assessment of public exposure to naturally occurring radioactive materials from mining and mineral...

Environ Monit Assess (2011) 180:15–29DOI 10.1007/s10661-010-1769-9

Assessment of public exposure to naturally occurringradioactive materials from mining and mineral processingactivities of Tarkwa Goldmine in Ghana

Augustine Faanu · James H. Ephraim ·Emmanuel O. Darko

Received: 23 July 2010 / Accepted: 21 October 2010 / Published online: 12 November 2010© Springer Science+Business Media B.V. 2010

Abstract Studies have been carried out in a Gold-mine in Ghana to determine the exposure of thepublic to naturally occurring radioactive materialsfrom processing of gold ore. Direct gamma spec-trometry and neutron activation analysis tech-niques were used to analyse soil, rock, water anddust samples from the mining environment. Themean activity concentrations measured for 238U,232Th and 40K in the soil/rock samples were 15.2,26.9 and 157.1 Bq kg−1, respectively. For the watersamples, the mean activity concentrations were0.54 and 0.41 Bq l−1) and 7.76 Bq l−1 for 226Ra,232Th and 40K, respectively. The mean activityconcentrations measured in the dust samples were4.90 and 2.75 μBq m−3 for 238U and 232Th, re-spectively. The total annual effective dose to thepublic was estimated to be 0.69 mSv. The resultsin this study compared well with typical world av-erage values. The results indicate an insignificantexposure of the public from the activities of theGoldmine.

A. Faanu (B) · E. O. DarkoRadiation Protection Institute, Ghana Atomic EnergyCommission, P.O. Box LG 80, Legon, Accra, Ghanae-mail: [email protected]

J. H. EphraimDepartment of Chemistry, Kwame NkrumahUniversity of Science and Technology,Kumasi, Ghana

Keywords Goldmine · Natural radioactivity ·Mineral processing · Banket series ·Gamma spectroscopy · Neutron activation

Introduction

Natural radioactivity constitutes the largest sourceof population dose (UNSCEAR 2000). The majorsources of external gamma radiation are due to238U, 232Th and their decay products and 40K.The 238U and its daughters rather than 226Ra andits daughter products are responsible for the ma-jor fraction of the internal dose receive by hu-mans from naturally occurring radionuclides (deOliveira et al. 2001). Even though the concentra-tions of these radionuclides are widely distributedin nature, they have been found to depend onthe local geological conditions and as a resultvary from place to place (Xinwei et al. 2006).The specific levels are related to the type of rockfrom which the soil originates. Higher radioactiv-ity levels are associated with igneous rocks such asgranite and lower levels with sedimentary rocks.There are exceptions however, as some shales andphosphate rocks have relatively high content ofradionuclides (Uosif 2007). Higher concentrationsmay arise as a result of human activities such asmining and mineral processing (UNSCEAR 2000;IAEA 2005).

16 Environ Monit Assess (2011) 180:15–29

Mining has been identified as one of the po-tential sources of exposure to naturally occur-ring radioactive materials (NORM) (UNSCEAR2000). However, mining companies are not beingregulated for NORM in most countries includingGhana. In Ghana, there are over 200 registeredmining companies operating small, medium andlarge scale mining. Radionuclide concentrationsresulting from industrial activities such as miningand mineral processing has not been evaluatedin almost all the mines in Ghana. Data on ra-dionuclide concentrations in raw materials, minetailings, scales in pipes, waste streams and publicexposures in Ghana are scanty (Darko et al. 2005;Darko and Faanu 2007). Consequently, there isgeneral lack of awareness and knowledge of theradiological hazards and exposure levels by legis-lators, regulators and operators.

A recent study on two mines in Ghana havereported average values for 238U, 232Th and 40Kin soil for the two mines as follows 28.7 ± 4.8,25.4 ± 4.0, 581.8 and 34.5, 20.7, 682.4 Bq kg−1

respectively (Darko et al. 2010). The reportedaverage annual effective dose for the two mineswas 0.3 ± 0.06 mSv (Darko et al. 2010). Similarstudies carried out on different types of mines inother countries have been reported (UNSCEAR2000).

The study area is located in Tarkwa, in theWestern Region of Ghana and the surroundingcommunities. Tarkwa Goldmine is one of thelargest gold mining companies in Ghana and hasbeen in operation for the past 10 years. One of themain reasons for the choice of the study area isthat the geological formation of the Tarkwa Gold-mine is similar to the gold bearing conglomeratesof the Witwatersrand basin in South Africa wherecommercial quantities of uranium are processedfrom the gold tailings (GFGL 2007).

The primary objective of this study is to assessthe radiation exposure to the public living in thevicinity of the Tarkwa Goldmine to natural ra-dioactivity as a result of the gold mining activities.The study focused on the determination of theactivity concentration and distribution of the nat-urally occurring radionuclides of the U/Th decayseries and 40K in soil/rock, water, tailings, and oredust by gamma spectrometry. The availability ofdata from such studies is very vital to all stake-

holders involved. It is envisaged that, at the endof the study, the knowledge and awareness on theissue of NORM will be addressed.

Description of study area

Location of the study area

The study area is Tarkwa Goldmine and itssurrounding communities including TarkwaTownship within the mines area of concession.The concession of the mines covers an area of294.606 km2 and located in the Western Regionof Ghana. The Tarkwa township is approximately300 km west of the capital city of Ghana, Accraby road at latitude 5◦15′ N and longitude 2◦00′ W.The mine is about 4 km from Tarkwa Townshipwith good access roads and well established infra-structure. Figure 1 shows the concession of themine and the surrounding communities wheresampling was carried out. Tarkwa is the adminis-trative capital of the study area and subsistencefarming is the main occupation of the people andmining being the main industrial activity (Avotriet al. 2002). The Tarkwa area lies within the maingold belt of Ghana that stretches from Aximin the southwest, to Konongo in the northeast(Kortatsi 2004). The total population of theTarkwa Township is about 80,000 (Kumah 2007)with an estimated population of the District being236,000 (IFC 2003; Darko et al. 2010). In additionthere are eight communities dotted around themines (Table 1).

