Environ Monit Assess (2015) 187:339 DOI 10.1007/s10661-015-4580-9
Distribution of natural and artificial radioactivity in soils, water and tuber crops Godfred Darko & Augustine Faanu & Osei Akoto & Akwasi Acheampong & Eric Jude Goode & Opoku Gyamfi
Received: 10 October 2014 / Accepted: 30 April 2015 # Springer International Publishing Switzerland 2015
Abstract Activity concentrations of radionuclides in water, soil and tuber crops of a major food-producing area in Ghana were investigated. The average gross alpha and beta activities were 0.021 and 0.094 Bq/L, respectively, and are below the guidelines for drinking water and therefore not expected to pose any significant health risk. The average annual effective dose due to ingestion of radionuclide in water ranged from 20.08 to 53.45 μSv/year. The average activity concentration of 238 U, 232Th, 40K and 137Cs in the soil from different farmlands in the study area was 23.19, 31.10, 143.78 and 2.88 Bq/kg, respectively, which is lower than world averages. The determined absorbed dose rate for the farmlands ranged from 23.63 to 50.51 nGy/year, which is within worldwide range of 18 to 93 nGy/year. The activity concentration of 238U, 232Th, 40K and 137Cs in cassava ranges from 0.38 to 6.73, 1.82 to 10.32, 17.65 to 41.01 and 0.38 to 1.02 Bq/kg, respectively. Additionally, the activity concentration of 238U, 232Th, 40K and 137 Cs in yam also ranges from 0.47 to 4.89, 0.93 to 5.03, 14.19 to 35.07 and 0.34 to 0.89 Bq/kg, respectively. The average concentration ratio for 238U, 232Th and 40K in yam was 0.12, 0.11 and 0.17, respectively, and in cassava was 0.11, 0.12 and 0.2, respectively. None of the G. Darko (*) : O. Akoto : A. Acheampong : E. J. Goode : O. Gyamfi Department of Chemistry, Kwame Nkrumah University of Science and Technology, Kumasi, Ghana e-mail: [email protected] A. Faanu Radiation Protection Institute, Ghana Atomic Energy Commission, Accra, Ghana
radioactivity is expected to cause significant health problems to human beings. Keywords Radioactivity . Activity . Dose . Concentration
Introduction Natural radionuclides are present at low concentrations in the soil, water, air and food. Whereas natural radioactivity comes mainly from long-lived primordial radionuclides that are widespread in the Earth’s crust (Alharbi and El-Taher 2013), human activities such as mining and generation of nuclear power add artificial radionuclides to the environment. Human beings are therefore exposed to radiations from sources outside the body and from radionuclides that are ingested through consumption of food and water or inhaled radioactive gases such as radon (Ehsanpour et al. 2014; Taskin et al. 2009). Radionuclides with relatively long half-lives are considered human health risk as they can get into the human system through the food chain and thereby increase the radiation burden for many years (Abu-Khadral et al. 2008). Ingestion of food crops loaded with naturally occurring radioactive materials (NORMs) has the potential of exposing human beings to high level of radiation doses (Bengtsson et al. 2012). Ingested radionuclides may accumulate in organs such as bones from where they exert toxic effects. About 66 % of uranium ingested is rapidly eliminated via urine (Pakade et al. 2012), while the rest is distributed and stored in the kidneys (12–
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25 %), bone (10–15 %), and soft tissue (Canu et al. 2011; Evensen et al. 2002; Wrenn et al. 1985). Radium deposits mostly in the bones (Canu et al. 2011), and radon gas diffuses into the stomach wall (Hopke et al. 2000). Since exposure doses are strongly related to the amount of radionuclides present, accurate evaluation of dietary intake is an important parameter for the radiological protection of the population (Cevik et al. 2006). Moreover, measuring the levels of natural and artificial radiation in the environment is crucial in implementing appropriate controls for the sake of radiological protection (Kinyua et al. 2011). Water is an important matrix in environmental studies because of its daily use for human consumption and its ability to transport pollutants (Degerlier and Karahan 2010). Measurement of natural radioactivity in soil is very important to determine because it helps in monitoring changes in natural background activity with time as a result of any radioactivity release. Many developed countries have taken steps to measure the levels of background radiations in their environments (Appl et al. 1998; Bozkurt et al. 2007; Madruga 2008; Palomo et al. 2007) in order to reduce human exposure. However, not much research work has been carried out on the radiological quality of the environment in most developing countries. There are limited detailed radiological data on the environment, water and food items in Ghana (Adu et al. 2011; Faanu et al. 2011a, b; Nguelem et al. 2013), and the public, regrettably, is unaware of the potential radiological hazards associated with the soil, food and water that they use. Undoubtedly, higher levels of radionuclide concentration in food and water have an adverse effect on the health of people exposed to these radionuclides. Knowledge of the levels and distribution of radionuclides in the environment is necessary if levels of human exposure to radiation from radionuclides are to be controlled. In this study, we determined the activity concentrations of natural radionuclides (238U, 232Th and 40K) as well as an artificial radionuclide (137Cs) in water, root tubers (cassava, yam, and cocoyam) and soil samples from the Tano-North District of Ghana. Our results were used to compute the radiation doses and compared with recommended dose limits in water, soil, and tuber crops in order to assess the public health impact from the activity concentrations and calculated effective dose rate. This study will provide pioneering and baseline data for the district. This is an important requirement for establishing and maintaining standards regarding activity concentration of soil, tuber crops and water.
Materials and methods Study area The Tano-North district of Ghana lies between latitude 7°00′ and 7° 25′ N and longitude 2° 3′ and 2° 15′ W covering a total land area of 876 km2. Agriculture is the main economic activity in the district. The main source of water in the district is borehole. However, some of the communities depend on streams, rivers, springs and wells as their source of water. The district is located in the moist semi-deciduous forest zone of Ghana. The geology of the district is made of the middle Precambrian formation (Milési et al. 1991). Generally, the soil in the district is fertile with abundant arable land which favours the cultivation of a wide range of both food and cash crops. The district which is a major food basket in the country is experiencing increasing mining activities including galamsey, while most of the communities depend on borehole and surface water sources for domestic use. Like in many developing countries, the levels of natural and artificial radionuclides in groundwater, soil and tuber crops grown in the district are not known. Figure 1 shows the map of the area and the sampling sites. Sampling and sample preparation Forty water samples from boreholes, wells and streams were collected from random sites across the district. Similarly, tuber crops (yam, cassava and cocoyam), which were ready for harvest, and the top soils (up to 10 cm) in which they grow were also collected. Sampling was carried out between August–December 2013 (first batch) and March–May 2014 (replicate). Physical parameters of the water samples were determined on site using a Hanna combometer coupled to an HI 98129 glass electrode (Hanna, Mauritius) and 1.5-L aliquots were taken into pre-cleaned bottles for radioactivity measurements in the lab. Measurement of gross-α and gross-β Water samples were filtered to remove undesirable solid particles and transferred into 1-L Marinelli beakers for gamma spectrometry. Aliquots of 300 mL of filtered samples acidified with 3 mL of HNO3 were evaporated to 10 mL on a hot plate. These were transferred into pre-
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Fig. 1 Map of Tano-North District showing the sampling points
weighed 25-mL stainless steel planchet and evaporated to dryness. The planchets were left to cool in desiccators and re-weighed. Measurements were performed on a gross α-β spectrometry coupled to a low-background gasless automatic α/β counting system (Canberra iMatic™, USA) and a solid-state passivated implanted planar silicon detector. Calibration was done using 241 Am standard for α and 90Sr for β emission. Efficiency was found to be 36.39±2.1 % for α and 36.61±2.2 % for β, while the background counting rates were 0.04± 0.01 and 0.22±0.03 counts/min for α and β, respectively. The minimum detectable activity was 0.0007 Bq for the α and 0.0037 Bq for the β emitters.
were then oven-dried at 105 °C to a constant weight. They were ground into fine powder with a ball mill grinder and sieved through a 2-mm-pore-size mesh into a Marinelli beaker. The samples were transferred into a Marinelli beaker up to the recommended level and weighed to determine the mass and then sealed and stored in the laboratory for 30 days to allow for equilibration between long-lived parent radionuclides and their short-lived daughter radionuclides. For the food samples, edible parts of root tubers were chopped into small pieces, washed and freeze-dried (model: Christ LMCV-1). The dried samples were ground into fine powder, weighed and stored prior to analysis.
