With the exhausting on-land sand resources and increasing environmental pressure, it has become necessary to look for alternative sources. Marine sand resources now stand for a more promising alternative, and contribute significantly to the overall provision of sand material in many countries (e.g.,    ). It was widely utilized as the main construction materials for buildings, road, artificial islands, coastal reclamation and beach nourishment, etc. Today, marine sand mining has become the second most important marine mining activity after offshore oil extraction   . The annual global production of the aggregate is about 16.5 billion tons, of which approximately 10% is supplied by marine sand mining in coastal waters  .
Together with the rapid economic development of China, the unprecedented demand for the marine sand mining has increased greatly in the recent years. After nearly 10 years of extensive exploration of marine sand offshore China, there are five areas were assessed which are suitable for marine sand mining, namely Liaodong Bay (LB), North Yellow Sea (NYS), Zhoushan Area (ZS), Taiwan Shoal (TS) and Pearl River Mouth (PR) (Figure 1), where very large quantity of sandy sediments developed.
Figure 1. Locations of samples (solid yellow dots) and study areas offshore China in this study. A, Liaodong Bay (LDB); B, North Yellow Sea (NYS); C, Zhoushan Area (ZS); D, Taiwan Shoal (TS); E, Pearl River Mouth (PR). The geographic coordinate of the samples are shown in Table 1.
However, the quality of marine sand should be scientifically evaluated before the mining and utilization. The quality parameters include the grain-size composition and sorting, heavy metal constituents, the mineral components, and most importantly, the natural radioactive properties (226Ra, 232Th and 40K). The study of the concentrations of radionuclides and their distribution in sands enables the assessment of radiological risk due to external human exposure to gamma radiation outdoors and inhalation of airborne radioactivity emanating from building constructions and dwellings (e.g.,   ). Generally, the specific activities of 226Ra, 232Th and 40K in raw building materials and their products depend on their geological and geographical conditions as well as the geochemical features of those materials  . And the natural radioactivity of marine sand depends on the sediment formation and transport processes that were involved; chemical and biochemical interactions influence the distribution patterns of uranium, thorium and their decay products. However, many studies had been carried out on the radionuclide concentrations in sand beaches around the world, such as India   , Brazil   , Thailand  , Egypt  , Iran  , and China (Xiamen,  ) using the gamma ray spectrometry. In spite of the high number of works carried out around the world on the beach sands, there is a lack of studies about radionuclides of offshore sands.
The aim of this study is to determine natural radioactivity (226Ra, 232Th, 40K) levels in sandy sediments (potential marine sand resources) collected from offshore China. Also, the average radium equivalent activity (Raeq), the total absorbed dose rate (D), the indoor and external hazard index (Hin, Hex), the annual gonadal dose equivalent (AGDE) and the annual effective dose equivalent (AEDE), etc., which will be defined later have been calculated and compared with the results of beach sands in literature all over the world. The results of this study will provide background data on the natural radioactive isotopes and environmental pollution of marine sand deposits offshore China.
2. Materials and Methods
Totally 141 sandy sediments were collected in five potential marine sand deposits offshore China, in which 8 samples are from Liaodong Bay, 68 samples from North Yellow Sea, 12 samples from Zhoushan Area, 41 samples from Taiwan Shoal and 12 samples from Pearl River Mouth, respectively. The locations of each sample were shown in Figure 1 and Table 1. The water depths of the samples range from 10 m to 50 m.
After all samples were dried at room temperature, samples were pulverized by Retsch mill and sieved through a 100 mesh to be homogenized, then weighed and transferred to Marinelli beakers of 1000 ml volume. Each sample was sealed for 30 days to reach radioactive equilibrium where the decay rate of the daughters becomes equal to that of the parent  . Sample preparation and all radioactivity measurements were carried out by using a Gamma-ray spectrometer (BE3830, Canbarra Industries, Inc.) in Qingdao Institute of Marine Geology. Gamma-ray spectrometer was used to determine the activities of 226Ra, 232Th and
Table 1. Location and Activity concentration (Bq・kg−1) of 226Ra, 232Th, and 40K in marine sands offshore China.