Geology, hydrogeology and meteorologyof the mining area

Geologically, the gold ore are located within theTarkwaian system, which forms a significant por-tion of the stratigraphy of the Ashanti Belt insouth western Ghana. Intrusive igneous rockscontribute to about 20% of the total thickness ofthe Tarkwaian system in the Tarkwa area. The orebody consists of a series of sedimentary banketquartz reef units similar to those mined in theWitwatersrand of South Africa. The geologicalmap of the study area is shown in Fig. 2.

Environ Monit Assess (2011) 180:15–29 17

Fig. 1 Layout of thestudy area showing thesampling points

Abosso Town

Mine Village

Tarkwa Town

Atuabo

BrahabebomeTeberebie Pit

Akontasi

Plant OfficeMantraim

Boboobo

Pepe Pit

Kottraverchy Pit

Pepesa

Huniso

Asuma

Samahu

N

Soil Sample

Water Sample

Air Sample

The geological formation of the mine is suchthat the gold bearing ore is situated between wastebelts with the major rock type being sedimen-tary. The Ashanti Belt is a north-easterly strik-ing broadly synclinal structure made up of lowerProterozoic sediments and Volcanics underlainby the metavolcanics and metasediments of theBirimian system. The contact between the Bir-

imian and the Tarkwaian systems is commonlymarked by zones of intense shearing and is hostto a number of significant shear hosted depositsincluding Prestea, Bogoso and Obuasi. The localgeology is dominated by the Banket Series whichconsists of a well sorted conglomerates and peb-bly quartzites with clasts generally considered tobe Birimian in origin and containing significant


Table 1 Communitiesand the populationdistribution in the studyarea (GFGL 2007)

No Community Location coordinates Population(2004 estimates)

1 Abekoase N 5◦22′24.39′′ W 2◦01′ 07.49′′ 4002 Brahabebom N 5◦18′47.44′′ W 1◦59′56.72′′ 1500-18003 Huniso N 5◦22′59.51′′ W 2◦03′55.51′′ 1,500–2,0004 New Atuabo N 5◦19′22.34′′ W 1◦58′36.40′′ 5,500–6,0005 Pepesa N 5◦19′56.60′′ W 2◦00′11.36′′ 1500-18006 Samahu N 5◦21′54.82′′ W 1◦ 59′58.46′′ 15007 Tebe N 5◦22′55.97′′ W 2◦01′48.32′′ 3008 Tarkwa Township N 5◦17′13.58′′ W 1◦59′55.31′′ 80,000

Fig. 2 Geological mapof the study area


gold mineralization, hosting the Tarkwa ore body.The rocks of the Tarkwaian system consist of theKawere Group, the Banket series, the TarkwaPhyllite and the Huni Sandstone. Most of therocks that resemble sandstone at the surface areweathered equivalents of parent quartzites (Kumaand Younger 2001). Two main methods are usedby the Tarkwa Goldmine to recover gold from theore. Gold ore is the alluvial type and non-sulphidicpaleplacer associated with the conglomerates ofthe Tarkwaian formations. The carbon in leachand the heap leach methods are being employedby the Tarkwa Goldmine to recover the gold fromthe ore (GFGL 2007).

Hydrogeologically, most of the major townsand villages except Tarkwa Township in theWassa West District depend on groundwater asthe main source of water supply through bore-holes and hand-dug wells (Kortatsi 2004). Thegroundwater occurrence is associated with the de-velopment of secondary porosity through fissuringand weathering since the area lacks primaryporosity due to the consolidated nature of therocks. Two types of soils exist in the Tarkwa-Prestea area, and these are forest oxysol in thesouth and forest ochrosol–oxysol integrates inthe north (Kortatsi 2004). The characteristics ofthe soils in the area are shown in Table 2 (Kumaand Younger 2001).

The climate of Tarkwa is the tropical typecharacterised by two wet seasons; March–July andSeptember–November. Data obtained from themines Environmental Department shows that thetotal annual rainfall figures measured for the year2008 was 1,744 mm with an average of 145 mm.The rainfall figures for August 2008 during whichthe first sampling was carried out was 85 mm when

Table 2 Characteristics of soils in the study area takenfrom literature (Kuma and Younger 2001)

Soil type Texture Percentage, %

Gravel Sand Silt Clay

Banket series Silty-sand 2 59 29 10Laterite 69 14 10 7

Huni Silty-sand 2 55 33 10Kawere Silt sand 0 47 40 13Tarkwa phyllite laterite 62 9 13 16Weathered dyke Silt 3 20 64 13

there was reduction in rainfall. The rainfall figuresduring the second sampling period in July 2009was 256.6 mm, and this period was very wet. Therelative humidity for the area was in a range of73–98% with an average of 86%. The averageatmospheric pressure was about 100.2 kPa in arange of 99.0–100.7 kPa and outdoor temperaturesin the range of 28–39◦C and an average value of34◦C.

Materials and methods

Sampling and sample preparation

Seventy-two samples were randomly collectedwithin selected areas of the mine concession andthe surrounding communities. They included 38soil/rock samples, 29 water samples and five dustsamples.

In the laboratory, each of the soil/rock sampleswere air dried on trays for 7 days and then oven-dried at a temperature of 105◦C for between 3and 4 h until all moisture was completely lost.The samples were ground into fine powder usinga ball mill and sieved through a 2-mm mesh sizepore into one 1-l Marinelli beakers. The Marinellibeakers with the samples were completely sealedfor 1 month to allow the short-lived daughters of238U and 232Th decay series to attain equilibriumwith their long-lived parent radionuclides (ASTM1983, 1986).

