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 Table of Contents 
ORIGINAL ARTICLE
Year : 2016  |  Volume : 39  |  Issue : 4  |  Page : 222-232  

Radiological assessment and geochemical characterization of the sediments of Awba Dam, University of Ibadan, Nigeria


Department of Physics, Radiation and Health Physics Research Laboratory, University of Ibadan, Ibadan, Nigeria

Date of Web Publication13-Feb-2017

Correspondence Address:
Nnamdi Norbert Jibiri
Department of Physics, Radiation and Health Physics Research Laboratory, University of Ibadan, Ibadan
Nigeria
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0972-0464.199972

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  Abstract 

The distribution of natural radionuclides and geochemical parameters in the sediments of Awba Dam, University of Ibadan, Nigeria has been determined. The mean absorbed dose rates obtained were 72.3 nGy/h (upstream), 53.9 nGy/h (middle stream) and 50.29 nGy/h (downstream) with the corresponding mean annual effective dose rates of 0.09, 0.07 and 0.06 mSv/year, respectively. The geochemical analysis showed that Si, Al, Fe, Mn, Mg, Ca, Na, K, and Ti were the major elements and Zn, Cu, Cr, Rb, Ni, Ba, Ga, Ce, and Sr as heavy metals in the sediments. The moderate enrichment elements were Zn, Rb, Ba and Ga, whereas elements deficient in enrichment were Cr, Sr, Ce, Cu, and Ni. The geo-accumulation index was from unpolluted to moderately polluted environment. The enrichment factors (EF) observed in the elements indicate that these metals are entirely from crustal material or natural origin while EF >1.5 observed in Zn, Rb, Ba, and Ga suggests that the sources are more likely to be anthropogenic. The values of the radiological hazard parameter indices were below the recommended safe limits; an indication that the sediments can be used safely.

Keywords: Awba Dam, enrichment factor, geo-accumulation index, heavy metals, natural radionuclides, pollution, University of Ibadan


How to cite this article:
Jibiri NN, Akomolafe IR. Radiological assessment and geochemical characterization of the sediments of Awba Dam, University of Ibadan, Nigeria. Radiat Prot Environ 2016;39:222-32

How to cite this URL:
Jibiri NN, Akomolafe IR. Radiological assessment and geochemical characterization of the sediments of Awba Dam, University of Ibadan, Nigeria. Radiat Prot Environ [serial online] 2016 [cited 2020 Feb 22];39:222-32. Available from: http://www.rpe.org.in/text.asp?2016/39/4/222/199972


  Introduction Top


The aquatic environments consist of an aqueous phase and a solid phase which is mainly sediment (particulates) in surface environments and the host bedrock in groundwater.[1] Since sediment is an integral part of an aquatic ecosystem, naturally occurring primordial radionuclides are found to trap in the sediment. Concerted attention should be given to the distribution pattern of naturally occurring radionuclides and thorough study of environmental materials that serve as reservoir of the natural radioactivity. Due to weathering and other environmental processes, radionuclides in rock and soil may accumulate in sediment and dissolve into drinking water, thereby leading to human exposure.[2] The earth crust also contains small amount of 238 U,232 Th,40 K and other heavy and major elements such as Cs, Cd, Pb, Fe, Mg, Mn, Cu, Zn, Cr, As, and Ni. The concentrations of all these elements depend on the geology of a local environment as well as other natural and anthropogenic processes.[3] Pollution of natural environment by metals has been a major problem because these metals are non-destructible and many of them have toxic effects on living organisms, especially when they exceed certain threshold of safety limits.[4]