40K. The gamma ray transitions of energies 186.3 keV and 1460 keV gamma-ray transition were used to determine the concentration of 226Ra and 40K, respectively, while the gamma-ray lines at 911.0 keV (228Ac) and 583.3 keV (208Tl) were used to determine the concentration of the 232Th series. The activity levels of the samples obtained for 226Ra, 232Th and 40K are expressed in Bq・kg−1.
3. Results and Discussion
3.1. Specific Radioactivity
The activity concentrations of the detected radionuclide 226Ra, 232Th and 40K in the five offshore sand deposits are presented in Table 1.
In the Liaodong Bay, the concentrations of 226Ra, 232Th and 40K vary from 14.7 Bq・kg−1 to 36.1 Bq・kg−1, 25.3 Bq・kg−1 to 56.8 Bq・kg−1 and 405.8 Bq・kg−1 to 919.4 Bq・kg−1, with the mean values of 26.5 Bq・kg−1, 41.8 Bq・kg−1 and 691.9 Bq・kg−1, respectively.
In the North Yellow Sea, the concentrations of 226Ra, 232Th and 40K vary from 7.2 Bq・kg−1 to 66.3 Bq・kg−1, 15.7 Bq・kg−1 to 175.4 Bq・kg−1 and 635 Bq・kg−1 to 1044 Bq・kg−1, with the mean values of 17.5 Bq・kg−1, 39.4 Bq・kg−1 and 857.1 Bq・kg−1, respectively.
In the Zhoushan Area, the concentrations of 226Ra, 232Th and 40K vary from 15.0 Bq・kg−1 to 24.0 Bq・kg−1, 33.0 Bq・kg−1 to 40.0 Bq・kg−1 and 665.0 Bq・kg−1 to 836.0 Bq・kg−1, with the mean values of 19.5 Bq・kg−1, 36.6 Bq・kg−1 and 728.1 Bq・kg−1, respectively.
In the Taiwan Shoal, the concentrations of 226Ra, 232Th and 40K vary from 5.6 Bq・kg−1 to 39.9 Bq・kg−1, 6.5 Bq・kg−1 to 53.1 Bq・kg−1 and 176 Bq・kg−1 to 658.8 Bq・kg−1, with the mean values of 20.7 Bq・kg−1, 36.3 Bq・kg−1 and 458.8 Bq・kg−1, respectively.
In the Pearl River Mouth, the concentrations of 226Ra, 232Th and 40K ranges from 9.4 Bq・kg−1 to 20.0 Bq・kg−1, 9.0 Bq・kg−1 to 22.0 Bq・kg−1 and 152.0 Bq・kg−1 to 319.0 Bq・kg−1, with the mean values of 13.8 Bq・kg−1, 13.5 Bq・kg−1 and 245.7 Bq・kg−1, respectively.
Obviously, the mean concentrations of 226Ra, 232Th and 40K radionuclides from these five sandy deposits are much comparable to those of the world average values of 35 Bq・kg−1, 30 Bq・kg−1 and 400 Bq・kg−1 for 226Ra, 232Th and 40K, respectively  and beach sands around the world (Table 2). The 226Ra concentrations from the five offshore sandy deposits in this study are all lower than that of world average value. However, the 40K concentrations in Liaodong Bay, North Yellow Sea, Zhoushan Area and Taiwan Shoal are all much higher than that of the world mean value, and the 232Th concentrations from Taiwan Shoal and Pearl River Mouth are lower than that of the world average (Figure 2).
3.2. Radium Equivalent Activity (Raeq)
Since the distribution of natural radionuclides in the samples is not uniform, a common radiological index has been introduced to evaluate the actual activity level of 226Ra, 232Th and 40K in the samples and the radiation hazards where
Table 2. Comparison of activity concentrations of 226Ra, 232Th, and 40K dose rates of sandy sediments found in present study and different areas around the world.
