ²³²Th and ²³⁸U are the two radionuclides that contribute the most to heat production (around 85% each), while ⁴⁰K contributes less. Daughter radionuclides present in the rocks of the Earth’s crust also create RHP; the most geologically significant decay chains in terms of heat production are those of ²³⁸U and ²³²Th. Equation (2) will be used to convert the activity concentrations of the different samples into parts per million (ppm) in order to compute the radiogenic heat production [22].

The heat generation constant (i.e., the quantity of heat released per unit time and per gram of ²³⁸U, 2³²Th and ⁴⁰K) and the concentrations of uranium, thorium, and potassium (C_U, C_Th, and C_K, respectively) in a sediment will be taken into consideration when calculating the radiogenic heat production of sediments (RHP in μW·m⁻³) using Equation (3) [19].

where ρ is the density of the sediment sample (kg·m⁻³) and C_U, C_Th, and C_K are the concentrations of uranium (weight ppm), thorium (weight ppm), and potassium (weight %), respectively.

A number of radiological risk indices were calculated in order to completely comprehend the radiation hazard posed by ²²⁶Ra, ²³²Th, and ⁴⁰K in the sediments from River Kamukuywa. These indices were used to assess the level of risk that naturally occurring radioactive nuclides posed to miners and the public at large.

3.3.1. Radium Equivalent (Ra_eq)

When determining how natural ionizing radiation from rivers sediments affects radiological health, it is essential to consider the radium equivalent. This guarantees uniformity in the evaluation of radiation exposure and helps to account for the irregular distribution of naturally occurring radionuclides in samples. With a suggested limit of 370 Bq·kg⁻¹, the radium equivalent is measured in Bq·kg⁻¹. Equation (4) was used to determine the Ra_eq [20].

where A_Ra, A_Th and A_K represent ²²⁶Ra, ²³²Th and ⁴⁰K specific activity concentration.

3.3.2. External Hazard Index (H_ex) and Internal Hazard Index (H_in)

The values of H_ex and H_in were determined using Equations (5) and (6), respectively, to evaluate the external and internal exposure to radiation from ²²⁶Ra, ²³²Th and ⁴⁰K in the samples under examination [31].

where A_Ra, A_Th and A_K represent the activity concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K in the samples respectively. For the radiation hazards to be considered negligible, the values of H_ex and H_in must be ≤1, which aligns with the dose equivalent limit of 1 mSv/yr (Mbonu and Ben, 2021).

3.3.3. Absorbed Dose Rate (D)

The absorbed dose rate is the total amount of energy that ionizing radiation deposits per unit mass between two extremes. The specific activity concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K in the samples were measured in order to determine the absorbed dose rate from terrestrial gamma radiation sources [9][24]. Equation (7) was used to determine absorbed dose rate.

where; A_Th, A_Ra and A_K are the activity concentrations of ²³²Th, ²²⁶Ra and ⁴⁰K, respectively. The global average limit for absorbed dose rate is 60 nGy·h⁻¹ [32].

3.3.4. Annual Effective Dose (AED)

The yearly effective dose equivalent to the population due to radioactivity in rocks, soils and plants will be calculated using a conversion factor of 0.7 Sv/Gy [33]. For the Kenyan scenario, adult indoor and outdoor dose occupancy factors of 0.4 and 0.6 will be evaluated, respectively. AED for both indoor and outdoor will be computed using Equations (8) and (9), respectively [8].

where D represents the absorbed dose rate in air in nGy/h, T is 8760 signifies the total hours in a year, 0.7 SvGy⁻¹ is the dose conversion factor and the factor 10⁻⁶ converts nano scale to mill scale. The AEDE recommended average limit is 1 mSv·y⁻¹.

3.3.5. Annual Gonadal Equivalent Dose (AGED)

Equation (10) was used to evaluate the Annual Gonadal Equivalent Dose (AGED) sand harvesters and the general population obtained [34]

where C_U, C_Th, and C_K represent the activity concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K, respectively (Wanyama et al., 2020). 300 µSv·y⁻¹ is the AGED suggested limit.

3.3.6. Excess Lifetime Cancer Risk (ELCR)

Excess Lifetime Cancer Risk (ELCR) is the term used to describe the potential for radiation exposure to cause cancer in the human body when exposure beyond a certain threshold within a certain time frame. Equation (11) was used to calculate the ELCR in order to evaluate the cancer risk among mining workers [15]. This computation is used to determine the probability that a person will get cancer as a result of radiation exposure during their lifetime.

where RF is the risk factor, DL is the mean length of life (70 years) representing the fatal cancer risks per Sievert, RF denotes the risk factor of 0.05 Sv⁻¹ and AEDE refers to the Annual Effective Dose Equivalent [30].