The water samples were homogenised andtransferred into one litre Marinelli beaker withoutany special preparation. In order to maintain ra-dioactive equilibrium between 226Ra and its short-live daughters, the homogenised samples werehermetically sealed in 1,000-ml Marinelli beakers,weighed and stored for a period of 1 month. Thesamples were each counted using a high puritygermanium detector.

Airborne particulate samples were collectedonto 0.45 μm pore size filter paper usingRADECO, model SAIC H-809C air sampler witha flow rate of 350 l/min. The sampling was carriedout for 4 h/day resulting in a throughput of 84 m3.At the end of the sampling period, the filters werelabelled and sealed in plastic bags to prevent theescape of gaseous radionuclides and transported


to the laboratory for analysis. The determinationof 238U and 232Th in the dust samples were carriedout by direct gamma spectrometry and neutronactivation analysis (NAA) using Ghana ResearchReactor-1 (GHARR-1).

Prior to the analysis by NAA, the air filterswere each folded into a rabbit capsule of diame-ter 1.6 cm and height 5.5 cm. The capsules wereplugged with cotton wool and sealed with a sol-dering rod and labelled with the sample code.Rock reference materials for uranium and tho-rium GBW07106-GSR-4 and GBW07107-GSR-5were prepared in a filter paper using 0.005 g of thestandard as the samples. The prepared samples,standards and blank were irradiated in GHARR-1 at the Ghana Atomic Energy Commission, op-erating at 15 kW with a thermal flux of 5 ×1011 n cm−2 s−1. The samples were transferredinto the irradiation sites via pneumatic transfersystem at a pressure of 0.6 Mpa. The sampleswere irradiated with a scheme for medium to longradionuclides for 1 h and allowed to decay for 48 hto 2 weeks until a suitable dead time was achievedbefore they were counted on the detector. Theconcentration of U and Th were measured inmicrograms per gram.

Instrumentation and calibration

Direct instrumental analysis without pre-treatment (non-destructive) was used for themeasurement of gamma rays for the soil, waterand air (dust) samples using a High PurityGermanium detector (HPGE). The gamma spec-trometry system consists of an n-type HPGEdetector coupled to a computer based multi-channel analyser. The relative efficiency of thedetector is 20% with energy resolution of 1.8 keVat gamma ray energy of 1,332 keV of 60Co. Theidentification of individual radionuclides wasperformed using their gamma ray energies, andthe quantitative analysis of radionuclides wasperformed using gamma ray spectrum analysissoftware, ORTEC MAESTRO-32.

The detector is mounted in a cylindrical leadshield (100 mm) lined with copper, cadmiumand plexiglass (3 mm each) to reduce the back-ground radiation. The detector is cooled in liq-uid nitrogen at a temperature of −196◦C (77 k).

In order to determine the background distribu-tion in the environment around the detector, tenempty Marinelli beakers were thoroughly cleanedand filled with distilled water and counted for36,000 s in the same geometry as the samples.The background spectra were used to correct thenet peak area of gamma rays of measured iso-topes. The background spectra were also used todetermine the minimum detectable activities of238U (0.12 Bq kg−1), 232Th (0.11 Bq kg−1) and 40K(0.15 Bq kg−1) of the detector.

The efficiency calibrations were carried outby counting standard radionuclides of known ac-tivities with well defined energies in the energyrange of 60 to ∼2,000 keV. For the analysis ofsoil/rock and water samples, the efficiency calibra-tion was carried out using standard radionuclidesuniformly distributed in solid water with volumeand density of 1,000 ml and 1.0 g m−3, respec-tively (source number, NW146 and manufacturedby QSA Global GmbH). For the dust samples,the efficiency calibrations were carried out usingmixed radionuclides standard uniformly distrib-uted on a plastic foil.

Calculation of activity concentrationand estimation of doses

For the soil/rock samples, the activity concentra-tion of 238U was calculated from the average peakenergies of 295.21 and 351.92 of 214Pb and 609.31of 214Bi. Similarly, the activity concentration of232Th was determined from the average energiesof 238.63 of 212Pb, 583.19 and 2,614.53 keV of 208Tland 911.21 keV of 228Ac. The activity concentra-tion of 40K was determined from the energy of1,460.83 keV. For the water samples, the activityconcentration of 226Ra was determined from thepeak energy of 609.31 keV of 214Bi.

The analytical expression used in the calcula-tion of the activity concentrations in Bq kg−1 forsoil/rock, Bq l−1 for water samples and Bq m−3 fordust samples is as shown in Eq. 1.

Asp = NDeλPtd

p.Tc.η (E) .m(1)

where; ND is the net counts of the radionuclide inthe samples, td is the delay time between samplingand counting, P is the gamma ray emission prob-


ability (gamma ray yield), η(E) is the absolutecounting efficiency of the detector system, Tc isthe sample counting time, m is the mass of thesample (kilogram) or volume (liter), exp (λptd)is the decay correction factor for delay betweentime of sampling and counting and λp is the decayconstant of the parent radionuclide.

The external gamma dose rate (Dγ ) at 1.0 mabove ground for the soil/rock samples was calcu-lated from the activity concentrations using Eq. 2(Uosif 2007).

Dγ

(n Gy h−1) = DCFK × AK + DCFU × AU

+ DCFTh × ATh (2)

where; DCFK, DCFU, DCFTh are the doseconversion factors for 40K, 238U and 232Th innSv/h/Bq kg−1 and AK, Au and ATh are theactivity concentrations for 40K, 238U and 232Th,respectively.

DCFK = 0.0417nSv/h/Bqkg−1

DCFU = 0.462nSv/h/Bqkg−1

DCFTh = 0.604nSv/h/Bqkg−1

The average annual effective dose was calcu-lated from the absorbed dose rate by applyingthe dose conversion factor of 0.7 Sv Gy−1 andan outdoor occupancy factor of 0.2 (UNSCEAR2000) represented by Eq. 3.

Eγ = Dγ× 0.2 × 8760 × 0.7 (3)

where Eγ is the average annual effective dose andDγ is the absorbed dose rate in air.

