ABSTRACT
The activity concentrations of the primordial radionuclides in potable water from 2 former mining areas (Bisichi and Bukuru) in Jos, Plateau state in Nigeria have been studied. The activities were determined by a non-destructive analysis using a computerized gamma ray spectrometry system with high purity germanium (HPGe). The results show the average activity concentrations for 226Ra, 232Th and 40K for Bukuru and Bisichi to be respectively 1.20 ±0.02, 1.93 ± 0.01, 4.75 ± 0.14 and 2.03 ± 0.14, 2.20 ± 0.13 and 3.26 ± 0.06 Bq/l. The corresponding annual effective doses for both locations are respectively 0.59 and 0.80 mSv/year which are much higher than the reference level of a dose of 0.1mSv/year from the intake of drinking water.
Key words: Activity concentration, radionuclides, drinking water.
Environmental radiation originates from a number of naturally occurring and man-made sources. The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) has estimated that exposure to natural sources contributes more than 98% of the radiation dose to the population (excluding medical exposure) (UNSCEAR, 1998)
The global average human exposure from natural sources is 2.4 mSv/year (UNSCEAR 1993). There are however, large local variations in this exposure depending on a number of factors, such as height above sea level, the amount and type of radionuclides in the soil, and the amount taken into the body in air, food and water.
Research reports on environmental radioactivity studies in the Jos Plateau have indicated high gamma radiation dose rates several orders of magnitude higher than world average value (Oresegun and Babalola, 1990, 1993; Farai and Jibiri, 2000). Majority of these reports attributed these high levels primarily on the influence of tin and its mining activities in the area (Farai and Jibiri, 2000; Jibiri et al., 2007a, b; Ademola, 2008).
The negative impact of tin mining activities such as occurred in Bukuru and Bisichi areas in Jos, Plateau, Nigeria, 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.
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 water, and to the soil in their vicinities (Hector et al., 2006).
The process of leaching as well as washing away of tailings due to erosion activities into surface and ground water is what goes on at Bukuru and Bisichi areas of Jos, Plateau in Nigeria. The tailings may accumulate to extents that could be detrimental to human lives, where this water is either drunk directly or used in processing foods.
The present research focuses on the assessment of radioactivity in potable water from dams and wells in Bukuru and Bisichi areas of Jos, Plateau, as well as the effective doses to the dwellers in these areas.
Sample collection and preparation
Drinking water samples were collected from dams and wells from two former tin-mining locations –Bukuru and Bisichi, in Jos, Plateau state, Nigeria. Other water bodies in these areas were discarded once it was established that they were not sources of drinking water. The map of the locations in question is shown in Figure 1.
The bottles were filled to the brim without any head space to prevent trapping of radon gas. For activity concentration measurement, the water samples were also transported to the laboratory and prepared into 1 L Marinelli beakers. The samples were filtered prior to preparation and measurements.
The beakers were thick enough to prevent the permeation of radon. The beakers were closed by screw caps and plastic tape was wrapped over the caps and then stored for measurement. This step was necessary to ensure that radon gas is confined within the volume and that the daughters will also remain in the sample. The samples were sealed for thirty days in order to allow for Radon and its short-lived progenies to reach secular radioactive equilibrium prior to gamma spectroscopy.
Sample measurement and analysis of spectra
All measurements were carried out at the Ghana Atomic Energy Commission, Accra. The activity concentrations of the water samples were determined by a non-destructive analysis using a computerized gamma ray spectrometry system with high purity germanium (HPGe). The relative efficiency of the detector system was 25%, and resolution of 1.8 keV at 1.33 MeV of 60Co. The gamma spectrometer is coupled to conventional electronics connected to a multichannel analyzer card (MCA) installed in a desk top computer. A software program called MAESTRO- 32 was used to accumulate and analyze the data manually using spread sheet (Microsoft Excel) to calculate the natural radioactivity concentrations in the samples. The detector is located inside a cylindrical lead shield of 5 cm thickness with internal diameter of 24 cm and height of 60 cm. The lead shield is lined with various layers of copper, cadmium and Plexiglas, each 3 mm thick.
A counting time of 36,000 s (10 h) was used to acquire spectral data for each sample. The activity concentrations of the uranium-series were determined using γ-ray emissions of 214Pb at 351.9 keV (35.8%) and 214Bi at 609.3 keV (44.8%) for 226Ra, and for the 232Th-series, the emissions of 228Ac at 911 keV (26.6%), 212Pb at 238.6 keV (43.3%) and 208Tl at 583 keV (30.1%) were used. The 40K activity concentration was determined directly from its emission line at 1460.8 keV (10.7%).
