Radioactivity level of coastal sediments and water across the Sangihe Island as the outlying islands of Indonesia

Indonesia as an archipelagic country is very concerned about maritime development, the outlying islands are the entrance for potential threats of radioactive distribution from global waters. Furthermore, the outlying islands have functions such as territorial defence, security, and sovereignty, economic and ecological. As environmental protection, level of natural and anthropogenic radioactivity was performed due to potential threats. The activity concentration of natural (226Ra, 232Th, and 40K) radionuclides inshore sediments samples are range from 3.44 ± 0.01 Bq kg−1, 5.13 ± 0.03 Bq kg−1 and 151.42 ± 0.04 Bq kg−1 respectively and average concentration level of anthropogenic of 137Cs in sediment and seawater obtained were 0.35 Bq kg−1 and 0.83 Bq m3 respectively. The concentrations value of NORM was lower compare to another region but the ratio sequence 226Ra<232Th<40K relatively have same pattern. The presence of fission product 137Cs in this region comes from global fallout since no nuclear activities in Indonesia and the level of 137Cs are almost same as the estimation the concentration 137Cs in the Pacific Ocean and comparable with other studies. Radioactivity levels in all samples are under the value of radiological hazard index (0.004-0.039, trace hold h-index=1) and annual effective dose rate are in save level (0.01 mSv/y, recommended limit AED=1mSv/year). The level of NORM radioactivity is locally and varied for each region meanwhile anthropogenic radioactivity are global and comparable since there are no nuclear activity in Indonesia.


Introduction
Indonesia is very concern in the development of maritime resources. As an archipelagic country the outers island potentially to be the first area to be contaminated by radioactive from global waters. Meanwhile, the outers islands have functions such as territorial defence, security and sovereignty, economic and ecological. Furthermore, maritime is civilization, but the establishment of maritime sector depending on how the sea is managed and utilized for prosperity and sustainability.
Sangihe Island is part of the North Sulawesi province located between 2⁰ 4' 13‖ -4⁰ 44' 22‖ N and 125⁰ 9' 28‖ -125⁰ 56' 57‖ E has boundaries as follows Republic of Phillipines and Talaud regency at northside, Pacific Ocean and maluku sea at eastside and Sulawesi sea at westside. As a consequence of being passed by global currents, Sangihe has marine biodiversity and became a tourist destination. Moreover, based on Sangihe economic growth data, one of the biggest contributors are marine 2nd International Conference on Fisheries and Marine IOP Conf. Series: Earth and Environmental Science 890 (2021) 012008 IOP Publishing doi: 10.1088/1755-1315/890/1/012008 2 products. As a consequence of the continuous flow and mixing of seawater, contaminants that enter in one place continuously spreading throughout the world's oceans [1]. The concentration of anthropogenic radionuclides in the Ocean began in 1945 from nuclear weapons tests, Nuclear Power Plant accidents, accidents and loss of nuclear material, and the disposal of industrial waste [1] Sangihe region was located in a subduction zone that create volcanic-magmatic activity and it has been suggested there are orogenic gold deposits and associated minerals [2]. Baru Gold Corporation (formerly East Asia Minerals) and local company recently granted operating license within area of 42.000 ha from Indonesian government (February 2021) but is awaiting final land acquisition and the project has not yet started the construction according to company announcement on official website. Mining activities in Sangihe actually have been around traditionally by native people. Nowadays Sangihe became a mining industry has potentially threatening land then transported to the coastal and marine environment.
There are 2 types of radioactivity based on the origin of their production, natural and artificial radionuclides. Artificial radionuclides are products from thermonuclear reactions of nuclear installation both normally or accident conditions and nuclear weapon testing which fall directly or indirectly into marine ecosystems from the atmosphere. Natural radionuclides in particular have been formed since the beginning of the earth's creation. Around 80% of human radiation exposure comes from naturally occurring radioactive sources, which can have a variety of negative consequences for humans, animals, and the environment. [3]. Natural materials that emit radiation are called naturally occurring radioactive materials (NORM). Sources of natural radiation that we receive come from terrestrial and extra-terrestrial (cosmic) [4]. Terrestrial background radiations are the total emitted radiations from the NORM in the earth's crust and vary with space and time.

