Lumoro dumpsite is geographically located at latitude 0.3360 N and longitude 34.3300 E, within Muanda village, South Bukusu Ward, Bumula Sub-County, Bungoma County, Kenya. The surrounding village has a total population of 14,320, with approximately 257 households situated near the dumpsite [9], which serves as a primary solid waste disposal site for Bungoma town and nearby settlements. The area is characterized by a tropical climate with bimodal rainfall, influencing soil composition and leaching processes. Waste deposited at the site includes domestic refuse, agricultural residues, ash, and construction debris, many of which are potential sources of naturally occurring radioactive materials (NORMs). The proximity of residential areas and daily human activity within the dumpsite makes it a relevant site for assessing possible radiological risks. (See Figure 1Figure 1)

Soil sampling was conducted at 15 administrative locations within the study area. A random sampling technique was employed to enhance the statistical representativeness of the samples [10]. At each location, three soil samples were collected using a trowel at depths ranging from 10 to 50 cm. The sampling depth of 10 - 50 cm was adopted following IAEA [11] and UNSCEAR (2000) guidelines, and similar approaches by Xinwei et al. [12] and Taskin et al. [13], to minimize surface contamination and capture stable radionuclide concentrations representative of undisturbed soil layers. The 0 - 10 cm surface soil, though contributing more to external exposure, was excluded to avoid variability arising from anthropogenic and environmental disturbances near the Lumoru dumpsite. The samples from each site were combined to form a single composite sample weighing approximately 700 g. In the laboratory, the soil samples were initially air-dried on plastic sheets at room temperature for one week, followed by oven drying at 110˚C for 8 hours. Stones and organic matter were removed, and the dried soil was pulverized to a particle size appropriate for gamma spectroscopic analysis. The soil was then sieved through a 2 µm mesh to ensure homogeneity [10]. Finally, the homogenized soil was treated with concentrated hydrochloric acid and allowed to sit for at least one month to achieve secular equilibrium between radon and its progeny prior to gamma spectroscopic measurement [14].

Figure 1. Study area. (Source: the authors)

The activity concentrations of ²³⁸U, ²³²Th, and ⁴⁰K in the soil samples were determined using a 7.6 cm × 7.6 cm NaI (TI) detector crystal optically coupled to a Photomultiplier Tube (PMT). The detector was enclosed in a 6 cm lead shield with cadmium and copper sheets. This arrangement is to minimize the effects of background and scattered radiation. It has an Oxford Multichannel Analyzer (MCA) card that is coupled with software for spectral data acquisition and analysis. It also consisted of a charge sensitive pre-amplifier, a shaping amplifier, high voltage, 80,000 kHz Wilkinson Analogue to Digital Converter (ADC) with MCA. Data acquisition and analysis are carried out by MCA, which consists of a software package and a Multichannel Buffer (MCB). The samples were placed in the detector cavity and measured for a period of 29,000 seconds for each radionuclide. [15]. The 295.22 keV, 351.93 keV for ²¹⁴Pb and 609.32 keV, 1120.29 keV and 1764.49 keV for 214Bi gamma lines were used in the assessment of activity concentration of ²³⁸U, while 911.21 keV for ²²⁸Ac and 968.97 keV and 238.63 keV for ²¹²Pb were used for ²³²Th. The isotope of ⁴⁰K was obtained from the single 1460 keV Gamma-line of ⁴⁰K [5][10]. The peak area of each energy in the spectrum was used to compute the specific activity concentration in the sample as well as other radiological hazards. The net count rates were converted into activity concentrations (Bq/kg) using detector efficiency and sample mass.

The calculation for specific activity concentration was done by procedure given in Equation (1) [16][17].

C a —Net gamma counting rate (counts per second), n —Detector efficiency of the specific gamma ray, P r —The absolute transition probability of gamma decay and m s —The mass of the sample (kg).

This radiological parameter will be estimated using empirical Equation (2) [18].

where C_U is the mean activity of ²³⁸U, C_Th is the mean activity ²³²Th and C_K is the mean activity concentrations of ⁴⁰K in soil samples expressed in Bq∙kg⁻¹.

It is a measure of radiation dose intensity or it can be defined as a measure of the chemical or physical effect created by given radiation exposure or physical effect created by given radiation exposure to a living matter. The absorbed gamma dose rates were calculated from activity concentration of ²³²Th, ²³⁸U and ⁴⁰K using the activity concentration-dose (nGy⁻¹ per Bq/kg) conversion coefficients of 0.622, 0.462 and 0.0432 as provided by UNSCEAR reports. Equation (3) shows the model used to estimate the dose rate from the activity concentrations [11].

where C U , C Th _, and C K are the average activities concentration of ²³⁸U, ²³²Th and ⁴⁰K respectively.