For the water samples, the committed effectivedoses were estimated from the activity concentra-tions of each individual radionuclide and apply-ing the yearly water consumption rate for adultsof 730 l/year (WHO 2004). The dose conversionfactors of 238U, 232Th and 40K were taken from theBSS (IAEA 1996) and using Eq. 4 to calculate.

Sing (w) = Iw

3∑

j=1

DCFing (U, Th, K) Asp (w) (4)

where, Asp (w) is the activity concentration ofthe radionuclides in a sample in Bq l−1, Iw is theintake of water and DCFing is the ingestion dosecoefficient in Sv/Bq l−1.

The elemental concentrations of U and Th de-termined in the dust samples in micrograms pergram using NAA were converted to activity con-centrations of 238U and 232Th in Bq m−3 accordingto the following expression (Eq. 5) (Tzortzis andTsertos 2004).

AE = FE.λE.NA. fA.E

ME.C(5)

Where AE is the activity concentration of ra-dionuclide, FE is the elemental concentrationof uranium or thorium, ME is the atomic mass(kg mol−1), λE is the decay constant (s−1), fA,E isthe fractional atomic abundance in nature, NA isAvogadro’s constant (6.023 × 1023 atoms mol−1)and C is a constant value of 1,000,000 for U andTh.

The inhalation effective dose from 238U and232Th in ore dust was calculated from Eq. 6 andthe dose conversion factors taken from the BSS(IAEA 1996).

Einh (dust) = T × Br × Fr

×2∑

j=1

DCF j, inh (U, Th) . C j (6)

where T is the exposure period in hours, Br is thebreathing rate for adult members of the public, Fr

is the respirable fraction of dust, C j is the activityconcentration of U and Th in Bq m−3.

DCFj,inh (U, Th) is the dose conversion factorfor U and Th in Sv/Bq m−3.

At each location, five measurements of theambient gamma dose rates were made at 1 mabove the ground and the average value takenin μGy h−1. The annual effective dose (Eγ,ext)was then estimated from the measured aver-age outdoor external gamma dose rate from theEq. 7.

Eγ,ext = Dγ,extTexpDCFext (7)

Where Dγ,ext is the average outdoor externalgamma dose rate μGy h−1, Texp is the expo-sure duration per year, 8,760 h and applying anoutdoor occupancy factor of 0.2, DCFext is theeffective dose to absorbed dose conversion factorof 0.7 Sv Gy−1 for environmental exposure togamma rays (UNSCEAR 2000).


Radon measurements

Air borne radon activity concentrations weremeasured directly with a Genitron Alpha Guard,Model PQ 2000/mp50. The measurements werecarried out outdoor in the field and indoor inresidential areas. The temperature, atmosphericpressure and relative humidity were also recordedduring the measurement. The Alpha Guard is pro-vided with a large surface glass fibre filter whichallows only the gaseous 222Rn to pass throughwhilst the radon progeny are prevented fromentering the ionisation chamber. The filter alsoprotects the interior of the chamber from contami-nation by dusty particles. The data were evaluatedusing Alpha View/Expert Software, which auto-matically transforms radon daughter concentra-tions from working level to equilibrium equivalentconcentration in Bq m−3.

The annual effective dose from radon gas in airwas estimated from Eq. 8.

Einh (Rn) = DCFRn FRnCRnTexp (8)

where Einh (Rn) is the annual effective dose frominhalation of radon, DCFRn is the dose per unitintake of radon via inhalation in nSv/Bq h m−3,(9 nSv/Bq h m−3) (UNSCEAR 2000), FRn isequilibrium factor for outdoor occupancy, 0.6(UNSCEAR 2000), CRn is the radon activityconcentration in Bq m−3 and Texp is the expo-sure period of 1 year for outdoor occupancy,which is 1,760 h. Radon concentrations in the soil(kBq m−3) were calculated using a proposal inUNSCEAR report (UNSCEAR 2000).

Estimation of total annual effective dose

The total annual effective dose (ET) to membersof the public was calculated using ICRP dosecalculation method (ICRP 1991, 2007). The an-alytical expression for the total effective dose isprovided in Eq. 9.

ET = Eγ (U, Th, K) + Eing (W) + Einh (Rn)

+ Einh (U, Th) (9)

where ET is the total effective dose in Sievert (Sv),Eγ (U, Th, K) is the external gamma effectivedose from the soil/rock samples, Eing (W) is theeffective dose from the consumption of water,Einh (Rn) is the effective dose from inhalation ofradon gas in air, Einh (U, Th) is the effective dosefrom the inhalation of dust containing 238U and232Th.

Results and discussion

Table 1 shows the communities around the minesinvestigated. The position coordinates and thepopulation distribution are also provided. InTable 2, the characteristics of the various types ofsoil are provided.

The results of the absorbed dose rate measuredin air at 1 m above the ground at the soil andwater sampling points in the mine site and thevarious communities investigated are presented inTable 3. As can be observed, measured absorbeddose rates at the soil sampling points varied in

Table 3 Averageabsorbed dose rate in airat 1 m above soil andwater sampling points inthe various communitiesof the study areas andestimated annualeffective dose

Sampling location Soil sampling Water sampling

Absorbed dose Annual effective Absorbed dose Annual effectiverate, nGy h−1 dose, μSv rate, nGy h−1 dose, μSv

Abekoase 40.0 42.9 25.0 30.7Brahabebom 30.0 36.8 40.0 49.1Huniso 15.0 18.4 10.0 12.6New Atuabo 55.0 67.5 50.0 61.3Pepesa 30.0 36.8 65.0 79.7Samahu 40.0 49.1 33.0 39.9Tarkwa 57.0 69.5 68.0 83.4Minesite 38.0 46.5 49.0 60.1Range 15.0–57.0 18.4–69.5 10.0–68.0 12.6–83.4Mean 38.1 45.9 42.5 52.1Standard deviation 13.7 16.8 19.7 24.1


a range of 15.0–57.0 nGy h−1 with a mean valueof 38.1 ± 13.7 nGy h−1. The corresponding meanannual effective dose was estimated to be 45.9 ±16.8 μSv in a range of 18.4–69.5 μSv. At the watersampling points absorbed dose rates varied in arange of 10.0–68.0 nGy h−1 with a mean valueof 42.5 ± 19.7 nGy h−1. The corresponding meanannual effective dose was estimated to be 52.1 ±24.1 μSv year−1.

