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ORIGINAL ARTICLE
Year : 2012  |  Volume : 35  |  Issue : 2  |  Page : 84-89  

Measurement of radon activity in soil samples of some selected towns across the Lake Bosumtwi basin, Ghana


1 Radiation Protection Institute, Ghana Atomic Energy Commission, Box LG 80, Legon, Accra, Ghana
2 Graduate School of Nuclear and Allied Sciences, University of Ghana, Kumasi, Ghana
3 Graduate School of Nuclear and Allied Sciences, University of Ghana, Kumasi; Department of Physics, Kwame Nkrumah University of Science and Technology, Kumasi, Ghana
4 Radiation Protection Institute, Ghana Atomic Energy Commission, Box LG 80, Legon, Accra; Graduate School of Nuclear and Allied Sciences, University of Ghana, Kumasi, Ghana

Date of Web Publication21-May-2013

Correspondence Address:
Charles Kansaana
Radiation Protection Institute, Ghana Atomic Energy Commission, P. O. Box LG80, Legon, Accra
Ghana
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0972-0464.112350

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  Abstract 

Background: The potential hazard of radiation exposures to radon and its daughter products from natural background has been highlighted in the world of scientific press and has become a matter of concern. The Lake Bosumtwi is one of the interesting research areas in Ghana due to the geological nature of the area, and also of its great importance based on the different uses of the lake and its surroundings. There is therefore the need to investigate the levels of radon activity in soil samples around Lake Bosomtwi basin as part of the national effort to establish base line data of radon levels in Ghana. Materials and Methods: The study was conducted to measure the levels of radon activity in soil samples within the lake Bosomtwi basin. Samples were collected from five selected villages around the lake at depths of 10 cm and 20 cm. The Role's Method was employed and measurements were made using specific cell counters. The measurements were performed with a Radon Degassing Unit (RDU-200) and a Radon Detector Analyzer (RDA-200). The calculated cell efficiency was obtained as 0.55 cpm/dpm. Results: The average radon concentrations at the depths were calculated and the maximum concentration for the 10 cm depth was obtained from Tepaso with a value of 4801.71±678 Bq/m 3 and the minimum concentration was obtained from Abonu with a value of 3887.07±815 Bq/m 3 . The maximum and minimum concentrations for the 20 cm depth were obtained from Tepaso and Kusuasi with values of 5602.10±943 Bq/m 3 and 4877.93±404 Bq/m 3 respectively. The overall average radon concentration obtained was 4745.31±559 Bq/m 3 . The results obtained were high when compared with results from previous studies. The values obtained are less than the World Health Organization's acceptable level for outdoor radon activity which is quoted as 9250 Bq/m 3 and hence the public are not exposed to any significant radiological health hazard in these areas. Conclusions: The radon concentration at the depth of 20 cm was found to be higher than the 10 cm depth and this shows that radon gas in soil increases with depth. The values obtained are less than the World Health Organization's acceptable level for outdoor radon activity and life activities would not be at risk in these areas.

Keywords: Lake Bosumtwi, maximum concentration, public exposure, radon activity, soil


How to cite this article:
Kansaana C, Aba AB, Addision ET, Darko EO, Adukpo OK, Faanu A. Measurement of radon activity in soil samples of some selected towns across the Lake Bosumtwi basin, Ghana. Radiat Prot Environ 2012;35:84-9

How to cite this URL:
Kansaana C, Aba AB, Addision ET, Darko EO, Adukpo OK, Faanu A. Measurement of radon activity in soil samples of some selected towns across the Lake Bosumtwi basin, Ghana. Radiat Prot Environ [serial online] 2012 [cited 2020 Jun 6];35:84-9. Available from: http://www.rpe.org.in/text.asp?2012/35/2/84/112350