Sediment forms a major component of an ecosystem and is the most endangered due to the influence of various human activities. Sediment is considered contaminated when chemicals are present or other alterations have been made to its natural environment.[5] Considering this fact, radioactivity in a water body can remain at significant levels as a result of secondary contamination processes. Due to gravitational and other depositional phenomena, the highest proportion of the radioactive materials is mainly found in the sediment compartment of the aquatic ecosystem.[6] Thus, river sediment is considered a durable and reliable register of the river pollution by radionuclides.[7] Studies on natural radioactivity and heavy metals are necessary, not only because of their radiological and pollutant impacts to human health, but they also serve as excellent biochemical and geochemical tracers in the environment and environmental monitoring.[8]

The Awba Dam, since 1964 provides water for the domestic needs of the university, opportunities for fish culture and to facilitate fisheries research and also serves as source of revenue. There has not been effective and regular control of environmental and human waste entering the dam and this has been a concern to many due to danger that may arise from these pollutants. No study on radionuclide and heavy metal distributions has been carried out on the Awba Dam of the University of Ibadan, Nigeria despite the level of discharges into the dam, from laboratories, runoffs from farms and discharges from national institute for radiation protection and research. This study is, therefore, aimed to determine the level of natural radionuclides and heavy metals distribution in the sediments of the dam with a view to characterizing the geochemical parameters and the radiological implications to the population of the University of Ibadan community.


  Materials and Methods Top


Description of sampling location

The Awba Reservoir in the university lies between the latitudes 7°26'–7°27'N and longitudes 3°53'–3°54'E located in the South Western region of Nigeria about 160 km from the Atlantic Ocean coast at an altitude of 185 m above sea level. With a maximum depth of 5.5, 140 m long with a crest of 12.2 m, the reservoir can hold about 230 million liters of water. The stream throughout its course in the university has a length of 975 m.[9] The dam is of enormous importance to the University of Ibadan community as a source of water, fish, and sand for building, recreation, and tourism purposes.

Collection of samples

A total of 31 sediment samples were collected across the dam. The samples were collected at a depth of 0–15 cm from the upstream, the downstream and the middle locations of the dam. The map of University of Ibadan showing the dam and the locations from where samples were collected is shown in [Figure 1]. After collection, the samples were placed in polythene bags and carefully labeled to prevent sample contamination. The samples were analyzed for their natural radioactivity content at Radiation and Health Physics Research Laboratory at the Department of Physics, University of Ibadan, while geochemical analyses were performed at geology department of the university.
Figure 1: The map of the University of Ibadan campus showing the Awba Dam and sampling points

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Samples preparation for radionuclides determination

The samples were air-dried, homogenized, crushed, and sieved to pass through a 2 mm mesh. The samples were further oven-dried to remove moisture at 110°C until a constant weight was reached. Thereafter, approximately 200 g of each of the samples was transferred to a cleaned, washed and uncontaminated cylindrical plastic container of size 8.5 cm length, and 7.5 cm diameter and sealed. The sealed samples were counted after a minimum period of 28 days to achieve secular equilibrium between 226 Ra and its short-lived progenies.

Sample preparation for geochemical parameters

Twenty representative samples out of the 31 samples used for the radionuclides analysis were used in the elemental analysis. Each sample was first pulverized and 0.5 g digested with 5 ml HF and a mixture of prepared solution of nitric acid and perchloric acid (ratio 3:2) according to USEPA method 3052.[10] The sample was stirred and heated inside a fume cupboard. The digested sample was diluted with distilled water and made up to 20 ml mark. A volume of 1 ml was taken from the solution and further diluted with distilled water to 10 ml mark. The major elements Si, Al, Fe, Mn, Mg, Ca, Na, K, and Ti were then determined using a Perkin Elmer 305 Atomic Absorption Spectrophotometer. Similarly, the heavy metals Zn, Cu, Cr, Rb, Ni, Ba, Ga, Ce, and Sr were also determined.