According to UNSCEAR report, the averageabsorbed dose rate in air outdoor from terres-trial gamma radiation is 59 nGy h−1 (UNSCEAR2000). Comparing the results of the gamma ab-sorbed dose rates in this study with the data inUNSCEAR report, the results of the absorbeddose rates in this study are by a factor one lowerthan the range of dose rates reported by othercountries (UNSCEAR 2000). The highest ab-sorbed dose rate values of 57.0 and 68.0 nGy h−1

recorded at the soil and water sampling pointsin Tarkwa Township respectively also comparedquite well with the worldwide average value. Theresults of the study in this mine are also lower thanthe results of similar studies carried out in othermines in Ghana (Darko et al. 2010).

Table 4 shows the activity concentrations of238U, 232Th and 40K in the soil/rock samples as wellas the absorbed dose rate and the estimated an-nual effective doses. The percentage contributionsof 238U, 232Th and 40K to the absorbed dose ratesare also provided. The mean value of the activity

concentrations of 238U is 15.2 ± 5.7 Bq kg−1 in arange of 7.7–25.5 Bq kg−1. For 232Th, the meanactivity concentration is 26.9 ± 19.5 Bq kg−1 inrange of 8.5–67.2 Bq kg−1 and that of 40K is157.0 ± 68.2 Bq kg−1 in a range of 60.4–248.9 Bq kg−1. The mean values of the activityconcentrations of 238U, 232Th and 40K are by abouttwo times lower than the world average values innormal areas (UNSCEAR 2000). The worldwideaverage activity concentration of 238U, 232Th and40K in soil samples from various studies aroundthe world have values reported by UNSCEAR as35, 30 and 400 Bq kg−1, respectively (UNSCEAR2000). The mean gamma dose rate from terrestrialgamma rays calculated from soil activity concen-trations was 29.9 nGy h−1 in a range of 11.5–62.7 nGy h−1 which is by a factor of two lowerthan the dose rate measured in air at 1 m abovethe ground. The difference between the measuredambient gamma dose rate in air and gamma doserate calculated from the soil concentrations maybe attributed to contributions from cosmic raysto the air measurements. The absorbed dose ratedue to the soil concentrations is also about twotimes lower than the worldwide average valueof 60 nGy h−1 (UNSCEAR 1993, 2000). Thisdifference could be attributed to differences ingeology and geochemical state of the samplingsites. The corresponding mean annual effectivedose estimated from the soil concentrations is0.19 mSv. In the determination of these values, a

Table 4 Average activity concentrations, absorbed dose rates and annual effective doses due to 238U, 232Th and 40K in soilin the study area

Sample location Activity concentrations, Bq kg−1 Absorbed Annual Percentage contribution of238U 232Th 40K dose rate, effective radionuclides to absorbed

nGy h−1 dose, mSv dose rates (%)238U 232Th 40K

Abekoase 16.5 ± 1.5 13.7 ± 1.2 125.8 ± 10.7 21.2 0.14 37.1 39.5 23.5Brahabebom 18.6 ± 1.7 37.5 ± 2.4 163.8 38.1 0.24 24.2 57.8 18.0Huniso 7.7 ± 0.9 8.5 ± 0.9 76.2 ± 6.7 11.5 0.08 30.3 42.7 27.0New Atuabo 13.3 ± 1.5 35.2 ± 2.5 194.6 ± 15.5 35.5 0.23 17.3 59.0 23.7Pepesa 12.2 ± 1.15 10.5 ± 1.0 60.4 ± 5.6 14.5 0.09 39.0 43.6 17.4Samahu 17.9 ± 1.6 23.3 ± 1.8 153.3 ± 12.7 28.7 0.18 29.0 44.2 22.1Tarkwa township 25.5 ± 2.0 67.2 ± 4.8 248.9 ± 19.5 62.7 0.38 19.1 61.9 18.9Minesite 9.6 ± 1.2 19.1 ± 1.54 233.3 ± 18.4 26.6 0.16 17.9 44.6 37.5Range 7.7–25.5 8.5–67.2 60.4–248.9 11.5–62.7 0.08–0.38 17.3–39.0 39.5–61.9 17.4–37.5Mean 15.2 26.9 157.0 29.9 0.19 26.7 49.2 23.5Standard deviation 5.7 19.5 68.2 16.2 0.1 8.5 8.8 6.5


dose conversion factor of 0.7 Sv Gy−1 and outdoorand indoor occupancy factors of 0.2 and 0.8, re-spectively, were applied (UNSCEAR 1993, 2000).Also from Table 4, it can be seen that 232Th con-tributes more significantly to the total absorbeddose rate with a mean value of 49.2% followedby 238U with a mean value of 26.7% and 40K withmean value of 23.5%.