  Introduction Top


Radon ( 222 Rn) is a radioactive gas generated by the decay of 226 Ra in the naturally occurring 238 U series. Radon is present in soil, rocks, building materials, and waters and escapes to the atmosphere. Typical radon concentration outdoors are low (approximately 10 Bq/m 3 ) [1] and depend on the composition of the underlying soil and rock formation and on meteorological parameters. In indoor environment, radon concentrates may accumulate significantly. This accumulation depends additionally on ventilation, heating, and on water use. [2]

Radon is the most significant natural source of human radiation exposure amongst all natural sources, delivered mainly indoors. Exposure to radon is the second most frequent cause of lung cancer after cigarette smoking and therefore considered a significant health hazard all over the world. [1] The potential hazard of exposures to radon and its daughter products from natural background sources has been highlighted in the world of scientific press and has become a matter of concern. Estimation of the incidence of lung cancers due to the decay products associated with indoor 222 Rn exposure is significant among the US population. [3]

There has been some a few radon research works conducted in Ghana [4],[5],[6],[7] and even though the levels are low, studies are inconclusive on the possible effects from low doses of radiation. In this study, Lake Bosumtwi is chosen because it is one of the most interesting research areas in Ghana due to the geological nature of the area, and also of its great importance based on the different uses of the lake and its surroundings. The lake has economic, social, and environmental significance. The people in the surrounding communities are mostly fisher folks and farmers. The lake also serves as a recreational ground where people go on holidays for fun and as a tourist site. It is therefore important to monitor the levels of radon activity in soils around the lake. The assessment of the levels of radon in the soils around towns of the lake is of particular importance as natural radiation is the largest contributor to the external dose of the world population. [1],[8]

The objectives of the study included assessing the levels of radon activity in soil samples from five selected villages around the lake. The second objective was to educate the average Ghanaian on the hazardous effect of radon, and how to handle such hazards, through organized forums and symposiums . Finally, we aimed to offer some recommendations based on the findings on possible ways of the hazard reduction.

This work therefore investigated the levels of radon activity in soil samples around Lake Bosumtwi basin as part of the national effort to establish base line data of radon levels in Ghana.


  Materials and Methods Top


Materials and equipment

The following materials and equipment were used for the research: Cutlass, polythene bags, tape measure, measuring cylinder, beaker, radon degassing unit-200 (RDU-200), radon detector analyzer-200 (RDA-200), stop watch, cells, plastic bottles with lids, and vacuum pump.

Description of the study area

Lake Bosumtwi is a natural inland fresh water lake in the Ashanti Region of Ghana. It is located about 30 km south-east of Kumasi in the Northern tip of the Adansi mountains in the forest zone of Ghana. (Koeberl et al.) [9] It lies between longitudes 01° 25′ W and latitudes 06° 32′ N and is excavated in 2 Ga old metasediments and metavolcanics of the Birimian Super group. [10] The 10.5 km diameter crater is almost completely filled by Lake Bosumtwi, with the crater rim rising about 250-300 m above the lake level [Figure 1]a. It is surrounded by a slight and irregular circular depression and an outer ring of minor topographic highs. The impact origin of the crater has been established from the presence of suevite breccia (containing shocked minerals) around the crater, as well as from the occurrences of coesite, Ni-rich iron spherules, and baddeleyite in the vesicular glasses within the suevites (Shanahan et al.) [11]
Figure 1: (a) The location of Lake Bosumtwi in southern Ghana. dark lines represent the approximate location of the intertropical convergence zone in raining and dry seasons. (b) Bathymetric map of Lake Bosumtwi, from Brooks et al .[16]

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The lake exhibits a radial drainage system of 106 km 2 and a maximum depth of 70 m; 99 m above sea level [Figure 1]b. Lake Bosumtwi covers an area of about 52 km 2 (Turner et al.) [12] It is hydrologically closed, with no connection to the regional groundwater aquifer, no river or stream inflow originating outside of the crater, and no surface water outflow except when the lake reaches a spillway located 110 m above the present lake surface (Turner et al.) [12] Mean annual temperature is 26.7°C, and the lake receives 1260 mm of annual rainfall (based on 1990-2000 averages at Kumasi), with most occurring during the two rainy seasons: April-July and September-October. The mean climatology has changed significantly over the last half century, with substantial decrease in precipitation and increase in temperature. The lake level is very sensitive to small changes in rainfall and other climatic parameters, such as annual mean temperature and evaporation (Awudu et al.) [13]