Activity concentration determination

The radioactivity counting was done using an ORTEC Coaxial n-type high purity germanium detector with 20% relative efficiency coupled to ORTEC multichannel analyzer. The detector is placed inside a cylindrical lead shield of 5 cm thickness with internal diameter of 24 cm and height of 60 cm. The lead shield is lined with various layers of copper, cadmium and Plexiglas each of 3 mm thick. The efficiency of the detector system was determined using standard reference, IAEA sediment sample (IAEA-315) containing 40 K,210 Pb,226 Ra,228 Ra,228 Th,232 Th,234 U, and 238 U. The standard reference sample and sediment samples having identical size, shape, density, spatial distribution of material, etc., were counted for 10 h under similar condition. The activity concentration of each radionuclide in the samples was obtained using the Equation 1:[11]



Where is the activity concentration of measured samples, is the net peak area at energy E of radionuclides, is the detector efficiency, T is the counting time (10 h), is the number of gamma per nuclear transformation, and M is the mass in kilograms of the measured sample and is the detector efficiency which is given by:



Where Cn is the net area of the standard sample, C is the activity of the standard sample in Bq/kg, T, and Where LLD is lower limit of detection an

given in Equation 1. The gamma lines 186.211 keV of 226 Ra, 609.316, 1120.287 and 1764.539 keV of 214 Bi were used to determine 226 Ra (238 U) while the gamma lines 911 keV of 228 Ac, 583.187 and 2614.511 keV of 208 Tl were used to determine 232 Th and that of 40 K was determined from the gamma line of 1460.822 keV. Using Equation 1, the activity concentrations of the radionuclides is presented in [Table 1].
Table 1: Activity concentrations (Bq/kg), absorbed dose and annual effective dose of primordial radionuclides in up, middle and down streams locations of the dam

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The minimum detectable activity (MDA) for 40 K,226 Ra and 232 Th in the sediment were determined using Equation 3:



Where,

Where LLD is lower limit of detection and ε, β, m, and t assumed the same value as given in Equation 1. The values of the MDA of the detector for 40 K,226 Ra, and 232 Th are 23.4, 12.0 and 16.4 Bq/kg, respectively.

Absorbed dose rate

The absorbed dose rate at 1 m above the ground (nGy/h) was calculated using the expression given by UNSCEAR 2000:[12]



Where DR is the absorbed dose rate in nGy/h and ARa, ATh and AK are the activity concentrations of 226 Ra,232 Th, and 40 K, respectively. The annual effective dose (AED) to the public due to the absorbed dose rate in air was calculated in unit of mSv/year by converting the total absorbed dose in nGy/h using 0.7 Sv/Gy as conversion factor (CF) and multiplying by occupancy factor (OF) using Equation 5:



Where, 0.7 Sv/Gy is the CF [13] for absorbed dose in air to external effective dose, and 0.2 represents the outdoor OF. This shows that the people in the study area spend approximately 20% of their time outdoors. The result of the absorbed dose rate and the AED rate are presented in [Table 1].

Radiation hazard indices

Radium equivalent activity

Radium equivalent is a common radiological index in comparing the specific activities of sediment samples containing 226 Ra,232 Th, and 40 K by a single quantity, which takes into account the radiation hazards associated with them.[14] The radium equivalent activity (Raeq) is mathematically defined as below.[15]



The value of Raeq must be <370 Bq/kg for the radiation hazard to be negligible. The results obtained are presented in [Table 2].
Table 2: Radium equivalent, internal, external hazard index and gamma representative index for upstream, middle and downstream in the sediments of Awba Dam

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Internal radiation hazard index

The internal radiation hazard index (Hin) is used to reduce the maximum permissible concentration of 226 Ra to half the values appropriate for the external exposure alone.[16] The inhalation of gaseous radionuclide, radon-222 which is the daughter product of 226 Ra might result in obtaining internal exposure. The internal exposure to 222 Rn and its radioactive progeny can be estimated in terms of Hin as below.



The results obtained are presented in [Table 2].

External radiation hazard index

The external hazard index (Hex) due to gamma radiation is calculated using Equation 8.[17]



To limit the external gamma dose of materials to 1.5 mGy/year for the radiation hazard to be negligible or insignificant, the Hex must be in conformity with the criterion of Hex ≤1.[18] The results obtained are presented in [Table 2].