The mean activity concentrations of 226Ra,232Th and 40K in the water samples are shown inTable 5. The mean values of 226Ra, 232Th and 40Kare 0.54 ± 0.23 in a range of 0.11–1.03 Bq l−1,0.41 ± 0.10 in a range of 0.21–0.56 Bq l−1 and7.76 ± 2.70 Bq l−1 in a range of 1.65–11.99 Bq l−1,respectively. The mean annual effective dosefrom the water concentrations is calculated to be0.21 mSv in a range of 0.10–0.34 mSv. It is impor-tant to note that all the water sources investigatedare underground water taken from boreholes andmine pits except the water from the streams andrivers. The main source of water supply of themines is underground for both domestic uses aswell as in the plant. The highest activity con-centration of 1.03 and 0.52 Bq l−1 of 226Ra and232Th, respectively, were recorded in a water from

the mine tailings dam which is a mixture of bothunderground and processed water discharged tothe tailings dam (not for domestic use). The lowestvalues of 0.11 and 0.21 Bq l−1 were recorded fromRiver Bonsa at Bonsaso which is about 30 kmat a remote location from the discharge pointsfrom the Mine. This sample was taken as con-trol to compare with the results from the studyarea. In this study, the results of the 232Th ac-tivity concentrations are quite high even thoughin most cases, they are lower than the activityconcentrations of 226Ra. The reasons for the high232Th concentrations in the water samples couldbe because they are underground water and theslightly acidic conditions with pH values in a rangeof 4.48–8.24 with an average value of 6.10 couldalso facilitate the dissolution of the radionuclides.The Ghana Standards Board (GSB) and WorldHealth Organisation (WHO) required pH rangesuitable for drinking water to be 6.5–8.5 (GSB2005; WHO 2004). The concentration of radionu-clides in groundwater depends on the kind of min-erals derive from the rock aquifers, the chemicalcomposition of the water and the soil ion retentiontime (Andreo and Carrasco 1999). As shown in

Table 5 Statistical summary of activity concentrations and estimated annual effective doses from water consumed from thestudy area

Sample location Type of water sample pH Activity concentration, Bq l−1 Annual effective226Ra 232Th 40K dose, mSv

Abekoase SW (stream) 5.84 0.55 ± 0.03 0.34 ± 0.03 7.46 ± 0.04 0.20UW (borehole) 5.32 0.76 ± 0.03 0.37 ± 0.02 11.14 ± 0.04 0.27

Brahabebom UW (borehole) 5.18 0.32 ± 0.03 0.52 ± 0.01 8.86 ± 0.04 0.19Huniso SW (river) 6.49 0.76 ± 0.03 0.41 ± 0.02 9.26 ± 0.04 0.27

UW (borehole) 4.48 0.51 ± 0.06 0.21 ± 0.04 1.65 ± 0.08 0.15New Atuabo UW (borehole) 5.45 0.36 ± 0.04 0.40 ± 0.03 7.87 ± 0.05 0.18Pepesa SW (stream) 6.40 0.75 ± 0.02 0.39 ± 0.06 8.69 ± 0.05 0.16

UW (borehole) 5.26 0.41 ± 0.03 0.31 ± 0.02 5.93 ± 0.05 0.26Samahu SW (stream) 6.92 0.37 ± 0.04 0.31 ± 0.03 9.93 ± 0.08 0.17

UW (borehole) 6.10 0.39 ± 0.04 0.45 ± 0.02 8.61 ± 0.05 0.20Tarkwa township SW (rainwater) 5.91 0.29 ± 0.05 0.49 ± 0.02 11.99 ± 0.04 0.20

TW (tap water) 6.33 0.58 ± 0.03 0.45 ± 0.02 8.69 ± 0.04 0.24UW (borehole) 5.48 0.46 ± 0.03 0.56 ± 0.01 5.13 ± 0.10 0.21

Minesite SW (tailings) 6.75 1.03 ± 0.05 0.52 ± 0.04 8.91 ± 0.05 0.34PW (plant) 8.24 0.55 ± 0.04 0.28 ± 0.02 5.65 ± 0.05 0.19UW (mine pits) 6.00 0.76 ± 0.03 0.51 ± 0.05 9.07 ± 0.06 0.28

Bonsaso (control) SW (river) 6.82 0.11 ± 0.09 0.51 ± 0.04 3.11 ± 0.06 0.10Range 4.48–8.24 0.11–1.03 0.21–0.56 1.65–11.99 0.10–0.34Mean 6.1 0.54 0.41 7.76 0.21Standard deviation 0.9 0.23 0.10 2.70 0.06


Table 5, most of the water samples were slightlyacidic whilst others were near neutral or slightlybasic. The pH is a very important water qualityparameter that has an important influence on thesolubility and mobility of metals or radionuclidesin water with solubility increasing with decreasingpH. At pH of approximately 7, the solubility ofuranium and thorium is extremely low and at pHless than 5, the concentration increases gradually.The results of the activity concentration of 226Ra(238U) and 232Th in this study were compared withthe WHO Guideline Levels and all the resultsincluding their mean values were lower than the1.00 Bq l−1 recommended values in drinking wa-ter. The WHO guideline value of annual effectivedose in water has been set at 0.10 mSv year−1

(WHO 2004). The mean annual effective dose inthis study is about twice the recommended an-nual effective dose in drinking water. Generally,the mean annual effective doses of all the watersources investigated in the various communitieshad values above the WHO recommended valueof 0.10 mSv year−1.

Table 6 shows the results of the comparisonof the mean absorbed dose rate calculated fromthe soil activity concentrations with the absorbeddose rates measured directly in air at 1 m abovethe ground in this work to similar studies inGhana and other countries. The mean absorbeddose rate calculated from the soil concentrations

Table 6 Comparison of absorbed dose rates estimatedfrom activity concentrations in soil/rocks and direct out-door measurements with published data (UNSCEAR2000a; Darko et al. 2010b)

Country Absorbed dose rate, nGy h−1

Estimation from Direct airground sources measurements

China 58 62USA 55 47Algeria 54 70Egypt 32 32Iran 53 71Malaysia 93 92Ghana 65 68World average 60 59Ghana (This work) 30 40aUNSCEAR (2000) Report for (China, USA, Algeria,Egypt, Iran, Malaysia, World Average Values)bDarko et al. (2010) for Ghana value

is 30 nGy h−1 whilst that measured directly inair is 40 nGy h−1. The mean absorbed dose ratemeasured in air was by 1.3 times higher than thatestimated from the soil activity concentrations.The difference could be attributed to the contri-bution of cosmic rays and statistical uncertaintiesin the measurements. The results in this study alsocompared quite with results from other countrieswhilst in some cases they are lower.