Lake Bosumtwi is a fresh closed lake playing an important role in the lives of more than 24 surrounding communities. It is used for commercial fishery and also for recreation and has a great potential for future agricultural development. The lake also serves as one of the primary tourist sites for the country. The lake is one of the main sources of livelihood for 24 communities living around and they heavily depend on the fish catch for their income and food (protein). Besides fishing, they depend on the aquatic resource for drinking water and irrigation water for agricultural activities.

The lake also provides the basis of other social and economic opportunities such as transportation and tourism. The Bosumtwi Forest Reserve, which is near the Ankaase community at Lake Bosumtwi [Figure 2], and has an area of 140 km 2 , is a legally protected area that consists of semi-deciduous tropical rainforest and provides a typical natural environment of the sort that attracts eco-tourists (Adu et al.) [14]
Figure 2: Lake Bosumtwi and surrounding villages

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Selection of the study area

The Lake Bosumtwi basin is one of the most interesting research areas in Ghana due to the geological nature of the area. Radon surveys already conducted show very interesting developments and hence the need for an in-depth study so that further extrapolation will be made to cover the whole Ghana.

Selection of sampling areas

The geological map of the study area was examined in order to select sampling areas which would give possibly an accurate representation of the real situation. The sampling areas were five villages selected around the lake namely: Abonu, Adwafo, Tepaso, Kusuasi, and Ataa Junction [Figure 2]. The sampling sites were chosen from various geological formations within the study areas. It is, therefore, anticipated that the type of geological formation upon which a person farms or lives within the vicinity will have a significant effect on the exposure indicating the extent of the exposure range of the inhabitants.

Sampling and sample preparation

The sampling was carried out in the month of November 2011, where the weather conditions were fairly stable. A total number of 50 samples were collected from the five selected villages around the lake. For each village, five areas at intervals of 5 m each were marked. Two soil samples at each marked area were collected at depths of 10 cm and 20 cm, and then transferred into polyethylene bags and labeled accordingly.

At the laboratory, about 200 g of each sample was soaked in 500 ml of distilled water in plastic bottles and tightly covered to prevent the gas from escaping into the environment. The prepared samples were stored for a minimum of 1 month to achieve secular equilibrium (Tufail et al.) [15]

Measurement of samples

The measurements were performed with a RDU-200. The heavy rubber tubing from the vacuum pump provided with the RDU-200 was connected to the pump input port on the console. The RDX-012 (cell) was connected to the scintillator cell connector on the RDU-200, thus making sure it fits properly. The vacuum valve was opened and making sure the flash valves were closed, the cell was evacuated using the pump to a gauge reading between 25 and 30 inches of mercury. The valve was then closed. The bubbler tube was first filled with water, and after generating a vacuum pressure, the flow regulator knob was adjusted to a position where it took exactly 3 min to reduce the vacuum gauge to zero. This time interval ensured the complete degassing of the radon gas. The flush valve was fully closed when the flow valve opened. Slide suction at the top of the bubbler was provided by slightly opening and closing the regulator. The cell was evacuated until the background level was very low. The background count was measured with the RDA detector for 10-min count.

Radiation counts

RDA-200 was used in the radiation counting. The alpha emitting properties of the short-lived radon daughters were used to determine the radioactivity counting ability of the RDA-200 using the fixed 3 min to reduce gauge to zero. The glass bubbler tube was then filled with 200 cm 3 of the dissolved solution and the procedure was repeated to slowly degas the radon in the solution into the cell. Each cell after being filled with the radon gas was quickly put into the photomultiplier tube chamber of the RDA-200. A 1-min interval was allowed for light dissipation in the photomultiplier tube. The power switch was put on and the time knob set to 10 min. The sample button was pressed to start count for 10 min after the 1-min interval. Value on display screen was recorded after the buzzer sound went. The process was repeated for each sample and the subsequent readings were recorded.

