The Activity Concentrations, Radiation Contamination, and Hazards from Wastes and Soil Samples in Nasirabad Industrial Area, Chattogram, Bangladesh

Soils and other solid wastes from industrial areas of Nasirabad, Chattogram are usually dumped or used for land development. Information about the radioactivity level presented on these soil and wastes enables one to assess any possible radiological hazard to humankind by the use of such materials. A total of 37 (31 soils and 6 solid waste) samples near from different types of industries along four kilometre range were collected. The presence and activity concentrations of naturally occurred radioactive materials (NORM) and anthropogenic radionuclides in the samples were estimated using HPGe detector of 40% relative efficiency. The activity concentration of 226 Ra, 232 Th and 40 K were found to be ranging from 8 ± 2 to 131 ± 18.33 with an average of 21 Bq∙kg −1 , 10 ± 2.69 to 133 ± 15.96 with an average of 40 Bq∙kg −1 and 81 ± 22.68 to 930 ± 260.40 with an average of 449 Bq∙kg −1 respectively. Besides this, some hazard indices like, the radium equivalent activity (Ra eq ), external hazard index (H ex ), internal hazard index (H in ), and the activity concentration index ( I γ ) were calculated to assess the radiation hazard in this region. The averages of calculated hazard indices were within the normal limits, except the activity concentration index, which shows elevated values. The outcomes of this study could serve as important baseline radiological data for future epidemi-ological studies and environmental monitoring initiatives in the study area.


Introduction
Human exposure to ionizing radiation is one of the scientific subjects that attract public attention over the time since the natural radioactivity from primordial origins is the vast majority to detect on the earth and is responsible for most of the total radiation exposure of the mankind (UNSCEAR, 2000). Among these 226 Ra is the most abounded progeny of 238 U (99%) that exists in the geo-surface media, generally in soil, in water and/or in the earth-born materials with appearing activity [1]. The radioactive substance of thorium occurs naturally in the earth's crust in combination with other minerals such as silica, and almost 99% of natural thorium exists in the form of 232 Th [2]. Underground mine rocks usually contain a more intense form of thorium. Wind, water action and other natural weathering break these rocks, and make the thorium and all other components of the rocks become a part of the soil [3]. Moreover, the soil containing a high amount of mud materials shows elevated contents of thorium [4]. 39 K (93.2581%) and 41 K (6.7302%), the two natural isotopes of potassium are compulsory for living species; however, various health hazards are cause of the strong gamma radiation (1460 keV) emitted from the primordial 40 K (0.0118%) [5] [6]. Different physicochemical forms of the aforementioned radionuclides trigger up their mobility and biological uptake. For instance, radium especially bonds to organic matter and has an affinity for hard tissue because of its chemical similarity to calcium [7]. While potassium and thorium favourably bond to clays, and hence these are more radioactive than the other sedimentary rocks.
Thorium enters the body mainly through inhalation of contaminated dust and swallowed in water and/or with food. Most of its portion ejects out, if some portion remains it will be deposited to bones, where it may remain for many years.
On the other hand, the potassium ingested through the food chain and finally deposited on muscular cells.
Chattogram, the commercial capital of Bangladesh is famous for its industrial affluence. Our study region, Nasirabad Industrial area has around 25 industries of a wide variety, like steel industries, metal & plastic processing industries, food processing industries, paint & chemical production factories, oil refining plants, silver recycling, woodcutting and textile mills [8]. These factories are illegitimately discharging a huge amount of pollutants to the ambient continuously.
Despite being the industrial area, around 100 thousand inhabitants live permanently in the span of 8 square kilometre of this study area [9]. Following the radiological health hazards particularly from the external exposure via gamma radiation, the concentrations of NORMs and contamination due to anthropogenic radionuclides in this area should be the subject of periodic monitoring.
This study has been conducted to investigate the level of natural radionuclides ( 226 Ra, 232 Th, and 40 K) and artificial radionuclides ( 137 Cs) in the soils of Nasirabad Industrial area due to its importance in commerce and radiological assessment to the working and living population.

Sample Collection and Preparation
Nasirabad Industrial area is in the northern side of Chattogram City Corpora-

Measurement by Gamma-Ray Spectrometry
The activity concentrations of NORM's and TENORM's in the samples were detected using a liquid nitrogen-cooled coaxial p-type HPGe gamma ray detector proach [11] was applied using the aforementioned gamma lines.
The activity concentration of each radionuclide of interest in each sample was evaluated using the following equation [12]: where "A" the activity concentration of the radionuclide in the sample, in Bq/kg, "N" the net counts under the corresponding photo-peak, " γ ε " the detection efficiency corresponding to each specific gamma ray, "I" the intensity of the corresponding gamma ray energy, " s t " the counting time in seconds and "w" the mass of the sample in gram.
The radioactivity of each sample is being reported with the counting error of one sigma. The analysis software Spectra Line GP calculates the standard deviation (SD) from the several values of uncertainty impute by the operator and their average for a radionuclide using Equation (2).
where, "n" is the total number of samples, " x " is the arithmetic average and " n x " indicates individual samples value.