Table 7 shows the results of the activity con-centrations of the radionuclides from dust samplesdetermined by direct gamma ray spectrometryand NAA respectively. The activity concentra-tions of 238U ranged from <0.12–11.10 μBq m−3

with a mean value of 4.90 μBq m−3. The activityconcentration of 232Th in the dust samples alsovaried in the range of 0.65–4.29 μBq m−3 with amean value of 2.75 μBq m−3. The correspondingestimated mean absorbed dose rate and meanannual effective dose were 3.51 × 10−6 nGy h−1

and 2.70 nSv respectively, which are in the rangeof normal background doses. The contribution tothe radiation exposure of the population in thestudy area can be considered to be insignificant.

Table 8 also gives the results of elementaluranium and thorium concentrations determinedfrom the dust samples by NAA in micrograms pergram. The table also contains the activity concen-trations calculated from the elemental uraniumand thorium concentrations in μBq g−1. The meanconcentration of uranium and thorium in the dustsamples are 1.91 ± 0.23 and 1.31 ± 0.20 μg g−1,respectively. The corresponding activity concen-trations are 23.52 and 5.31 μBq g−1, respectively.These results does not contribute significantly tothe exposure of members of the public in the studysince the values are below the exemption crite-ria specified in the Basic Safety Standard (IAEA1996).

The activity concentration of 222Rn measuredin air and the component estimated from the soilconcentrations are given in Table 9. For the 222Rnconcentration measured in air, the results variedin a range of 27.5–32.7 Bq m−3 with a meanvalue of 30.0 ± 1.7 Bq m−3. The calculated annualeffective dose ranged from 0.26–0.31 mSv with amean value of 0.29 ± 0.02 mSv. The results inthis study compared well with results publishedin UNSCEAR 1996 and 2000 reports for normal


Table 7 The activityconcentrations of 238Uand 232Th in dust/airsamples using directgamma ray analysis,absorbed dose rate andannual effective doses

UMAT University ofMines and Technology

Sample location Activity concentration, Absorbed dose AnnualμBq m−3 rate ×10−6, effective238U 232Th nGy h−1 dose, nSv

New Atuabo 3.62 4.29 4.40 4.05Mine staff club house <0.12 0.65 0.43 0.60Boboobo (Tarkwa) 4.07 3.74 4.20 3.56Agricultural Hill (Tarkwa) 0.82 2.08 1.70 1.93UMAT lecturers residential 11.10 3.00 6.80 3.12

area (Tarkwa)Range <0.12–11.10 0.65– 4.29 0.43–6.80 0.60–4.05Mean 4.90 2.75 3.51 2.65Standard deviation 4.37 1.45 2.23 1.39

Table 8 Concentration ofU and Th in dust samplesusing NAA

Sample location Concentration

Uranium 238U Thorium 232Th(μg g−1) (μBq g−1) (μg g−1) (μBq g−1)

New Atuabo <0.01 <0.12 0.68 ± 0.11 2.77Mine staff club house 3.82 ± 0.37 47.10 1.40 ± 0.21 5.70Boboobo (Tarkwa) 2.22 ± 0.31 27.30 1.16 ± 0.18 4.70Agricultural Hill (Tarkwa) 0.90 ± 0.13 11.15 <0.01 <0.004UMAT residential area (Tarkwa) 0.69 ± 0.10 8.52 1.99 ± 0.30 8.08Mean 1.91 ± 0.23 23.52 1.31 ± 0.20 5.31

Table 9 Rn-222 concentration in air and soil and the corresponding estimated airborne annual effective doses

Sample location Temperature Atmospheric Relative Radon concentration Airborne 222Rn

(◦C) pressure (kPa) humidity (%) Airborne 222Rn Soil 222Rn annual effective(Bq m−3) (kBq m−3) dose, mSv

Abekoase 33.5 100.6 90.5 30.0 26.8 0.29Brahabebom 30.0 100.3 98.0 31.5 30.2 0.30Huniso 35.5 100.7 84.0 27.5 12.5 0.26New Atuabo 29.5 100.5 95.0 29.5 21.5 0.28Pepesa 38.0 100.8 84.0 30.0 19.8 0.29Samahu 33.5 100.6 94.5 30.5 28.9 0.29Tarkwa Township 34.0 99.9 89.7 32.7 41.3 0.31Mine site 34.6 100.1 83.1 27.9 15.5 0.27Range 29.5–38.0 99.9–100.8 83.1–98.0 27.5–32.7 12.5–41.3 0.26–0.31Mean 33.6 100.4 89.9 30.0 24.6 0.29Standard deviation 2.8 0.3 5.7 1.7 9.2 0.02


Table 10 Comparison ofactivity concentrations of238U, 232Th and 40K insoils in the study area andpublished data(UNSCEAR 2000a;Darko et al. 2010b)

aUNSCEAR (2000)Report for (Algeria,Egypt, USA, India,Malaysia, Lithuania, UK,Hungary, Spain andWorld Average values)bDarko et al. (2010) forGhana (Mine 1 and Mine2 values)

Country Concentration in soil, Bq kg−1

238U 232Th 40K

Range Mean Range Mean Range Mean

Ghana (this work) 8–26 15 9–67 27 60–249 157Ghana (Mine 1) 29 25 582Ghana (Mine 2) 35 21 682Algeria 2–110 30 2–140 25 66–1150 370Egypt 6–120 37 2–96 18 29–650 320USA 4–140 35 4–130 35 100–700 370India 7–81 29 14–160 64 38–760 400Malaysia 49–86 66 63–110 82 170–430 310Lithuania 3–30 50 9–46 25 350–850 600UK 2–330 1–180 0–3200Hungary 12–66 29 12–45 28 79–570 370Spain 2–210 33 25–1650 470World average 33 45 420

areas around the world with values in a rangeof 2–30 Bq m−3 in air (UNSCEAR 2000). Theresults are also below the action level of radonconcentration in air of 1,000 Bq m−3 for whichintervention is required. The corresponding an-nual effective dose is 6 mSv year−1 using an as-sumed outdoor occupancy of 1,760 h/year (ICRP1991; UNSCEAR 2000). This means that the areastudied does not have significant levels of 222Rngas. The environmental conditions under whichthe measurements of 222Rn activity concentrationswere carried out are also shown in Table 9. Theseinclude the temperatures, atmospheric pressure,and relative humidity with mean values of 33.6◦C,0.0104 kPa and 89.9%, respectively.

