Monitoring of specific activities of long-lived radioactive elements along the Mini-Okoro/Oginigba Creek, Port Harcourt

: Presence and concentrations of radionuclides could be as a result of natural and human activities. This study examined the associations and differences among soil, sediment and water specific activities of long-lived radioactive element (LLRE). Gamma spectroscopy was used to measure the concentration of the LLRE along the Mini Okoro/Oginigba Creek, Port Harcourt. Specific activities of three selected LLRE were derived. Correlation analysis was carried out to examine associations among the specific activities across different substrates. A strong and a significant negative correlation exists between the specific activities of Water 40 K and Soil 232 Th (r =-0.721, p<0.05); Water 238 U and Soil 238 U (r = -0.717, p<0.05) and Water 40 K and Sediment 238 U (r=-0.69, p<0.05). Comparison using Mann-Whitney U test shows that, soil and sediment are similar in their specific activities with Z values of -0.408, -1.209 and -1.021 (p > 0.05) for 40 K, 232 Th and 238 U respectively. The concentration of solid samples (soil and sediment) is different from the liquid (water) samples. These associations can be attributed to some specific underlying factors. And in other to understand them there is need for more studies.


Introduction
Human settlements such as cities and other urban settlements, create one of the most intense human interactions between humans and nature (Lawal, 2009). These interactions could be seen in the form of the continuous flow of resources and materials through the economy, which create both useful products and also create wastes which is often the cause of many of our environmental (Lawal, 2014) and health issues.
Radioactive materials could be naturally occurring radioactive materials (NORM) and technologically enhanced NORM (TENORM). Knowledge and understanding of the distribution of these is very important in assessing their potential health and environmental impacts.
In nature, NORM concentration are usually dependent on the location and geological characteristics (United Nations. Scientific Committee on the Effects of Atomic Radiation, 2000). Processes (such as weathering, rainfall, and runoff) within the natural environment acting on rocks facilitate the enrichment and distribution of radionuclides in the soils. Therefore, their specific activity in the soil can be used as the natural background radiation. But, within the urban environment and other areas where human activities have impacted, such could be increased as a results of anthropogenic factors. These factors could be in the form of mining and combustion of coal; oil and gas production; metal mining and smelting; mineral sands, fertiliser industry; building industry and recycling-all of which tend to enhance NORM in the environment (World Nuclear Association, 2014).
The dominance of the oil and gas industry in Nigeria, makes it a very important industry along the coastal region of Nigeria. With this dominance is the attendant impacts on the environment, especially in relation to the potential enhancement of NORM across the coastal region. Thus, it could be said that the industries potentially increases TENORM concentration across this region. Looking at the peculiarities of the region, high annual rainfall, high water table (Nwankwoala and Omunguye, 2013) and shallows borehole/wells for potable water, minimal coverage of pipe borne water utility (Water and Sanitation Department, 2014) the risk to public health is quite significant. The industries and activities previously mentioned, utilises a considerable amount of NORM, which often end up in effluents originating from them all of which has a very high probability of ending up in water bodies (and or groundwater). This potentially has a negative impact on the quality and ecology of the water bodies. This understanding has resulted in the monitoring and regulation of some of these industries, although, there is a great deal of inconsistency in these regulations across countries and industries (World Nuclear Association, 2014). In Nigeria, for example, there are significant issues with respect to public health and environmental management. These issues include lack of enforcement (zoning of industrial activities and effluent standards) and loose/inefficient regulatory framework as well as costs, equipment, coverage problems in monitoring the environment. Therefore, to address some of these issues there is a need to understand the dynamics and relationship among some of the environmental health indicators (in this case the specific activities of NORM/TENORM). This understanding can help in filling gaps in data and thus allow for local, regional and national modelling of NORM across the country, thereby supporting initiatives that could enhance environmental quality and quality of life across the country.
To this end, the objectives of the study is to:  Examine the association among soil, sediment and water specific activity of selected radionuclides.  Determine the differences among the specific activities of each radionuclide across substrates within the study area.