Sampling area
The research was carried out in the area on the coast of Sangihe Island in August 2018. Sediment and Seawater samples were collected from the 5 Beach location in the Northern part of Sangihe. Sampling locations were chosen random method due to no nuclear activity, however, the location was chosen considered representative of the area condition were passed by global ocean currents.
Collected samples are required to be packed properly to avoid cross contamination during transfer to the laboratory and sample identity and collection information shall accompany the samples during transfer.

Sediment sampling
Sampling was performed on the upper beach by collect the sea sediments about 1Kg each. When determining the activity of gamma radionuclide concentrations in sediment, it does not require chemical preparation. Sediment only need to be dried and make sure the water and biota content were not there. In NORM measurements samples to be sealed for certain days before measure by gammaray spectrometry to prevent 222 Rn (noble gas) escape from container and ensure sample reach secular equilibrium between daughter and parent activity. When samples were sealed after about 30 days it will achieve ≈ 99.56% of full secular equilibrium ( 226 Ra, 222 Rn and 214 Pb, 214 Bi) [5]. Sediment samples were measure by using HPGe coaxial detector GC2020 type with 20% relative efficiency and GENIE 2000 as analysis software by Canberra.

Seawater sampling
Approximately 80 L of surface seawater were collected as same as sediment sampling location. Seawater samples were taken to determine the activity concentration of radionuclide anthropogenic 137 Cs. Altogether seawater samples were performed pre-concentration processes due to their very low concentration. Pre-concentration procedure was conducted according to IAEA standard procedures for individual radionuclides with minor modification. This procedure uses the solution of 10 gram of K 4 Fe (CN) 6 and 10 gram of Cu(NO 3 ) 2 for each sample until homogenous. The slurry produced from precipitation were collected using filter paper. Furthermore, filter paper was dried and packed in plastic container following the radiometric standard. Seawater samples were measure and determine the activity concentration using window analysis and full spectrum analysis as well as sediments [6].

Radioactivity analysis
Determination of radionuclides in sediment and seawater using HPGE gamma spectrometry procedure was follow in this study. The type of detector used for sample analysis in this study is an HPGe detector made in Canberra equipped with a 15cm thick Pb shielding to prevent interference from outside radiation. The GENIE2000 software was used to operate the gamma spectrometer system by Canberra. The use of the GENIE2000 software includes energy calibration, efficiency calibration, correction, as well as displaying the spectrum and interpreting the analysis results of the spectrum. Energy and Efficiency calibration of detector HPGe performed using Soil 152Eu E&Z standard and IAEA RG Th-232 reference materials. The sediment samples were counted for 3 days to achieve more than 95% confidence level and reduce counting error. The activity concentration and uncertainty of radionuclides was calculated following formula [7]  where a is the activity concentration (Bq kg), ng is net peak area samples, n0 is net peak area background, tg is time of measurements (second) (2) where w is conversion factor, ms is mass of the sample (kg), ε is the efficiency of measurement, correction factor as the effect of geometry, element composition, density etc between samples and standard, is correction factor of coincidence summing, P is the gamma abundance and D is decay time correction. Continue with uncertainty of activity (u(A)) calculation following formula.

Result and discussion
Sangihe Island is composed of volcanic rocks that erupted from at least four volcanic centres, which progressively young from south to north [8]. The radioactivity level of the sediment that has been analysed for its NORM content reflects the activities on land because the sampling location is not far from the land. Sediment play important role in the recycling process as well as serve as reservoir of various pollutants.  [9,10,11,12] while 40 K activity concentration dominates is soil all over the world.