To evaluate the effective dose to the population attributable to radioactivity in the soil a conversion factor of 0.7 Sv/Gy was used [19]. The evaluation of the indoor and outdoor doses used the occupancy factors of 0.6 and 0.4 respectively [6]. The corresponding AED was evaluated using Equations (4) and (5).

where; A E D I N and A E D O U T are Annual effective dose rate for indoor and outdoor environments respectively, A D R ( n G y h − 1 ) is the absorbed dose rate in air, 8760 is the time in hours for one year, 0.7 (SvGy⁻¹) is the conversion factor which converts the absorbed dose rate in the air to an effective dose, 0.6 is the indoor occupancy factor and 0.4 is the outdoor occupancy factor [5][10]. These occupancy factors (indoor and outdoor) were selected to reflect the semi-rural lifestyle of communities around Lumoru dumpsite, where residents divide their time between indoor domestic life and outdoor livelihood activities such as farming, trading, and waste handling. These values align with UNSCEAR [11] standards while realistically representing local exposure conditions.

The soil samples were then assessed to determine the safety of human beings around the Lumoru dumpsite. For this reason, the representative level index ( I ϒ ) was calculated using Equation (6) [20].

According to UNSCEAR, bone marrow and bone surface cells are considered organs of interest for dose measurements. They are the most sensitive body organs to radiation. [21], it affects the most sensitive parts of the body like the bone marrow, destroying red bone marrow that produces red blood cells [3]. Excess white blood cells attack and destroy red blood cells, causing fatal leukemia, a blood cancer. [21] AGDE was calculated by the equation.

The internal hazard index (H_in) and external hazard index were determined using Equation (7) and Equation (8 [3]).

This shows the probability for an individual to develop cancer over a lifetime at a given exposure. It was determined using Equation (9) [5].

where AEDR is Annual Effective Dose Rate, DL is duration of life or life expectancy (70 yrs) and RF is the total risk factor per Sievert given as 0.05 as per ICRP-60.

The mean activity concentrations at Lumoru were 7 ± 0.4 Bq/kg for ²³⁸U, 57 ± 2.0 Bq/kg for ²³²Th, and 25 ± 1.3 Bq/kg for ⁴⁰K, as seen in Table 1. Compared with UNSCEAR global averages of 33 Bq/kg for ²²⁶Ra, 45 Bq/kg for ²³²Th, and 420 Bq/kg for ⁴⁰K [11].

Table 1. Activity concentration and radium equivalent in Bq/kg.

The distribution of radionuclides across the sampled soils showed distinct spatial variability. Uranium activity ranged from 2 ± 0.1 Bq/kg in sample D12 to 17 ± 0.6 Bq/kg in D15, reflecting localized increases possibly linked to buried construction debris or compacted waste zones. The relatively narrow range of uranium concentrations suggests limited input from industrial or phosphate waste materials, which are typical uranium carriers in other urban dumpsites [22]. Thorium, however, exhibited the widest variation, with activity spanning from 35 ± 1.6 Bq/kg in D9 and D10 to 90 ± 3.1 Bq/kg in D15, Table 1. This pronounced difference points to irregular deposition of ceramics, tiles, and sanitary ware, which often contain thorium-bearing minerals. Potassium activity also varied moderately, from 14 ± 0.70 Bq/kg in D8 to 68 ± 3.2 Bq/kg in D9, likely reflecting the uneven distribution of organic waste and ash residues across the site. Such variability is consistent with open, un-engineered dumpsites where waste layers are neither uniform nor segregated [4].

The comparative findings in Table 2 show clear differences in the levels of natural radionuclides reported across dumpsites in Kenya and Nigeria. In this study, uranium and potassium levels were generally low, while thorium showed moderate enrichment. A contrasting trend was noted at the Machinjoni dumpsite in Kitale, where higher thorium activity is linked to the accumulation of building and construction debris [6]. Urban dumpsites such as Ijebu-Ode and those in Lagos-Ogun recorded higher uranium and potassium activities, likely due to the mix of industrial, organic, and metallic waste typical of larger cities [22][23]. In comparison, the Ikorodu market dumpsites exhibited much lower activity levels, consistent with their smaller waste volumes and less complex waste profiles [4]. Overall, these observations suggest that variations in radioactivity across dumpsites are shaped more by the type and management of waste than by underlying geology [2][24]. The results from this study therefore indicate a moderately impacted site, where the radiological characteristics reflect mainly domestic and construction waste inputs rather than industrial or natural geological sources.

Table 2. Comparison with other studies.