MATERIALS AND METHODS
Study Area: The study was carried out around the Mini-Okoro/Oginigba creek ( Figure 1) located within the Trans-Amadi Industrial Area of Port Harcourt Metropolis. According to the Köppen-Geiger climate classification (Rubel and Kottek, 2010), the study area belongs to the Tropical Monsoonal climate. This is characterised by a short dry season and a pronounced wet season, with one or more months having less that 60mm of rain. All months have mean monthly temperature greater than 18 o C and the highest annual temperature is usually experienced just before the onset of the rainy season.   (Short and Stauble, 1967). The terrain has slopes ranging between an average of 3 and 5 degrees towards the NW-SE direction, thus impacting on the drainage of the area (poorly drained, low relief and gentle slopes -consequently low river flow). Data Collection: Sodium-Iodide Scintillation detector was used to measure the specific activity concentration for the LLRE selected in the samples collected from the study area. This method was adopted because there is no need for chemical separation before taking reading. It is also highly efficient and relatively cost effective (however energy resolution in not very good). Sampling started from the Rumuobiakani Bridge down to the Tran-Amadi Slaughter bridge (behind Port Harcourt Zoo). 30 samples were collected along this route (ten each for soil, water and sediment).
Water samples were collected using two litres plastic bottles. Prior to collection, plastic bottles were rinsed with the sample water to minimise contamination. Samples were collected on the banks of the stream (less dilution of the washout). Acidification was carried out using 0.1M of Dilute HCl at the rate of 10ml per litre of water sample. This minimises precipitation on and prevent absorption by the walls of the plastic bottle used. Sample bottles were tightly capped and labelled accordingly with space left for expansion. Sample were transferred in the Marinelli sample containers and stored for four weeks. After this storage gamma spectroscopy was then carried out on the samples.
At a depth of between 10 and 15cm, soil and sediment were collected. Samples collected, were placed in black cellophane bags and labelled.
Masking tape was used to seal the sample bags. In preparing for analysis, soil and sediment sample were air dried and crushed afterwards and made to pass through a 2mm sieve. These were then weighed and stored (four weeks) before they were eventually analysed. The storage allows for Radon and other short lived progenies to attain secular radioactive equilibrium before gamma spectroscopy (Prakash et al, 2007). The samples were stored in Marinelli sample containers as well.
All samples were placed in the Sodium-Iodide Detector and counted for 10 hours, analysis was carried out at the National Institute of Radiation Protection and Research (NIRPR) at the University of Ibadan, Ibadan. The instrument is a Model 802, coupled to Canberra Lead cylindrical shield of 10cm thickness. Energy calibration used a set of International Atomic Energy Agency (IAEA) standard (known) sources of radionuclides within a well-defined energy range (0.511 -2.615MeV). Computation of the photo-peak of each of the radionuclide was done using the firmware algorithm of the Multichannel Analyser. Specific activities were indirectly determined through the activities of their decay products. Net area after the correction for background in each photo peak was adopted in the computation of the activity concentration of each radionuclide of interest. Rumubiakani dump site 10 Mini-Okoro bridge area Data Analysis: The specific activities of the three long-lived radioactive elements (LLRE) will be subjected to non-parametric tests. This was because the sample set for this study area is small. The implication from this is that it would be wrong to assume that the dataset will be normally distributed, thus parametric test would be inappropriate. Spearman Rank Correlation was used to examine the relationship among the LLRE examined across the three substrates (Soil, Sediment and Water). Spearman rank R is similar to the Pearson productmoment correlation coefficient in relation to the variability accounted for by them. However, Spearman R is computed from ranks, details of the computation, power and efficiency can be found in the works of Gibbons and Chakraborti (2014) and Siegel and Castellan (1988).