Activity measurement in solid samples
Description of gamma spectrometer
Soil samples were air-dried for 5 days and homogenized after removing extraneous materials from them. They
The samples were measured with a high-resolution gamma-ray spectrometer consisting of an n-type
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ORTEC high-purity germanium detector (model GMX40P4, USA) in a vertical configuration. The detector crystal of 63.0 mm Ø and the length of about 65.0 mm have a resolution of 1.95 keV and relative efficiency of 40 % for 1.33 MeV 60Co. The system is coupled to a PCA-MR 8192 Multi-Channel Analyzer (MCA) provided with ORTEC Maestro 32 MCB configuration software for spectrum acquisition, evaluation and analysis. The detector was operated at a voltage of 4800 V (reverse biased) and cooled with liquid nitrogen at −196 °C. A cylindrical lead shield (20 mm) with a fixed bottom and a movable lid is used to shield the detector in order to reduce background gamma radiation. Within the lead shield are also copper, cadmium and plexiglass (3 mm each) to absorb X-rays and other photons that might be produced in the lead as result of the interaction with the photons. Calibration of the gamma spectrometer A 1.0 g/m3 multi-nuclide standard solution (IAEA, Vienna) containing 241Am (59.54 keV), 109Cd (88.03 keV), 57 Co (122.06 keV), 139 Ce (165.86 keV), 203 Hg (279.20 keV), 113Sn (391.69 keV), 85Sr (514.01 keV), 137 Cs (661.66 keV), 60Co (1173.2 and 1332.5 keV) and 88 Y (898.04 and 1836.1 keV) was used to calibrate the instrument for energy and efficiency prior to sample analysis. However, only four radionuclides (241Am, 57 Co, 137Cs and 60Co) were selected for the energy calibration because the short-lived radionuclides did not give significant peaks. Standards and samples were measured in the 1.0-L Marinelli beaker (Deutsher Kalibrierdient-3 QSA Global GmBH, Germany) using a counting time of 36,000 s to acquire spectra data. Energy calibration was performed by matching the energy of the principal γ-rays in the spectrum of the standard reference material to the channel number of the spectrometer (Faanu et al. 2011a, b). The efficiency calibration was performed by acquiring a spectrum of the standard until the count rate at the peak of total absorption can be calculated with statistical uncertainty of less than 1 % at a confidence level of 95 %. The net count rate was determined at the photo peaks for all the energy values to be used for the determination of the efficiency at the time of measurement. The efficiency at each energy value was plotted as a function of the peak energy and extrapolated to determine the efficiency at other peak energy values for the measurement geometry used (Nguelem et al. 2013).
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Calculation of activity concentration The activity concentration of 238U and 232Th was calculated from the average peak energy values of 1764.49 of 214 Bi, 609.31 keV of 214Bi, 238.63 keV of 212Pb and 911.21 keV of 228Ac, respectively. The activity concentrations of 137Cs and 40K were determined from the energy values of 662 and 1460.83 keV, respectively. The analytical expression used to calculate the activity concentrations for soils and foodstuff (Bq/kg) and water (Bq/L) is described elsewhere (Oresengun et al. 2010).
Estimation of doses The external gamma dose rate at 1.0 m above ground for soil samples was calculated from the activity concentrations using conversion factors of 0.0417, 0.462 and 0.604 nGy/h/Bq/kg, respectively, for 40K, 238U and 232 Th (Amin et al. 2011). The annual effective dose was calculated from the absorbed dose rate by applying the dose conversion factor of 0.7 Sv/Gy and an outdoor occupancy factor of 0.2 (UNSCEAR 2000). In the case of the water samples, the committed effective doses were estimated from the activity concentrations of each individual radionuclide and application of the yearly water consumption rate for adults of 730 L/year (2 L/ day by 365 days) and the dose conversion factors of 238 U, 232Th and 40K taken from the BSS and United Nations Scientific Committee on the Effect of Atomic Radiation (UNSCEAR) report (IAEA 2001).
Committed annual effective dose The annual effective dose due to ingestion of radionuclides in water (mSv/year) was computed on the basis of the activity concentrations of the radionuclides, assuming an average daily water intake of 2 L for an adult (WHO 2006) and ingestion dose coefficient of 4.5× 10−5 mSv/Bq for 238U, 2.3×10−4 mSv/Bq for 232Th and 6.2×10−6 mSv/Bq for 40K (Alam et al. 1999). The committed effective dose owing to ingestion of 238U, 232 Th, 40K and 137Cs in foodstuffs was calculated as the sum of the products of activity concentration of radionuclides, annual foodstuff intake of 170 kg/year (Angelucci 2013) and the dose coefficients of 4.5× 10−5, 7.2×10−5, 6.2×10−6, 1.3×10−8 mSv/Bq for 238U, 232 Th, 40K and 137Cs, respectively (ICRP 2007).
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Results and discussion Gross-α and gross-β analyses generally serve as a means of screening water samples to assess the radiological quality. The WHO recommended guidelines on gross-α and gross-β in drinking water are 0.5 and 1.0 Bq/L, respectively. Table 1 presents the physical parameters and the gross-α and gross-β activities in the water samples. The gross alpha activity concentration in water varied between 0.007 and 0.039 Bq/L. The highest gross alpha value was recorded at Duayaw/Nkwanta with a value of 0.039 Bq/L, while the lowest gross alpha concentration was recorded from the main underground water in Bomaa with a value of 0.007 Bq/L, respectively. The gross-β activity concentration in water varied between 0.011 and 0.181 Bq/L with an average value of 0.091 Bq/L. The highest gross beta activity of 0.181 Bq/L was recorded at Boukrukruwa, while the lowest gross beta value was recorded at Duayaw/ Nkwanta with a value of 0.011 Bq/L. All the water samples measured gave gross-α and gross-β values lower than the WHO guidelines. The measured values compare well with data from Turkey (Degerlier and Karahan 2010), Tarkwa Ghana (Faanu et al. 2010), Spain (Pujol and Sanchez-Cabeza 2000) and Kumasi, Ghana (Darko et al. 2014). Duayaw/Nkwanta showed activity concentrations of 238 U, 232Th and 40K in ranges of 0.21 to 0.53, 0.23 to 1.41 and 2.49 to 11.89 Bq/L with averages of 0.30± 0.02, 0.53±0.04 and 4.87±0.73 Bq/L, respectively. The maximum activity concentration of 238U, 232Th and 40K measured for Duayaw/Nkwanta was 0.53±0.05, 1.41± 0.08 and 11.89±0.74 Bq/L, respectively. Activities of 238 U, 232Th and 40K measured for Boukrukruwa had concentration ranges of 0.19 to 0.26, 0.21 to 0.26 and 2.59 to 4.15 Bq/L, respectively, and had averages of 0.23±0.01, 0.23±0.01 and 3.12±0.32 Bq/L, respectively. The maximum activity due to 238U, 232Th and 40K in
Boukrukruwa is 0.26± 0.02, 0.26 ± 0.02 and 4.15 ± 0.42 Bq/L, respectively. The lowest activities due to 238 U, 232Th and 40K were also measured to be 0.19± 0.01, 0.21±0.01 and 2.59±0.26 Bq/L, respectively. Bomaa recorded activity concentration for 238U, 232Th and 40K in ranges of 0.13 to 0.99, 0.13 to 1.36 and 1.84 to 12.00 Bq/L with average activities of 0.38±0.03, 0.41±0.03 and 4.29±0.32 Bq/L, respectively. The maximum activities for 238U, 232Th and 40K measured were 0.99±0.08, 1.36±0.09 and 12.00±1.12 Bq/L, respectively, while the lowest activities are 0.13±0.01, 0.11± 0.011 and 1.84±0.06 Bq/L. The activity concentrations of 238U, 232Th and 40K measured at Techire varied from 0.18 to 0.73, 0.18 to 1.47 and 2.47 to 12.38 Bq/L with average values of 0.35 ±0.02, 0.54±0.03 and 5.05±0.59 Bq/L, respectively. The maximum activities for 238U, 232Th and 40K recorded for Techire were 0.73±0.06, 1.47±0.08 and 12.38±1.27 Bq/ L, whereas the lowest activity values are 0.18±0.01, 0.18 ±0.01 1 and 2.47±0.12 Bq/L, respectively. At Tanoso, the activity concentrations for the radionuclides varied from 0.28 to 0.46, 0.25 to 0.35 and 2.77 to 3.87 Bq/L for 238 U, 232Th and 40K, respectively. The maximum activity due to 238U and 40K was 0.46±0.05 and 3.87±0.35 Bq/L, respectively, while the maximum value for 232Th was recorded as a value of 0.35±0.02 Bq/L. The lowest activity for 238U was measured as 0.28±0.02. Finally, the minimum activity for 40K was 2.77±0.19 Bq/L. The activity concentration of 238U in all the water samples was about ten times lower than the WHO guideline level of 10 Bq/L in drinking water. The activity concentration of 232Th in about 86 % of water samples was below the WHO guideline level of 1.0 Bq/L. The average activity concentrations of 40K were higher than those of all the other radionuclides in all the samples. The maximum average activities of 40K, 238 U and 232Th were 5.05±0.59, 0.38±0.03 and 0.54± 0.03 Bq/L, respectively. The maximum average activities for 40K and 232Th were measured in water samples
Table 1 Physical parameters and gross-α and gross-β activities of water samples Location
Temperature (°C)
Conductivity (μS/cm)
pH
TDS (ppm)
Gross-α (Bq/L)
Gross-β (Bq/L)
Duayaw/Nkwanta
24.55±0.13
110.00±42.93
5.79±0.13
56.50±12.01
0.03±0.01
0.10±0.06