Table 1 and Figure 2Figure 2 display the findings of the radioactivity analysis conducted on sediment samples from River Kamukuywa, Bungoma County. The average activity for ²³⁸U, ²³²Th and ⁴⁰K was 31 ± 2, 51 ± 3, and 57 ± 3 Bq·kg⁻¹, respectively. The activity concentrations varied from 19 ± 1 to 48 ± 2, 17 ± 1 to 89 ± 4, and 35 ± 2 to 90 ± 5 Bq·kg⁻¹. With the exception of ²³²Th, which surpassed the UNSCEAR, 2000, published average of 45 Bq·kg⁻¹, the values were below the global averages of 32 Bq·kg⁻¹ and 400 Bq·kg⁻¹ for ²³⁸U and ⁴⁰K respectively. The highest activity concentrations for ²³²Th (89 ± 4 Bq·kg⁻¹), ²³⁸U (48 ± 2 Bq·kg⁻¹) and ⁴⁰K (90 ± 5 Bq·kg⁻¹) were found in sample 9 and 15 respectively. The region’s geology, which is linked to granite rocks, is responsible for this high activity. The ²³⁸U activity concentrations in about 95% of the sediments were lower than the global mean of 35 Bq·kg⁻¹. The activity concentrations of ⁴⁰K in all the sediment samples collected were below the global average of 400 Bq·kg⁻¹ [35]. Generally, the variability is dependent on local weather circumstances, soil characteristics, geological composition, and geographic considerations.

Table 1. Activity concentration, radioelement concentration and radiogenic heat in the collected samples.

Figure 2. The activity concentration of radionuclides in sediment samples.

A mix of natural geological elements and human activity (excavation and quarrying of rocks) may be responsible for the elevated levels of ²³²Th radionuclide activity found in sediment samples (Table 1). Due to their geological makeup, some areas naturally have larger concentrations of radioactive materials. Radionuclides may be mobilized and redistributed in water bodies as a result of natural processes like weathering and erosion. Higher levels of pollution may arise from the buildup of radioactive elements in rivers.

It should be noted that determining the precise causes of the elevated radionuclide levels in the area would require a comprehensive scientific examination. A thorough investigation would include a thorough examination of the area geology, soil composition, past land usage near the river, and possible radioactive material sources.

The three radionuclides’ activity concentrations (²³⁸U, ²³²Th and ⁴⁰K) revealed a pattern of regional distribution. The radiation levels of ⁴⁰K, ²³²Th and ²³⁸U varied from 11% ± 1% to 28%, 4% ± 1% to 21% ± 1%, and 1.5 ppm to 3.9 ppm, respectively (See Figure 3Figure 3).

Figure 3. Radioelement concentrations in the collected samples.

The three radionuclides had average concentrations of 2.5 ppm, 12 ± 1 ppm and 18% ± 1% for ²³⁸U, ²³²Th and ⁴⁰K, respectively (Table 1), which were higher than the global average (UNSCEAR, 2000). The greatest difference from the global average was found in ²³²Th and ⁴⁰K. The use of phosphate fertilizers for higher yields and other anthropogenic activities, particularly in agriculture, may be the cause of the higher activity concentrations. Figure 4Figure 4 displays the individual radioelement radiogenic production from the collected samples.

From Figure 4Figure 4, ²³⁸U, ²³²Th and ⁴⁰K corresponding individual RHP values were 0.001 µW/m³ to 0.002 µW/m³, 0.003 to 0.016 µW/m³, and 0.008 µW/m³ to 0.021 µW/m³, respectively. For ²³⁸U, ²³²Th and ⁴⁰K, the average radiogenic heat output was 0.001 µW/m³, 0.009 µW/m³ and 0.013 µW/m³, respectively. The average RHP for the three radionuclides among all samples gathered is displayed in Figure 5Figure 5.

Figure 4. Radiogenic heat production in the collected samples.

Figure 5. Average radiogenic heat production in the samples.