Mann-Whitney U test was employed in testing the difference between the substrates for each of the LLRE. This test is an alternative to the parametric ttest analysis for independent samples. It is the most powerful of all the non-parametric t-test analysis for independent samples, i.e. more sensitive than Wald-Wolfowitz runs test and Kolmogorov-Smirnov twosample test (StataCorp, 2011). This analysis allows testing whether there are differences between two groups without the assumption of normal distribution.

RESULTS AND DISCUSSION
Exploration of parameter association: Analysis of the soil, water and sediments radiometric parameters shows that four statistically significant associations could be found for the study area (Table 2). A strong negative correlation was observed between specific activity of 40 K in water samples and that of 232 Th in the soil across the study area. Similarly, specific activity of 238 U in water samples and that of soil samples displayed strong negative correlation across the area. There was also relationship between water and sediment related parameters. Specific activity of 40 K (water) and that of 238 U (sediment) showed a strong negative correlation.
From the foregoing, it is evident that there is an inverse relationship between the specific activity of some of the LLRE in water and that of soil (mostly) and sediment. It must be noted that this observation only pointed at association and not causation. Thus, from the study area, LLRE species 40 K in the water decreases, 232 Th (soil) and 238 U (sediment) specific activity increases (and vice versa). However, 238 U specific activity is quite unique with, water and soil showing inverse relationship. This correspondence was absent for any other species of LLRE.
Overall, it is obvious that there are other factors at play, i.e. affecting the fate of radionuclide in soils. For example, Essignton et al (1981) showed that changes in sorption due to, alteration of solubility and charge speciation influence the short or long term fate of radionuclides in soils. Edgington and Nelson (1986) also showed that interactions of radionuclides in the oceans or marine sediments could be described in terms of adsorption equilibrium. While effective equilibrium constant is a function of the redox properties, the complex forming ability, concentration of the stable element of the element considered as well as properties of the water body (concentration of ligands, surface and chemical properties). Moreover, Dar and El Saman (2014), shows that around the Jubal Strait (an important oil production and processing area along the Red Sea), there is a positive correlation between dissolved oxygen and the activity patterns of some LLRE while there is a negative correlation between these and pH and salinity.
To fully understand the interaction among the LLRE, there is need to collect more samples as well as data on environmental condition under which such data were collected. However, this result indicates that there are conditions present in the study area which favours inverse relationship as observed.
Exploration of differences across substrate specific activity: Water and Soil specific activities for the LLRE were compared to confirm if there is a difference between them. The mean rank for all the three LLRE were found to be higher for soil in comparison to that of water (Table 3). This is an indication that the specific activities of LLRE in water samples are generally lower than that of soil samples. This could be attributed to the ability of soils to hold on to the radionuclides (just like heavy metals) while in the case of water, there may be constant renewal and depletion due to the flow of the water body. . Result of the Mann-Whitney U test (Table 4) shows that there is a significant difference between the LLRE specific activities in the soil and that of water. There is a highly significant different in specific activity of 232 Th (Z = -2.646, p =0.007) between water and soil sample. Similarly, a highly significant different was also observed for 238 U (Z = -3.674, p = 0.000).
Monitoring of specific activities of long-lived radioactive elements 29 1 GREGORY O. AVWIRI, 2 *OLANREWAJU LAWAL, 1 EDITH I. NWOKEOJI This gave an indication that, across the study area the specific activities of these LLRE vary considerably across different substrates. While it could be argued that, the LLRE specific activity in the soils (are related to that of the geology of the area) should be similar to that of the water in the same water (comparatively). The result indicates that the shape of the distribution is different for all the LLRE across the two substrates.