Boukrukruwa
25.00±1.07
151.75±57.97
5.97±0.34
75.75±27.82
0.02±0.01
0.13±0.04
Bomaa
24.56±0.25
249.60±16.57
6.16±0.45
124.61±57.99
0.01±0.00
0.09±0.01
Techire
24.55±0.06
281.25±49.00
6.48±0.58
111.50±73.46
0.01±0.00
0.07±0.01
Tanoso
24.45±0.06
181.50±19.33
6.72±0.85
104.25±41.70
0.02±0.01
0.08±0.01
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from Techire and its environs with values 5.05±0.59 and 0.54±0.03 Bq/L, respectively. However, the maximum average activity for 238U was measured in water samples collected from Bomaa and its environs with a value of 0.38±0.03 Bq/L. The differences in activities of the water samples could be attributed to the differences in NORMs present in the bedrocks. The occurrence and distribution of radioactivity in water largely depend on factors such as the local geological characteristics of the source (Shashikumar et al. 2011). Radiological safety of drinking water is an important water quality parameter of concern. The regulation and guidelines used by most countries are based on the WHO limits. The annual committed effective dose of radionuclides ingested through water is presented in Table 2. The average annual committed effective dose for Duayaw/Nkwanta, Boukrukruwa, Bomaa, Techire and Tanoso was 40.43± 2.17, 20.08± 1.89, 33.58± 1.91, 53.45±3.77 and 24.90±2.61 μSv/year, respectively. It indicates that the inhabitants of Techire receive the highest radiation dose upon water ingestion, the inhabitants of Duayaw/Nkwanta followed, whereas the people of Boukrukruwa received the least effective dose due to NORMS in drinking water. The WHO ceiling on annual committed effective dose due to ingestion of radionuclides in water is 100 μSv/year (WHO 2006). This guideline is accepted by member states, the European Commission and the Food and Agriculture Organization. The average annual committed effective dose in this study did not exceed the WHO guideline level of 100 μSv/year. Therefore, no harmful radiological health effects are expected from the consumption of drinking water from the sampled sources. The findings in this study show that the activity concentrations determined are higher
than those found in Spain (Pujol and Sanchez-Cabeza 2000), Morocco (Hakam et al. 2001) and Lake Bosomtwi in Ghana (Adu et al. 2011) but are lower than the findings in Yemen (El-Mageed et al. 2013) and Cape Coast in Ghana (Faanu et al. 2011a, b). The activity concentration of the radionuclides, their absorbed dose rate and the annual effective dose with soil samples are given in Table 3. The activity concentration of 238U, 232Th, 40K and 137Cs in soil ranges from 14.56 to 34.47, 18.88 to 48.17, 83.11 to 191.76 and 0.183 to 1.46 Bq/kg, respectively. The low concentration of 137Cs could be attributed to the fact that 137Cs is not naturally found in the soil. Since Ghana has no nuclear plants, the traces of 137Cs detected are to be presumed to have been deposited through atmospheric transfer from a nuclear power plant somewhere or from the testing of a nuclear weapon. The concentrations of 40K in the soil samples were lower compared with the global average value of 400 Bq/kg (Manigandan and Chandar 2014; Ndontchueng et al. 2014). About 80 % of the soil samples analyzed recorded lower concentrations of 238 U compared to the global average of 30 Bq/kg (UNSCEAR 2010), while 70 % of the soil samples had 232Th less than the global averages of 35 Bq/kg (UNSCEAR 2010). However, soil samples from farms located in Bomaa and Koforidua recorded average activity concentration of 33.73 and 34.47 Bq/kg, respectively, for 238U that is higher compared with the world average for 238U. Furthermore, soil samples collected from farms located in Bomaa, Subriso, and Koforidua recorded average activities of 46.42, 42.64 and 48.17 Bq/kg for 232Th, respectively, which were also higher than the world average of 35.0 Bq/kg. Those soil samples with higher concentrations of 238U and 232Th greater than the global average could
Table 2 Average activity concentration and effective dose due to 40K, 238U and 232Th in water samples Sample location
Activity concentration (Bq/L) 40
K
238
U
Committed effective dose (μSv/year) 232
Th
Duayaw/Nkwanta
4.87±0.36
0.30±0.07
0.53±0.13
40.43±2.17
Boukrukruwa
3.12±0.33
0.23±0.02
0.23±0.02
20.08±1.89
Bomaa
4.24±0.32
0.38±0.02
0.41±0.02
33.58±1.91
Techire
5.05±0.39
0.35±0.02
0.54±0.03
53.45±3.77
Tanoso
3.15±0.27
0.36±0.03
0.29±0.02
24.90±2.61
Guideline level (WHO 2004)
N/A
10
1
100
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Table 3 Activity concentration, absorbed dose rate, and annual effective dose due to 238U, 232Th, 40K and 137Cs in soil Sample ID
Activity concentration (Bq/kg) 238
U
232
Th
40
K
Absorbed dose rate, nGy/h 137
Cs
Annual Percentage contribution of radionuclide effective to absorbed dose rate (%) dose, mSv 238 232 40 U Th K
Bomaa
33.77±2.35 46.42±3.79 164.76±6.09 2.53±0.12 50.51±2.32 0.06±0.01
30.89±1.01
55.51±0.41
13.60±0.00
Subriso
29.92±2.89 42.64±3.13 162.38±5.49 3.51±0.14 46.34±2.01 0.06±0.00
29.82±0.21
55.57±1.05
14.61±1.91
Techire
19.79±1.79 22.52±1.90 163.40±6.35 1.64±0.04 29.56±1.21 0.04±0.01
30.93±1.00
46.02±2.01
23.05±0.54
Bredi no. 1
21.38±1.86 33.89±2.69 132.49±4.29 3.64±0.14 35.87±2.00 0.04±0.01
27.53±2.14
47.07±2.00
15.40±0.00
Koforidua
34.47±2.72 48.17±3.80 159.29±5.53 3.92±0.15 51.66±2.12 0.06±0.00
30.82±1.09
56.32±2.10
12.86±0.51
Afrisipa
22.14±1.89 24.89±2.01 103.07±3.02 2.91±0.12 29.56±0.99 0.04±0.00
34.61±0.04
50.86±0.95
14.54±0.32
Bredi no. 2
18.59±1.23 18.89±1.11
87.07±2.13 1.91±0.10 23.63±1.11 0.03±0.00
36.85±0.23
48.28±0.07
15.37±1.11
Benchem
14.56±1.18 20.72±1.84 191.76±7.20 3.92±0.15 27.24±1.09 0.03±0.00
24.70±2.07
45.94±1.05
29.35±0.40
Boukrukruwa
17.33±1.99 23.22±2.01 190.48±7.11 3.39±0.12 29.97±1.52 0.04±0.00
26.71±1.00
46.79±0.74
26.50±0.18
Duayaw/Nkwanta 19.89±1.99 29.66±2.19
83.11±2.24 1.46±0.07 30.57±2.00 0.04±0.00
30.06±1.91
58.60±0.06
11.34±0.11
Min.
14.56±2.18 18.89±2.76
83.11±3.24 1.46±0.07 23.63±1.98 0.04±0.00
24.70±0.97
45.94±0.32
11.34±1.01
Max.
34.47±3.72 48.17±5.40 191.76±8.23 3.92±0.18 50.51±3.20 0.06±0.00
36.35±2.09
58.60±1.00
29.35±2.10
Average
23.19±1.99 31.10±2.45 143.78±4.23 2.88±0.12 35.49±2.51 0.04±0.00
30.24±1.31
52.10±0.99
17.66±0.04
primarily be attributed to the presences of rocks bearing high concentration of those radionuclides in the area. The presence of radionuclides in food crops is mainly due to the uptake of radionuclides from the soil by the root system of plants. Table 4 shows the activity concentration of radionuclides in cassava from different farms in the study area. Concentrations of 238U, 232Th, 40 K and 137Cs varied from 0.38 to 6.73, 1.82, 17.65 to 41.01 and 0.38 to 1.02 Bq/kg, respectively. The average values of the radionuclides are 2.674±0.14, 3.799± 0.23, 29.207±2.36 and 0.70±0.04 Bq/kg, respectively. The activity concentration of 238U, 232Th, 40K and 137Cs in yam ranges between 0.47 and 4.89, 0.93 and 5.03, 14.19 and 35.07 and 0.34 and 0.89 Bq/kg, respectively. The average values for the radionuclides are 2.776± 0.17, 3.162±0.21, 23.879±1.55 and 0.714±0.05 Bq/ kg in that order. It can be observed that the concentration of all the radionuclides in cassava and yam is less than the amount in the corresponding farmlands. For radionuclides to be absorbed by the root system of plants, they must be in the soluble form in the soil (IAEA 2005; KabataPendias 2000). Since all radionuclides in the soil cannot be in the soluble state, coupled with variations in the solubility of radionuclides, it is expected that the radionuclide concentration in the soil will be higher compared with that in food plants. Additionally, there was a greater uptake of 40K than the other radionuclides by
cassava and yam. This can be attributed to the fact that 40 K has a higher solubility than the other radionuclides. Additionally, it is an essential element which is required for plant growth and metabolism. Moreover, 40K can be added to the soil through the application of fertilizer, which is a common practice in the area. The concentration of 137Cs in cassava and yam was very low, and this may be due to the very low concentration of 137Cs in soil which subsequently affects the absorption and utilization by plants. Generally, cassava accumulated slightly higher concentration of radionuclides compared to yam. Cassava and yam form the major constituent of the staple food of the people of the Tano-North area which is fufu and ampesi, respectively. The average annual committed effective dose received by the population due to consumption of cassava and yam in the study area is 29.19 and 9.90 μSv/year. This means that consumption of cassava and yam from the study area does not pose any radiological health risk as the estimated committed effective doses were much less than the WHO guideline of 100 μSv/year. Concentration ratio (Fv) which is the ratio of the activity concentration of radionuclides in the plant to that in the soil (IAEA 2009) is an indication of uptake of any radionuclides in a layer of specified soil to plants that grow in the soil. Table 5 shows the extent of transfer of 238U, 232Th and 4K from the soil to cassava and yam.