Results and Discussion
In this study, 37 samples of soils and industrial wastes were analysed to detect the level of radiation contamination and NORM's. Among them, six samples are throw-out solid (mud, slug, and rust) waste collected near the metal processing industries. Table 1 shows the distribution of detected radionuclides and their specific activity (in Bq•kg −1 ) on dry-weight basis. Among them, 10 samples have the 226 Ra bellow the detection limit, whereas only in 4 samples 232 The was undetected and apart from this, 40 K was present as dominating amount over the other radionuclides in every sample.
were, Ra C , Th C and K C are the activity concentration (Bq•kg −1 ) of 226 Ra, 232 Th, and 40 K respectively.
Ten samples from all studied data show the higher value of dose rate compared to world average (57 nGy•h −1 ) reported by UNSCEAR with the maximum value of 157 nGy•h −1 in sample s-05. This implies that the area of the collected sample will not be a good choice for living.
Annual effective dose equivalent (AEDE) received by an individual was also estimated from the dose rate. In this case, the quotient of effective dose rate to absorbed dose rate in air is taken as 0.7 Sv•Gy −1 and 20% occupancy for outdoor while 80% for indoor as per suggestion of UNSCEAR report for environmental exposure to gamma rays of moderate energy [13].
Most of the samples have AEDE (outdoor) bellow the world average (0.07 mSv•y −1 ), except ten samples having the higher. On the other hand, only the sample S-05 and S-17 exhibits the AEDE (indoor) higher than world average (0.41 mSv•y −1 ) mentioned in UNSCEAR 2000. But in combined annual effective dose (indoor + outdoor), sample S-05 (0.963 ± 0.129 mSv•y −1 ), S-17 (0.576 ± 0.090 mSv•y −1 ) and S-24 (0.492 ± 0.072 mSv•y −1 ) show the value higher than the world average (0.48 mSv•y −1 ), whereas the average annual dose (0.29 ± 0.05 mSv•y −1 ) for all samples is still below the world average. Table 2 listed the examined value of absorbed dose rate, annual effective dose equivalent for indoor and outdoor.
Radium equivalent activity (Ra eq ) concentration in Bq•kg −1 , is the widely used hazard index which implies the comparison of activity concentration of radium, thorium and potassium presented in the samples. It is assumed that 370 Bq•kg −1 of 226 Ra, 259 Bq•kg −1 of 232 Th and 4810 Bq•kg −1 of 40 K produced the same gamma-ray dose. Based on this, Ra eq was calculated by using Equation (5) for this study [14].   [13].
In order to assess the safety level of radiation, the European Commission proposed an index called gamma activity concentration index ( I γ ). I γ is calculated by using the following formula, Equation (6). [15] 300 200 3000 where, Ra C , Th C and K C are the activity concentration of 226 Ra, 232 Th, and 40 K in Bq•kg −1 . If the samples are used as building materials and/or land development, the activity concentration index shall not exceed the following values shows in Table 3 depending on the dose limit criterion [15].
The distribution of the value of I γ for the collected samples from Nasirabad industrial area, that are used for land development and house construction was studied and presented in   Figure 2. External (H ex ) and internal (H in ) hazard index with compared to allowable limit.
where, Ra C , Th C and K C are the activity concentration of 226 Ra, 232 Th, and 40 K in Bq•kg −1 respectively. The H ex must not exceed the limit of unity for the radiation hazard to be negligible. As well as, the H in must also be less than unity to have minimal hazardous effect of radon and its progeny to the respiratory organs [17]. In our study all of the samples from both soil and solid waste category having the H ex bellow the unity and only one soil sample S-05 shows the H in value 1.30 ± 0.181, which is higher than unity. Nevertheless, the average of H ex and H in is still within limit. Figure 2 is representing the analysis and result of these indices.

Conclusion
The soil and solid waste are nearby the industries in Nasirabad industrial area.
Chattogram was collected and investigated through determining the activity concentration of terrestrial radionuclides ( 226 Ra, 232 Th, and 40 K) using HPGe detected in any of the 37 samples i.e., this area is free from unwanted radioactive contamination. The solid waste samples are containing those primordial radionuclides in a noticeably low amount compared to other soil samples. This may due to the source of origin of these two category samples. Besides this, the dose rate from the samples was also determined, few samples show a slightly higher value compared to the world average. The indoor and outdoor annual effective dose equivalent was also studied. In this case, it was clearly found that the average result was below the safety limit except three samples having elevated value.
Average of Radium equivalent activity concentration was found below the recommended level, which sounds good. The external, internal hazard index, and activity concentration index value were also measured for further evaluation. It's seen that the averages of these indices are lower than the allowed safety level.
The studied soil and solid waste samples from a growing industrial area will help to establish a baseline radiological data for further environmental safety precaution.