Calculation of corrected net count per minute

The corrected net count per minute was calculated using the relation as shown in Equation 1.



where the net count per minute = radon count per minute - background radiation count of cell per minute.



where λ is the decay constant of radon and t is the total time delay.

Calculation of cell efficiency

The cell efficiency was calculated to convert counts per minute into activity unit of pico Curie per liter since the method was standardized. This was done by filling the cells with radon from the pylon radon source under count per minute.



where CPM = the pylon radon count
                  = pylon radon count per minute - cell background pylon count per minute
                  =254.73 cpm.
SC = the sequential correction = 1.00;
C = correction factor for radon = 1.0094;
DA = dispensing activity = 2.66 Bq;
A = correction factor for radon decay from time of collection to start of counting = 0.97;
Number of alpha emitters = 3;
Conversion factor from cpm to cps = 60.

Calculation of radon activity in soil samples

The analytical expression used in the calculation of the radon concentrations in pCi/l for the soil samples is shown in Equation 3.



The calculated radon concentration in unit of pCi/l was converted to Bq/m 3 using the relation as shown in Equation 4.




  Results and Discussion Top


Soil is the basic ingredient used in construction materials and also for agricultural purposes in Ghana. Thus, it is quite important to find the radon emanation potential to have an estimation of radiation risk to the habitants. The values of radon activity in soil samples at the selected villages around the lake at the depths of 10 cm and 20 cm are presented in [Table 1].
Table 1: Radon concentration in soil samples at the selected villages around the Lake Bosumtwi basin

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For Abonu, the concentration ranged between a maximum of 4382.50 Bq/m 3 and a minimum of 3239.10 Bq/m 3 with an average of 3887.07 ± 815 Bq/m 3 for the 10 cm depth. The 20 cm depth indicated an average of 5068.87 ± 1,213 Bq/m 3 in a range of concentration between 4191.97 Bq/m 3 and 5906.87 Bq/m 3 . For Adwafo, the concentration ranged between a maximum of 4954.15 Bq/m 3 and a minimum of 4329.80 Bq/m 3 with an average of 4230.08 ± 1226 Bq/m 3 at the 10 cm depth. The 20 cm depth gave an average of 5113.38 ± 678 Bq/m 3 in a range of concentration between 5980.85 Bq/m 3 and 4245.90 Bq/m 3 . For Tepaso, the maximum concentration was 5335.23 Bq/m 3 and the minimum concentration was 4382.51 Bq/m 3 with an average of 4801.71 ± 678 Bq/m 3 for the 10 cm depth. The 20 cm depth indicated an average of 5602.10 ± 943 Bq/m 3 in a range of concentration between 4954.15 Bq/m 3 and 6287.96 Bq/m 3 . Also for Kusuasi, the concentration ranged between a maximum of 4291.90 Bq/m 3 and a minimum of 3620.33 Bq/m 3 with an average of 3956.12 ± 474 Bq/m 3 for the 10 cm depth. The 20 cm depth indicated an average of 4877.93 ± 404 Bq/m 3 in a range of concentration between 4573.06 Bq/m 3 and 5144.69 Bq/m 3 . Finally for Ataa Junction, the concentration ranged between a maximum of 5464.64 Bq/m 3 and a minimum of 4001.43 Bq/m 3 with an average of 4733.04 ± 1034 Bq/m 3 at the 10 cm depth. The 20 cm depth gave an average of 5182.80 ± 541 Bq/m 3 in a range of concentration between 4763.60 Bq/m 3 and 5525.78 Bq/m 3 .