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Table 4 Activity concentration of radionuclide in cassava and yam samples Location
Sample
Activity concentration (Bq/kg (dry weight) 238
232
U
Bomaa Subriso Techire Bredi no. 1 Koforidua Afrisipa Bredi no. 2 Benchem Boukrukruwa Duayaw/Nkwanta Average
Th
Committed effective dose (μSv/year)
40
137
K
Cs
Cassava
1.40±0.10
2.43±0.11
38.21±3.76
0.55±0.03
Yam
3.45±0.21
3.28±0.24
35.07±3.34
0.47±0.03
23.50±1.46 8.64±0.21
Cassava
2.40±0.12
3.87±0.29
28.03±1.40
0.79±0.05
23.30±1.08
Yam
2.43±0.15
3.35±0.25
29.58±2.43
0.34±0.02
25.10±1.10 22.70±1.32
Cassava
2.74±0.22
1.82±0.08
29.87±2.46
0.54±0.03
Yam
1.72±0.09
1.74±0.04
24.59±1.43
0.47±0.03
5.03±0.01
Cassava
0.38±0.02
4.85±0.35
19.89±1.11
0.48±0.03
32.60±1.30
Yam
4.76±0.29
3.26±0.22
19.24±0.89
0.85±0.05
8.08±0.01
Cassava
6.73±0.42
10.32±0.64
27.76±2.42
1.02±0.06
63.70±2.24
Yam
4.89±0.29
4.99±0.31
25.79±1.37
0.84±0.05
10.50±0.56
Cassava
4.49±0.31
6.48±0.42
41.01±4.02
0.94±0.06
48.00±2.12
Yam
4.54±0.28
5.03±0.41
17.79±0.66
0.77±0.05
9.67±0.00
Cassava
1.75±0.06
0.84±0.04
17.65±0.89
0.88±0.05
12.80±0.23
Yam
2.74±0.23
3.17±0.20
14.19±0.62
0.78±0.05
6.26±0.01 22.30±0.11
Cassava
0.97±0.04
2.75±0.14
30.19±2.51
0.86±0.05
Yam
0.74±0.03
0.93±0.08
24.19±1.42
0.75±0.04
3.52±0.00
Cassava
0.77±0.03
1.84±0.09
39.08±3.85
0.57±0.03
21.10±1.01
Yam
0.47±0.02
2.85±0.18
27.99±2.38
0.89±0.06
5.70±0.06
Cassava
2.13±0.11
2.77±0.14
20.39±1.22
0.38±0.03
21.90±1.07
Yam
2.02±0.14
3.01±0.19
20.34±0.95
0.97±0.07
6.54±0.23
Cassava
2.67±0.14
3.99±0.23
29.21±2.36
0.70±0.04
29.19±2.19
Yam
2.78±0.17
3.16±0.21
23.88±1.55
0.71±0.05
9.90±0.09
Table 5 Transfer of radionuclides from soil to cassava and yam
Bomaa
Transfer due to 238U
Transfer due to 232Th
Transfer due to 40K
Cassava
Yam
Cassava
Yam
Cassava
Yam
0.04±0.00
0.10±0.00
0.05±0.00
0.07±0.00
0.23±0.00
0.21±0.00
Subriso
0.08±0.00
0.08±0.01
0.09±0.00
0.08±0.00
0.17±0.00
0.18±0.00
Techire
0.14±0.00
0.09±0.00
0.08±0.00
0.08±0.00
0.18±0.00
0.15±0.00
Bredi no. 1
0.16±0.00
0.22±0.00
0.14±0.00
0.10±0.00
0.15±0.00
0.15±0.00
Koforidua
0.19±0.00
0.14±0.00
0.21±0.01
0.10±0.00
0.17±0.00
0.16±0.00
Afrisipa
0.20±0.00
0.20±0.00
0.26±0.01
0.20±0.00
0.40±0.00
0.17±0.00
Bredi no. 2
0.09±0.00
0.15±0.00
0.04±0.00
0.17±0.00
0.20±0.00
0.16±0.00
Benchem
0.07±0.00
0.05±0.00
0.13±0.00
0.04±0.00
0.16±0.00
0.13±0.00
Boukrukruwa
0.04±0.00
0.03±0.00
0.08±0.00
0.13±0.00
0.21±0.00
0.15±0.00
Duayaw/Nkwanta
0.11±0.00
0.10±0.00
0.09±0.00
0.10±0.00
0.25±0.00
0.24±0.00
Average
0.11±0.00
0.12±0.00
0.12±0.00
0.11±0.00
0.21±0.00
0.17±0.00
Maximum limit (IAEA 2009)
4.3×10−2
4.3×10−2
3.5×10−5
3.5×10−5
–
–
Environ Monit Assess (2015) 187:339
The estimated concentration ratio for 238U in cassava ranges from 0.04 to 0.20 with an average value of 0.11, while that for yam ranges from 0.03 to 0.22 with an average value of 0.12. It is realized that the concentration ratio for 238U in yam is slightly higher in yam compared to that of cassava. The calculated concentration ratio for 232Th in cassava ranges from 0.04 to 0.26 with an average of 0.12, while that of yam ranges from 0.04 to 0.20 with an average of 0.11. On the contrary, the concentration ratio for 232Th in cassava is higher compared with that of yam. Finally, the estimated concentration ratio for 40K in cassava ranges from 0.15 to 0.40 with an average value of 0.2. However, that of yam ranges from 0.13 to 0.24 with an average value of 0.17. Again, cassava registers a higher average concentration ratio for 40K in cassava than in yam. Generally, it can be observed that 40K recorded the highest concentration ratio pointing to the fact that it is an essential plant nutrient. The averaged soil-to-plant uptake of 232 Th and 238U was nearly three times more than the recommended in both cassava and yam. Soil-to-plant transfer of radionuclides is influenced by factors such as physicochemical characteristics of the radionuclides, soil properties, contact time, type of crops and soil management practices (Ng et al. 1982).
Conclusion The radionuclide concentration in water, soil, cassava and yam grown in the Tano-North District of Ghana has been determined. Furthermore, the risk due to exposure to radiation to an adult member of the public through ingestion of tuber crops and water was estimated. The estimated average annual effective dose due to the injection of the NORMS in water by human population was about half the 100 μSv/year recommended by WHO. However, borehole water from two sites (Bomaa and Techire Amangoase) recorded values slightly above the recommended limits. It can be stated with certainty that the ingestion of water by the inhabitants of the Tano-North District will not result in any significant radiological threat in the near future. The average activity concentration of 238U, 232Th, 40 K, and 137Cs in soil samples from various farmlands in the district was estimated to be 23.19, 31.10, 143.78 and 2.88 Bq/kg, respectively. The established absorbed dose rate for soil samples from selected farms ranges between 23.63 and 50.51 μSv/year, which is within
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allowable range of 18 to 93 μSv/year established by the UNSCEAR (2000). Internal exposure due to the ingestion of radionuclides in cassava and yam has been established in the study. The average activity concentration of 238U, 232Th, 40K and 137Cs in cassava was determined to be 2.67, 3.99, 29.21 and 0.70 Bq/kg, respectively. Furthermore, the mean activity concentration of 238U, 232Th, 40K and 137Cs in yam was also determined to be 2.78, 3.16, 23.58 and 0.71 Bq/kg, respectively. These concentrations correspond to a mean committed effective dose of 29.19 and 9.90 μSv/year for cassava and yam, respectively. These values of 29.19 and 9.90 μSv/year due to the consumption of cassava and yam from the district cannot be linked to any radiation hazard to the population. The availability of data from such a study is very useful as it serves as vital information to all stakeholders concerned with food and drinking water quality. It complements data required for setting of guidelines on radiological safety for food and drinking water worldwide. Finally, levels of activity concentration obtained will serve as a reference for future studies in the district and the surrounding areas. The study is important and timely because radiological assessment of our environment is necessary especially when the environment is of economic and social importance. The database on radioactivity in drinking water and associated radiation dose to the population of many communities is not available and is required to maintain human drinking water standards globally.
