RADIOLOGICAL HAZARD LEVELS OF CONSTRUCTION ROCKS EXCAVATED FROM QUARRIES IN KERICHO COUNTY, KENYA

ABSTRACT


I. INTRODUCTION
Mineral mining or the extraction of rocks exposes workers to ionizing radiation from the primordial radioisotopes found in the rocks [1].The radioactive isotopes 238 U and 232 Th, as well as their decay products, as well as the radioactive isotope 40 K which are found in the earth's crust that is soil, rocks and water expose people to ionizing radiation at doses that are either outdoor or indoor [2].Indoor exposure results from radiations emitted from the building materials such as soil, rocks and even the ground where the building is constructed.Radiations from the primordial radionuclides present in these construction rocks may pose a risk to miners, masons, transporters, the populace around the quarries and inhabitants of the houses built from these rocks if the radiation is beyond the permissible levels [3].The annual effective dose rate (AEDR) in both the outdoor and indoor settings should not exceed 1mSvy -1 and the average radium equivalent (Raeq) should be less than the permitted maximum of 370 BqKg -1 in both the outdoor and indoor environments [4].Natural radioactivity exposure arising from radiations depends on the geological conditions, and therefore the levels in rocks are different.Quite a few the building materials (soil and rocks) have natural primordial radioisotopes of 238 U and 232 Th series [5].The decay series/chain of 238 U and 232 Th.Potassium-40 adds to the external and internal radiation exposure for residents of rock-built homes where gamma radiation is emitted from walls, ceilings, and floors when radon, thorium and progeny are ingested [6].Ionizing radiations are emitted when an unstable nucleus decays to be stable [7].The earth's crust is made of sedimentary rocks, metamorphic rocks, and igneous rocks [8].Documentations from radiometric surveys show that igneous rocks have elevated levels of 40 K, 238 U and 232 Th [9].Apart from inhaling radionuclides (radon gases and dust particles), these radionuclides get into our body systems through food uptake found in plants, notably 40 K. Human consumption of food grown in regions with potentially elevated levels of background radionuclides is at significant risk [10].Radium, uranium, and thorium in their natural states and their decay products as well and potassium are examples of NORM materials that have not been altered by human action [11].The combination of radiation from highly energetic particles, such as the sun and stars that enter the earth's atmosphere (cosmic radiations) and radionuclides, such as 238 U and 232 Th, their decay products, and the naturally occurring 40 K found on the earth's surface forms background radiation that exposes human beings either internally or externally to radioactive substances [12].Because 40 K and 14 C are present in food, soil and water, they form internal radiations in human bodies [13].The current research examined the hazard levels of 238 U, 232 Th, and 40 K on the construction rocks excavated within Kericho county.

II.1 STUDY AREA
The study took place in Kericho County's quarries.It is situated in the Great Rift Valley's southern rift region, 35 0 02` and 35 0 40` east longitude and the equator and latitude 0 0 23` south, with a height of roughly 2002m above sea level, it covers an area of 2111 square kilometres.The county has an estimated population of 901,777 per the 2019 Kenya population and housing census [14].Samples were collected from the following quarries: Tunnel and Kimondui in Kipkelion West, Kedowa, Jagoror, Kipsirichet and United in Kipkelion East, Rockland, Kisumu Concrete and Rai cement in Soin-Sigowet, Kibingei in Kericho West, Laliat and Chepsetion in Kericho East, Agisiek, Kibugat and Maburo in Bureti sub-county.Figure 1 shows the map of the study area.Source: [15].
The selected areas were chosen because most residents and others from neighboring counties source rocks used for construction from Kericho County.Therefore, there is a need for research to determine the safety of materials used for the building of either domestic or commercial houses.

II.2 SAMPLE PREPARATION
Two rocks' samples, each weighing 200g, were collected from each quarry using a 1000ml plastic container, packed and labelled using unique codes.The 30 collected samples were crushed, sieved in 1mm wire mesh, and dried up in an oven at a temperature of 105 0 C for 24hrs to enable direct calculations of specific activity concentrations of radioisotopes.The samples were weighed and packed in 200ml plastic containers, tightly closed, and wrapped with aluminum foil to prevent the leakage of uranium, radon, and thorium.The samples were kept for four weeks to achieve secular equilibrium between 226 Ra and 232 Th [16].

II.3 GAMMA RAY CALIBRATION
Each sample was placed in a detector and monitored for a period 8 hours making it possible to run two samples per day.After background correction on the obtained spectra, for 40 K, the gamma peak with a centroid at 1460keV was used, 214 Bi (609 keV), 1765 keV for 238 U, 208 Ac and 208 Ti (583keV), 2615 keV for 232 Th while 214 Pb at 352keV and 212 Pb at 239keV photopeak's were used for activities of 238 U and 232 Th respectively.

II.4.1 Natural Activity Concentrations
The ready to measure rock samples were placed in the lead shielded NaITI gamma detector for an average of 8 hours to accumulate gamma counts and achieve well-formed photo-peaks.The activity of the NORMs was determined using the method of comparison given by equation 1 [17].
Where; M, A, I, r and s represent mass, activity, intensity, reference and sample respectively.