The activity concentrations of 222Rn in the soilmatrix calculated from the activity concentrationof 226Ra in the soil samples are also given inTable 9. The mean activity concentration of 222Rnin the soil was 24.6 kBq m−3 in a range of 12.5–41.3 kBq m−3.

Table 10 shows comparison of the activity con-centration of 238U, 232Th and 40K in soil in thestudy area with similar studies in Ghana and pub-lished reports from other countries (UNSCEAR2000; Darko et al. 2010).

Figure 1 is the layout of the map of thestudy area showing locations and the communitieswhere the sampling was carried out. The differenttypes of samples are represented with differentsymbols. The geological map of the study area isshown as Fig. 2. Figure 2 also shows the concession

of the mines and the different geological forma-tions of the mines.

Figure 3 shows the comparison of the resultsof the absorbed dose rate measured at the soil,water and dust sampling locations. The figureshows that absorbed dose rates at the samplinglocations varied in a range of 38–60 nGy h−1.These results compared well with published re-sults (UNSCEAR 2000). A comparison of theannual effective doses calculated from the soil,water, dust and airborne radon gas is shown inFig. 4. It shows that the airborne radon and watercontributes most significantly whilst the contribu-tion from dust is the least to the exposure in thepopulation of the study area. In general, however,the annual effective doses calculated from the var-ious samples are considered insignificant. The ac-

0

10

20

30

40

50

60

70

Soil Water Dust

Ab

sorb

ed d

ose

rat

e, n

Gy/

h

Type of sample

Fig. 3 Comparison of absorbed dose rate from direct airmeasurement at 1 m above the ground at soil, water anddust sampling points


0

0.05

0.1

0.15

0.2

0.25

0.3

Soil Water Dust Radon gas

An

nu

al e

ffec

tive

do

se, m

Sv

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Fig. 4 Comparison of annual effective doses due to ex-posure in soil, water and dust samples as well as airborneradon

tivity concentrations of the soil, rock, waste rack,ore and tailings in the study area are also com-pared and the results shown in Fig. 5. The activityconcentrations in the different types of samplesare quite uniform and do not show any significantvariations. Figure 6 shows the comparison of theactivity concentrations of the radionuclides in thedifferent types of water sources investigated. Itshows clearly that the activity concentrations of238U in the different types of water samples arehigher than that of 232Th which is in line withliterature values. However, in the water taken as acontrol in a river, the 232Th activity concentrationis higher than 238U. This could be attributed todeposition of particulate matter into the water

Act

ivit

y co

nce

ntr

atio

n, B

q/k

g

50

100

150

200

250

300

0SOIL ROCK

U-238

Type of sampleWASTE

Th-232

ORE

K-40

TAILINGS

Fig. 5 Comparison of the activity concentration indifferent types of samples in the study area

Act

ivit

y co

nce

ntr

atio

n, B

q/L

4321

56

87

910

0

U-238 Th-232 K-40

SW UW

Type of water sample

PW TW Control

Fig. 6 Comparison of activity concentrations of differentwater sources

body or transportation by sediments containing232Th since it is a surface water.

Conclusions

The study considered public exposure in the min-ing environment due to four exposure pathways:namely, direct external gamma ray exposure fromnatural radioactivity concentrations in soil/rocks,internal exposure from drinking water containingnatural radioactivity, inhalation of radon gas andinhalation of dust containing 238U and 232Th.

The mean activity concentrations of 238U, 232Thand 40K in the soil were estimated to be 15.2, 26.9,157 Bq kg−1, respectively. For the water samples,the mean activity concentrations of 226Ra, 232Thand 40K water samples are 0.54, 0.41, 7.76 Bq l−1,respectively. The mean activity concentrations of238U and 232Th in dust samples are 4.90 and2.75 μBq m−3, respectively. The results in thisstudy compared well with other studies carried outin other countries and with the worldwide averageactivity concentrations (UNSCEAR 2000). Themean annual effective doses estimated from directexternal gamma ray exposure from natural ra-dioactivity concentrations in soil/rocks, exposurefrom drinking water containing natural radioac-tivity, inhalation of airborne radon and inhalationof dust containing 238U and 232Th were 0.19, 0.21and 0.29 mSv and 2.65 nSv, respectively. The cor-responding total annual effective dose for all theexposure pathways was 0.69 mSv. Even though


the airborne radon contributed more significantly,42% to the total annual effective dose, the ac-tivity concentrations measured are far below theICRP recommended level of 1,000 Bq m−3 forwhich remedial action is needed. However, it isrecommended that the mining company estab-lishes a periodic monitoring programme especiallyfor the control of airborne radon. The total an-nual effective dose is lower than the 1 mSv/yeardose limit recommended by the ICRP for publicradiation exposure control. The results indicateinsignificant levels of the natural radionuclides,implying that the mining activities do not pose anysignificant radiological hazard to the communitiesin this area.

Acknowledgements The authors would like to thank theRadiation Protection Institute of Ghana Atomic EnergyCommission and the Tarkwa Goldmine Ltd for use of facil-ities and other forms of support to carry out this study. Theauthors are also grateful to the staff of the Environmentaland Safety Departments of Tarkwa Goldmine Ghana Ltdand to Mr. David Kpegloh, Oscar Adukpo, Rita Kpordzro,Henry Lawluvi, Bernice Agyeman, Ali Ibrahim all of theRadiation Protection Institute for their assistance in thisstudy.

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