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Distribution of natural and artificial radioactivity in soils, water and tuber crops Godfred Darko & Augustine Faanu & Osei Akoto & Akwasi Acheampong & Eric Jude Goode & Opoku Gyamfi
Received: 10 October 2014 / Accepted: 30 April 2015 # Springer International Publishing Switzerland 2015
Abstract Activity concentrations of radionuclides in water, soil and tuber crops of a major food-producing area in Ghana were investigated. The average gross alpha and beta activities were 0.021 and 0.094 Bq/L, respectively, and are below the guidelines for drinking water and therefore not expected to pose any significant health risk. The average annual effective dose due to ingestion of radionuclide in water ranged from 20.08 to 53.45 μSv/year. The average activity concentration of 238 U, 232Th, 40K and 137Cs in the soil from different farmlands in the study area was 23.19, 31.10, 143.78 and 2.88 Bq/kg, respectively, which is lower than world averages. The determined absorbed dose rate for the farmlands ranged from 23.63 to 50.51 nGy/year, which is within worldwide range of 18 to 93 nGy/year. The activity concentration of 238U, 232Th, 40K and 137Cs in cassava ranges from 0.38 to 6.73, 1.82 to 10.32, 17.65 to 41.01 and 0.38 to 1.02 Bq/kg, respectively. Additionally, the activity concentration of 238U, 232Th, 40K and 137 Cs in yam also ranges from 0.47 to 4.89, 0.93 to 5.03, 14.19 to 35.07 and 0.34 to 0.89 Bq/kg, respectively. The average concentration ratio for 238U, 232Th and 40K in yam was 0.12, 0.11 and 0.17, respectively, and in cassava was 0.11, 0.12 and 0.2, respectively. None of the G. Darko (*) : O. Akoto : A. Acheampong : E. J. Goode : O. Gyamfi Department of Chemistry, Kwame Nkrumah University of Science and Technology, Kumasi, Ghana e-mail: [email protected] A. Faanu Radiation Protection Institute, Ghana Atomic Energy Commission, Accra, Ghana
radioactivity is expected to cause significant health problems to human beings. Keywords Radioactivity . Activity . Dose . Concentration
Introduction Natural radionuclides are present at low concentrations in the soil, water, air and food. Whereas natural radioactivity comes mainly from long-lived primordial radionuclides that are widespread in the Earth’s crust (Alharbi and El-Taher 2013), human activities such as mining and generation of nuclear power add artificial radionuclides to the environment. Human beings are therefore exposed to radiations from sources outside the body and from radionuclides that are ingested through consumption of food and water or inhaled radioactive gases such as radon (Ehsanpour et al. 2014; Taskin et al. 2009). Radionuclides with relatively long half-lives are considered human health risk as they can get into the human system through the food chain and thereby increase the radiation burden for many years (Abu-Khadral et al. 2008). Ingestion of food crops loaded with naturally occurring radioactive materials (NORMs) has the potential of exposing human beings to high level of radiation doses (Bengtsson et al. 2012). Ingested radionuclides may accumulate in organs such as bones from where they exert toxic effects. About 66 % of uranium ingested is rapidly eliminated via urine (Pakade et al. 2012), while the rest is distributed and stored in the kidneys (12–
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25 %), bone (10–15 %), and soft tissue (Canu et al. 2011; Evensen et al. 2002; Wrenn et al. 1985). Radium deposits mostly in the bones (Canu et al. 2011), and radon gas diffuses into the stomach wall (Hopke et al. 2000). Since exposure doses are strongly related to the amount of radionuclides present, accurate evaluation of dietary intake is an important parameter for the radiological protection of the population (Cevik et al. 2006). Moreover, measuring the levels of natural and artificial radiation in the environment is crucial in implementing appropriate controls for the sake of radiological protection (Kinyua et al. 2011). Water is an important matrix in environmental studies because of its daily use for human consumption and its ability to transport pollutants (Degerlier and Karahan 2010). Measurement of natural radioactivity in soil is very important to determine because it helps in monitoring changes in natural background activity with time as a result of any radioactivity release. Many developed countries have taken steps to measure the levels of background radiations in their environments (Appl et al. 1998; Bozkurt et al. 2007; Madruga 2008; Palomo et al. 2007) in order to reduce human exposure. However, not much research work has been carried out on the radiological quality of the environment in most developing countries. There are limited detailed radiological data on the environment, water and food items in Ghana (Adu et al. 2011; Faanu et al. 2011a, b; Nguelem et al. 2013), and the public, regrettably, is unaware of the potential radiological hazards associated with the soil, food and water that they use. Undoubtedly, higher levels of radionuclide concentration in food and water have an adverse effect on the health of people exposed to these radionuclides. Knowledge of the levels and distribution of radionuclides in the environment is necessary if levels of human exposure to radiation from radionuclides are to be controlled. In this study, we determined the activity concentrations of natural radionuclides (238U, 232Th and 40K) as well as an artificial radionuclide (137Cs) in water, root tubers (cassava, yam, and cocoyam) and soil samples from the Tano-North District of Ghana. Our results were used to compute the radiation doses and compared with recommended dose limits in water, soil, and tuber crops in order to assess the public health impact from the activity concentrations and calculated effective dose rate. This study will provide pioneering and baseline data for the district. This is an important requirement for establishing and maintaining standards regarding activity concentration of soil, tuber crops and water.
Materials and methods Study area The Tano-North district of Ghana lies between latitude 7°00′ and 7° 25′ N and longitude 2° 3′ and 2° 15′ W covering a total land area of 876 km2. Agriculture is the main economic activity in the district. The main source of water in the district is borehole. However, some of the communities depend on streams, rivers, springs and wells as their source of water. The district is located in the moist semi-deciduous forest zone of Ghana. The geology of the district is made of the middle Precambrian formation (Milési et al. 1991). Generally, the soil in the district is fertile with abundant arable land which favours the cultivation of a wide range of both food and cash crops. The district which is a major food basket in the country is experiencing increasing mining activities including galamsey, while most of the communities depend on borehole and surface water sources for domestic use. Like in many developing countries, the levels of natural and artificial radionuclides in groundwater, soil and tuber crops grown in the district are not known. Figure 1 shows the map of the area and the sampling sites. Sampling and sample preparation Forty water samples from boreholes, wells and streams were collected from random sites across the district. Similarly, tuber crops (yam, cassava and cocoyam), which were ready for harvest, and the top soils (up to 10 cm) in which they grow were also collected. Sampling was carried out between August–December 2013 (first batch) and March–May 2014 (replicate). Physical parameters of the water samples were determined on site using a Hanna combometer coupled to an HI 98129 glass electrode (Hanna, Mauritius) and 1.5-L aliquots were taken into pre-cleaned bottles for radioactivity measurements in the lab. Measurement of gross-α and gross-β Water samples were filtered to remove undesirable solid particles and transferred into 1-L Marinelli beakers for gamma spectrometry. Aliquots of 300 mL of filtered samples acidified with 3 mL of HNO3 were evaporated to 10 mL on a hot plate. These were transferred into pre-
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Fig. 1 Map of Tano-North District showing the sampling points
weighed 25-mL stainless steel planchet and evaporated to dryness. The planchets were left to cool in desiccators and re-weighed. Measurements were performed on a gross α-β spectrometry coupled to a low-background gasless automatic α/β counting system (Canberra iMatic™, USA) and a solid-state passivated implanted planar silicon detector. Calibration was done using 241 Am standard for α and 90Sr for β emission. Efficiency was found to be 36.39±2.1 % for α and 36.61±2.2 % for β, while the background counting rates were 0.04± 0.01 and 0.22±0.03 counts/min for α and β, respectively. The minimum detectable activity was 0.0007 Bq for the α and 0.0037 Bq for the β emitters.
were then oven-dried at 105 °C to a constant weight. They were ground into fine powder with a ball mill grinder and sieved through a 2-mm-pore-size mesh into a Marinelli beaker. The samples were transferred into a Marinelli beaker up to the recommended level and weighed to determine the mass and then sealed and stored in the laboratory for 30 days to allow for equilibration between long-lived parent radionuclides and their short-lived daughter radionuclides. For the food samples, edible parts of root tubers were chopped into small pieces, washed and freeze-dried (model: Christ LMCV-1). The dried samples were ground into fine powder, weighed and stored prior to analysis.