II.4.2 Radium Equivalent (Raeq)
The phrase radium equivalent refers to the proportionate total of 226  Ra, 232 Th, and 40 K activities based on the premise that 1BqKg -1 of 226 Ra, 0.7BqKg -1 of 232 Th, and 13BqKg -1 of 40K generate the same gamma dose rate as 1Bqkg -1 of 226 Ra [18].
Calculating the radium equivalent was based on the empirical relationship shown in Equation 2.

II.4.3 External Hazard Index (Hex)
Radiation from naturally occurring radionuclides found in construction rocks may cause external exposure to gamma radiation, measured using an external hazard index.If the external danger index of the radiation is less than one, the consequences are insignificant [19].Hex was determined by employing equation 3.
ACTh, ACRa, and ACK represent the average activity concentrations of 232 Th, 226 Ra, and 40 K.
ACRa, ACTh, and ACK are the mean activity concentrations of 226 Ra, 232 Th, and 40 K in BqKg -1 respectively.If the value is less than a unit, then the effects of the radiation are negligible to human organs [5].

III.1. NATURAL ACTIVITY CONCENTRATIONS
Due to unequal distribution of primordial radionuclides and geological location, activity concentrations varied from one quarry to another.The average activity concentration of 232 Th, 283 U, and 40 K were 102 ± 5 Bq/kg, 56 ± 3 Bq/kg and 1845 ± 92 Bq/kg and ranged from 41 ± 2.09 to 138 ± 7 Bq/kg, 27 ± 1 to 116 ± 6 Bq/kg and 1042 ± 52 to 2690 ± 135 Bq/kg respectively. 238U, 232 Th and 40 K average activity concentrations were above the world's average value of 50Bq/kg, 50Bq/kg and 500Bq/kg respectively.All the quarries showed a higher value for activity concentration of potassium-40 compared to world's average value.Seven quarries had a value of activity concentrations of 238 U being below the world average value.The average activity concentration of 40 K was higher compared to that of 232 Th and 238 U and is attributed to the minerals such as phosphate, quartzite, sandstone, and granite rich radioactive materials.Table 3.1 shows the activity concentration of the radionuclides from the collected samples.Though the reported findings on activity concentrations being higher than the world's recommended values, they are far much below the exemption limits of 1000Bq/kg, 1000 Bq/kg and 100000 Bq/kg for 232 Th, 238 U and 40 K respectively, thus making the rocks excavated in Kericho county quarries safe for use.

III.2. RADIUM EQUIVALENT
The radium equivalent was determined using equation 2 and the results were graphically illustrated in Figure 2. The mean radium equivalent for the samples was 344 ± 17 Bq/kg which does not exceed the proposed radioactivity criterion levels hence below the recommended maximum value of 370 Bqkg −1 [9].Radium equivalent for all the individual samples was graphically represented in Figure 2. All the collected samples recorded a radium equivalent which was less than the world's recommended maximum value of 370 Bq/kg as shown in Figure 2. The radium equivalent values shown in Figure 3.1 ranged between 165 ± 8 Bq/kg and 309 ± 15 Bq/kg.Therefore, the construction rocks pose minimal radiological risk since all the radium equivalent values at the study area were less than the world's permissible maximum value of 370 Bq/kg [20].

III.3. HAZARD INDICES
External exposure due to gamma radiations was determined using equation 3. The values obtained averaged at 0.8 mSv/y, which was less than the worlds recommended value of 1 mSv/y [21].Determination of internal exposure due to inhalation and ingestion of primordial radionuclides was done by employing equation 4. Hin averaged at 0.9 mSv/y, which was less than the worlds recommended value of 1 mSv/y.Figure 3 shows the hazard indices values obtained from the analysis of the collected samples.Source: Authors, (2022).
From Figure 3, the minimum and maximum external hazard index were 0.4 ± 0.02 and 0.8 respectively.Hin obtained ranged from 0.5 mSv/y to 1.1 mSv/y.Most samples posted hazard indices below a unit while the rest of the samples were within the recommended criterion limit of 100 mSv/y.This was because of varying activity concentration of 238 U, 232 Th and 40 K.This made some samples hazard indices to be higher than a unit but still below the recommended exceptional value of 100 mSv/y hence posing minimal significant potential health threat to the general population.

IV. CONCLUSIONS
Determination of the natural radioactivity levels in rocks used for construction in Kericho County from selected quarries has been done using NaI(TI) gamma ray spectrometer.The average activity concentration of 232 Th, 238 U, and 40 K were 101 ± 5 Bq/kg, 56 ± 3 Bq/kg and 1100 ± 55 Bq/kg respectively.The radium equivalent ranged from 165 ± 8 Bq/kg to 309 ± 15 Bq/kg with an average value of 285 ± 14 Bq/kg which is below the permissible limit of 370Bq/kg.The average external hazard index was 0.8 mSv/y while the average internal hazard index was 0.9 mSv/y.They were less than acceptable limit of 1mSv/y.Though the activity concentrations were above the worlds recommended values, they were below the exemption limits, thus, rocks excavated from Kericho county are safe for use.Since this research work did not take into consideration Radon concentration in the underground water sources around the quarries where rocks are excavated, there is need for determination of Radon -222 concentration levels in underground water sources around the studied quarries.

Table 1 :
Activity Concentration of the Collected Samples.Average