Calibration of gamma spectrometry system
Prior to the measurements, the detector and measuring assembly were calibrated for energy and efficiency to enable both qualitative and quantitative analysis of the samples to be performed. The energy and efficiency calibrations were performed using mixed radionuclide calibration standard homogenously distributed in the form of solid water, serial number NW 146 with approximate volume 1000 ml and density 1.0 g cm-3 in a 1.0 L Marinelli beaker. The standard was supplied by DeutscherKalibrierdienst (DKD-3), QSA Global GmBH, Germany and contains radionuclides with known energies (241Am (59.54 keV), 109Cd (88.03 keV), 57Co (122.06 keV), 139Ce (165.86 keV), 203Hg (279.20 keV), 113Sn (391.69 keV), 85Sr (514.01 keV), 137Cs (661.66 keV), 60Co (1173.2 keV and 1332.5 keV) and 88Y (898.04 keV and 1836.1 keV) and activities in a 1000 ml Marinelli beaker was used.
Calculation of activity concentration
The specific activity concentrations (Asp) of 226Ra, 232Th, 40K in Bq l-1 for the water were determined using the following expression (Beck et al., 1972):
Where; Nsam = net counts of the radionuclide in the sample; PE = gamma ray emission probability (gamma yield); ε = total counting efficiency of the detector system; Tc = sample counting time; M = mass of sample (kg) or volume (L)
Minimum detectable activity
The minimum detectable activity (MDA) of the γ-ray measurements were calculated according to the formula:
Where is the statistical coverage factor equal to 1.645 confidence level 95%, B is the background counts for the region of interest of a certain radionuclide, T is the counting time in seconds, P is the gamma yield for any particular element, W is the weight of the empty Marinelli beaker and ε is the efficiency of the detector.
The minimum detectable activity (MDA) derived from background measurements was approximately 0.11 Bq kg-1 for 226Ra, 0.10 Bq kg-1 for 232Th and 0.15 Bq kg-1 for 40K. Concentration values below these detection limits have been taken in this work to be below the minimum detection limit (MDL).
Primordial radionuclide activity in water
The results for the primordial radionuclide activity in the drinking water samples are shown in Table 1 and Figures 2 to 4.
The highest activities for the radionuclides of interest were noticed in the water sample from the dam 3 in Bukuru. This could only suggest that mining activities was very much pronounced in that vicinity.
The water sample from well 1 in Bitsichi showed a similar trend except that Ra-226 activity in the sample was lower compared to that in well water 2. The least activities were observed in water samples from dam 1 in Bukuru, where Ra-226 was below minimum detection limit. The average activity concentrations for 226Ra, 232Th and 40K for Bukuru and Bisichi are respectively 1.20 ±0.02, 1.93 ± 0.01, 4.75 ± 0.14Bq/l and 2.03 ± 0.14, 2.20 ± 0.13, 3.26 ± 0.06 Bq/l.
It follows from the results that the activities of 226Ra and 232Th on the average are higher in Bitsichi than in Bukuru, unlike 40K which shows the reverse. Furthermore, the concentration of 226Ra on the average is more in well water compared to the dams. The reason for the latter is due to the fact that ground water (well water) flows through fractured rock carrying radioactive materials and other elements from the solid to the liquid phase.
The annual effective dose from radionuclide in drinking water was computed using the following equation, assuming a daily water intake of 2 L/day (EPA, 2000-2005):
The dose conversion factors of 2.8×10-7(Sv/Bq), 2.3×10-7(Sv/Bq) and 6.2×10-9(Sv/Bq) were respectively used for 226Ra, 232Th and 40K (ICRP, 1996) (Table 2).
A comparison of the results with those for different European countries (Table 3) shows that the values obtained in this work are higher. The results obtained in this work are approximately 0.59 and 0.80 mSv/year for Bukuru and Bitsichi respectively which is much higher than the reference level of a dose of 0.1 mSv/year from the intake of potable water.
The activity concentrations of the primordial radionuclides in potable water from Bitsichi and Bukuru, former tin-mining areas in Jos, Plateau were investigated. The results show that internationally recommended minimum acceptable values were exceeded; the contributory factor being likely as a result of the tin- mining activities that had been carried out in the area in the past. There is need for proper water treatment in the areas in order to reduce health risks due to ingested radionuclides.
The authors have not declared any conflict of interest.
The author acknowledges the help of Pastor Dickson of Deeper Life Bible Church, Jos who assisted with the transportation to the former tin mining areas as well as the collection of samples from the dams in Bitsichi and Bukuru areas of Jos, Plateau state. The efforts of David Okoh and Nicholas Sackitey of the Radiation Protection Institute, Ghana Atomic Energy Commission, Accra in the measurement of the samples is also appreciated.
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