Activity measurement in solid samples
Description of gamma spectrometer
Soil samples were air-dried for 5 days and homogenized after removing extraneous materials from them. They
The samples were measured with a high-resolution gamma-ray spectrometer consisting of an n-type
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ORTEC high-purity germanium detector (model GMX40P4, USA) in a vertical configuration. The detector crystal of 63.0 mm Ø and the length of about 65.0 mm have a resolution of 1.95 keV and relative efficiency of 40 % for 1.33 MeV 60Co. The system is coupled to a PCA-MR 8192 Multi-Channel Analyzer (MCA) provided with ORTEC Maestro 32 MCB configuration software for spectrum acquisition, evaluation and analysis. The detector was operated at a voltage of 4800 V (reverse biased) and cooled with liquid nitrogen at −196 °C. A cylindrical lead shield (20 mm) with a fixed bottom and a movable lid is used to shield the detector in order to reduce background gamma radiation. Within the lead shield are also copper, cadmium and plexiglass (3 mm each) to absorb X-rays and other photons that might be produced in the lead as result of the interaction with the photons. Calibration of the gamma spectrometer A 1.0 g/m3 multi-nuclide standard solution (IAEA, Vienna) containing 241Am (59.54 keV), 109Cd (88.03 keV), 57 Co (122.06 keV), 139 Ce (165.86 keV), 203 Hg (279.20 keV), 113Sn (391.69 keV), 85Sr (514.01 keV), 137 Cs (661.66 keV), 60Co (1173.2 and 1332.5 keV) and 88 Y (898.04 and 1836.1 keV) was used to calibrate the instrument for energy and efficiency prior to sample analysis. However, only four radionuclides (241Am, 57 Co, 137Cs and 60Co) were selected for the energy calibration because the short-lived radionuclides did not give significant peaks. Standards and samples were measured in the 1.0-L Marinelli beaker (Deutsher Kalibrierdient-3 QSA Global GmBH, Germany) using a counting time of 36,000 s to acquire spectra data. Energy calibration was performed by matching the energy of the principal γ-rays in the spectrum of the standard reference material to the channel number of the spectrometer (Faanu et al. 2011a, b). The efficiency calibration was performed by acquiring a spectrum of the standard until the count rate at the peak of total absorption can be calculated with statistical uncertainty of less than 1 % at a confidence level of 95 %. The net count rate was determined at the photo peaks for all the energy values to be used for the determination of the efficiency at the time of measurement. The efficiency at each energy value was plotted as a function of the peak energy and extrapolated to determine the efficiency at other peak energy values for the measurement geometry used (Nguelem et al. 2013).
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Calculation of activity concentration The activity concentration of 238U and 232Th was calculated from the average peak energy values of 1764.49 of 214 Bi, 609.31 keV of 214Bi, 238.63 keV of 212Pb and 911.21 keV of 228Ac, respectively. The activity concentrations of 137Cs and 40K were determined from the energy values of 662 and 1460.83 keV, respectively. The analytical expression used to calculate the activity concentrations for soils and foodstuff (Bq/kg) and water (Bq/L) is described elsewhere (Oresengun et al. 2010).
Estimation of doses The external gamma dose rate at 1.0 m above ground for soil samples was calculated from the activity concentrations using conversion factors of 0.0417, 0.462 and 0.604 nGy/h/Bq/kg, respectively, for 40K, 238U and 232 Th (Amin et al. 2011). The annual effective dose was calculated from the absorbed dose rate by applying the dose conversion factor of 0.7 Sv/Gy and an outdoor occupancy factor of 0.2 (UNSCEAR 2000). In the case of the water samples, the committed effective doses were estimated from the activity concentrations of each individual radionuclide and application of the yearly water consumption rate for adults of 730 L/year (2 L/ day by 365 days) and the dose conversion factors of 238 U, 232Th and 40K taken from the BSS and United Nations Scientific Committee on the Effect of Atomic Radiation (UNSCEAR) report (IAEA 2001).
Committed annual effective dose The annual effective dose due to ingestion of radionuclides in water (mSv/year) was computed on the basis of the activity concentrations of the radionuclides, assuming an average daily water intake of 2 L for an adult (WHO 2006) and ingestion dose coefficient of 4.5× 10−5 mSv/Bq for 238U, 2.3×10−4 mSv/Bq for 232Th and 6.2×10−6 mSv/Bq for 40K (Alam et al. 1999). The committed effective dose owing to ingestion of 238U, 232 Th, 40K and 137Cs in foodstuffs was calculated as the sum of the products of activity concentration of radionuclides, annual foodstuff intake of 170 kg/year (Angelucci 2013) and the dose coefficients of 4.5× 10−5, 7.2×10−5, 6.2×10−6, 1.3×10−8 mSv/Bq for 238U, 232 Th, 40K and 137Cs, respectively (ICRP 2007).
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Results and discussion Gross-α and gross-β analyses generally serve as a means of screening water samples to assess the radiological quality. The WHO recommended guidelines on gross-α and gross-β in drinking water are 0.5 and 1.0 Bq/L, respectively. Table 1 presents the physical parameters and the gross-α and gross-β activities in the water samples. The gross alpha activity concentration in water varied between 0.007 and 0.039 Bq/L. The highest gross alpha value was recorded at Duayaw/Nkwanta with a value of 0.039 Bq/L, while the lowest gross alpha concentration was recorded from the main underground water in Bomaa with a value of 0.007 Bq/L, respectively. The gross-β activity concentration in water varied between 0.011 and 0.181 Bq/L with an average value of 0.091 Bq/L. The highest gross beta activity of 0.181 Bq/L was recorded at Boukrukruwa, while the lowest gross beta value was recorded at Duayaw/ Nkwanta with a value of 0.011 Bq/L. All the water samples measured gave gross-α and gross-β values lower than the WHO guidelines. The measured values compare well with data from Turkey (Degerlier and Karahan 2010), Tarkwa Ghana (Faanu et al. 2010), Spain (Pujol and Sanchez-Cabeza 2000) and Kumasi, Ghana (Darko et al. 2014). Duayaw/Nkwanta showed activity concentrations of 238 U, 232Th and 40K in ranges of 0.21 to 0.53, 0.23 to 1.41 and 2.49 to 11.89 Bq/L with averages of 0.30± 0.02, 0.53±0.04 and 4.87±0.73 Bq/L, respectively. The maximum activity concentration of 238U, 232Th and 40K measured for Duayaw/Nkwanta was 0.53±0.05, 1.41± 0.08 and 11.89±0.74 Bq/L, respectively. Activities of 238 U, 232Th and 40K measured for Boukrukruwa had concentration ranges of 0.19 to 0.26, 0.21 to 0.26 and 2.59 to 4.15 Bq/L, respectively, and had averages of 0.23±0.01, 0.23±0.01 and 3.12±0.32 Bq/L, respectively. The maximum activity due to 238U, 232Th and 40K in
Boukrukruwa is 0.26± 0.02, 0.26 ± 0.02 and 4.15 ± 0.42 Bq/L, respectively. The lowest activities due to 238 U, 232Th and 40K were also measured to be 0.19± 0.01, 0.21±0.01 and 2.59±0.26 Bq/L, respectively. Bomaa recorded activity concentration for 238U, 232Th and 40K in ranges of 0.13 to 0.99, 0.13 to 1.36 and 1.84 to 12.00 Bq/L with average activities of 0.38±0.03, 0.41±0.03 and 4.29±0.32 Bq/L, respectively. The maximum activities for 238U, 232Th and 40K measured were 0.99±0.08, 1.36±0.09 and 12.00±1.12 Bq/L, respectively, while the lowest activities are 0.13±0.01, 0.11± 0.011 and 1.84±0.06 Bq/L. The activity concentrations of 238U, 232Th and 40K measured at Techire varied from 0.18 to 0.73, 0.18 to 1.47 and 2.47 to 12.38 Bq/L with average values of 0.35 ±0.02, 0.54±0.03 and 5.05±0.59 Bq/L, respectively. The maximum activities for 238U, 232Th and 40K recorded for Techire were 0.73±0.06, 1.47±0.08 and 12.38±1.27 Bq/ L, whereas the lowest activity values are 0.18±0.01, 0.18 ±0.01 1 and 2.47±0.12 Bq/L, respectively. At Tanoso, the activity concentrations for the radionuclides varied from 0.28 to 0.46, 0.25 to 0.35 and 2.77 to 3.87 Bq/L for 238 U, 232Th and 40K, respectively. The maximum activity due to 238U and 40K was 0.46±0.05 and 3.87±0.35 Bq/L, respectively, while the maximum value for 232Th was recorded as a value of 0.35±0.02 Bq/L. The lowest activity for 238U was measured as 0.28±0.02. Finally, the minimum activity for 40K was 2.77±0.19 Bq/L. The activity concentration of 238U in all the water samples was about ten times lower than the WHO guideline level of 10 Bq/L in drinking water. The activity concentration of 232Th in about 86 % of water samples was below the WHO guideline level of 1.0 Bq/L. The average activity concentrations of 40K were higher than those of all the other radionuclides in all the samples. The maximum average activities of 40K, 238 U and 232Th were 5.05±0.59, 0.38±0.03 and 0.54± 0.03 Bq/L, respectively. The maximum average activities for 40K and 232Th were measured in water samples
Table 1 Physical parameters and gross-α and gross-β activities of water samples Location
Temperature (°C)
Conductivity (μS/cm)
pH
TDS (ppm)
Gross-α (Bq/L)
Gross-β (Bq/L)
Duayaw/Nkwanta
24.55±0.13
110.00±42.93
5.79±0.13
56.50±12.01
0.03±0.01
0.10±0.06