Figure 2. Mean values of 226Ra, 232Th and 40K concentrations in marine sands of Liaodong Bay (LDB), North Yellow Sea (NYS), Zhoushan Area (ZS), Taiwan Shoal (TS), and Pearl River Mouth (PR). The world average values were also shown for comparisons  , shown as the dashed line for 226Ra (green), 232Th (red) and 40K (blue), respectively.
associated with these radionuclides, the radium equivalent activity (Raeq), which can be calculated from the relation   :
Raeq = CRa + 1.43CTh + 0.077CK (1)
CRa, CTh, and CK are the specific activities of 226Ra, 232Th and 40K, respectively, in units of Bq・kg−1. In the definition of the radium equivalent, it is assumed that 10 Bq/kg of 226Ra, 7 Bq/kg of 232Th and 130 Bq/kg of 40K each produce an equal gamma-ray dose rate   .
The calculated values of Raeq for the five sand deposits in investigation are shown in Figure 3. The calculated values of Raeq range from 52.0 (Pearl River Mouth) to 139.8 (North Yellow Sea), with a trend that the Raeq is much higher in North Part of the Chinese Seas than that in the South. All values of Raeq in the studied samples are found to be lower than the criterion limit of 370 Bq/kg  , and therefore, do not pose any radiological hazard when used for construction of buildings.
3.3. Representative Level Index (RLI)
In order to estimate the level of gamma radioactivity associated with different concentrations of certain specific radionuclides, known as the representative level index   , the formula is given as:
RLI = CRa/150 + CTh/100 + CK/1500 (3)
where CRa, CTh and CK are the average activity concentrations of 226Ra, 232Th and 40K, respectively, in units of Bq・kg−1. The mean RLI values varied from 0.39 (Pearl River Mouth) to 1.08 (North Yeloow Sea) (Figure 4). It is clear that these values do not exceed the upper limit for RLI, which is unity  .
Figure 3. Different marine sands vs. their mean values of radium equivalent (Bq・kg−1). n = sample numbers.
Figure 4. Different marine sand deposits vs. mean (bar) and maximum (line) values of representative level index (Bq・kg−1).
3.4. Absorbed Gamma Dose Rate (DR), and Annual Effective Dose Rate (HR)
The absorbed dose rates in indoor air (DR) and the corresponding annual effective doses (HR) attributed to gamma-ray emission from the radionuclides (226Ra, 232Th and 40K) in building materials were evaluated using data and formula provided by UNSCEAR (2000)  and the EC (1999)  . In the UNSCEAR and EC reports, the dose conversion coefficients were calculated for the center of a standard room with the dimension of 4 m * 5 m * 2.8 m. The thickness of the walls, floors, ceiling and the density of the structure are 20 cm and 2350 kg/m3 (concrete), respectively. The resulting dose coefficients were found to be 0.92 nGy/h per Bq/kg for 226Ra, 1.1 nGy/h per Bq/kg for 232Th, and 0.080 nGy/h per Bq/kg for 40K:
DR (nGy/h) = 0.92CRa + 1.1CTh + 0.080CK (4)
where CRa, CTh and CK are the activity concentrations of 226Ra, 232Th and 40K, respectively.
To estimate the annual effective dose rates, it is necessary to use the conversion coefficient from the absorbed dose in air to the effective dose (0.7 Sv/Gy) and the outdoor occupancy factor (0.2) proposed by UNSCEAR (2000). Therefore, the effective dose rate is determined as follows:
Outdoor (mSv/y) = DR (nGY/h) * 24 h * 365.25 d * 0.2 (outdoor occupancy factor) * 0.7Sv・Gy−1 (Conversion factor) * 10−6
HR = DR * 8766 * 0.2 * 0.7 * 10−6 = DR * 0.00123 (5)
where DR is given by Equation (4).
The estimated results of DR and HR for all the studied marine sands range from 31.4 nGy・h−1 (Taiwan Shoal) to 327.1 nGy・h−1 (North Yellow Sea) and from 0.04 mSv・y−1 (Taiwan Shoal) to 0.20 mSv・y−1 (North Yellow Sea), respectively (Figure 5). And the estimated mean value of DR in all of studied samples is 107.9 nGy・h−1, which is little bit higher than world average indoor absorbed gamma dose rate of 84 nGy・h−1  . Additionally, the estimated mean value of the annual effective dose rate of 0.13 mSv・y−1 is also higher than the world average value (0.07 mSv・y−1,  ).