The average radon concentrations at various depths were compared. The maximum concentration for the 10 cm depth was obtained from Tepaso and the minimum concentration was obtained from Abonu. The maximum and minimum concentrations for the 20 cm depth were obtained from Tepaso and Kusuasi, respectively. The overall radon activity obtained from the Lake Bosumtwi basin was 4745.31 ± 559 Bq/m 3 . [Figure 3] shows the comparison of average radon concentrations for the 10 cm and 20 cm depths for the five selected villages around the Lake Bosumtwi basin.
Figure 3: Comparison of average radon concentration for 10 cm and 20 cm depths for the five selected villages around Lake Bosomtwi

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The radon concentration for the 10 cm depth is found to be low when compared to the 20 cm depth. The results obtained from the study prove that soil radon increases with depth. Radon emanation has important influence on the radon flux from the soil. A possible reason for the difference in radon concentrations at the depths of 10 cm and 20 cm in the soil could be that the pressure on the surface influences the measurements at these lower depths. Another possible reason could be that, the measurements at small depths suffer a leakage of air down the side of the probe when taking measurements. The possible difference in compaction of the soil (difference in soil profile) could also explain the more rapid increase in the soil-gas concentration at different sites. This would mean that less radon would diffuse to the surface as more and more radon is potentially trapped in the interstitial pore space of the soil. Another reason that might influence the difference in the radon concentrations at the two depths in the soil is the air pressure at the surface.

Comparison of radon activity in soil with other studies from Lake Bosumtwi

Radon analysis in the vicinity of Lake Bosumtwi has been an ongoing activity for quite some time now. Over the years, radon activity levels from the results obtained have been fluctuating with minimum and maximum values and this fluctuation may be attributed to various reasons such as change in climatic condition, inhabitation/settlement, and human activities (farming, building, etc.).

In 2001, research on radon was conducted around Lake Bosumtwi to compare the soil depth to radon concentration. The average value for the 10 cm and 20 cm depths where the samples were collected gave a minimum value of 3418.80 Bq/m 3 at 10 cm and a maximum value of 8143.70 Bq/m 3 at 20 cm with an overall average of 4123.20 Bq/m 3 . [7]

In 2002, research work conducted was an improvement of the results obtained in 2001; the expected result was to ascertain the effect of soil depth on radon emanation. The results obtained showed a minimum of 3873.90 Bq/m 3 at 10 cm and a maximum of 4015.30 Bq/m 3 at 20 cm with an overall value 3792.50 Bq/m 3 . The results proved that depth really had an effect on radon emanation. [7]

Further work in 2003 saw a different turn in the radon research. Samples were collected from six different villages around the lake and the results showed that Duasi Village had the maximum value both for the sample count and the average, with Mmorontuo Village having the minimum for the sample count and the average. A minimum value of 1440.67 Bq/m 3 and a maximum of 1300.18 Bq/m 3 were recorded with an overall average of 3922.74 Bq/m 3 . [7]

A confirmation analysis was done in 2006 on the results obtained in 2003. In this analysis, four of the six villages which recorded high values in 2003 were sampled again. Of all the villages, a minimum value of 1180.30 Bq/m 3 was recorded by Adwafo village and Duasi got the maximum value of 70,841.43 Bq/m 3 and the overall calculated average was 3696.30 Bq/m 3 . [7]

Comparison of the results indicated variation in the parameters. The results obtained for this year's survey were compared with results from previous years and [Figure 4] shows a variation in radon concentration with passing years. This study has the highest radon level followed by the year 2001 with the year 2006 obtaining the lowest value. These values are high compared to other years and this may be due to increased human activities such as farming, building, and other related activities.
Figure 4: Variation in radon concentration with passing years

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  Conclusion Top


Radon levels in soil samples around Lake Bosumtwi basin have been investigated using the Role's method. The radon concentration at the depth of 20 cm was found to be higher than the 10 cm depth and this shows that radon gas in soil increases with depth. Comparing the results of this study to other research works carried in some geological fault zones in Ghana, it may be concluded that the lake and its environs may be branded as active fault zones in the future. The values obtained are less than the World Health Organization's acceptable level for outdoor radon activity which is quoted as 9250 Bq/m 3 . Hence, human activities would not be at risk in these areas. The results will provide data and information for dose assessment and further studies.