The total radiogenic heat ranged from 0.056 µW/m³ to 0.14 µW/m³ (Figure 5Figure 5). All the three radionuclides combined provided an average RHP of 0.08 µW/m³. This study’s findings are less than those of [20], which found an average of 0.540764 µW/m³. Additionally, the results differ from those of Ahero,s rice field sediments [22], who found an average RHP of 4.5 ± 1.1 µW/m³. The estimated average radiogenic heat production rate was 0.3634 µW/m³ that ranged from 0.2430 to 0.6453 µW/m³ from river Himalayan [36]. Given the varied geological configurations of the study regions and the kinds of rocks that the soils were formed from, the discrepancy in the results can be explained. Table 2 compares the activity concentrations in River Kamukuywa sediments with data from various countries across the world. The activity concentrations obtained from this study were lower than those obtained from Himalayan river [36]. It is evident that the activity concentrations found in previous research conducted globally are comparable to the findings from the study area.

Table 2. Comparative results for activity concentration levels of radionuclides in various regions.

Table 3. Summary of the correlation of activity concentration and radiological hazards.

The relationship between naturally occurring radionuclides and radiological features in sediment samples was investigated using a correlation matrix. Table 3 displays the corresponding relationships between naturally occurring radionuclides and radiological properties. According to the findings, there is a stronger association between the chosen pairings when the positive correlation coefficient is large. This implies that each of the three identified radionuclides contributed significantly to radiation exposure. The activity concentrations of uranium, potassium, and thorium mostly affect the changes of the radiological dangers, especially D, AED, H_in, H_ex, AGED, ELCR, and Ra_eq [39].

The mean values of the radiological hazards specifically Ra_eq, H_ex, H_in, D, AEDE, AGED, and ELCR parameters are displayed in Table 4. Assessing the activity levels of ²²⁶Ra, ²³²Th and ⁴⁰K is the traditional way of determining the radiological danger related to natural radioactivity in sediments. Ra_eq must be less than 370 Bq·kg⁻¹ in order for sediments or sand to be deemed safe. The average Ra_eq value was 109 ± 5 Bq·kg⁻¹, with a range of 55 ± 3 Bq·kg⁻¹ to 173 ± 9 Bq·kg⁻¹. Interestingly, every sediment sample was below the global average of 370 Bq·kg⁻¹ [40]. The external and internal hazard indices should be kept below a threshold limit of one in order to protect human health from radiation hazards (Figure 6Figure 6).

Figure 6. Hazard values calculated from the measured samples.

The H_ex values ranged from 0.20 to 0.60 (Figure 6Figure 6), with an average of 0.30, while the H_in values ranged from 0.10 to 0.40 (Figure 6Figure 6), with an average of 0.40. All these values fall below the global threshold value of 1 [1].

With a range of 24 ± 1 to 76 ± 4 nGy·h⁻¹, the average absorbed dose rate was 47 ± 2 nGy·h⁻¹, which was less than the average limit of 60 nGy·h⁻¹ [35]. Every sediment sample examined for AEDE had AED_in values ranging from 0.09 to 0.28 mSv·y⁻¹ (Figure 7Figure 7), with an average of 0.18 mSv·y⁻¹ (Table 4).

From Figure 7Figure 7, AED_out values ranged from 0.06 to 0.19 mSv·y⁻¹. The average AED_out was 0.12 mSv·y⁻¹ (Table 4). The predicted doses should not surpass 1 mSv·y⁻¹ for the general population and 100 mSv·y⁻¹ for occupational workers, according to statistics from the (ICRP, 2007) report. There is little chance of radon-related health risks in the area. With a mean value of 330 ± 17 μSv·y⁻¹, the values for the (AGED, μSv·y⁻¹) ranged from 170 ± 9 μSv·y⁻¹ to 523 ± 26 μSv·y⁻¹, exceeding the global average of 300 μSv·y⁻¹. Increased AGED values indicate higher amounts of gamma radiation exposure, which could be brought on by nearby geology, substantial industrial or man-made activities and natural background radiation.

Figure 7. Annual effective dose from the collected samples.

Table 4. Radiological hazards from the sediment samples.

It is clear from the calculated radiological parameters that every sediment sample hazard was below the recommended average limit. The mean value of the estimated excess lifetime cancer risk (ELCR) for outdoor exposure was 0.50 × 10⁻⁴, with a range of 0.20 × 10⁻⁴ to 0.80 × 10⁻⁴ mSv·y⁻¹ (Table 4). This value is below the threshold limit when compared to the global average of 0.2900 × 10⁻³ mSv·y⁻¹, which is regarded as the acceptable level [1]. These results imply that ionizing radiation exposure in the investigated area is not linked to a high risk of cancer.