In this study, the radium equivalent activity (Ra_eq) varied across the samples, ranging from 61 ± 2.8 Bq/kg in D11 to 148 ± 4.4 Bq/kg in D15, with an average of 91 ± 3.4 Bq/kg. These values are well below the safety limit of 370 Bq/kg recommended by UNSCEAR [11], confirming that the soils from this dumpsite are radiologically safe for human exposure. The observed variability likely reflects the heterogeneous composition of waste materials deposited at different sections of the site. When compared with similar investigations, the mean Ra_eq obtained in this study is slightly lower than that reported for the Machinjoni dumpsite in Kitale (118 ± 5.93 Bq/kg) [6] and Kaduna dumpsites in Nigeria (135.54 ± 18.00 Bq/kg) [3]. However, it is somewhat higher than values recorded in Okakarara, Namibia (57.80 ± 0.98 Bq/kg) [5]. The differences observed among sites may be attributed to variations in local geology, waste type, and disposal practices, all of which influence the concentration of natural radionuclides in soil. The relatively low Ra_eq values in this study therefore indicate a moderate presence of uranium- and thorium-bearing materials, consistent with general domestic and municipal waste rather than industrial residues. Overall, the results affirm that the site poses no significant radiological hazard, and that background radiation remains the main contributor to overall exposure in the surrounding area.

The Absorbed Dose Rate (ADR) across the Lumoru dumpsite soils showed measurable variation, ranging from 26 ± 1.32 nGy/h in sample D11 to 63 ± 3.18 nGy/h in D15, with an overall mean of 39 ± 1.96 nGy/h as seen in Table 3. These values are comfortably below the worldwide outdoor average of 59 nGy/h recommended by UNSCEAR [11], indicating that external gamma radiation from the site remains within safe exposure levels. The variation among samples reflects spatial differences in radionuclide distribution, likely influenced by the uneven deposition of waste materials. Slightly elevated dose rates in samples such as D13 and D15 correspond to higher concentrations of thorium and uranium, while lower values, as seen in D7 and D11, are associated with soil zones containing predominantly organic or decomposed material.

Table 3.Derived radiological parameters from activity concentrations.

The corresponding annual effective dose equivalent for outdoor exposure (AEDEout) ranged from 0.06 ± 0.01 mSv/y in D11 to 0.15 ± 0.01 mSv/y in D15, with an average of 0.09 ± 0.01 mSv/y. These results fall below the global average of 0.48 mSv/y for outdoor terrestrial radiation, implying that radiation doses received by individuals living or working near the dumpsite are well within permissible limits. The relatively small spread in AEDE values across the samples indicates minimal movement or buildup of radionuclides in the surrounding soil environment, suggesting stable waste-radiation interaction over time. In summary, the observed ADR and AEDE values confirm that radiation exposure at the Lumoru dumpsite is primarily due to natural background sources. The measured doses are too low to pose any significant health risk, underscoring the radiological safety of the surrounding environment.

The AGDE values for Lumoru soils averaged 290 ± 10 μSv/year, ranging from 178 ± 7 to 436 ± 14 μSv/year. This parameter is particularly important because gonadal tissues are among the most radiosensitive organs, and prolonged exposure could have hereditary implications. While no strict international limit exists for AGDE, values reported in Lumoru are within the same order of magnitude as those observed in other Kenyan studies. These values, though modest, underscore the need to monitor long-term exposure, particularly for waste handlers and scavengers who spend extended hours at the dumpsite.

The radiological hazard indices derived from soils at the Lumoru dumpsite indicate that radiation exposure levels are within safe limits. The Representative Level Index (RLI) ranged from 0.42 ± 0.02 to 1.03 ± 0.05, with a mean of 0.64 ± 0.03, suggesting that the overall gamma radiation level remains below the permissible threshold of unity. The external hazard index (Hₑₓ) and internal hazard index (Hᵢₙ) recorded means of 0.25 ± 0.01 and 0.28 ± 0.01, respectively, both comfortably below the international limit of one [11]. Slightly higher values observed in samples such as D13 and D15 reflect area-based variations in radionuclide concentration, while the majority of samples show uniformly low radiation levels, confirming overall radiological stability across the site.

The Excess Lifetime Cancer Risk (ELCR) values ranged from 0.21 × 10⁻³ to 0.53 × 10⁻³, averaging 0.30 × 10⁻³, which lies within the globally acceptable range of 0.29 × 10⁻³ to 1.0 × 10⁻³ for outdoor exposure. These low risk levels indicate that prolonged contact with or residence near the dumpsite is unlikely to result in significant health effects. Taken together, the radiological hazard indices demonstrate that the Lumoru dumpsite presents no measurable carcinogenic or radiological threat. The findings underscore that natural background radiation remains the principal source of exposure, confirming that the site is radiologically safe for ongoing human and environmental activities.