Boukrukruwa
25.00±1.07
151.75±57.97
5.97±0.34
75.75±27.82
0.02±0.01
0.13±0.04
Bomaa
24.56±0.25
249.60±16.57
6.16±0.45
124.61±57.99
0.01±0.00
0.09±0.01
Techire
24.55±0.06
281.25±49.00
6.48±0.58
111.50±73.46
0.01±0.00
0.07±0.01
Tanoso
24.45±0.06
181.50±19.33
6.72±0.85
104.25±41.70
0.02±0.01
0.08±0.01
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from Techire and its environs with values 5.05±0.59 and 0.54±0.03 Bq/L, respectively. However, the maximum average activity for 238U was measured in water samples collected from Bomaa and its environs with a value of 0.38±0.03 Bq/L. The differences in activities of the water samples could be attributed to the differences in NORMs present in the bedrocks. The occurrence and distribution of radioactivity in water largely depend on factors such as the local geological characteristics of the source (Shashikumar et al. 2011). Radiological safety of drinking water is an important water quality parameter of concern. The regulation and guidelines used by most countries are based on the WHO limits. The annual committed effective dose of radionuclides ingested through water is presented in Table 2. The average annual committed effective dose for Duayaw/Nkwanta, Boukrukruwa, Bomaa, Techire and Tanoso was 40.43± 2.17, 20.08± 1.89, 33.58± 1.91, 53.45±3.77 and 24.90±2.61 μSv/year, respectively. It indicates that the inhabitants of Techire receive the highest radiation dose upon water ingestion, the inhabitants of Duayaw/Nkwanta followed, whereas the people of Boukrukruwa received the least effective dose due to NORMS in drinking water. The WHO ceiling on annual committed effective dose due to ingestion of radionuclides in water is 100 μSv/year (WHO 2006). This guideline is accepted by member states, the European Commission and the Food and Agriculture Organization. The average annual committed effective dose in this study did not exceed the WHO guideline level of 100 μSv/year. Therefore, no harmful radiological health effects are expected from the consumption of drinking water from the sampled sources. The findings in this study show that the activity concentrations determined are higher
than those found in Spain (Pujol and Sanchez-Cabeza 2000), Morocco (Hakam et al. 2001) and Lake Bosomtwi in Ghana (Adu et al. 2011) but are lower than the findings in Yemen (El-Mageed et al. 2013) and Cape Coast in Ghana (Faanu et al. 2011a, b). The activity concentration of the radionuclides, their absorbed dose rate and the annual effective dose with soil samples are given in Table 3. The activity concentration of 238U, 232Th, 40K and 137Cs in soil ranges from 14.56 to 34.47, 18.88 to 48.17, 83.11 to 191.76 and 0.183 to 1.46 Bq/kg, respectively. The low concentration of 137Cs could be attributed to the fact that 137Cs is not naturally found in the soil. Since Ghana has no nuclear plants, the traces of 137Cs detected are to be presumed to have been deposited through atmospheric transfer from a nuclear power plant somewhere or from the testing of a nuclear weapon. The concentrations of 40K in the soil samples were lower compared with the global average value of 400 Bq/kg (Manigandan and Chandar 2014; Ndontchueng et al. 2014). About 80 % of the soil samples analyzed recorded lower concentrations of 238 U compared to the global average of 30 Bq/kg (UNSCEAR 2010), while 70 % of the soil samples had 232Th less than the global averages of 35 Bq/kg (UNSCEAR 2010). However, soil samples from farms located in Bomaa and Koforidua recorded average activity concentration of 33.73 and 34.47 Bq/kg, respectively, for 238U that is higher compared with the world average for 238U. Furthermore, soil samples collected from farms located in Bomaa, Subriso, and Koforidua recorded average activities of 46.42, 42.64 and 48.17 Bq/kg for 232Th, respectively, which were also higher than the world average of 35.0 Bq/kg. Those soil samples with higher concentrations of 238U and 232Th greater than the global average could
Table 2 Average activity concentration and effective dose due to 40K, 238U and 232Th in water samples Sample location
Activity concentration (Bq/L) 40
K
238
U
Committed effective dose (μSv/year) 232
Th
Duayaw/Nkwanta
4.87±0.36
0.30±0.07
0.53±0.13
40.43±2.17
Boukrukruwa
3.12±0.33
0.23±0.02
0.23±0.02
20.08±1.89
Bomaa
4.24±0.32
0.38±0.02
0.41±0.02
33.58±1.91
Techire
5.05±0.39
0.35±0.02
0.54±0.03
53.45±3.77
Tanoso
3.15±0.27
0.36±0.03
0.29±0.02
24.90±2.61
Guideline level (WHO 2004)
N/A
10
1
100
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Table 3 Activity concentration, absorbed dose rate, and annual effective dose due to 238U, 232Th, 40K and 137Cs in soil Sample ID
Activity concentration (Bq/kg) 238
U
232
Th
40
K
Absorbed dose rate, nGy/h 137
Cs
Annual Percentage contribution of radionuclide effective to absorbed dose rate (%) dose, mSv 238 232 40 U Th K
Bomaa
33.77±2.35 46.42±3.79 164.76±6.09 2.53±0.12 50.51±2.32 0.06±0.01
30.89±1.01
55.51±0.41
13.60±0.00
Subriso
29.92±2.89 42.64±3.13 162.38±5.49 3.51±0.14 46.34±2.01 0.06±0.00
29.82±0.21
55.57±1.05
14.61±1.91
Techire
19.79±1.79 22.52±1.90 163.40±6.35 1.64±0.04 29.56±1.21 0.04±0.01
30.93±1.00
46.02±2.01
23.05±0.54
Bredi no. 1
21.38±1.86 33.89±2.69 132.49±4.29 3.64±0.14 35.87±2.00 0.04±0.01
27.53±2.14
47.07±2.00
15.40±0.00
Koforidua
34.47±2.72 48.17±3.80 159.29±5.53 3.92±0.15 51.66±2.12 0.06±0.00
30.82±1.09
56.32±2.10
12.86±0.51
Afrisipa
22.14±1.89 24.89±2.01 103.07±3.02 2.91±0.12 29.56±0.99 0.04±0.00
34.61±0.04
50.86±0.95
14.54±0.32
Bredi no. 2
18.59±1.23 18.89±1.11
87.07±2.13 1.91±0.10 23.63±1.11 0.03±0.00
36.85±0.23
48.28±0.07
15.37±1.11
Benchem
14.56±1.18 20.72±1.84 191.76±7.20 3.92±0.15 27.24±1.09 0.03±0.00
24.70±2.07
45.94±1.05
29.35±0.40
Boukrukruwa
17.33±1.99 23.22±2.01 190.48±7.11 3.39±0.12 29.97±1.52 0.04±0.00
26.71±1.00
46.79±0.74
26.50±0.18
Duayaw/Nkwanta 19.89±1.99 29.66±2.19
83.11±2.24 1.46±0.07 30.57±2.00 0.04±0.00
30.06±1.91
58.60±0.06
11.34±0.11
Min.
14.56±2.18 18.89±2.76
83.11±3.24 1.46±0.07 23.63±1.98 0.04±0.00
24.70±0.97
45.94±0.32
11.34±1.01
Max.
34.47±3.72 48.17±5.40 191.76±8.23 3.92±0.18 50.51±3.20 0.06±0.00
36.35±2.09
58.60±1.00
29.35±2.10
Average
23.19±1.99 31.10±2.45 143.78±4.23 2.88±0.12 35.49±2.51 0.04±0.00
30.24±1.31
52.10±0.99
17.66±0.04
primarily be attributed to the presences of rocks bearing high concentration of those radionuclides in the area. The presence of radionuclides in food crops is mainly due to the uptake of radionuclides from the soil by the root system of plants. Table 4 shows the activity concentration of radionuclides in cassava from different farms in the study area. Concentrations of 238U, 232Th, 40 K and 137Cs varied from 0.38 to 6.73, 1.82, 17.65 to 41.01 and 0.38 to 1.02 Bq/kg, respectively. The average values of the radionuclides are 2.674±0.14, 3.799± 0.23, 29.207±2.36 and 0.70±0.04 Bq/kg, respectively. The activity concentration of 238U, 232Th, 40K and 137Cs in yam ranges between 0.47 and 4.89, 0.93 and 5.03, 14.19 and 35.07 and 0.34 and 0.89 Bq/kg, respectively. The average values for the radionuclides are 2.776± 0.17, 3.162±0.21, 23.879±1.55 and 0.714±0.05 Bq/ kg in that order. It can be observed that the concentration of all the radionuclides in cassava and yam is less than the amount in the corresponding farmlands. For radionuclides to be absorbed by the root system of plants, they must be in the soluble form in the soil (IAEA 2005; KabataPendias 2000). Since all radionuclides in the soil cannot be in the soluble state, coupled with variations in the solubility of radionuclides, it is expected that the radionuclide concentration in the soil will be higher compared with that in food plants. Additionally, there was a greater uptake of 40K than the other radionuclides by
cassava and yam. This can be attributed to the fact that 40 K has a higher solubility than the other radionuclides. Additionally, it is an essential element which is required for plant growth and metabolism. Moreover, 40K can be added to the soil through the application of fertilizer, which is a common practice in the area. The concentration of 137Cs in cassava and yam was very low, and this may be due to the very low concentration of 137Cs in soil which subsequently affects the absorption and utilization by plants. Generally, cassava accumulated slightly higher concentration of radionuclides compared to yam. Cassava and yam form the major constituent of the staple food of the people of the Tano-North area which is fufu and ampesi, respectively. The average annual committed effective dose received by the population due to consumption of cassava and yam in the study area is 29.19 and 9.90 μSv/year. This means that consumption of cassava and yam from the study area does not pose any radiological health risk as the estimated committed effective doses were much less than the WHO guideline of 100 μSv/year. Concentration ratio (Fv) which is the ratio of the activity concentration of radionuclides in the plant to that in the soil (IAEA 2009) is an indication of uptake of any radionuclides in a layer of specified soil to plants that grow in the soil. Table 5 shows the extent of transfer of 238U, 232Th and 4K from the soil to cassava and yam.