Between water and sediment, a comparison of the rank mean (Table 5), shows that the mean rank of specific activities for the 3 LLRE were consistently higher in the sediment samples in comparison to the water samples. This observation is similar to what was obtained for soil and water -LLRE specific activity in water is about half of that recorded for soil samples. Examination of the differences between these substrates (Table 6), shows that there are significant differences between water and sediment samples in relation to the specific activities of LLRE in the study area. This is contrary to the logical argument which suggests that, as sediments were carried by water (runoff) into the water body there should be a similarity in the distribution of the specific activity of the two. This is could be expected since samples were collected at the bank of the stream where there is less dilution of the effluent from the surrounding enterprises and industries. Mean ranks result for comparison between the soil and sediment samples' specific activities for the 3 LLRE are presented in Table 7. The mean rank of the specific activity of 40 K across the soil samples was found to be lower than that of the soil. However, the opposite was recorded for 232 Th and 238 U i.e. mean ranks for soil is higher than that of sediment. This result shows that the difference between soil and sediment specific activity is dynamic and radionuclide specific even though they are all LLRE. However, with the mean rank showing differences between the two substrates, it is thus necessary to confirm whether these differences are statistically significant. Result of Mann-Whitney U test (Table 8) shows that there are no significant difference between the soil and the sediment irrespective of the species of the radionuclide. This is an indication that while sediments are deposited and soils are more or less resident there is no difference between the gamma specific activities of LLRE for both across the study area.
Overall the results shows, that soil and water specific activities for LLRE across the study are different. This is contrary to expectation that the concentration of LLRE in the soil and water at same sampling point/area should be similar if not in values but in shape of the distribution. Sediments are carried by water, and it could be expected that the trend and pattern of LLRE in sediments and water should be similar. The opposite was observed, that there is a significant difference between the concentration of the LLRE in soil and water. This further shows that while there are differences in the chemical properties and half-lives of the radionuclides and as such leaching into the environment also occurs at different rates (Ames et al, 1978;Gäfvert and Faerevik, 2005). 40 K solubility in sediment is known to be higher than other LLRE (measured in the study) and this solubility varies with respect to the amount of organic matter and particle size of the soil (Harrison, 1999). Moreover, the concentration in surface water may be affected by climatic conditions, the oxidation state of the water (Ahmed et al, 2001). As such, we could attribute this significant difference in the concentration between solids (sediment and soils) and liquid (water) across the study area to this differences in the factors influencing their concentration.
A look at the difference between the solids (sediment and soils) show that soil and sediment are similar in their concentration of LLRE across the study area, the difference can be said to be least pronounced in 40 K followed by 238 U and then 232 Th. It is well established that the more fine-grained materials are in the soil or sediment, the high the concentration of radionuclides (Dar and El Saman, 2014;International Atomic Energy Agency, 1988). Furthermore, Ames et al., (1978) showed that ions in solution exist in dynamic equilibrium in soils, sediment and rocks, therefore any factor inherent in the solid matrix and the solution can influence the concentration of elements and their species in solution. And according to them pH, cation exchange capacity, type and amount of soil minerals competing ion etc. influence this concentration. In light of this, it is no surprise that there are no differences between the concentration of these LLRE in the soil and sediment samples.

Conclusion:
The study seeks to examine the association in specific activities of selected LLRE in the soil, water and sediment across an intensively used creek in an urban setting in Niger Delta. Furthermore, it seeks to examine differences among soil, water and sediment concentration of the LLRE in the study area. The result shows that there is a strong negative correlation between soil and water concentration of 238 U similarly strong negative association was found between Water 40 K and Soil 232 Th as well as between Water 40 K and Sediment 238 U. This shows that there are underlying factors within the study area which brought about this association. And this requires further investigation.
In relation to the second research question, we can conclude that the solid samples (soil and sediment) are different from the liquid (water) samples with respect to their concentration of LLRE. This could be attributed to the differences in factors influencing the concentration of LLRE. Even though the samples are taken from areas with very little dilution effect, there are still differences between these two. As such, we posit that even though there is interaction between the two, the inherent attributes of the substrates confer differences in their ability to harbour LLRE. However, in the case of soil and sediment, no difference was found and thus the conclusion is, depending on the particle size, soil and sediment are not likely to be different in the concentration of LLRE especially around our study area. This study was carried out with a small dataset, it is thus suggested that more evidence will be required to further confirm the relationship and difference observed in this study.