Representative gamma index

The representative gamma index (Iγr) used to estimate the level of γ - radiation hazard associated with the natural radionuclides in specific investigated samples was determined using the expression proposed by NEA-OECD [19] and given by Equation 9.



The results obtained are presented in [Table 2].





Activity concentrations of the radionuclides in the sediment

The activity concentrations of 40 K,232 Th, and 226 Ra in the sediments samples are presented in [Table 1]. The average concentration of 40 K,232 Th, and 226 Ra in the upstream are 266.4 ± 20.0, 63.3 ± 8.7 and 49.6 ± 14.9 Bq/kg, respectively. The concentration of 40 K,232 Th and 226 Ra in Bq/kg at the middle stream are 110.8 ± 9.9, 52.5 ± 7.3, and 38.2 ± 11.6, respectively. Similarly, the same pattern was observed at downstream of the dam, the average concentration of 40 K,232 Th, and 226 Ra are 103.3 ± 9.5, 49.0 ± 6.8 and 35.2 ± 10.9 Bq/kg, respectively.

The pattern of distribution observed in the radionuclides at the upstream of the dam may be due to the fact that more pollutants are channeled into the dam from this end such as effluents from domestic, industrial, and laboratory waste from students' hostels, National Institute for Radiation Protection and Research and Faculty of Science Demonstration Laboratories. Furthermore, the waste products from aquarium and runoffs of fertilizer from nearby farms find their way to the dam. However, the activity concentrations obtained in this study were in agreement with other research works [16],[20],[21] conducted on sediments in different part of Southwest Nigeria. Furthermore, in comparison with results obtained from other parts of the world, we can infer from [Table 3] that 40 K activity is much lower whereas 232 Th and 226 Ra activity levels are comparatively higher. The range of values of 40 K and 226 Ra observed in this study are within the range of values for crustal concentrations of 140–820 Bq/kg for 40 K, 11–64 Bq/kg for 232 Th (exception for 232 Th which is slightly high) and 16–110 Bq/kg for 238 U (226 Ra) as reported in UNSCEAR 2000[12] for area with high background radiation levels around the world. From [Table 3], the mean activity concentrations of the radionuclides decrease in the order 40 K >232 Th >226 Ra. The higher radioactive levels of 40 K seen in the study area may be associated with the granite rocks which break down as a result of weathering and transport to the dam to form sediment [Figure 2] and [Figure 3].
Table 3: Activity concentrations of naturally occurring radionuclides obtained by different authors in other parts of the world

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Figure 2: The annual effective dose in the upstream, middle and down streams location

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Figure 3: The radium equivalent for up, middle and down streams

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Dose assessment

As could be observed from [Table 1] the absorbed dose rates obtained ranged from 54.1 to 79.2 nGy/h, 40.0 to 70.5 nGy/h, and 38.3 to 63.5 nGy/h for upstream, middle, and downstream respectively, whereas the mean was 72.3 nGy/h for upstream, 54.0 nGy/h for middle stream, and 50.2 nGy/h for downstream. The AED rate ranged from 0.07 to 0.10 mSv/year for upstream, 0.05 to 0.09 mSv/year for middle stream, and 0.05 to 0.08 mSv/year for downstream. The average value of 0.07 mSv/year obtained from the study compared well with recommended annual outdoor dose limit for members of the public far from nuclear facilities by the UNSCEAR.[13]

Radiological assessment

Radium equivalent gives the useful guideline in the regulation of the safety standards on radiation protection for the general public.[10] As shown in [Table 2], all the values (up, middle, and down streams) are within the recommended value of 370 Bq/kg. When these values were compared with the values obtained for Osun river,[21] it could be observed that the value for Osun river also decreased down the river having its highest value at the upper region and the least at the lower region. Both hazard indices, external and internal, are much below unity. These results suggest that radiation hazard associated with the sediment samples when used as building material would not pose any health effects. The sediment samples at the Upstream exceed the upper limit for Iγr which is unity, whereas the sediment samples at the middle and down streams are below one. This suggests that at the upper stream of the dam, there are relatively higher concentrations of the natural radionuclides, albeit low, as may be seen [Table 1] when compared with middle and lower streams of the dam.