  Acknowledgment Top


The authors are grateful to the Department of Physics of the Kwame Nkrumah University of Science and Technology for making their laboratories available for the research.[16]

 
  References Top

1.UNSCEAR. Sources and effects of ionizing radiation. United nations scientific committee on the effects of atomic radiation, United Nations, New York; 2000.  Back to cited text no. 1
    
2.Nazaroff WW, Nero AV. Radon and its decay products in indoor air. USA: Wiley; 1988. p. 518.  Back to cited text no. 2
    
3.Nero AV, Sextro RG, Doyle SM, Moed BA, Nazaroff WW, Revzan KL, et al. Characterizing the sources, range, and environmental influences of radon 222 and its decay products. Sci Total Environ 1985;45:233-44.  Back to cited text no. 3
    
4.Aba BA, Amo S. Contribution of radon to population dose from natural radiation in Ghana. J Sci Technol 1993;13:99-104.  Back to cited text no. 4
    
5.Aba BA. Monitoring of natural background radiation in some Ghanaian homes. Journal of Science and Technology 1994a;14:181.  Back to cited text no. 5
    
6.Aba BA. Radon measurements in deep mines in Ghana. Journal of Science and Technology 1994b;14:105-8.  Back to cited text no. 6
    
7.Badoe LG, Andam A, Sosu EK. Measurement of radon level in Ashanti region and design of a radon vulnerability map for Ghana. Res J Environ Earth Sci 2012;4:76-81.  Back to cited text no. 7
    
8.UNSCEAR. Sources and effects of ionizing radiation. UNSCEAR 1993 Report to the general assembly, with scientific annexes. United Nations, New York: United Nations Sales Publication; 1993. E.94.I X.2.  Back to cited text no. 8
    
9.Koeberl C, Bottomley RJ, Glass BP, Storzer D. Geochemistry and age of ivory coast tektites and microtektites. Geochim Cosmochim Acta 1997;61:1745-72.  Back to cited text no. 9
    
10.Talbot MR, Johannessen T. A high resolution paleoclimatic record for the last 27,000 years in tropical West Africa from the carbon and nitrogen isotopic composition of lacustrine organic matter. Earth Planet Sci Lett 1992;110:23-37.  Back to cited text no. 10
    
11.Shanahan TM, Overpeck JT, Peck J, King J, Scholz C, Hughen K, et al. Paleoenvironmental changes in West Africa since the last glacial maximum from a geochemical and modeling study of Lake Bosumtwi, Ghana. San Francisco: American Geophysical Union Fall Meeting; 2005.  Back to cited text no. 11
    
12.Turner BF, Gardner LR, Sharp WE. The hydrology of Lake Bosumtwi, a climate sensitive lake in Ghana, West Africa. J Hydrol 1995;183:243-61.  Back to cited text no. 12
    
13.Awudu AR, Adu S, Adukpo1 OK, Obeng MK, Sarsah LA, Quarshie E, et al. Preliminary study of natural radioactivity in soils of some selected towns along the Bosumtwi Lake, Ghana. Elixir J 2011;41:5761-5.  Back to cited text no. 13
    
14.Adu S, Darko EO, Awudu AR, Adukpo OK, Emi-Reynolds G, Obeng M, et al. Preliminary study of natural radioactivity in the lake Bosumtwi Basin. Res J Environ Earth Sci 2011;3:463-8.  Back to cited text no. 14
    
15.Tufail M, Iqbal M, Mirza MS. Radiation doses due to natural radioactivity in Pakistan marbles. Radioprotection 2000;34:355-9.  Back to cited text no. 15
    
16.Brooks K, Scholz CA, King JW, Peck J, Overpeck JT, Russell JM, et al. Late-quaternary lowstands of Lake Bosumtwi, Ghana: Evidence from high-resolution seismic-reflection and sediment-core data. Palaeogeogr Palaeoclimatol Palaeoecol 2005;216:235-49.  Back to cited text no. 16
    


    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4]
 
 
    Tables

  [Table 1]



 

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