3.5. Alpha Index (Ia)
The alpha index was developed as an assessment of the excess alpha radiation exposure caused by inhalation originating from building materials. The alpha index (Ia) is determined by the following formula  :
Figure 5. Different marine sand deposits vs. mean (bar) and maximum (line) values of absorbed dose rate (DR, nGy・h−1) and annual effective dose equivalent (HR, mSv・y−1).
Ia = CRa/200 (Bq/kg) (6)
where CRa is the 226Ra activity concentration (Bq・kg−1) in the building materials. The recommended exemption level and recommended upper level for the 226Ra activity concentration in building materials as are 100 Bq・kg−1 and 200 Bq・kg−1, respectively, as suggested by the Radiation Protection Authorities in Denmark, Finland, Iceland, Norway and Sweden and the upper level is in agreement with the action level given by the ICRP in Publication 65 (1994) and by the European Commission (1990)  . It was observed that the values of the alpha index in the studied marine sand samples are below the recommended unity (Figure 6(a)).
3.6. Gamma Index (Ig)
Another radiation hazard index, the gamma activity concentration index, Ig, has been defined by the European Commission (1990)  and Righi and Bruzzi (2006)  , which is given as:
Ig = CRa/300 + CTh/200 + CK/3000 (7)
The index Ig is corrected with the annual dose rate attributed to excess external gamma radiation caused by superficial material. Values of Ig ≤ 2 correspond to the dose rate criterion of 0.3 mSV・y−1, whereas 2 < Ig ≤ 6 correspond to a criterion of 1 mSv・y−1 (  ). Therefore, the activity concentration index should be used only as a screening tool for identifying materials that might be of concern when used as construction materials; although material with Ig > 6 should be avoided, in that these values correspond to dose rates higher than 1 mSv・y−1  , which is the highest dose rate value recommended for the population  .
The gamma index Ig for the marine sands varies between 0.13 (Taiwan Shoal) and 0.62 (Taiwan Shoal) with an average of 0.45 (Figure 6(b)). Therefore, these marine sands can be exempted from all restrictions concerning radioactivity.
Figure 6. Mean (bar) and Maximum (line/data) values of alpha index (Ia) and gamma index (Ig) for different marine sand deposits.
3.7. Radiation Hazard Indices
The external radiation hazard (Hex) and the internal radiation hazard, (Hin) was developed for the additional criteria to assess the radiological suitability of a building material  . And they are defined as follows:
Hex = CRa/370 + CTh/258 + CK/4810 (8)
Hin = CRa/185 + CTh/259 + CK/4810 (9)
where CRa, CTh and CK are the activities of 226Ra, 232Th and 40K, respectively, in units of Bq・kg−1.
The determined values of Hex vary from 0.09 (Taiwan Shoal) to 1.05 (North Yellow Sea) with an average of 0.32 (Figure 7). The Hin values range between 0.11 (Taiwan Shoal) and 1.23 (North Yellow Sea) with an average of 0.37 (Figure 7). However, the highest values of Hex and Hin indices are both found in the same sample from North Yellow Sea, with highest value of 226Ra activity in all samples, which make them larger than 1 (the criterion,  ).
3.8. Excess Lifetime Cancer Risk (ELCR)
The excess lifetime cancer risk (ELCR) was determined using the following equation  :
ELCR = HR × DL × RF 10)
where HR, DL and RF are the annual effective dose equivalent, duration of life (70 years) and risk factor (0.05 Sv−1), respectively. The risk factor is defined as the fatal cancer risk per Sievert. For stochastic effects, the ICRP 60 uses a value of 0.05 for the public  .
The calculated range of ELCR is from 0.14 × 10−3 (Taiwan Shoal) to 1.41 × 10−3 (North Yellow Sea). The average ELCR values for the five marine sand
Figure 7. Mean (bar) and maximum (line) values of Hex and Hin of different marine sand deposits.
deposits are 0.54 × 10−3 (Liaodong Bay), 0.55 × 10−3 (North Yellow Sea), 0.50 × 10−3 (Zhoushan), 0.36 × 10−3 (Taiwan Shoal) and 0.20 × 10−3 (Pearl River Mouth), respectively (Figure 8). And it is very clear that most of the marine sands offshore China is slightly higher than the world average (0.29 × 10−3)  .