Geochemical assessment

The concentrations of the trace elements in the sediment samples from the selected sampling points are presented in [Table 4]. The highest mean concentration of the trace elements was Ba while the lowest was Ce. The order of increased magnitude was Ce < Ga < Cu < Ni < Rb < Sr < Cr < Zn < Ba. From [Table 4], it is observed that the concentration of Cu was between 20.38 and 39.56 ppm while that of Pb and Zn range from 12.60 to 23.14 ppm and 210.00 to 393.10 ppm, respectively. The high relative abundance of these elements may suggest possible replacement during and after diagenesis. In general, Rb, Cu, and Ba were clearly associated with clay minerals.
Table 4: Concentrations of heavy metals in sediments of Awba Dam

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The results of the analysis of the major elements are presented in [Table 5]. The result showed a slight variation in elemental composition of all the sample sets. This variation was due to different components constituting the sediment samples; reflecting the homogeneity of the sediment suite and indicating constancy of provenance and sedimentary environment of the material. The pattern observed in the sediments was SiO2> Al2O3> Fe2O3> MgO > CaO > TiO2> K2O > Na2O > MnO. Chemical composition is closely dependent on grain size, with Al2O3 increasing toward finer sediments and SiO2 toward sands. This strongly has control on mineralogical composition, in which clay minerals dominate the fine fraction, and quartz the coarse fraction. The SiO2 content is relatively high as shown in [Table 5]. Mineralogical studies have shown that SiO2 is mainly present as quartz, both fine as well as coarse-grained. Cursory examinations have revealed the rare presence of diatoms and sponge spicules, suggesting that these are not important sources of Si in these sediments. The CaO content generally >1% indicates slight calcareous sediments. This is derived from skeletal components and therefore is biogenous. The distribution of Al2O3, TiO2, and Fe2O3 helps in understanding the sources of elements and geochemical environment of sedimentation. Al is mainly held in the clay mineral lattices as an essential constituent. This is also evident from the similarity of the distribution pattern of Al2O3 with respect to textural distribution. The contribution of Al2O3 by resistant detrital minerals is considerably less than that of clay mineral contribution. The TiO2 content of these sediments mainly comes from the detrital minerals as seen from its higher concentration (average 1.21%) in the sample sets. Corroborative investigations have shown the presence of large quantities of opaques in these sediments.
Table 5: Concentrations of major elements in sediments of Awba Dam

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Distribution patterns of MgO, Na2O, and K2O and their correlations with Al2O3 [Table 5] indicate that clay minerals are important host minerals for these elements. In addition to their initial presence in crystal lattices of clay minerals, exchange for Ca and absorption processes cause fixation of Mg, Na, and K in river-borne clays when these are brought into contact with seawater.

Enrichment factor and geo-accumulation index

In the assessment of the degree of pollution of the dam sediments by heavy metals; there is need to evaluate the enrichment factor (EF) value. To distinguish the heavy metals originating from human activities from those originating from natural weathering, calculation of EF is an essential part of geochemical studies.[34] As proposed by Simex and Helz,[35] EF as employed to assess the degree of contamination and to understand the distribution of the elements of anthropogenic origin from sites by individual elements in sediments. Fe was chosen as the normalizing element while determining EF-values since in wetlands it is mainly supplied from sediments and is one of the widely used reference elements.