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Table 4 Activity concentration of radionuclide in cassava and yam samples Location
Sample
Activity concentration (Bq/kg (dry weight) 238
232
U
Bomaa Subriso Techire Bredi no. 1 Koforidua Afrisipa Bredi no. 2 Benchem Boukrukruwa Duayaw/Nkwanta Average
Th
Committed effective dose (μSv/year)
40
137
K
Cs
Cassava
1.40±0.10
2.43±0.11
38.21±3.76
0.55±0.03
Yam
3.45±0.21
3.28±0.24
35.07±3.34
0.47±0.03
23.50±1.46 8.64±0.21
Cassava
2.40±0.12
3.87±0.29
28.03±1.40
0.79±0.05
23.30±1.08
Yam
2.43±0.15
3.35±0.25
29.58±2.43
0.34±0.02
25.10±1.10 22.70±1.32
Cassava
2.74±0.22
1.82±0.08
29.87±2.46
0.54±0.03
Yam
1.72±0.09
1.74±0.04
24.59±1.43
0.47±0.03
5.03±0.01
Cassava
0.38±0.02
4.85±0.35
19.89±1.11
0.48±0.03
32.60±1.30
Yam
4.76±0.29
3.26±0.22
19.24±0.89
0.85±0.05
8.08±0.01
Cassava
6.73±0.42
10.32±0.64
27.76±2.42
1.02±0.06
63.70±2.24
Yam
4.89±0.29
4.99±0.31
25.79±1.37
0.84±0.05
10.50±0.56
Cassava
4.49±0.31
6.48±0.42
41.01±4.02
0.94±0.06
48.00±2.12
Yam
4.54±0.28
5.03±0.41
17.79±0.66
0.77±0.05
9.67±0.00
Cassava
1.75±0.06
0.84±0.04
17.65±0.89
0.88±0.05
12.80±0.23
Yam
2.74±0.23
3.17±0.20
14.19±0.62
0.78±0.05
6.26±0.01 22.30±0.11
Cassava
0.97±0.04
2.75±0.14
30.19±2.51
0.86±0.05
Yam
0.74±0.03
0.93±0.08
24.19±1.42
0.75±0.04
3.52±0.00
Cassava
0.77±0.03
1.84±0.09
39.08±3.85
0.57±0.03
21.10±1.01
Yam
0.47±0.02
2.85±0.18
27.99±2.38
0.89±0.06
5.70±0.06
Cassava
2.13±0.11
2.77±0.14
20.39±1.22
0.38±0.03
21.90±1.07
Yam
2.02±0.14
3.01±0.19
20.34±0.95
0.97±0.07
6.54±0.23
Cassava
2.67±0.14
3.99±0.23
29.21±2.36
0.70±0.04
29.19±2.19
Yam
2.78±0.17
3.16±0.21
23.88±1.55
0.71±0.05
9.90±0.09
Table 5 Transfer of radionuclides from soil to cassava and yam
Bomaa
Transfer due to 238U
Transfer due to 232Th
Transfer due to 40K
Cassava
Yam
Cassava
Yam
Cassava
Yam
0.04±0.00
0.10±0.00
0.05±0.00
0.07±0.00
0.23±0.00
0.21±0.00
Subriso
0.08±0.00
0.08±0.01
0.09±0.00
0.08±0.00
0.17±0.00
0.18±0.00
Techire
0.14±0.00
0.09±0.00
0.08±0.00
0.08±0.00
0.18±0.00
0.15±0.00
Bredi no. 1
0.16±0.00
0.22±0.00
0.14±0.00
0.10±0.00
0.15±0.00
0.15±0.00
Koforidua
0.19±0.00
0.14±0.00
0.21±0.01
0.10±0.00
0.17±0.00
0.16±0.00
Afrisipa
0.20±0.00
0.20±0.00
0.26±0.01
0.20±0.00
0.40±0.00
0.17±0.00
Bredi no. 2
0.09±0.00
0.15±0.00
0.04±0.00
0.17±0.00
0.20±0.00
0.16±0.00
Benchem
0.07±0.00
0.05±0.00
0.13±0.00
0.04±0.00
0.16±0.00
0.13±0.00
Boukrukruwa
0.04±0.00
0.03±0.00
0.08±0.00
0.13±0.00
0.21±0.00
0.15±0.00
Duayaw/Nkwanta
0.11±0.00
0.10±0.00
0.09±0.00
0.10±0.00
0.25±0.00
0.24±0.00
Average
0.11±0.00
0.12±0.00
0.12±0.00
0.11±0.00
0.21±0.00
0.17±0.00
Maximum limit (IAEA 2009)
4.3×10−2
4.3×10−2
3.5×10−5
3.5×10−5
–
–
Environ Monit Assess (2015) 187:339
The estimated concentration ratio for 238U in cassava ranges from 0.04 to 0.20 with an average value of 0.11, while that for yam ranges from 0.03 to 0.22 with an average value of 0.12. It is realized that the concentration ratio for 238U in yam is slightly higher in yam compared to that of cassava. The calculated concentration ratio for 232Th in cassava ranges from 0.04 to 0.26 with an average of 0.12, while that of yam ranges from 0.04 to 0.20 with an average of 0.11. On the contrary, the concentration ratio for 232Th in cassava is higher compared with that of yam. Finally, the estimated concentration ratio for 40K in cassava ranges from 0.15 to 0.40 with an average value of 0.2. However, that of yam ranges from 0.13 to 0.24 with an average value of 0.17. Again, cassava registers a higher average concentration ratio for 40K in cassava than in yam. Generally, it can be observed that 40K recorded the highest concentration ratio pointing to the fact that it is an essential plant nutrient. The averaged soil-to-plant uptake of 232 Th and 238U was nearly three times more than the recommended in both cassava and yam. Soil-to-plant transfer of radionuclides is influenced by factors such as physicochemical characteristics of the radionuclides, soil properties, contact time, type of crops and soil management practices (Ng et al. 1982).
Conclusion The radionuclide concentration in water, soil, cassava and yam grown in the Tano-North District of Ghana has been determined. Furthermore, the risk due to exposure to radiation to an adult member of the public through ingestion of tuber crops and water was estimated. The estimated average annual effective dose due to the injection of the NORMS in water by human population was about half the 100 μSv/year recommended by WHO. However, borehole water from two sites (Bomaa and Techire Amangoase) recorded values slightly above the recommended limits. It can be stated with certainty that the ingestion of water by the inhabitants of the Tano-North District will not result in any significant radiological threat in the near future. The average activity concentration of 238U, 232Th, 40 K, and 137Cs in soil samples from various farmlands in the district was estimated to be 23.19, 31.10, 143.78 and 2.88 Bq/kg, respectively. The established absorbed dose rate for soil samples from selected farms ranges between 23.63 and 50.51 μSv/year, which is within
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allowable range of 18 to 93 μSv/year established by the UNSCEAR (2000). Internal exposure due to the ingestion of radionuclides in cassava and yam has been established in the study. The average activity concentration of 238U, 232Th, 40K and 137Cs in cassava was determined to be 2.67, 3.99, 29.21 and 0.70 Bq/kg, respectively. Furthermore, the mean activity concentration of 238U, 232Th, 40K and 137Cs in yam was also determined to be 2.78, 3.16, 23.58 and 0.71 Bq/kg, respectively. These concentrations correspond to a mean committed effective dose of 29.19 and 9.90 μSv/year for cassava and yam, respectively. These values of 29.19 and 9.90 μSv/year due to the consumption of cassava and yam from the district cannot be linked to any radiation hazard to the population. The availability of data from such a study is very useful as it serves as vital information to all stakeholders concerned with food and drinking water quality. It complements data required for setting of guidelines on radiological safety for food and drinking water worldwide. Finally, levels of activity concentration obtained will serve as a reference for future studies in the district and the surrounding areas. The study is important and timely because radiological assessment of our environment is necessary especially when the environment is of economic and social importance. The database on radioactivity in drinking water and associated radiation dose to the population of many communities is not available and is required to maintain human drinking water standards globally.
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