Human exposure to radiation from naturally occurring radionuclides belonging to ²³⁵U, ²³⁸U and ²³²Th series and the single ⁴⁰K have been an issue of concern to radiation health officers worldwide [1]. The soil-to-plant transfer factor is one of the key parameters extensively used for the estimation of internal radiation dose from radionuclides through food consumption [2]. The soil-to-plant Transfer factor, TF, also known as concentration ratio, CR, is defined as the ratio of the activity concentration (in Bqkg^-1) in the dry weight of the plant to the activity concentration (in Bqkg^-1) in dry weight of the soil [10]. Among the various kinds of food, rice is regularly consumed worldwide as well as countrywide like Nigeria. Consequently, human radiation exposure owing to the ingestion of radionuclide via rice consumption is a global concern. Rice is the major food crop and being the principal staple food for Nigeria in their basic daily diet, therefore the consumption of rice is treated as the most important pathways for the transfer of radionuclides into humans [2]. Radioactivity contaminated land can arise as a result of industrial processes, including past waste disposal or accident. Radionuclide could be present on land as a result of a number of past land uses. Contamination of land could occur during the extraction and handling of materials containing high levels of naturally occurring radionuclide materials (NORM) [3]. The continuous extraction and Processing of Lead/Zinc solid minerals in Ikwo Local Government Area of Ebonyi state enhances the environment radioactivity of the area. The advent of excavation and other human activities coupled with poor environmental management systems have resulted to the release of various forms of toxic, corrosive and radioactive contaminants or pollutants into the environment. When harmful substances are introduced into the system and the environment becomes polluted, it affects man and his environment adversely [4,5]. Ikwo is a local government area where mining, farming and education are their major occupation. These include lead/zinc mining and cultivation of farm products. Rice is produced mainly in Ikwo Local Government Area of Ebonyi State. Other crops produced in the state are yam, cassava, maize, groundnut, cocoyam, sweet potato, plantain, banana, oil palm and melon. Rice is considered the world’s third largest crop, which plays a significant role in human nutrition [6]. Since humans cannot do without food, the habitual consumption of food can thus result to accumulation of the radionuclides in the internal organs of the body. This situation can lead to various radiation damages, the degree of which is proportional to the radiation doses delivered to the organs. The ubiquitous and persistence nature of natural radionuclides in soil is the starting point of food contamination. It has been investigated that the content of radionuclides in the food chain has a direct linear correlation with that of the soil where they were grown. While it is known that the radionuclides make their way into the soil through natural processes, agricultural practices have continued to make substantial additions to the radionuclides contents of the cultivated soil, thus leading to elevated levels in the soil and consequent transfer to plants [1]. This research work therefore is evaluate the transfer factor of radionuclides from soil to rice around solid mineral mining areas in Ikwo Local Government Area of Ebonyi State, and also serve as baseline data.

The Study Area

The study area is located in Ikwo Local Government Area of Ebonyi State in South Eastern Nigeria. The area lies within Longitude 8⁰ 00’E - 8⁰20’E and Latitude 6⁰00’N - 6⁰20’N [7]. Ikwo is the largest local government area in Ebonyi State. The nature of activities in the study area includes but not limited to the following: excavation of solid minerals like lead/zinc, salt and limestone, farming of rice, cassava, yam, groundnut and cocoyam.

Figure 1: Modified Google Map showing the Study Area [7].

Sample Collection

Samples were collected in five (5) different farm fields located around the mining area. At each farmland, soil samples and corresponding rice grains samples were collected. Twenty five (25) surface soil samples and twenty five (25) rice grains samples were collected from five different rice farms in Ikwo local government (five soil and five rice samples from each farm). The soil samples were collected using a hand trowel tool to a depth of about 150mm with each spot separated far enough from each other. 5g of soil from each spot was taken from the field. The soil and rice grain samples collected were packaged separately in a cellophane bag well labelled, it was taken to the laboratory for analysis [8].

Sample Preparation

The soil samples and their corresponding rice grain samples were transferred into a polythene bag and taken for preparation at the radiation Physics laboratory, University of Ibadan, Nigeria [8]. Each soil sample from each spot was mixed thoroughly as a composite sample that is representative of the spot. Extraneous materials like plant materials, roots, pebbles were removed from the soil samples. The samples were then separately dried at 110°C in a temperature controlled oven until there was no detectable change in the mass of the samples. The dried soil samples were thoroughly crushed, grounded and pulverized to powder. The powder was passed through a 2 mm sieve. Due to the limited space of the detector shield only 200g of the soil samples (dry weight) were used for analysis since, this is the quantity it could conveniently take.  The samples after weighing were transferred to radon-impermeable cylindrical plastic containers of uniform size (60 mm height by 65 mm diameter) and were sealed for a period of about 30 days. This was done in order to allow for Radon and its short-lived progenies to reach secular radioactive equilibrium prior to gamma spectroscopy [9]. The samples were analysed using NaI (Tl) detector (Model No. 80 series, Canberra Inc.) at NNRA, Ibadan Nigeria [1]. The instrument setting and operational conditions were done in accordance with the manufacturers’ specifications. The activity concentration of Uranium ²³⁸U, thorium ²³²Th and potassium ⁴⁰K was determined using the software. The radiological transfer factors were calculated from the activity concentration values obtained [10].

Radium Equivalent Activity (Raeq)

Is the specific activities of materials containing ²³⁸U, ²³²Th and ⁴⁰K are compared using a common index known as Radium equivalent which takes into account the radiation hazards associated with them as a single quantity [11]. It is calculated using the equation below:

Raeq (BqKg^-1) = A_U + 1.43 A_Th + 0.077 A_K                                (1)

Where A_U, A_Th, and A_K are the activity concentrations of ²³⁴U, ²³²Th and ⁴⁰K respectively. Using materials whose Radium equivalent concentration exceeds 370 Bqkg^-1 is not encouraged to circumvent radiation hazards [12].

Estimation of Soil-to-Plant Transfer Factor (TF)

The parameter, transfer factor (TF), is used to express the soil-to-plant transfer mechanism of radionuclides to various components of plants through the rooting systems [9]. Within the same rooting zone, the TF is expressed, mathematically, as the ratio of the concentration of radionuclide of interest in plant parts (Bqkg−1, dry weight) to the concentration of the radionuclide of interest in the soil (Bqkg^-1, dry weight) [1]. That is

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