Where, Cn is the concentration of element “n” in the sediment sample examined, Cref is the concentration in the earth crust,[36] Bn denote the reference element and Bref represents the concentration of the element in continental crust. The world average elemental concentrations reported by Turekian and Wedepohl 1961[33] in the earth's crust were used as a reference in this study because regional geochemical background values for these elements were not available. Five contamination categories are recognized on the basis of the EF as given by Sutherland.[37]

The result of the EF for heavy metals and major elements is presented in [Table 6]. The higher EF values are observed in the order of Zn > Rb > Ba > Ga > Cr > Sr > Ce > Cu > Ni for trace elements (heavy metals). Based on the classification given above the moderate enrichment elements are Zn, Rb, Ba, and Ga while elements that are deficient in enrichment are Cr, Sr, Ce, Cu, and Ni. The minimum EFs less than unity observed in these samples imply that these elements are depleted in some of the phases relative to crustal abundances in the study area. The EFs value observed in Cr, Sr, Ce, Cu, and Ni indicates that these metals are entirely from crustal material or natural origin while EF > 1.5 as observed in Zn, Rb, Ba, and Ga suggests that the sources are more likely to be anthropogenic.[38] Similar observation was made for the major elements as could be seen in [Table 6]. The higher values in order of Al2O3 > TiO2 > SiO2 > MgO with the exception of K2O > Na2O > CaO > MnO having EF <1.5 indicating crustal material or natural origin.
Table 6: The enrichment factor of heavy metals in the sediments

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A common approach to estimating the enrichment of metal concentrations above background or baseline concentrations is to calculate the geo-accumulation index (Igeo) as proposed by Müller.[39] The method assesses the degree of metal pollution in terms of seven enrichment classes based on the increasing numerical values of the index. This index is calculated as follows: The classification is shown in [Table 7][34]
Table 7: Classification table for geo.accumulation index

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Where Cn is the concentration of the element in the enriched samples, and Bn is the background or pristine value of the element. The factor 1.5 is introduced to minimize the effect of possible variations in the background values attributed to lithologic variations in the sediments. The result of the Igeo for both heavy metals and major elements is shown in [Table 6] and [Table 8]. The pollution level of the dam with heavy metals and major elements ranged from unpolluted to moderately polluted as shown in [Table 5] and [Table 6].
Table 8: Enrichment factor for major elements

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The results of the Pearson correlation coefficients between the activity concentration and trace metals and major elements are presented in [Table 9] and [Table 10]. From [Table 9] the analysis revealed that Ce had significant positive correlation with the primordial radionuclides while Sr and Rb had negative correlation with 238 U and 232 Th, respectively, and the remaining of the trace elements did not correlate with primordial radionuclides. This may suggest that they have similar source. Similarly, a significant correlation was observed between the activity concentration of the primordial radionuclides and some of the major elements. Na2O correlated with all the three radionuclides at 0.01 and 0.05 level. Furthermore, TiO2 had a significant correlation with 232 Th and 238 U while MgO correlated with 40 K and 232 Th.
Table 9: Pearson correlation matrix of activity concentrations and heavy metals of Awba Dam sediments

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Table 10: Pearson correlation matrix of activity concentrations and major elements of Awba Dam sediments

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The activity concentrations of naturally occurring radionuclides and geochemical parameters in sediments of Awba Dam have been determined. The mean AED due to sediments of the dam was well compared with recommended safe limit, and the radiological hazard indices were lower than the recommended international safe limits. The results obtained showed that the environment of Awba Dam is radiologically safe. This is an indication that the sediments of Awba Dam can be used for the construction of buildings without causing any radiation hazard. The Igeo showed that the degree of pollution in the dam was from unpolluted to the moderately polluted environment. The EFsfor Cr, Sr, Ce, Cu, and Ni indicates that they are from crustal material while EF >1.5 for Zn, Rb, Ba and Ga suggests that the sources are more likely to be anthropogenic. These calculated parameters, however, could serve as baseline information for future reference and epidemiological studies of the dam.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

 
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