Uranium and thorium in soils, mineral sands, water and food samples in a tin mining area in Nigeria with elevated activity

Abstract

The activity concentrations of uranium and thorium have been determined in soils and mineral sands from the Nigerian tin mining area of Bisichi, located in the Jos Plateau, and from two control areas in Nigeria (Jos City and Akure) using high-purity germanium detectors (HPGe). High resolution sector field inductively coupled plasma mass spectroscopy (HR-SF-ICP-MS) was used to determine uranium and thorium in liquids and foodstuffs consumed locally in the mining area. The activities of uranium and thorium measured in the soils and mineral sands from Bisichi ranged from 8.7 kBq kg⁻¹ to 51 kBq kg⁻¹ for ²³⁸U and from 16.8 kBq kg⁻¹ to 98 kBq kg⁻¹ for ²³²Th, respectively. These values were significantly higher than those in the control areas of Jos City and Akure and than the reference values reported in the literature. They even exceeded the concentrations reported for areas of high natural radioactive background. Radionuclide concentrations in samples of the local foodstuffs and in water samples collected in Bisichi were found to be higher than UNSCEAR reference values. The results reveal the pollution potential of the mining activities on the surrounding areas.

Introduction

The average activities of ²³⁸U and ²³²Th in the undifferentiated earth crust are in the range of 25–50 Bq kg⁻¹, but, due to their large ion radius, both elements may be especially concentrated in late crystallising rocks such as granites and other alkaline magmatic ores, often accompanied by other incompatible elements like Rare Earth Elements (REE) (UNSCEAR, 2000).

Uranium is characterised by both radiotoxicity and chemical toxicity, but it is the latter which limits its exposure to humans (Oeh et al., 2007a) whereas thorium is to be considered as only radiotoxic. The health hazards associated with these radionuclides stem from their ability to accumulate in human tissues. During the nuclear transformation processes, the radionuclides emit gamma rays as well as high-LET charged particles, thereby causing intensive damage to the tissues where they are localized and, to a lesser extent, to the neighbouring organs.

Radionuclides of both the uranium and thorium decay series can be often present to a high degree in the materials occurring in frame of tin mining activities, which are then to be considered as TENORM (“technologically enhanced naturally occurring radioactive materials”).

The negative impact of tin mining activities on the environment is mainly due to the excavation of large amounts of sand and the eventual accumulation of a large volume of tailings (Banat et al., 2005, Remon et al., 2005, Akinlua et al., 2006, Birkefeld et al., 2006, Nyarko et al., 2006), which significantly alter the natural constituents of radionuclides in the soil and thus affect the terrestrial ecosystem. It has been observed that mining, milling and processing of uranium- and thorium-bearing minerals lead to enhanced radiation exposures not only to the workers but also to the inhabitants of the mining and processing sites (UNSCEAR, 2000, Lipsztein et al., 2001).

Indiscriminate and improper deposition of tailings, especially on steep slopes, increases their mobility and hence the risk of being transported to large inhabited areas (Henriques and Fernandes, 1991). Due to leaching and re-suspension processes, ²³⁸U and ²³²Th from abandoned dumping sites find their way in surface and ground water (Ragnarsdottir and Charlet, 2000). Consequently, this makes mine tailings a source of pollution to the ground and surface waters, and to the soil in their vicinities (Hector et al., 2006). The effect to man is of particular importance when lands that are used for crops production also serve as repository to tailings, thus increasing the risk of human exposure to TENORM. This may occur by inhalation of suspended dust in the air, direct dermal contact and/or by consumption of crops grown on the affected lands.

The tin mining site in Bisichi is located in the Jos Plateau, in the north-central part of Nigeria, and is about 25 km south of Jos City. The Jos Plateau is on the Nigerian basement complex, at about 1100 m above the sea level. The tin ore consists particularly of the mineral cassiterite and can be assigned to two geological epochs (Buchanan et al., 1971). The Pre-Cambrian mineralization is associated with the Older (Pan African) Granite Province and the cassiterite is stored in the pegmatites of the Basement Complex (Jacobson and Webb, 1946). The Jurassic intrusive sequences are represented by non-orogenic Younger Granites, which are outlined in circular or elliptical shape (Bowden et al., 1979). There, the cassiterite is preferably held in the biotite-bearing phases either as disseminations at the roof zones of the granite bodies or as lodes and fracture controlled greisen veins (Bowden and Kinnard, 1984). The mining activities are exclusively focused on the Younger Granite Complexes due to easier access and started around 1904, shortly after airborne radiometric mapping revealed high deposit of cassiterite and columbite (niobium) ores. Accessory minerals like tantalite, zirconium, monazite, xenotime, and thorite, which have been known to have high concentration of uranium and thorium, were later discovered in the mid-1920s. This discovery led to increased mechanized mining and large mining pits were created due to soil excavation that resulted in ecological devastation of the region. At present, mechanized mining is no longer operational; however, illegal mining activities are present in the area. The method usually employed by illegal miners involves digging of wells to a depth ranging from 10 to 20 m before tunnelling to different directions. Other methods usually employed by local villagers during the rainy season include surface and sub-surface mining processes in which drainage channels are created. The mineral ore, which is heavier than sand, will be left behind as water is allowed to run through the channel. This method also helps to detect areas where these minerals are located. The mining pits left behind by the mechanized activities are also being re-mined by the local people by channelling the accumulated water pond to other locations as the minerals are mined. The water collected in the dams created as a result of the mining activities is used as drinking water and for irrigation purposes by the communities living around the mining site. Agricultural practices have been operational and the main staple foodstuffs grown in the area are root tubers, cereals and vegetables. Moreover, the neighbouring residents have direct and regular access to these sites, as tailings are being used as building materials (Ademola and Farai, 2006). Hence, enhanced radiation burden posed by these practices remains a source of concern in both occupational and public radiation protection programs.

In view of the unregulated mining activities in the Jos Plateau, internal radiological impact and risk to the inhabitants and workers are of major interest. In some locations in Nigeria, radionuclides have been found in foodstuff and have been reported to be a potential health risk to the public through the dietary pathway (Arogunjo, 2003a, Arogunjo, 2003b, Arogunjo et al., 2005). Recently, extremely high concentrations of natural radionuclides were reported in foodstuffs from a mining site of the Jos Plateau called Bisitchi (Jibiri et al., 2007), and rough estimates of the resulting internal dose were performed. The map presented in Fig. 1 of that manuscript locates the site of Bisitchi north-west of Jos City.

In order to monitor incorporation pattern of radionuclides, reliable knowledge of the daily intake from natural radionuclide sources and their metabolic behaviour is indispensable. In Nigeria, the ingestion rate of these radionuclides has not been studied as it has been done in other parts of the globe. The United Nations Scientific Committee on the Effects of Atomic Radiation summarizes reference values of thorium and uranium in environmental materials from other parts of the world with little or no data from Africa (UNSCEAR, 2000). The majority of the measurements on environmental and biological materials were carried out in the North Temperate Zone and may not truly reflect the global average (Santos et al., 2002).

The present study was aimed at evaluating the activity concentration of radionuclides of the uranium and thorium series in soils and mineral samples of the tin mining area of Bisichi and the eventual contamination in well and surface waters as well as in selected foodstuffs and liquids (including alcoholic and soft drinks) consumed by people working and living in that region.