Tabel 2. Radioactivity concentration in coastal sediment of Sangihe
Comparing results from the collecting sites, its clearly the ST5 location shown a value that is significantly different from other sites. This shows that the dynamic character of coastal areas is able to influence the comparable radioactivity levels in both sediment and seawater. This, most probably due to the tidal effect causing continuous leaching of sediment. Furthermore, the sampling location number 5 is the side of the Sangihe island that faces directly global waters. Geologically the oldest part of the island is in the southeast and the youngest in the northwest; there are forming new volcanoes off the north western tip and western shores of Sangihe Island [13,14]. Th have a good positive correlation ( 0.69 ) compared to 40 K ( 0.26 ) because both of radionuclide are from the same parents radionuclide 238 U meanwhile 40 K is not connected and also a parent radionuclide.
Further analysis, we can determine the external dose rate in the air at 1 m above using the following formula (4) where dose unit in nGy h -1 and Act unit following the activity concentration were obtained. We can use the dose rate data to estimated the annual effective dose (AED) following formula [15] ( 5) According to the table the ratio of 226 Ra and 232 Th was 1.4 and in all location, thorium is higher than radium probably this area pegmatite rocks [17]. The radiological data of NORM in soil and sediments are compared with other author in table 2. Base on the table, the dose rate and radiation hazard index in Sangihe were below all over data from other. According to the table 2, the annual effective dose rate in Sangihe were the lowest among the other and very low under ICRP recommendation dose for public 1 mSv year -1 . NORM/TENORM data collection is required due to increasing mining activity on land and plans to make Sangihe a mining industry. Marine radioecology studies include research on radionuclide content both natural and anthropogenic so that the level of radioactivity in the area can be known.
The concentration of seawater and sediment activity was plotted using ODV and then converted into contours. The contours show the distribution of anthropogenic radionuclides following the pattern of the ITF current flowing to the south. Meanwhile, the sediment counter shows sediment in the middle of the island and south of Sangihe. So that further research is needed to conduct radioecological research in the southern part of Sangihe Island to confirm the activity concentration distribution of sediment distribution. It can be concluding the level of NORM radioactivity is locally and varied for each region meanwhile anthropogenic radioactivity is global because there is no nuclear activity in Indonesia.   In addition, the concentration of 137 Cs is also used as a tracer to predict the transfer of global water mass [18,19] because it is easily soluble in water so that when mixed with local seawater masses make it dispersed into various water areas [20]. Indonesia has the Indonesian Through Flow (ITF) bring water mass system of water that crosses Indonesian local waters that connects the Pacific Ocean to the Indian Ocean. The ARLINDO phenomenon has become one of the characteristics of the current system in Indonesia. Circulation of seawater pass through Indonesian waters originating from the Pacific Ocean to the Indian Ocean due to the difference in density [21].

Conclusion
The level of radioactivity from 5 stations in Northern Sangihe have been determine by Spectrometry Gamma. The average concentration level of NORM in this study were lower compared to another region. The average activity concentration of 226 Ra, 232 Th and 40 K results were 3.44 ± 0.01 Bq kg -1 ,5.13 ± 0.03 Bq kg -1 and 151.42 ± 0.04 Bq kg -1 respectively. The data that has been taken can be compared when the mining industry is running by seeing whether the NORM level is increasing or not. The average concentration level of anthropogenic of 137 Cs in sediment and seawater obtained were 0.35 Bq kg -1 and 0.83 Bq m 3 respectively. It can be concluding the level of NORM radioactivity is locally and varied for each region meanwhile anthropogenic radioactivity is global because there is no nuclear activity in Indonesia. On other hand the anthropogenic radionuclide 137 Cs level comparable and will be stable as carried from the pacific or declined very slowly until Twenty-thirty years after, levels of 137 Cs in the Pacific and Indian Ocean are expected to be below 0.3 and 0.1 Bq/m 3 [22].