ANALYSIS OF TERRESTRIAL RADIONUCLIDES ( 232TH, 238U, 40K) AND SELECTED HEAVY METALS IN SEDIMENT, WATER, FISH AND FOOD CROPS FROM THE LAKE TURKANA REGION

Abstract

Human activities such as mining and natural processes like earthquakes can release radionuclides and heavy metals into the environment, which are then transported by water into reservoirs, sometimes exceeding safe levels. Elevated concentrations of these elements threaten aquatic ecosystems, reduce water quality, and can bioaccumulate in food chains, posing risks to human health. The Lake Turkana Basin, an important source of water, and livelihoods for surrounding communities, is particularly vulnerable, making baseline data critical for environmental monitoring and public health protection. Previous studies on RiftValley lake systems have indicated that geological formations often contribute to elevated background radiation and mineral leaching into the water column. The objective of this research was to establish a radiological and elemental baseline for the Lake Turkana Basin by quantifying the activity concentrations of 232Th, 238U, and 40K, and assessing heavy metal toxicity in both geological and biological matrices to evaluate the associated public health risks. The objective was realized by studying sediment and water samples collected from the eight centers found along the Weatern shores of Lake Turkana namely Kerio, Eliye, Kalokol, Long’ech, Kataboi, Nachukui, Nareng’ewoi, and Nariokotome. Sediment and water samples were also collected from River Turkwel at Lodwar town and Kang’atosa. Sorghum and cassava were obtained from ten farms at Kang’atosa, and fish were sourced from Kalokol and Long’ech fish markets. The samples for radiometric analysis were dried to remove moisture, homogenized to create a fine, uniform powder, and sealed in standardized counting containers to establish secular equilibrium before measurement. Samples were analyzed for radionuclide concentrations of 232Th, 238U and 40K using a High Purity Germanium (HpGe) detector while Microwave Plasma–Atomic Emission Spectroscopy (MP-EAS) was used to analyze concentrations of the metals lead, copper, arsenic, cadmium, chromium, nickel, zinc and barium in water and fish samples. Before heavy metal analysis, water samples were filtered, immediately acidified with concentrated nitric acid to prevent metal precipitation, fish sampleswere dried, homogenized, and then underwent wet acid digestion (usually using nitric acid to dissolve the organic matrix and fully release the heavy metals into a liquid solution suitable for MP-AES nebulization. The activity concentrations of 232Th, 238U and 40K were used to determine the radiation hazard indicators in sediment samples such as Absorbed Dose Rate (ADR), Annual Effective Dose (AED), Excess Lifetime Cancer Risk (ELCR), External Hazard Index (Hex), Internal Hazard Index (Hin) and the Gamma Index. Also, the activity concentrations of the three radionuclides was used to determine AED in cassava and sorghum samples. Descriptive statistics was used to statistical variables such as the mean, standard error, standard deviation, variance, skewness, kurtosis and range in the data. Analysis of Variance (ANOVA) between groups for different variables in sediment samples was done. Heavy metal concentrations were used to determine water quality indices as well as health risk parameters in fish samples. Pearson correlation was used to determine the correlation between activity concentrations of 232Th, 238U and 40K in fish samples and concentrations of As, Ni, Ba, Cu, Zn, Pb, Cd and Cr in fish samples. Results found sediments from Kalokol to have the highest activity concentration levels of 232Th (75.5 Bq/kg) and 238U (64.5 Bq/kg), while sediments from Nareng’ewoi had the highest activity concentration level of 40K (811.7 Bq/kg). Overall average activity concentrations in sediments from all regions were 38.7 ± 7.53 Bq/kg for 232Th, 30.1 ± 5.80 Bq/kg for 238U, and 664.4 ± 65.43 Bq/kg for 40K. Although average concentration levels of 232Th and 238U were below world averages, 40K exceeded them. All radium equivalent activity values for sediments were safely below the 370 Bq/kg limit. In agricultural products, activity concentration of 40K was notably high in sorghum (699.34 Bq/kg) and cassava (804.42 Bq/kg). Fish species also showed varying radionuclide concentration levels, with Nile Perch having the highest 238U (49.56 ± 2.81 Bq/kg). Radiological hazard indicators in sediments were generally within permissible safety limits. Although the ingestion dose from cassava (0.152 mSv/yr) remained within safe limits, the average dose from sorghum (1.110 mSv/yr) exceeded the ICRP recommended limit. Surface water analysis revealed Arsenic as the most prominent heavy metal (0.756 ± 0.033 mg/L average). The Metal Contamination Index (0.0002–0.0026) was low, but the Heavy Metal Evaluation Index (46.207–122.431) indicated significant pollution, mainly from arsenic (As) and zinc (Zn). Cadmium (Cd) was undetected in fish, while Nile perch showed the highest concentrations of As (0.732 ± 0.045 mg/kg) and Zn (0.769 ± 0.057 mg/kg), as well as the greatest bioaccumulation. Health risk assessment from fish consumption revealed negligible non-carcinogenic risks, with hazard index values below 1. Correlation analysis showed strong positive relationships between ²³⁸U and As (r = 0.900, p = 0.001) and between ²³⁸U and Zn (r = 0.792, p = 0.011). This research establishes a vital geochemical baseline for the Lake Turkana Basin, revealing a complex interplay between natural geological background radiation and anthropogenic heavy metal enrichment. While the radiometric profile characterized by elevated 40K but generally safe levels of 232Th and 238U reflects the unique volcanic lithology of the East African Rift, the significant Arsenic and Zinc signatures in surface waters point toward localized pollution that necessitates active environmental oversight. Crucially, the discovery that sorghum consumption in certain areas may lead to an ingestion dose exceeding ICRP limits (1.110 mSv/yr) underscores a specific public health vulnerability that simple water testing would miss. Ultimately, while the basin remains largely resilient to non-carcinogenic risks at present, these findings serve as a critical early-warning system, advocating for integrated water-soil-food monitoring to ensure that the region’s mining and agricultural sectors do not compromise the long-term biological integrity of the ecosystem.