3.9. Annual Gonadal Dose Equivalent (AGDE)
The activity of bone marrow and bone surface cells are considered to be origins of interest by UNSCER (1988). Therefore, the annual gonadal dose equivalent (AGDE) arising from the specific activities of 226Ra, 232Th and 40K was calculated using the following formula  :
AGDE (μSv・y−1) = 3.09 CRa + 4.18 CTh + 0.314 CK (10)
The mean AGDE values for each marine sand deposit are presented in Figure 9. The values of AGDE varied from 119.0 (Taiwan Shoal) to 1225.0 mSv・y−1
Figure 8. Mean (bar) and maximum (line) values of excess lifetime cancer (*10−3) of different marine sands.
Figure 9. Mean (bar) and Maximum (line) values of annual gonadal dose equivalent (mSv・y−1) for different marine sands.
(North Yellow Sea) and the average value was found to be 409.0 mSv・y−1. The average values do not generally exceed the permissible recommended limits, indicating that the hazardous effects of the radiation are negligible. In the literature, the average AGDE value for the Eastern Desert of Egypt was found to be 2398 mSv・y−1  , for the Tamilnadu of 350.63 mSv・y−1  , for the Fırtına Valley (Turkey) of 550.5 mSv・y−1  .
3.10. Multivariate Statistical Analysis
Correlation analysis was carried out in terms of bivariate statistics to determine the mutual relations and strengths of association between pairs of variables through the calculation of the linear Pearson correlation coefficients. The results for Pearson correlation coefficients between all the studied radioactive variables of the marine sand deposits offshore China are shown in Table 3.
A high positive correlation coefficient is observed between 232Th and 226Ra (Figure 10(a)), because the radium and thorium decay series occur together in nature  . In contrast, a very weak negative correlation coefficient was observed between these two nuclides and 40K (Figure 10(b) and Figure 10(c)), because 40K is from different origin  . In addition, all radioactive variables have strong positive correlation coefficients with 226Ra and 232Th, while they are weakly negatively correlated with 40K. All radioactive variables calculated are positively correlated with one another (Table 3).
Principal component analysis was performed on the whole data set (13 variables) to assess the relations between them. The rotated factor analysis was carried out via varimax rotation with Kaiser normalization. The rotated factor 1 and factor 2 values are shown in Table 4. Two principal components were yielded with eigenvalues > 1, explaining 98.26% of the total variance. From the
Table 3. Pearson correlation matrix among the variables.
Figure 10. Correlation scatter plots between the radionuclide concentrations of 226Ra, 232Th and 40K. A, 40K vs. 226Ra; B, 40K vs. 232Th; C, 226Ra vs. 232Th.
Table 4. Rotated factor loading of the variables.
rotation space of component 1 and the component 2 (Figure 11), the first component accounts for 88.22% of the total variance and is mainly characterized by high positive loadings of concentration of 40K, 232Th and most of the radioactive variables. While the second component accounts for 10.41% of the total variance and is mainly corresponds to positive loading of 226Ra and Ia. From the overall component analysis, it can be deduced that 40K and 232Th dominantly increase the radioactivity in the entire marine sand deposits offshore China.
The natural radionuclide content, radium equivalent activity (Raeq), indoor gamma absorbed dose rate (DR), annual effective dose (HR), alpha index (Ia), gamma index (Ig), external radiation hazard level index (RLI), excess lifetime cancer risk (ELCR) and annual gonadal dose equivalent (AGDE) of five marine sand deposits offshore China were determined. The values obtained in the present study are mostly within the recommended safety limits in spite of just one sample from North Yellow Sea, demonstrating that these marine sands will not pose any significant radiation hazard; thus, the use of these marine sands even in the construction of buildings can be considered safe for the human being. From the statistical analysis, the marine sands in the northern part of offshore China (Liaodong Bay, North Yellow Sea) have 226Ra, 232Th and 40K concentrations that are higher than those from southern part of offshore China (Zhoushan, Taiwan Shoal and Pearl River Mouth). In addition, 40K and 232Th are primarily responsible for radioactivity levels of marine sands offshore China.
Figure 11. Graphical representations of component 1 and Component 2 (a) and factor loadings for each sample from different marine sands (b).
This research was financially supported by the Natural Science Foundation of China (No. 41476052, 40976036, 41376096).