The earth is naturally radioactive, and about 90% of human radiation exposure arises from natural sources such as cosmic radiation, exposure to radon gas and terrestrial radionuclides (Lee and Lee, 2005). The most common terrestrial radionuclides that produce gamma-rays are member of the 238U and 232Th series and 40K, and their concentrations vary considerably depending on the soil type and local geology.
At dumpsites there are possibilities for radiation to be emitted due to the presence of radioactive waste in the landfills as well as naturally occurring radionuclides in the soil. The radioactive contamination of soil, water and air can be transferred to human through the soil via plants (40K) or through inhalation (222Rn and 220Rn). These radionuclides even at low concentrations can have potential impacts on the environmental quality and human health and may pose a long term risk. Various studies have been carried out in recent years in Nigeria and elsewhere to investigate radioactivity in dumpsite (Odunaike et al., 2008, Ogundiran and Afolabi, 2008, Okoronkwo et al., 2006 and Oladapo et al., 2012). Radioactivity measurements have also shown the existence of traces of radionuclide in the staple foods consumed in Nigeria. It was revealed that staple food stuffs consumed in Nigeria contain traces of radionuclide (Akinloye and Olomo, 2005 and Jibiri et al., 2007) and as a result of this, refuse dumpsites were identified as a liable recipient in containment of radioactive materials. Farmers cultivate and plant legumes, vegetables, etc in the field around the dumpsites, so the transportation of heavy metal as well as radionuclides in soil from this sites are possible via root-uptake and then to human through breathing and ingestion. Therefore, it is necessary to carry out an accurate measurement of elemental and radionuclide composition in soil sample from these dumpsites.
2. Sample collection and processing
Soil samples were collected from active sites at each dumpsite. 10 soil samples were collected from each dumpsite. The samples were collected to a depth of about 20–30 cm at the various points using auger and about 20 m away from each sampling point. The samples were processed following standard procedures (EML, 1983). Soil samples were well mixed, weighed and then dried in an oven at 105 °C overnight and re-weighed to find the water content. The samples were crushed and sieved through a 0.2-mm sieve. Sieved samples were weighed and a mass of 200 g of each sample was placed in a non-radioactive plastic container. The plastic containers were hermetically sealed with adhesive tape (AERB, 2003) for over a month for secular equilibrium to take develop (Olomo, Akinloye, & Balogun, 1994).
The results of the gamma-ray analysis of fifty soil samples from the 5 dumpsites surveyed are presented in Table 1. The radionuclides observed with reliable regularity belonged to the decay series chain headed by 226Ra and 232Th as well as the non-series 40K. From the results obtained in Table 1, the gamma doses rate D (nGyh−1) at 1 m above the ground due to 238U, 232Th and 40K in the soil samples was calculated from Eq. (4) and presented in column 2 in Table 2. The radium equivalent activity (Raeq) which gives a single index used to describe the gamma output from different mixtures of radium, thorium and potassium in the material was calculated from Eq. (5) and presented in column 3 in Table 2. The concentrations of heavy metals detected in the soil samples from the dumpsites surveyed are presented in Table 3. The heavy metals detected are iron (Fe), copper (Cu), magnesium (Mg), calcium (Ca), phosphorus (Ph), lead (Pb), Zinc (Zn) and cadmium (Cd). The mean concentrations of these metals are shown in Table 3. Fig. 1 presents the comparison of the heavy metals detected in the soil samples from the 5 different dumpsites surveyed.